Partial List of Abstracts, 1999

Anderson, D.E., Striegl, R.G., Stannard, D.I., Michmerhuizen, C.M., McConnaughey, T.A., and LaBaugh, J.W., 1999, Estimating lake atmosphere CO₂ exchange: Limnology and Oceanography, v. 44, n. 4, p. 988-1001.

Lake-atmosphere CO₂ flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake-water CO₂ storage and with flux predictions from boundary-layer and surface-renewal models. Over a 3-yr period, lake-atmosphere exchanges of CO₂ were measured over 5 weeks in spring, summer, and fall. Observed springtime CO₂ efflux was large (2.3-2.7 mmol m^-2s^-1) immediately after lake-thaw. That efflux decreased exponentially with time to less than 0.2 mmol m^-2s^-1 within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO₂ concentrations, lake CO₂ storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (-0.17 mmol m^-2s^-1) to the lake's surface, while late fall flux was trendless and smaller (-0.0021 mmol m^-2s^-1). Large springtime efflux afforded an opportunity to make direct measurement of lake-atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air-water gas-transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake-water storage change in CO₂, agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary-layer and surface-renewal models. Comparison of measured and modeled transfer velocities for CO₂, along with measured and modeled cumulative CO₂ flux, indicates that in most instances the surface-renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary-layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.

Amelung, Falk, Galloway, D.L., Bell, J.W., Zebker, H.A., and Laczniak, R.J., 1999, Sensing the ups and downs of Las Vegas: InSAR reveals structural control of land subsidence and aquifer-system deformation: Geology, v. 27, p. 483-486.

Land subsidence in Las Vegas, Nevada, United States, between April 1992 and December 1997 was measured using spaceborne interferometric synthetic aperture radar. The detailed deformation maps clearly show that the spatial extent of subsidence is controlled by geologic structures (faults) and sediment composition (clay thickness). The maximum detected subsidence during the 5.75 yr period is 19 cm. Comparison with leveling data indicates that the subsidence rates declined during the past decade as a result of rising ground-water levels brought about by a net reduction in ground-water extraction. Temporal analysis also detects seasonal subsidence and uplift patterns, which provide information about the elastic and inelastic properties of the aquifer system and their spatial variability.

Anderman, E.R., and Hill, M.C., 1999, A new multi-stage ground-water transport inverse method, Presentation, evaluation, and implications: Water Resources Research, v. 35, no. 4, p. 1053-1063.

More computationally efficient methods of using concentration data are needed to estimate groundwater flow and transport parameters. This work introduces and evaluates a three-stage nonlinear-regression-based iterative procedure in which trial advective-front locations link decoupled flow and transport models. Method accuracy and efficiency are evaluated by comparing results to those obtained when flow- and transport- model parameters are estimated simultaneously. The new method is evaluated as conclusively as possible by using a simple test case that includes distinct flow and transport parameters, but does not include any approximations that are problem dependent. The test case is analytical; the only flow parameter is a constant velocity, and the transport parameters are longitudinal and transverse dispersivity. Any difficulties detected using the new method in this ideal situation are likely to be exacerbated in practical problems. Monte-Carlo analysis of observation error ensures that no specific error realization obscures the results. Results indicate that, while this, and probably other, multistage methods do not always produce optimal parameter estimates, the computational advantage may make them useful in some circumstances, perhaps as a precursor to using a simultaneous method.

Andrews, E.D., Johnston, C.E., Schmidt, J.C., and Gonzales, M., 1999, Topographic evolution of sand bars, in Webb, R.H., Schmidt, J.C., 1999, topographic evolution of sand bars in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., The controlled flood in the Grand Canyon: American Geophysical Union Monograph 110, p. 117-130.

Sand bars deposited in lateral separation eddies are an essential biological and recreational resource of the Colorado River downstream from Glen Canyon Dam. Since 1986, however, sustained discharges substantially in excess of the power-plant capacity have not occurred, and approximately half of the sand bars that existed in 1986 have been degraded by erosion and encroachment of vegetation. A primary purpose of the 1996 controlled flood release from Glen Canyon Dam was to measure the rate of sand deposition and erosion as well as to observe the adjustment of sand bar topography during a period of sustained high flow. Repeated, detailed bathymetric surveys of 5 eddies were made before, during, and after a flood of 1275 m³/s for 7 days to determine the topographic evolution of sand bars. Two of the eddies are located upstream of the Little Colorado River, the primary source of sand to the Colorado River through Grand Canyon, and 3 eddies are located downstream of the Little Colorado River. The topography of sand bars in all 5 eddies adjusted rapidly during the first several hours of the flood. Sand bars aggraded and degraded by as much as 3.5 m within less than 24 hrs. The general pattern of deposition and erosion observed during the first day persisted to varying degrees throughout the flood, even though a few to several thousand cubic meters of sand were eroded and deposited from one day to the next. The area of sand exposed above the 565 m³/s for 7 water-surface elevation and available for camping, however, increased in all the eddies studied. Subaqueous mass failures of over steepened portions of the sand bar appeared to occur in all eddies.

Bargar, J.R., Reitmeyer, R., and Davis, J.A., 1999, Spectroscopic confirmation of uranium (VI)-carbonato adsorption complexes on hematite: Environmental Science and Technology, v. 33, p. 2481-2484.

Evaluating societal risks posed by uranium contamination from wasted management facilities, mining sites, and heavy industry requires knowledge about uranium transport in groundwater, often the most significant pathway of exposure to humans. It has been proposed that uranium mobility in aquifers may be controlled by adsorption of U(VI)- carbonato complexes on oxide minerals. The existence of such complexes has not been demonstrated, and little is known about their compositions and reaction stoichiometries. We have used attenuated total reflectance Fourier transform infrared (ATR-FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopies to probe the existence, structures and compositions of ºFeOsurface- U(VI)-carbonato complexes on hematite throughout the pH range of uranyl uptake under conditions relevant to aquifers. U(VI)-carbonato complexes were found to be the predominant adsorbed U(VI) species at all pH values examined, a much wider pH range than previously postulated based on analogy to aqueous U(VI)-carbonato complexes, which are trace constituents at pH < 6. This result indicates the inadequacy of the common modeling assumption that the compositions and predominance of adsorbed species can be inferred from aqueous species. By extension, adsorbed carbonato complexes may be of major importance to the groundwater transport of similar actinide contaminants such as neptunium and plutonium.

The computer program WTAQ calculates hydraulic-head drawdowns in a confined or water-table aquifer that result from pumping at a well of finite of infinitesimal diameter. The program is based on an analytical model of axial-symmetric ground-water flow in a homogenous and anisotropic aquifer. The program allows for well-bore storage and well-bore skin at the pumped well and for delayed drawdown response at an observation well; by including these factors, it is possible to accurately evaluate the specific storage of a water-table aquifer from early-rime drawdown data in observation wells and piezometers. For water-table aquifers, the program allows for either delayed or instantaneous drainage from the unsaturated zone. WTAQ calculates dimensionless or dimensional theoretical drawdowns that can be used with measured drawdowns at observation points to estimate the hydraulic properties of confined and water-table aquifers. Three sample problems illustrate use of WTAQ for estimating horizontal and vertical hydraulic conductivity, specific storage, and specific yield of a water-table aquifer by type-curve methods and by and automatic parameter-estimation method.

An important gap in the misunderstanding of the hydrology of the Middle Rio Grand Basin, central New Mexico, is the rate at which water from the Rio Grande recharges the Santa Fe Group aquifer system. Several methodologies - including use of the Glover-Balmer equation, food pulses, and channel permeameters - have been applied to this problem in the Middle Rio Grande Basin. In the work presented here, ground-water temperature profiles and ground-water levels beneath the Rio Grande were measured and numerically simulated at four sites. The direction and rate of vertical ground-water flux between the river and underlying aquifer was simulated and the effective vertical hydraulic conductivity of the sediments underlying the river was estimated through model calibration.

Seven sets of nested piezometers were installed during July and August 1996 at four sites along the Rio Grande in the Albuquerque area, though only four of the piezometer nests were simulated. In downstream order, these four sites are (1) the Bernalillo site, upstream from the New Mexico State Highway 44 bridge in Bernanlillo (piezometer nest BRN02); (2) the Corrales site, upstream from the Rio Rancho sewage treatment plant in Rio Rancho (COR01); (3) the Paseo del Norte site, upstream from the Paseo del Norte bridge in Albuquerque (PDN01); and (4) the Rio Bravo site, upstream from the Rio Bravo bridge in Albuquerque (RDR01). All piezometers were completed in the inner-valley alluvium of the Santa Fe Group aquifer system. Ground-water levels and temperatures were measured in the four piezometer nests a total of seven times in the 24-month period from September 1996 through August 1998.

The flux between the surface- and ground-water systems at each of the field sites was quantified by one-dimensional numerical simulation of the water and heat exchange in the subsurface using the heat and water transport model VS2DH. Model calibration was aided by the use of PEST, a model independent computer program that uses nonlinear parameter estimation.

Mean vertical hydraulic conductivities were estimated by model calibration and range from 1.5 x 10^-5 to 5.8 x 10^-6 meters per second (m/s). Mean simulated vertical ground-water flux for the BRN02 piezometer nest is 3.30 x 10^-7 m/s; for the COR01 piezometer nest is 3.58 x 10^-7 m/s; for the PDN01 piezometer nest is 4.22 x 10^-7 m/s; and for the RBR01 piezometer nest is 2.05 x 10^-7 m/s. Comparison of the simulated vertical fluxes and vertical hydraulic conductivities derived from this study with values from other studies in the Middle Rio Grande Basin indicate agreement between 1 and 3.5 orders of magnitude for hydraulic conductivity and within 1 order of magnitude for vertical flux.

Bekins, B.A., Baehr, A.L., Cozzarelli, I.M., Essaid, H.I., Haack, S.K., Harvey, R.W., Shaprio, A.M., Smith, J.A., Smith, R.L., 1999, Capabilities and challenges of natural attenuation in the subsurface: Lessons from the U.S. Geological Survey Toxics Substances Hydrology Program, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - A Proceeding of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3--Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 37-56

Bekins, B.A., Cozzarelli, I.M., Godsy, E.M., Warren, Ean, Tuccillo, M.E., Essaid, H.I., and Paganelli, V.V., 1999, Chemical and physical controls on microbial populations in the Bemidji Toxics Site crude-oil plume, in Morganwalp, D.W. and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3--Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 133-142.

Bekins, B.A., Godsy, E.M, and Warren, E., 1999, Distribution of microbial physiologic types in an aquifer contaminated by crude oil: Microbial Ecology, v. 36, p. 263-275.

High-resolution (decadal) records of climate change from the Owens, Mono, and Pyramid Lake basins of California and Nevada indicate that millennial-scale oscillations in climate of the Great Basin occurred between 52.6 and 9.2 14C ka. Climate records from the Owens and Pyramid Lake basins indicate that most, but not all, glacier advances (stades) between 52.6 and ~15.0 14C (~60.0 to ~14.0 14C ka), stadial/interstadial oscillations were recorded in Owens and Pyramid Lake sediments by the negative response of phytoplankton productivity to the influx of glacially derived silicates. During glacier advances, rock flour diluted the TOC fraction of lake sediments and introduction of glacially derived suspended sediment also increased the turbidity of lake water, decreasing light penetration and photosynthetic production of organic carbon. It is not possible to correlate objectively peaks in the Owens and Pyramid Lake TOC records (interstades) with Dansgaard-Oeschger interstades in the GISP2 ice-core d¹⁸O record given uncertainties in age control and difference in the shapes of the OL90, PLC92, and GISP2 records. In the North Atlantic region, some climate records have clearly defined variability/cyclicity with periodicities of 102 to 103 yr; these records are correlatable over several thousand km. In the Great Basin, climate proxies also have clearly defined variability with similar time constants, but the distance over which this variability can be correlated remains unknown. Globally, there may be minimal spatial scales (domains) within which climate varies coherently on centennial and millennial scales, but it is likely that the sizes of these domains vary with geographic setting and time. A more comprehensive understanding of the mechanisms of climate forcing and the physical linkages between climate forcing and system response is needed in order to predict the spatial scale(s) over which climate varies coherently.

The d¹³C values of individual trihalomethanes (THM) formed on reaction of chlorine with dissolved organic carbon (DOC) leached from maize (corn; Zea maize L.) and Scirpus acutus (an aquatic bulrush), and with DOC extracted from agricultural drainage waters were determined using purge and trap introduction into a gas chromatograph-combustion-isotope ratio monitoring mass spectrometer. We observed a 16.8^0/00 difference between the d¹³C values of THM produced from the maize and Scirpus leachates, similar to the isotopic difference between the whole plant materials. Both maize and Scirpus formed THM 12^0/00 lower in ¹³C than whole plant material. We suggest that the low value of the THM relative to the whole plant material is evidence of distinct pools of THM-forming DOC, representing different biochemical types or chemical structures, and possessing different environmental reactivity.

Humic extracts of waters draining an agricultural field containing Scirpus peat soils and planted with maize formed THM with isotopic values intermediate between those of maize and Scirpus leachates, indicating maize may contribute significantly to the THM-forming DOC. The difference between the d¹³C values of the whole isolate and that of the THM it yielded was 3.9^0/00, however, suggesting diagenesis plays a role in determining the d¹³C value of THM-forming DOC in the drainage waters, and precluding the direct use of isotopic mixing models to quantitatively attribute sources.

Technical and practical aspects of applying geostatistics are developed for individuals involved in investigations at hazardous-, toxic-, and radioactive-waste sites. Important geostatistical concepts, such as variograms and ordinary, universal, and indicator kriging, are described in general terms for introductory purposes and in more detail for practical applications. Variogram modeling using measured ground-water elevation data is described in detail to illustrate principles of stationarity, anisotropy, transformations, and cross validation. Several examples of kriging applications are described using ground-water-level elevations, bedrock elevations, and ground-water-quality data. A review of contemporary literature and selected public domain software associated with geo statistics also is provided, as is a discussion of alternative methods for spatial modeling, including inverse distance weighting, triangulation, splines, trend-surface analysis, and simulation.

The issues of development of alpine areas and the possible influences of climate change in the Rocky Mountain region have both increased the focus of understanding processes controlling the water quality of mountain streams. This report presents 10 years of water quality and stream flow data from two headwater basins in Summit County, Colorado. The upper reach of the Snake River is acidic and metal-enriched from the natural and anthropogenic weathering of pyrite in the watershed, whereas the water quality of Deer Creek is pristine with a circumneutral pH. The Snake River and Deer Creek watersheds have been sites of extensive research for the past 15 years, and the data in this report have been used in these interpretative studies. The data sets are one of the longest water quality records for streams in the Upper Colorado Rivers basin and provide a description of how water quality has varied with differences between years in snowpack and other climatic parameters.

Dissolved organic carbon (DOC) from terrestrial sources forms the major component of the annual carbon budget in many headwater streams. In high-elevation catchments in the Rocky Mountains, DOC originates in the upper soil horizons and is flushed to the stream primarily during spring snowmelt. To identify controls on the size of the mobile soil DOC pool available to be transported during the annual melt event, we measured soil DOC production across a range of vegetation communities and soul types together with catchment DOC export in paired watersheds in Summit County, Colorado. Both surface water DOC concentrations and watershed DOC export were lower in areas where pyrite weathering resulted in lower soil pH. Similarly, the amount of DOC leached from organic soils was significantly smaller (p<0.01) at sites having low soil pH. Scaling point source measurements of DOC production and leaching to the two basins and assuming only vegetated areas contribute to DOC production, we calculated that the amount of mobile DOC available to be leached to surface water during melt was 20.3 g C m^-2 in the circumneutral basin and 17.8 g C m^-2 in the catchment characterized by pyrite weathering. The significant (r² = 0.91 and p < 0.05), linear relationship between over-winter CO2 flux and the amount of DOC leached from upper soil horizons during snowmelt suggests that the mechanism for the difference in production of mobile DOC was heterotrophic processing of soil carbon in snow-covered soil. Furthermore, this strong relationship between over-winter heterotrophic activity and the size of the mobile DOC pool present in a range of soil and vegetation types provides a likely mechanism for explaining the interannual variability of DOC export observed in high-elevation catchments.

Seven nested headwater catchments (8 to 161 ha) were monitored during five summer rain events to evaluate storm runoff components and the effect of catchment size on water sources. Two-component isotopic hydrograph separation showed that event-water contributions near the time of peakflow ranged from 49% to 62% in the 7 catchments during the highest intensity event. The proportion of event water in stormflow was greater than could be accounted for by direct precipitation onto saturated areas. DOC concentrations in stormflow were strongly correlated with stream ¹⁸O composition. Bivariate mixing diagrams indicated that the large event water contributions were likely derived from flow through the soil O-horizon. Results from two-tracer, three-component hydrograph separations showed that the throughfall and O-horizon soil-water components together could account for the estimated contributions of event water to stormflow. End-member mixing analysis confirmed these results. Estimated event-water contributions were inversely related to catchment size, but the relation was significant for only the event with greatest rainfall intensity. Our results suggest that perched, shallow subsurface flow provides a substantial contribution to summer stormflow in these small catchments, but the relative contribution of this component decreases with catchment size.

Brown, G.K., MacCarthy, P., and Leenheer, J.A., 1999, Simultaneous determination of Ca, Cu, Ni, Zn, and Cd binding strengths with fulvic acid fractions by Schubert's method, Analytica Chimica Acta, v. 402, 169-181.

The equilibrium binding of Ca²⁺, Ni²⁺, Cd²⁺, Cu²⁺ and Zn²⁺ with unfractionated Suwannee river fulvic acid (SRFA) and an enhanced metal binding subfraction of SRFA was measured using Schubert's ion-exchange method at pH 6.0 and at an ionic strength (m) of 0.1 (NaNO₃). The fractionation and subfractionation were directed towards obtaining an isolate with an elevated metal binding capacity or binding strength as estimated by Cu²⁺ potentiometry (ISE). Fractions were obtained by stepwise eluting an XAD-8 column loaded with SRFA with water eluents of pH 1.0 to pH 12.0. Subfractions were obtained by loading the fraction eluted from XAD-8 at pH 5.0 onto a silica gel column and eluting with solvents of increasing polarity. Schuberts ion exchange method was rigorously tested by measuring simultaneously the conditional stability constants (K) of citric acid complexed with the five metals at pH 3.5 and 6.0. The log K of SRFA with Ca2+, Ni2+, Cd2+, Cu2+ and Zn2+ determined simultaneously at pH 6.0 follow the sequence of Cu²⁺ > Cd²⁺ > Ni²⁺ > Zn²⁺ > Ca²⁺ while all log K values increased for the enhanced metal binding subfraction and followed a different sequence of Cu²⁺ > Cd²⁺ > Ca²⁺ > Ni²⁺ > Zn²⁺. Both fulvic acid samples and citric acid exhibited a 1:1 metal to ligand stochiometry under the relatively low metal loading conditions used here. Quantitative ¹³C nuclear magnetic resonance spectroscopy showed increases in aromaticity and ketone content and decreases in aliphatic carbon for the elevated metal binding fraction while the carboxyl carbon, and elemental nitrogen, phosphorus, and sulfur content did not change. The more polar, elevated metal binding fraction did show a significant increase in molecular weight over the unfractionated SRFA.

Sr isotopes can be used in granitoid catchment studies to partition base cautions to mineral weathering and atmospheric sources it the ⁸⁷Sr/⁸⁶Sr ratio provided by each source can be quantified. Although the ⁸⁷Sr/⁸⁶Sr ratio of the atmospheric component is relatively easy to quantify, the ⁸⁷Sr/⁸⁶Sr ration of the weathering component is more difficult to quantify due mainly to incongruent weathering. To determine the ⁸⁷Sr/⁸⁶Sr ratio of the weathering component, we propose a graphical method in which the array of Nb/Sr and ⁸⁷Sr/⁸⁶Sr ratios of weathering residues in a rock or soil profile is regressed to the Nb-free component. We apply the technique to weathering profiles at a catchment in the Georgia (USA) Piedmont, and show that the calculated ⁸⁷Sr/⁸⁶Sr ratio of the weathering component is that of plagiocase in the bedrock weathering environment, and on average that of stream baseflow in the saprolite and colluvium weathering environments.

From July 1994 through May 1997, the U.S. Geological Survey in cooperation with the Department of Energy, sampled 86 wells completed in the Snake River Plain aquifer at and near the Idaho National Engineering and Environmental Laboratory (INEEL). The wells were sampled for a variety of constituents including one- and two-carbon halocarbons. Concentrations of dichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11), and trichlorotrifluoromethane (CFC-113) were determined. The samples for halocarbon analysis were collected in 62-milliliter flame sealed borosilicate glass ampoules in the field. The data will be used to evaluate the ages of ground waters at INEEL. The ages of the ground water will be used to determine recharge rates, residence time, and travel time of water in the Snake River Plain aquifer in and near INEEL. The chromatograms of 139 ground waters are presented showing a large number of halomethanes, haloethanes, and haloethenes present in the ground waters underlying the INEEL. The chromatograms can be used to qualitatively evaluate a large number of contaminants at parts per trillion to parts per billion concentrations. The data can be used to study temporal and spatial distribution of contaminants in the Snake River Plain aquifer. Representative compressed chromatograms for all ground waters sampled in this study are available on two 3.5-inch high density computer disks. The data and the program required to decompress the data can be obtained from the U.S. Geological Survey office at Idaho Falls, Idaho. Sulfur hexafluoride (SF6) concentrations were measured in selected wells to determine the feasibility of using this environmental tracer as an age dating tool of ground water. Concentrations of dissolved nitrogen, argon, carbon dioxide, oxygen, and methane were measured in 79 ground waters. Concentrations of dissolved permanent gases are tabulated and will be used to evaluate the temperature of recharge of ground water in and near the INEEL.

Metal concentrations of the soluble fraction of the cytoplasm (cytosol) and the whole body were determined in the caddisfly Hydropsyche spp. (Trichoptera). Metal accumulation in the cytosol and the whole body were compared in samples collected along 380 kms of a contamination gradient in the Clark Fork river in four consecutive years (1992-1995), and from a contaminated tributary (Flint Creek). Samples from the contaminated sites were compared to an uncontaminated tributary (Blackfoot River). Relations between cytosolic metal concentration and cytosolic protein (used as a general biomarker of protein metabolism) also were examined in 1994 and 1995. Relative to whole body concentrations, cytosolic metal concentrations varied among metals and years. Spatial patterns in whole body and cytosolic Cd, Cu and Pb concentrations were qualitatively similar each year, and these concentrations generally corresponded to contamination levels measured in bed sediments. The proportions of metals recovered in the cytosol of ranged from 12 to 64% for Cd and Cu and from 2 to 38% for Pb. Zinc in the whole body also was consistent with contamination levels, but cytosolic Zn concentrations increased only at the highest whole body Zn concentrations. As a result, the proportion of Zn recovered in the cytosol ranged from 16 to 63% and tended to be inversely related to whole body Zn concentrations. The proportions of cytosolic metals varied significantly among years and, as a result, interannual differences in metal concentrations were greater in the cytosol than in the whole body. The results demonstrated that Hydropsyche in the river were chronically exposed to biologically available metals. Some features of this exposure were not evident from whole body concentrations. In general, protein levels did not correspond to cytosolic metal concentrations. A variety of environmental factors could interact with metal exposures to produce complex responses in protein metabolism. Systematic study will be necessary to differentiate the effects of multiple environmental stressors on organisms living in contaminated ecosystems.

Campbell Rehmann, L.L., Welty, C., and Harvey, R.W., 1999, Stochastic analysis of virus transport in aquifers: Water Resources Research, v. 35, p. 1987-2006.

A large-scale model of virus transport in aquifers is derived using spectral perturbation analysis. The effects of spatial variability in aquifer hydraulic conductivity and virus transport (attachment, detachment, and inactivation) parameters on large-scale virus transport are evaluated. A stochastic mean model of virus transport is developed by linking a simple system of local-scale free-virus transport and attached-virus conservation equations from the current literature with a random-field representation of aquifer and virus transport properties. The resultant mean equations for free and attached viruses are found to differ considerably from the local-scale equations on which they are based and include effects such as a free-virus effective velocity that is a function of aquifer heterogeneity as well as virus transport parameters. Stochastic mean free-virus breakthrough curves are compared with local model output in order to observe the effects of spatial variability on mean one-dimensional virus transport in three-dimensionally heterogeneous porous media. Significant findings from this theoretical analysis include the following: (1) Stochastic model breakthrough occurs earlier than local model breakthrough, and this effect is most pronounced for the least conductive aquifers studied. (2) A high degree of aquifer heterogeneity can lead to virus breakthrough actually preceding that of a conservative tracer. (3) As the mean hydraulic conductivity is increased, the mean model shows less sensitivity to the variance of the natural-logarithm hydraulic conductivity and mean virus diameter. (4) Incorporation of a heterogeneous colloid filtration term results in higher predicted concentrations than a simple first-order adsorption term for a given mean attachment rate. (5) Incorporation of aquifer heterogeneity leads to a greater range of virus diameters for which significant breakthrough occurs. (6) The mean model is more sensitive to the inactivation rate of viruses associated with solid surfaces than to the inactivation rate of viruses in solution.

Carmody, R.W., and Seal II, R.R., 1999, Evaluation of that sulfur isotopic composition and homogeneity of the Soufre de Lacq reference material: Chemical Geology, v. 153, p. 289-295.

Sulfur isotopic analysis of the elemental sulfur reference material Soufre de Lacq, prepared as silver sulfide by chromous chloride reduction and as copper sulfide by sealed-tube synthesis, indicates that Soufre de Lacq is isotopically homogeneous across different size fractions to within analytical uncertainty (±0.15 parts per mil). The sulfur isotopic composition of aliquots of Soufre de Lacq prepared by these two techniques are identical to within analytical uncertainty. The mean sulfur isotopic composition for Soufre de Lacq prepared as silver sulfide and copper sulfide (relative to VCDT) is +16.20 ± 0.15 parts per mil (1sigma ).

Carter, V., Rybicki, N.B., Landwehr, J.M., Reel, J.T., and Ruhl, H.A., 1999, Summary of correlations among seasonal water quality, discharge, weather, and coverage by submersed aquatic vegetation in the tidal Potomac River and Potomac Estuary, 1983-96: U.S. Geological Survey Open-File Report 98-657, 81 p.

Carter, V., Ruhl, H.A., Rybicki, N.B., Reel, J.T., and Gammon, Patricia T., 1999, Vegetative resistance to flow in south Florida: Summary of vegetation sampling at sites NESRS3 and P33, Shark River Slough, April, 1996: U.S. Geological Survey Open-File Report 99-187, 73 p.

Carter, V., Ruhl, H.A., Rybicki, N.B., Reel, J.,T., and Gammon, Patricia T., 1999, Vegetative resistance to flow in south Florida: Summary of vegetation sampling at sites NESRS3 and P33, Shark River Slough, November, 1996: U.S. Geological Survey Open-File Report 99-218, 89 p.

Cayan, D.R., Redmond, K.T, and Riddle, L.G., 1999, ENSO and hydrologic extremes in the Western United States: J. Clim., v. 12, p. 2881-2893.

Frequency distributions of daily precipitation in winter and daily stream flow from late winter to early summer, at several hundred sites in the western United States, exhibit strong and systematic responses to the two phases of ENSO. Most of the stream flows considered are driven by snowmelt. The Southern Oscillation index (SOI) is used as the ENSO phase indicator. Both modest (median) and larger (90th percentile) events were considered. In years with negative SOI values (El Niño), days with high daily precipitation and stream flow are more frequent than average over the Southwest and less frequent over the Northwest. During years with positive SOI values (La Niña), a nearly opposite pattern is seen. A more pronounced increase is seen in the number of days exceeding climatological 90th percentile values than in the number exceeding climatological 50th percentile values, for both precipitation and stream flow. Stream flow responses to ENSO extremes are accentuated over precipitation responses. Evidence suggests that the mechanism for this amplification involves ENSO-phase differences in the persistence and duration of wet episodes, affecting the efficiency of the process by which precipitation is converted to runoff. The SOI leads the precipitation events by several months, and hydrologic lags (mostly through snowmelt) delay the stream flow response by several more months. The combined 6-12-month predictive aspect of this relationship should be of significant benefit in responding to flood (or drought) risk and in improving overall water management in the western states.

Lava Falls Rapid, which was created and is maintained by debris flows from Prospect Canyon, is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Debris flows enter the Colorado River at tributary junctures, creating rapids. The frequency of debris flows is an important consideration when management of regulated rivers involves maintenance of channel morphology. We used cosmogonic ³He, ¹⁴C, and historical photographs to date 12 late Holocene and historic debris flows from Prospect Canyon. The highest and oldest deposits from debris flows on the debris fan yielded a ³He date of about 3 ka, which indicates predominately late Holocene aggradation of one of the largest debris fans in Grand Canyon. The deposit, which has a 25-m escarpment caused by river reworking, crossed the Colorado River and raised its base level by 30 m for an indeterminate although likely short period. We mapped depositional surfaces of 11 debris flows that occurred after 3 ka. Two deposits inset against the highest deposit yielded ³He ages of about 2.2 ka, and at least two others followed shortly afterwards. At least one of these debris flows also dammed the Colorado River. The most recent prehistoric debris flow occurred no more than 0.5 ka. The largest historic debris flow, which constricted the river by 80%, occurred in 1939. Five other debris flows occurred after 1939; these debris flows constricted the Colorado River by 35-80%. Assuming the depositional volumes of late Holocene debris flows can be modeled using a lognormal distribution, we calculated recurrence intervals of 15 to more than 2000 years for debris flows from Prospect Canyon.

Recently, methods have been developed to analyze NO₃^- for d¹⁵N and d¹⁸O, improving our ability to identify NO₃^- sources and transformations. However, none of the existing methods are suited for waters with low NO₃^- concentrations (0.7-10 mM). We describe an improved method for collecting and recovering NO₃^- on exchange columns. To overcome the lengthy collection loading times imposed by the large sample volumes (7-70 L), the sample was prefiltered (0.45 mm) with a large surface area filter. Switching to AG2X anion resin and using a coarser mesh size (100-200) than previous methods also enhanced sample flow. Placement of a cation column in front of the anion column minimized clogging of the anion column by dissolved organic carbon (DOC) accumulation. This also served to minimize transfer of unwanted oxygen atoms from DOC to the ¹⁸O portion of the NO₃^- sample, thereby contaminating the sample and shifting d¹⁸O. The cat-AG2X method is suited for on-site sample collection, making it possible to collect and recover NO₃^- from low ionic strength waters with modest DOC concentrations (80-800 mM), relieves the investigator of transporting large volumes of water back to the laboratory, and offers a means of sampling rain, snow, snowmelt, and stream samples from access-limited sites.

We tested the accuracy of 95% individual prediction intervals for hydraulic heads, streamflow gains, and effective transmissivities computed by groundwater models of two Danish aquifers. To compute the intervals, we assumed that each predicted value can be written as the sum of a computed dependent variable and a random error. Testing was accomplished by using a cross-validation method and by using new field measurements of hydraulic heads and transmissivities that were not used to develop or calibrate the models. The tested null hypotheses are that the coverage probability of the prediction intervals is not significantly smaller than the assumed probability (95%) and that each tail probability is not significantly different from the assumed probability (23%). In all cases tested, these hypotheses were accepted at the 5% level of significance. We therefore conclude that for the groundwater models of two real aquifers the individual prediction intervals appear to be accurate.

The fact that dependent variables of groundwater models are generally nonlinear functions of model parameters is shown to be a potentially significant factor in calculating accurate confidence intervals for both model parameters and functions of the parameters, such as the values of dependent variables calculated by the model. The Lagrangian method of Vecchia and Cooley [Vecchia, A.V., and Cooley, R.L., 1987, Water Resources Research, v. 23(7), p. 1237-1250] was used to calculate nonlinear Scheffe-type confidence intervals for the parameters and the simulated heads of a steady-state groundwater flow model covering 450 km² of a leaky aquifer. The nonlinear confidence intervals are compared to corresponding linear intervals. As suggested by the significant nonlinearity of the regression model, linear confidence intervals are often not accurate. The commonly made assumption that widths of linear confidence intervals always underestimate the actual (nonlinear) widths was not correct. Results show that nonlinear effects can cause the nonlinear intervals to be asymmetric and either larger or smaller than the linear approximations. Prior information on transmissivities helps reduce the size of the confidence intervals, with the most notable effects occurring for the parameters on which there is prior information and for head values in parameter zones for which there is prior information on the parameters.

Anthropogenic nutrient enrichment of the coastal zone is now a well-established fact. However, there is still uncertainty about the mechanisms through which nutrient enrichment can disrupt biological communities and ecosystem processes in the coastal zone. For example, while some estuaries exhibit classic symptoms of acute eutrophication, including enhanced production of algal biomass, other nutrient-rich estuaries maintain low algal biomass and primary production. This implies that large differences exist among coastal ecosystems in the rates and patterns of nutrient assimilation and cycling. Part of this variability comes from differences among ecosystems in the other resource that can limit algal growth and production - the light energy required for photosynthesis. Complete understanding of the eutrophication process requires consideration of the interacting effects of light and nutrients, including the role of light availability as a regulator of the expression of eutrophication. A simple index of the relative strength of light and nutrient limitation of algal growth can be derived from models that describe growth rate as a function of these resources. This index can then be used as one diagnostic to classify the sensitivity of coastal ecosystems to the harmful effects of eutrophication. Here I illustrate the application of this diagnostic with light and nutrient measurements made in three California estuaries and two Dutch estuaries.

Periphyton samples from the Big Cypress National Preserve were analyzed for concentrations of copper, lead, zinc, mercury, and methylmercury. Concentrations of organic carbon, inorganic carbon, nitrogen, and phosphorus in periphyton samples also were determined. The samples were extracted with sodium acetate solution at a pH of 5.5 to determine exchangeable and carbonate phase metal concentrations in periphyton. Total metal concentrations in the periphyton were directly related to the degree of calcite saturation in the water column. Exchangeable and carbonate phase metal concentrations were directly related to the percent inorganic carbon in the samples. A connection between geochemistry of trace metals and calcite precipitation and dissolution is suggested.

Cozzarelli, I.M., Baedecker, M.J., Eganhouse, R.P., Tuccillo, M.E., Aiken, G.R., Bekins, B.A., and Jaeschke, J.B., 1999, Long-term geochemical evolution of the Bemidji Toxics site crude-oil plume, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3 - Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 123-132.

Cozzarelli, I.M., Herman, J.S., Baedecker, M.J., and Fischer, J.M., 1999, Geochemical heterogeneity of a gasoline-contaminated aquifer: Journal of Contaminant Hydrology, v. 40, p. 261-284.

The scale of biogeochemical reactions was studied in a physically and chemically heterogeneous surficial Coastal Plain aquifer contaminated by a gasoline spill. The physical heterogeneity of the aquifer is manifested in two hydrologic units, a shallow local aquifer of perched water and a regional sandy aquifer. Over the studied vertical interval of 21.3 ft (6.5 m), concentrations of reactive species varied by orders of magnitude, and the impact of biodegradation was expressed to widely varying degrees. A thin (3 ft thick) section of the perched-water zone was the most contaminated; total aromatic hydrocarbons were as high as 19.4 mg/l. Hydrocarbons were degraded by microbially mediated reactions that varied over short vertical distances and time. Anaerobic processes dominated within the low-permeability clay unit, whereas in the more permeable sandy layers nitrate reduction and aerobic degradation occurred. Hydrocarbons were more persistent over time in the low-permeability layer due to the limited availability of electron acceptors for degradation. The microbial degradation of hydrocarbons was linked to sulfate and iron reduction in the clay unit and led to alterations in the aquifer solids; electron microscopy revealed the presence of FeS minerals encrusting primary aquifer grains. High concentrations of Fe²⁺ in groundwater, up to 34.5 mg/l, persist in kinetic disequilibrium in the presence of elevated H₂S levels of 1.0 mg/l. Assessment of aquifer heterogeneities and groundwater contamination was possible due to sample discrimination at a scale of approximately 2 ft (similar to 0.6 m), a much finer resolution than is attempted in many remedial investigations of polluted aquifers. The information obtained in this type of study is essential to the development of models capable of estimating the fate of hydrocarbons at a site scale.

A regional-scale, steady-state, saturated-zone ground-water flow model was constructed to evaluate potential regional ground-water flow in the vicinity of Yucca Mountain, Nevada. The model was limited to three layers in an effort to evaluate the characteristics governing large-scale subsurface flow. Geoscientific information systems (GSIS) were used to characterize the complex surface and subsurface hydrogeologic conditions of the area, and this characterization was used to construct likely conceptual models of the flow system. Subsurface properties in this system vary dramatically, producing high contrasts and abrupt contacts. This characteristic, combined with the large scale of the model, make zonation the logical choice for representing the hydraulic-conductivity distribution. Different conceptual models were evaluated using sensitivity analysis and were tested by using nonlinear regression to determine parameter values that are optimal, in that they provide the best match between the measured and simulated heads and flows. The different conceptual models were judged based both on the fit achieved to measured heads and spring flows, and the plausibility of the optimal parameter values. One of the conceptual models considered appears to represent the system most realistically. Any apparent model error is probably caused by the coarse vertical and horizontal discretization.

Dean, W.E., 1999, The carbon cycle and biogeochemcial dynamics in lake sediments: Journal of Paleolimnology, v., 21, p. 375-393.

The concentrations of organic carbon (OC) and CaCO₃ in lake sediments are often inversely related. This relation occurs in surface sediments from different locations in the same lake, surface sediments from different lakes, and with depth in Holocene sediments. Where data on accumulation rates are available, the relation holds for organic carbon and CaCO₃ accumulation rates as well. An increase of several percent OC is accompanied by a decrease of several tens of percent CaCO₃ indicating that the inverse relation is not due to simple dilution of one component by another. It appears from core data that once the OC concentration in the sediments becomes greater than about 12%, the CO₂ produced by decomposition of that OC and production of organic acids lowers the pH of anoxic pore waters enough to dissolve any CaCO₃ that reaches the sediment-water interface. In a lake with a seasonally anoxic hypolimnion, processes in the water column also can produce an inverse relation between OC and CaCO₃ over time. If productivity of the lake increases, the rain rate of OC from the epilimnion increases. Biogenic removal of CO₂ and accompanying increase in pH also may increase the production of CaCO₃. However, the decomposition of organic matter in the hypolimnion will decrease the pH of the hypolimnion causing greater dissolution of CaCO₃ and therefore a decrease in the rain rate of CaCO₃ to the sediment-water interface.

A photolinear analysis of the Eldridge-Wilde well field, Pinellas County, Florida, was conducted to identify zones of fracture concentration. Photolinear fracture traces and lineaments correspond to vertical zones of fracture concentration. Forty-three photolinears, ranging in length from 269 meters to 2.53 kilometers were mapped on four aerial photographs. The dominant azimuthal orientation of the photolinears is in the range of 41 degrees to 50 degrees. The relation between well yield and well location relative to fracture trace location was investigated. Of the forty-eight production wells in the field study area for which specific yield data were available, seven are located on mapped fracture traces, one of which is on a fracture trace intersection. Boxplots of specific yields of wells "on" and "off" fracture traces show that the median productivity of the "on" group, 308 gallons per minute per foot of drawdown. However, nonparametric statistical analysis could not reject the null hypothesis that both samples were from the same statistical population. Application of this photolinear analysis to any subsequent water supply investigations would require supplementary inquiries, including field checking of the mapped photolinears and test drilling. In the course of the investigation, a list of sixteen hydrogeologic variables that can influence porosity and permeability at the Eldridge-Wilde well field was developed.

Dodge, K.A., Hornberger, M.I., and Bouse, R.M., 1999, Water quality, bed sediment and biological data (October 1997 through September 1998) and statistical summaries of data for streams in the Upper Clark Fork Basin, Montana: U.S. Geological Survey Open File Report 99-251.

Water, bed sediment, and biota were sampled in streams from Butte to below Missoula as part of a program to characterize aquatic resources in the upper Clark Fork basin of western Montana. Sampling stations were located on the Clark Fork and major tributaries. Water-quality data were obtained periodically at 15 stations during October 1997 through September 1998 (water year 1998). Data for 15 bed-sediment and 15 biological stations were obtained in August 1998. The primary constituents analyzed were trace elements associated with tailings from historical mining and smelting activities. Water-quality data include concentrations of selected major ions, trace elements, and suspended sediment in stream samples. Daily values of streamflow, suspended-sediment concentration, and suspended-sediment discharge are given for three stations. Bed-sediment data include trace-element concentrations in the fine-grained and bulk fractions. Biological data include trace-element concentrations in whole-body tissue of aquatic benthic insects. Quality-assurance data are reported for analytical results of water, bed sediment, and biota. Statistical summaries of water-quality, bed-sediment, and biological data are provided for the period of record at each station since 1985.

Eganhouse, R.P., Matthews, L.L., Cozzarelli, I.M., and Scholl, M.A., 1999, Evidence for natural attenuation of volatile organic compounds in the leachate plume of the Norman, Oklahoma landfill, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3 - Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 531-540.

Everett, C.R., Chin, Y., and Aiken, G.R., 1999, High pressure size exclusion chromatography analysis of dissolved organic matter isolated by tagential flow ultrafiltration: Limnology and Oceanography, v. 44, p. 1316-1322.

A 1,000-Dalton tangential-flow ultrafiltration (TFUF) membrane was used to isolate dissolved organic matter (DOM) from several freshwater environments. The TFUF unit used in this study was able to completely retain a polystyrene sulfonate 1,800-Dalton standard. Unaltered and TFUF-fractionated DOM molecular weights were assayed by high-pressure size exclusion chromatography (HPSEC). The weight-averaged molecular weights of the retentates were larger than those of the raw water samples, whereas the filtrates were all significantly smaller and approximately the same size or smaller than the manufacturer-specified pore size of the membrane. Moreover, at 280 nm the molar absorptivity of the DOM retained by the ultrafilter is significantly larger than the material in the filtrate. This observation suggests that most of the chromophoric components are associated with the higher molecular weight fraction of the DOM pool. Multivalent metals in the aqueous matrix also affected the molecular weights of the DOM molecules. Typically, proton-exchanged DOM retentates were smaller than untreated samples. This TFUF system appears to be an effective means of isolating aquatic DOM by size, but the ultimate size of the retentates may be affected by the presence of metals and by configurational properties unique to the DOM phase.

Performance Assessment (PA) is the use of mathematical models to simulate the long-term behavior of engineered and geologic barriers in a nuclear waste repository; methods of uncertainty analysis are used to assess effects of parametric and conceptual uncertainties associated with the model system upon the uncertainty in outcomes of the simulation. PA is required by the U.S. Environmental Protection Agency as part of its certification process for geologic repositories for nuclear waste. This paper is a dialogue to explore the value and limitations of PA. Two 'skeptics' acknowledge the utility of PA in organizing the scientific investigations that are necessary for confident siting and licensing of a repository; however, they maintain that the PA process is, at least as it is currently implemented, is an essentially unscientific process with shortcomings that may provide results of limited use in evaluating actual effects on public health and safety. Conceptual uncertainties in a PA analysis can be so great that results can be confidently applied only over short time ranges, the antithesis of the purpose behind long-term, geologic disposal. Two 'proponents' of PA agree that performance assessment is unscientific, but only in the sense that PA is an engineering analysis that uses existing scientific knowledge to support public policy decisions, rather than an investigation intended to increase fundamental knowledge of nature; PA has different goals and constraints than a typical scientific study. The 'proponents' describe an ideal, six-step process for conducting generalized PA, here called probabilistic systems analysis (PSA); they note that virtually all scientific content of a PA is introduced during the model-building steps of a PSA; they contend that a PA based on simple but scientifically acceptable mathematical models can provide useful and objective input to regulatory decision makers. The value of the results of any PA must lie between these two views and will depend on the level of knowledge of the site, the degree to which models capture actual physical and chemical processes, the time over which extrapolations are made, and the proper evaluation of health risks attending implementation of the repository. The challenge is in evaluating whether the quality of the PA matches the needs of decision makers charged with protecting the health and safety of the public.

Simmonsite, Na₂LiAlF₆, a new mineral of pegmatitic-hydrothermal origin, occurs in a late-stage breccia pipe structure that cuts the Zapot amazonite-topaz-zinnwaldite pegmatite located in the Gillis Range, Mineral Co., Nevada, U.S.A. The mineral is intimately intergrown with cryolite, cryolithionite and trace elpasolite. A secondary assemblage of other alumino-fluoride minerals and a second generation of cryolithionite has formed from the primary assemblage. The mineral is monoclinic, P2₁ or P2₁/m, a = 7.5006(6) Å, b = 7.474(1) Å, c = 7.503(1) Å, b= 90.847(9)^o, V = 420.6(1) ų, Z = 4. The four strongest diffraction maxima [d (Å), hkl, I/I₁₀₀ are (4.33, 111 and 111, 100); (1.877, 400 and 004, 90); (2.25, 131, 113, 131 and 311, 70); and (2.65, 220, 202, 022, 60). Simmonsite is pale buff cream with white streak, somewhat greasy, translucent to transparent, Mohs hardness of 2.5-3, no distinct cleavage, subconchoidal fracture, no parting, not extremely brittle, D_m is 3.05(2) g/cm³, and D_c is 3.06(1) g/cm³. The mineral is biaxial, very nearly isotropic, N is 1.359(1) for l = 589 nm, and birefringence is 0.0009. Electron microprobe analyses gave (wt%) Na = 23.4, Al = 13.9, Al = 13.9, F = 58.6, Li = 3.56 (calculated), with a total of 99.46. The empirical formula (based on 6 F atoms) is N_1.98Li_1.00Al_1.00F ₆. The crystal structure was not solved, presumably because of unit-cell scale twinning, but similarities to the perovskite-type structure exist. The mineral is named for William B. Simmons, Professor of Mineralogy and Petrology, University of New Orleans, New Orleans.

Fram, M.S., Fujii, J.L., Weishaar, B.A., Bergamaschi, B.A., and Aiken, G.R., 1999, How DOC composition may explain poor correlation between specific trihalomethane formation potential and specific UV absorbance, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 2 - Contamination of hydrologic systems and related ecosystems: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, p 423-430.

Sediment chronologies based on radioisotope depth profiles were developed at two sites in the San Francisco Bay estuary to provide a framework for interpreting historical trends in organic compound and metal contaminant inputs. At Richardson Bay near the estuary mouth, sediments are highly mixed by biological and/or physical processes. Excess ²³⁴Th penetration ranged from 2 to more than 10 cm at eight coring sites, yielding surface sediment mixing coefficients ranging from 12 to 170 cm²/year. At the site chosen for contaminant analyses, excess ²¹⁰Pb activity was essentially constant over the upper 25 cm of the core with an exponential decrease below to the supported activity between 70 and 90 cm. Both ¹³⁷Cs and ^239,240Pu penetrated to 57-cm depth and have broad subsurface maxima between 33 and 41 cm. The best fit of the excess ²¹⁰Pb profile to a steady state sediment accumulation and mixing model yielded an accumulation rate of 0.825 g/cm²/year (0.89 cm/year at sediment surface), surface mixing coefficient of 71 cm²/year, and 33-cm mixed zone with a half-Gaussian depth dependence parameter of 9 cm. Simulations of ¹³⁷Cs and ^239,240Pu profiles using these parameters successfully predicted the maximum depth of penetration and the depth of maximum ¹³⁷Cs and ^239,240Pu activity. Profiles of successive 1-year hypothetical contaminant pulses were generated using this parameter set to determine the age distribution of sediments at any depth horizon. Because of mixing, sediment particles with a wide range of deposition dates occur at each depth. A sediment chronology was derived from this age distribution to assign the minimum age of deposition and a date of maximum deposition to a depth horizon. The minimum age of sediments in a given horizon is used to estimate the date of first appearance of a contaminant from its maximum depth of penetration. The date of maximum deposition is used to estimate the peak year of input for a contaminant from the depth interval with the highest concentration of that contaminant. Because of the extensive mixing, sediment-bound constituents are rapidly diluted with older material after deposition. In addition, contaminants persist in the mixed zone for many years after deposition. More than 75 years are required to bury 90% of a deposited contaminant below the mixed zone. Reconstructing contaminant inputs is limited to changes occurring on a 20-year time scale. In contrast, mixing is much lower relative to accumulation at a site in San Pablo Bay. Instead, periods of rapid deposition and/or erosion occurred as indicated by frequent sand-silt laminae in the X-radiograph. ¹³⁷Cs, ^239,240Pu , and excess ²¹⁰Pb activity all penetrated to about 120 cm. The distinct maxima in the fallout radionuclides at 105-110 cm yielded overall linear sedimentation rates of 3.9 to 4.1 cm/year, which are comparable to a rate of 4.5 plus or minus 1.5 cm/year derived from the excess ²¹⁰Pb profile.

Sea level, velocity, temperature, and salinity (conductivity and temperature) data collected in Suisun Bay, California, from December 11, 1992, through May 31, 1993, by the U.S. Geological Survey are documented in this report. Sea-level data were collected at four locations and temperature and salinity data were collected at seven locations. Velocity data were collected at three locations using acoustic Doppler current profilers and at four other locations using point velocity meters. Sea-level and velocity data are presented in three forms (1) harmonic analysis results, (2) time-series plots (sea level, current speed, and current direction versus time), and (3) time-series plots of the low-pass filtered data. Temperature and salinity data are presented as plots of raw and low-pass filtered time series.

The velocity and salinity data collected during this study document a period when the residual current patterns and salt field were significantly altered by large Delta outflow (three peaks in excess of 2,000 cubic meters per second). Residual current profiles were consistently seaward with magnitudes that fluctuated primarily in concert with Delta outflow and secondarily with the spring-neap tide cycle. The freshwater inputs advected salinity seaward of Suisun Bay for most of this study. Except for a 10-day period at the beginning of the study, dynamically significant salinities (>2) were seaward of Suisun Bay, which resulted in little of no gravitational circulation transport.

Yucca Mountain straddles the west boundary of the Nevada Test Site in an arid, remote, and thinly populated region of southwestern Nevada. It is the potential site of a monitored geologic repository for the nation's commercial and military spent nuclear fuel, high-level radioactive waste derived from reprocessing of uranium and plutonium, surplus plutonium, and other nuclear-weapons materials. The fundamental rationale for a geologic repository for radioactive materials is to securely isolate them from the environment and its occupants to the greatest extent possible.

Hoch, A.R., Reddy, M.M., and Drever, J.I., 1999, Importance of mechanical disaggregation in chemical weathering in a cold alpine environment, San Juan Mountains, Colorado: Geological Society of American Bullentin, February 1999, v. III, no. 2, p. 304-314.

Weathering of welded tuff near the summit of Snowshoe Mountain (3660 m) in southwestern Colorado was studied by analyzing infiltrating waters in the soil and associated solid phases. Infiltrating waters exhibit anomalously high potassium to silica ratios resulting from dissolution of a potassium-rich glass that occurs as a trace phase in the rock. In laboratory experiments using rock from the field site, initial dissolution generated potassium-rich solutions similar to those observed in the field. The anomalous potassium release decreased over time (about 1 month), after which the dominant cation was calcium, with a much lower potassium to silica ratio. The anomalous potassium concentrations observed in the infiltrating soil solutions result from weathering of freshly exposed rock surfaces. Continual mechanical disaggregation of the rock due to segregation freezing exposes fresh glass to weathering and thus maintains the source of potassium for the infiltrating water. The ongoing process of creation of fresh surfaces by physical processes is an important influence on the composition of infiltrating waters in the vadose zone.

Although waste water discharge into San Francisco Bay has increased since the 1950's, this has been accompanied by investments in advanced treatment. Since 1976, near-monthly samples of sediments and the deposit feeding clam, Macoma balthica, have been collected near the Palo Alto Regional Water Quality Control Plant (PARWQCP) to determine how changes in waste water discharge affected trends in silver, copper, and zinc concentrations. Long-term reductions of metals in M. balthica and sediments were evident as waste water treatment improved at PARWQCP. Mean annual silver concentrations in M. balthica were 106 mg/g in 1978, 55.4 mg/g in 1987 and 3.6 mg/g in 1997. These declines coincided with improved treatment processes implemented by PARWQCP. Mean annual concentrations of copper in M. balthica declined from a maximum of 287 mg/g in 1980 to the minimum of 24 mg/g in 1991. Temporal changes in zinc concentrations were nearly bimodal, with the highest concentrations occurring during the years of high precipitation. Copper bioaccumulation was strongly correlated with copper loads from the PARWQCP until 1990, suggesting that effluents from the treatment plant were the primary source of copper to M. balthica during this period. Copper loadings from the PARWQCP continued to decline steadily after 1990, but copper in M. balthica continued to increase to a high of 71 mg/g in 1996, showing no significant correlation to copper loadings. Thus, as the localized sources of copper decreased, inputs from outside sources (for example, urban runoff), became more important in controlling metal bioaccumulation. Stream flow and precipitation were used as surrogate measures for metal loads entering into the Bay and show a strong correlation to copper bioaccumulation from 1989-97. The high concentrations of metals in the mudflat may have had and adverse affect on the resident population of organisms as measured by the low number of reproductively active individuals present between 1974-1983 (<20 percent). As metal concentrations began to decrease, 70-100 percent of the population were reproductively active. Reproductive patterns typical of less impacted sites in the Bay were not consistently observed at the Palo Alto mudflat until 1989, corresponding to mean annual copper concentrations of 35 mg/g in tissues and mean annual silver concentrations of 11 mg/g.

Concentrations of Ag, Al, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn were determined in six sediment cores from San Francisco Bay (SFB) and one sediment core in Tomales Bay (TB), a reference estuary. SFB cores were collected from between the head of the estuary and its mouth (Grizzly Bay, GB; San Pablo Bay, SP; Central Bay, CB; Richardson Bay, RB, respectively) and ranged in length from 150 to 250 cm. Concentrations of Cr, V and Ni are greater than mean crustal content in SFB and TB sediments, and greater than found in many other coastal sediments. However, erosion of ultramafic rock formations in the watershed appears to be the predominant source. Baseline concentrations of other metals were determined from horizons deposited before sediments were influenced by human activities and by comparing concentrations to those in TB. Baseline concentrations of Cu co-varied with Al in the SFB sediments and ranged from 23.7 ± 1.2 µg/g to 41.4 ± 2.4 µg/g. Baseline concentrations of other metals were less variable: Ag, 0.09 ± 0.02 µg/g; Pb, 5.2 ± 0.7 µg/g; Hg, 0.06 ± 0.01 µg/g; Zn, 78 ± 7 µg/g. The earliest anthropogenic influence on metal concentrations appeared as Hg contamination (0.3-0.4 µg/g) in sediments deposited at SP between 1850 and 1880, apparently associated with debris from hydraulic gold mining

Hostettler, F.D., Kvenvoiden, K.A., Rosenbauer, R.J., and Short, J.W., Aspects of the Exxon Valdez oil spill-a forensic study and a toxics controversy, 1999, in Morganwalp, D.W. and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Charleston, S. Carolina. March 8-12, 1999: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, v. 2, p. 135-144.

San Francisco Bay is one of the world's largest urbanized estuarine systems. Its water and sediment receive organic input from a wide variety of sources; much of this organic material is anthropogenically derived. To document the spatial and historical record of the organic contaminant input, surficial sediment from 17 sites throughout San Francisco Bay and sediment cores from two locations--Richardson Bay and San Pablo Bay--were analyzed for biomarker constituents. Biomarkers, that is, `molecular fossils', primarily hopanes, steranes, and n-alkanes, provide information on anthropogenic contamination, especially that related to petrogenic sources, as well as on recent input of biogenic material. The biomarker parameters from the surficial sediment and the upper horizons of the cores show a dominance of anthropogenic input, whereas the biomarker profiles at the lower horizons of the cores indicate primarily biogenic input. In the Richardson Bay core the gradual upcore transition from lower maturity background organics to a dominance of anthropogenic contamination occurred about 70-100 years ago and corresponds to the industrial development of the San Francisco Bay area. In San Pablo Bay, the transition was very abrupt, reflecting the complex depositional history of the area. This sharp transition, perhaps indicating a depositional hiatus or erosional period, dated at pre-1952, is clearly visible. Below, the hiatus the biomarker parameters are immature; above, they are mature and show an anthropogenic overlay. Higher concentrations of terrigenous n-alkanes in the upper horizons in this core are indicative of an increase in terrigenous organic matter input in San Pablo Bay, possibly a result of water diversion projects and changes in the fresh water flow into the Bay from the Delta. Alternatively, it could reflect a dilution of organic material in the lower core sections with hydraulic mining debris.

Geochemical correlation and differentiation of hydrocarbons from crude oils and coals is difficult. The complex mixture of the hydrocarbon constituents and the dynamic nature of these constituents in the environment as they weather contribute to this difficulty. A new parameter, the polycyclic aromatic hydrocarbon (PAH) refractory index, is defined here to help in this correlation. The PAH refractory index is a ratio of two of the most refractory constituents of most crude oils, namely triaromatic steranes and monomethylchrysenes. These are among the most persistent compounds in oil after deposition in the environment and thus retain reliably the signature of the original petroleum input. This index is utilized in Prince William Sound (PWS) to differentiate three different oils, as well as to provide evidence that coal, not oil, is the dominant source of the PAHs which are prominent constituents of marine sediments from PWS and the Gulf of Alaska.

The decrease in permeability (k) of the continental crust with depth (z), as constrained by geothermal data and calculated fluid flux during metamorphism, is given by log k = -14 - 3.2 log z, where k is in meters squared and z is in kilometers. At moderate to great crustal depths (> approximately 5 km), this curve is defined mainly by data from prograde metamorphic systems, and is thus applicable to orogenic belts where the crust is being thickened and/or heated; lower permeabilities may occur in stable cratonic regions. This k-z relation implies that typical metamorphic fluid flux values of approximately 10^-11 m/s are consistent with fluid pressures significantly above hydrostatic values. The k-z curve also predicts that metamorphic CO₂ flux from large orogens may be sufficient to cause significant climatic effects, if retrograde carbonation reactions are minimal, and suggests a significant capacity for diffuse degassing of Earth (10¹⁵ -10¹⁶ g/yr) in tectonically active regions.

Selenium stable isotope ratio measurements should serve as indicators of sources and biogeochemical transformations of Se. We report measurements of Se isotope fractionation during selenate reduction, selenite sorption, oxidation of reduced Se in soils, and Se volatilization by algae and soil samples. These results, combined with previous work with Se isotopes, indicate that reduction of soluble oxyanions is the dominant cause of Se isotope fractionation. Accordingly, Se isotope ratios should be useful as indicators of oxyanion reduction, which can transform mobile species to forms that are less mobile and less bioavailable. Additional investigations of Se isotope fractionation are needed to confirm this preliminary assessment.

We have developed a new method for measurement of natural Se isotope ratio variation which requires less than 500 ng Se per analysis and yields ±0.2^0/00 precision on ⁸⁰Se/⁷⁶Se. A double isotope spike technique corrects for isotopic fractionation during sample preparation and mass spectrometry. The small minimum sample size is important, as Se concentrations are often below 1 ppm in solids and 1 µg/L in fluids. The Se purification process is rapid and compatible with various sample matrices, including acidic rock or sediment digests.

Joye, S.B., Connell, T.L., Miller, L.G., Jellison, R., and Oremland, R.S., 1999, Oxidation of ammonia and methane in an alkaline, saline lake: Limnol. Oceanogr., v. 44, p. 178-188.

The oxidation of ammonia (NH₃) and methane (CH₄) was investigated in an alkaline saline lake, Mono Lake, California (USA). Ammonia oxidation was examined in April and July 1995 by comparing dark ¹⁴CO₂ fixation rates in the presence or absence of methyl fluoride (MeF), an inhibitor of NH₃ oxidation. Ammonia oxidizer-mediated dark ¹⁴CO₂ fixation rates were similar in surface (5-7m) and oxycline (11-15 m) waters, ranging between 70-340 and 89-186 nM d^-1, respectively, or 1-7% of primary production by phytoplankton. Ammonia oxidation rates ranged between 580-2,380 nM d^-1 in surface waters and 732-1,548 nM d^-1 in oxycline waters. Methane oxidation was examined using a 14CH₄ tracer technique in July 1994, April 1995, and July 1995. Methane oxidation rates were consistently higher in July, and rates in oxycline and anaerobic bottom waters (0.5-37 and 7-48 nM d^-1, respectively) were 10-fold higher than those in aerobic surface waters (0.04-3.8 nM d^-1). The majority of CH₄ oxidation, in terms of integrated activity, occurred within anoxic bottom waters. Measured oxidation rates and water column concentrations were used to estimate the biological turnover times of NH₃ and CH₄. The NH₃ pool turns over rapidly, on time scales of 0.8 d in surface waters and 10 d within the oxycline, while CH₄ is cycled on 10³-d time scales in surface waters and 10²-d time scales within oxycline and bottom waters. Our data suggest an important role for NH₃ oxidation in alkaline, saline lakes since the process converts volatile NH₃ to soluble NO₂^-, thereby reducing loss via lake-atmosphere exchange and maintaining nitrogen in a form that is readily available to phytoplankton.

Kerkhof, L., Voytek, M.A., Sherrell, R., Mille, D., and Schofield, O., 1999, Variability in bacterial community structure during upwelling in the coastal ocean: Hydrobiologia, v. 401, p. 139-148.

Kharaka, Y.K., Thordsen, J.J., Evans, W.C., 1999, Crustal fluids: CO2 of mantle and crustal origins in the San Andreas fault system, California, in Armannsson, H., ed., Proceedings 5th International Symposium on the Geochemistry of Earth Surface (GES-5): Rotterdam, A.A. Balkema, p. 515-518.

Chemical and isotopic analyses of water and gases were determined from 44 thermal and saline springs and wells located near the San Andreas fault system to investigate the role of fluids in the dynamics of this major plate-bounding fault. Results indicate that the waters are mainly of meteoric origin, with shallow to moderate circulation depths (down to 6 km). The chemical compositions of water and gases are highly variable, controlled mainly by the enclosing rock types. However, compositions and isotope abundances of noble gases and d¹³C values of HCO₃ indicate a significant (up to 50%) mantle component for volatiles. Because upper mantle gases are dominated by CO₂ (C/³He ~ 1010), model calculations yield high CO₂ flux values of 0.001-1 kg CO2/(km²-s). The highest CO₂ fluxes are from fluids in the Franciscan assemblage, where C and He isotopes indicate a dominantly crustal source for CO₂. Presently we are measuring the surficial and dissolved CO₂ fluxes and C isotopes over two large drainage areas where data from springs are available. Preliminary results yield total (uncorrected for soil CO₂) values ~0.001-1 kg CO₂/(km²-s). Numerical simulations indicate that the CO₂ flux values, extrapolated to seismogenic depths, are sufficient to generate lithostatic fluid pressures, and thus explain the mechanical weakness of this fault. Furthermore, the model times required to increase fluid pressures to lithostatic values are comparable to those of earthquake cycles.

Kile, D.E., and Eberl, D.D., 1999, Crystal growth mechanisms in miarolitic cavities in the Lake George ring complex, Colorado: American Mineralogist, v. 84, p. 718-724.

The Crystal Peak area of the Pikes Peak batholith, near Lake George in central Colorado, is world-renowned for its crystals of amazonite (the blue-green variety of microcline) and smoky quartz. Such crystals, collected from individual microlitic pegmatites, have a remarkably small variation in crystal size within each pegmatite, and the shapes of plots of their crystal size distributions (CSDs) are invariably lognormal or close to lognormal in all cases. There observations are explained by a crystal growth mechanism that was governed initially by surface-controlled kinetics, during which crystals tended to grow larger in proportion to their size, thereby establishing lognormal CSDs. Surface-controlled growth was followed by longer periods of supply controlled growth, during which growth rate was predominantly size-independent, consequently preserving the lognormal shapes of the CSDs and the small size variation. The change from surface- to supply controlled growth kinetics may have resulted from an increasing demand for nutrients that exceeded diffusion limitations of the system. The proposed model for crystal growth in this locality appears to be common in the geologic record, and can be used with other information, such as isotopic data, to deduce physico-chemical conditions during crystal formation.

Kile, D.E., Wershaw, R.L., and Chiou, C.T., 1999, Correlation of soil and sediment organic matter polarity to aqueous sorption of nonionic compounds: Environmental Science and Technology, v. 33, p. 2053-2056.

Polarities of the soil/sediment organic matter (SOM) in 19 soil and 9 freshwater sediment samples were determined from solid-state ¹³C-CP/MAS NMR spectra and compared with published partition coefficients (K_oc) of carbon tetrachloride (CT) from aqueous solution. Nondestructive analysis of whole samples by solid-state NMR permits a direct assessment of the polarity of SOM that is not possible by elemental analysis. The percent of organic carbon associated with polar functional groups was estimated from the combined fraction of carbohydrate and carboxylamide-ester carbons. A plot of the measured partition coefficients (K_oc) of carbon tetrachloride (CT) vs. percent polar organic carbon (POC) shows distinctly different populations of soils and sediments as well as a roughly inverse trend among the soil/sediment populations. Plots of K_oc values for CT against other structural group carbon fractions did not yield distinct populations. The results indicate that the polarity of SOM is a significant factor in accounting for differences in K_oc between the organic matter in soils and sediments. The alternate direct correlation of the sum of aliphatic and aromatic structural carbons with K_oc illustrates the influence of nonpolar hydrocarbon on solute partition interaction. Additional elemental analysis data of selected samples further substantiate the effect of the organic matter polarity on the partition efficiency of nonpolar solutes. The separation between soil and sediment samples based on percent POC reflects definite differences of the properties of soil and sediment organic matters that are attributable to diagenesis.

Dissolved sulfide concentrations in the water column and in sediment pore waters were measured by square-wave voltammetry (nanomolar detection limit) during three cruises to the Santa Barbara Basin in February 1995, November-December 1995, and April 1997. In the water column, sulfide concentrations measured outside the basin averaged 3 ± 1 nM (n = 28) in the 0 to 600 m depth range. Inside the basin, dissolved sulfides increased to reach values of up to 15 nM at depths >400 m. A suite of box cores and multicores collected at four sites along the northeastern flank of the basin showed considerable range in surficial (<0.5 cm) pore-water sulfide concentrations: <0.008, 0.01, 0.02, to as much as 0.4 mM at the 340, 430, 550, and 590 m sites, respectively. At a core depth of 10 cm, however, pore-water sulfides exhibited and even wider range: 0.005, 0.05, 0.1, and 100 mM at the same sites, respectively. The sulfide flux into the deep basin, estimated from water-column profiles during three cruises, suggests a fairly consistent input of 100-300 nmole m^-2 h^-1. In contrast, sulfide fluxes estimated from pore-water sulfide gradients at the sediment water interface were much more variable (-4 to 13,000 nmole m^-2 h^-1). Dissolved silicate profiles show clear indications of irrigation at shallow sites (340 and 430 m) in comparison to deeper basin sites (550 and 590 m) with low (<10 mM) bottom-water dissolved-oxygen concentrations. Pore-water profiles indicate ammonia generation at all sites, but particularly at the deep-basin 590 m site with concentrations increasing with sediment depth to >400mM at 10 cm. Decreases in water-column nitrate below the sill depth indicate nitrate consumption (-55 to -137 mmole m^-2 h^-1) similar to nearby Santa Monica Basin. Peaks in pore-water iron concentrations were generally observed between 2 and 5 cm depth with shallowest peaks at the 590 m site. These observations, including observations of the benthic microfauna, suggest that the extent to which the sulfide flux, sustained by elevated pore-water concentrations, reaches the water column may be modulated by the abundance of sulfide-oxidizing bacteria in addition to iron redox and precipitation reactions.

Landa, E.R. , 1999, Transferability of soil cleanup standards in remedial actions associated with technologically enhanced naturally occurring radioactive materials: Geochemical perspectives: International Atomic Energy Agency Symposium on Restoration of Environments with Radioactive Residues, Nov.29- Dec. 3, 1999; Arlington, VA., Contributed papers, IAEA-SM-359, p. 101-104.

During the 1980s, many new molecular biology techniques were developed, providing new capabilities for studying the genetics and activities of organisms. Biologists and ecologists saw the promise that these techniques held for studying different aspects of organisms, both in culture and in the natural environment. In less than a decade, these techniques were adopted by a large number of researchers studying many types of organisms in diverse environments. Much of the molecular-level information acquired has been used to address questions of evolution, biogeography, population structure and biodiversity. At this juncture, molecular ecologists are poised to contribute to the study of fundamental characteristics underlying aquatic community structure. The goal of this overview is to assess where we have been, where we are now and what the future holds for revealing the basis of community structure and function with molecular-level information.

Landa, E.R., Beals, D.M., Halverson, J.E., Michel, R.L., Cefus, G.R. 1999. Tritium and plutonium in waters from the Bering and Chukchi Seas: Health Physics, v. 77, p. 668-676.

During the summer of 1993, seawater in the Bering and Chukchi Seas was sampled on a joint Russian-American cruise [BERPAC] of the RV Okean to determine concentrations of tritium, ²³⁹Pu and ²⁴⁰Pu. Concentrations of tritium were determined by electrolytic enrichment and liquid scintillation counting. Tritium levels ranged up to 420 mBq L^-1 and showed no evidence of inputs other than those attributed to atmospheric nuclear weapons testing. Plutonium was recovered from water samples by ferric hydroxide precipitation, and concentrations were determined by thermal ionization mass spectrometry. ^239+240Pu concentrations ranged from <1 to 5.5 µBq L^-1. These concentrations are lower than those measured in water samples from other parts of the ocean during the mid-1960's to the late 1980's. The ²⁴⁰Pu:²³⁹Pu ratios, although associated with large uncertainties, suggest that most of the plutonium is derived from world-wide fallout. As points of comparison, the highest concentrations of tritium and plutonium observed here were about five orders of magnitude lower than the maximum permissible concentrations allowed in water released to the off-site environs from licensed nuclear facilities in the United States. This study and others sponsored by the International Atomic Energy Agency and the Office of Naval Research's Arctic Nuclear Waste Assessment Program are providing data for the assessment of potential radiological impacts in the Arctic regions associated with nuclear waste disposal by the former Soviet Union.

Landwehr, J.M., Reel, J.T., Rybicki, N.B., Ruhl, H.A., and Carter, V., 1999, Chesapeake Bay habitat criteria scores and the distribution and abundance of submersed aquatic vegetation in the tidal Potomac River and Potomac Estuary, 1983-1997: U.S. Geological Survey Open-File Report 99-219, 34 p.

Legates, D.R., and McCabe, G.J., 1999, Evaluating the "Goodness-of-Fit" of Hydrologic and Hydroclimatic Models: Water Resources Research v. 35, p. 233-241.

Correlation and correlation-based measures (e.g., the coefficient of determination) have been widely used to evaluate the "goodness-of-fit" of hydrologic and hydroclimatic models. These measures are oversensitive to extreme values (outliers) and are insensitive to additive and proportional differences between model predictions and observations. Because of these limitations, correlation-based measures can indicate that a model is a good predictor, even when it is not. In this paper, useful alternative goodness-of-fit or relative error measures (including the coefficient of efficiency and the index of agreement) that overcome many of the limitations of correlation-based measures are discussed. Modifications to these statistics to aid in interpretation are presented. It is concluded that correlation and correlation-based measures should not be used to assess the goodness-of-fit of a hydrologic or hydroclimatic model and that additional evaluation measures (such as summary statistics and absolute error measures) should supplement model evaluation tools.

The formation and spatial distribution of phytoplankton blooms in estuaries are controlled by (1) local mechanisms, which determine the production-loss balance for a water column at a particular spatial location (i.e. control if a bloom is possible), and (2) transport-related mechanisms, which govern biomass distribution (i.e. control if and where a bloom actually occurs). In this study, the first of a 2-paper series, we use a depth-averaged numerical model as a theoretical tool to describe how interacting local conditions (water column height, light availability, benthic grazing) influence the local balance between phytoplankton sources and sinks. We also explore trends in the spatial variability of the production-loss balance across the topographic gradients between deep channels and lateral shoals which are characteristic of shallow estuaries. For example, under conditions of high turbidity and slow benthic grazing the highest rates of phytoplankton population growth are found in the shallowest regions. On the other hand, with low turbidity and rapid benthic grazing the highest growth rates occur in the deeper areas. We also explore the effects of semidiurnal tidal variation in water column height, as well as spring-neap variability. Local population growth in the shallowest regions is very sensitive to tidal-scale shallowing and deepening of the water column, especially in the presence of benthic grazing. A spring-neap signal in population growth rate is also prominent in the shallow areas. Population growth in deeper regions is less sensitive to temporal variations in tidal elevation. These results show that both shallow and deep regions of estuaries can act as sources or sinks for phytoplankton biomass, depending on the local conditions of mean water column height, tidal amplitude, light-limited growth rate, and consumption by grazers.

The development and distribution of phytoplankton blooms in estuaries are functions of both local conditions (i.e. the production-loss balance for a water column at a particular spatial location) and large-scale horizontal transport. In this study, the second of a 2-paper series, we use a depth-averaged hydrodynamic-biological model to identify transport-related mechanisms impacting phytoplankton biomass accumulation and distribution on a system level. We chose South San Francisco Bay as a model domain, since its combination of a deep channel surrounded by broad shoals is typical of drowned-river estuaries. Five general mechanisms involving interaction of horizontal transport with variability in local conditions are discussed. Residual (on the order of days to weeks) transport mechanisms affecting bloom development and location include residence time/export, import, and the role of deep channel regions as conduits for mass transport. Interactions occurring on tidal time scales, i.e. on the order of hours) include the phasing of lateral oscillatory tidal flow relative to temporal changes in local net phytoplankton growth rates, as well as lateral sloshing of shoal-derived biomass into deep channel regions during ebb and back into shallow regions during flood tide. Based on these results, we conclude that: (1) while local conditions control whether a bloom is possible, the combination of transport and spatial-temporal variability in local conditions determines if and where a bloom will actually occur; (2) tidal-time-scale physical-biological interactions provide important mechanisms for bloom development and evolution. As a result of both subtidal and tidal-time-scale transport processes, peak biomass may not be observed where local conditions are most favorable to phytoplankton production, and inherently unproductive areas may be regions of high biomass accumulation.

Mandernack, K.W., Bazylinski, D.A., Shanks, W.C. and Bullen, T.D., 1999, Oxygen and iron isotope studies of magnetite produced by magnetotactic bacteria: Science, v. 285, p. 1892-1896.

A series of carefully controlled laboratory studies was carried out to investigate oxygen and iron isotope fractionation during the intracellular production of magnetite (Fe₃O₄) by two different species of magnetotactic bacteria at temperatures between 4° and 35°C under microaerobic and anaerobic conditions. No detectable fractionation of iron isotopes in the bacterial magnetites was observed. However, oxygen isotope measurements indicated a temperature-dependent fractionation for Fe₃O₄ and water that is consistent with that observed for Fe₃O₄ produced extracellularly by thermophilic Fe³⁺-reducing bacteria. These results contrast with established fractionation curves estimated from either high-temperature experiments or theoretical calculations. With the fractionation curve established in this report, oxygen-18 isotope values of bacterial Fe₃O₄ may be useful in paleoenvironmental studies for determining the oxygen-18 isotope values of formation waters and for inferring paleotemperatures.

In the upper crust, where hydraulic gradients are typically <1 MPa km-1, advective heat transport is often effective for permeabilities k ³ 10-16 m ² and advective mass (solute) transport for k ³ 10-20 m². Regional-scale analyses of coupled groundwater flow and heat transport in the upper crust typically infer permeabilities in the range of 10-17 to 10-14 m², so that heat advection is sometimes significant analyses of metamorphic systems suggest that a geochemically significant level of permeability can exist to the base of the crust. In active metamorphic systems in the mid to lower crust, where vertical hydraulic gradients are likely >10 MPa km-1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ~10-16 m² to ~10-18 m² between 5- and 12-km depth. Below ~12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ~10-18.5± 1 m². Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k » -3.2 log z – 14, where k is in meters squared and z is in kilometers. At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat. Such threshold value for significant advection of heat may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling.

Mariner, R.H. and Lowenstern, J.B., 1999, The geochemistry of water from springs, wells, and snowpack on and adjacent to Medicine Lake volcano, Northern California: Transactions of the 1999 Geothermal Resources Council Annual Meeting, v. 23, p. 319-326.

Chemical analyses of waters from cold springs and wells of the Medicine Lake volcano and surrounding region indicate small chloride anomalies that may be due to water-rock interaction or limited mixing with high-temperature geothermal fluids. The Fall River Springs (FRS) with a combined discharge of approximately 37 m³/s, show a negative correlation between chloride (Cl) and temperature, implying that the Cl is not derived from a high-temperature geothermal fluid. The high discharge from the FRS indicates recharge over a large geographic region. Chemical and isotopic variations in the FRS show that they contain a mixture of three distinct waters. The isotopic composition of recharge on and adjacent to the volcano are estimated from the isotopic composition of snow and precipitation amounts adjusted for evapotranspiration. Enough recharge of the required isotopic composition (-100 per mil delta D) is available from a combination of the Medicine Lake caldera, the Fall River basin and the Long Bell basin to support the slightly warmer components of the FRS (32 m³/s). The cold-dilute part of the FRS (approximately 5 m³/s) may recharge in the Bear Creek basin or at lower elevations in the Fall River basin.

Marinsky, J.A., Mathuthu, A., Ephraim, J.H., and Reddy, M.M., 1999,Calcium ion binding to a soil fulvic acid using a Donnan potential: Radiochimica Acta, v. 84, p. 205-211.

Calcium ion binding to a soil fulvic acid (Armadale Bh Horizon) was evaluated over a range of calcium ion concentrations, from pH 3.8 to 7.3, using potentiometric titrations and calcium ion electrode measurements. Fulvic acid concentration was constant (100 milligrams per liter) and calcium ion concentration varied up to 8 x 10^-4 moles per liter. Experiments discussed here included: (1) titrations of fulvic acid-calcium ion containing solutions with sodium hydroxide; and (2) titrations of fully neutralized fulvic acid with calcium chloride solutions. Apparent binding constants (expressed as the logarithm of the value, log b_app) vary with solution pH, calcium ion concentration, degree of acid dissociation, and ionic strength (from log b_app = 2.5 to 3.9) and are similar to those reported by others. Fulvic acid charge, and the associated Donnan Potential, influences calcium ion-fulvic acid ion pair formation. A Donnan Potential correction term allowed calculation of intrinsic binding constants vary from 1.2 to 2.5 (the average value is about log b = 1.6) and are similar to, but somewhat higher than, stability constants for calcium ion-carboxylic acid monodentate complexes.

Marzolf, G.R., Bowser, C.J., Hart, R.J., Stephens, D.W., and Vernieu, W.S., 1999, Photosynthetic and respiratory processes: an open approach, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., the 1996 controlled flood in Grand Canyon: American Geophysical Union, monograph no. 110, p. 205-216.

The investigation examined open-stream methods for detection of photosynthetically driven (light dependent) chemical change, and subsequently provided an estimate of flood effects on the photosynthetic community in the 25-km reach of the Colorado River below Glen Canyon Dam. Observations reported here confirm that the dynamics of oxygen concentration and pH are correlated because photosynthesis and respiration in the benthic community cause the diel patterns. Observations at the constant low flows immediately before and after the controlled flood permitted measurement of changes in stream chemistry caused by biological activity and provided a test of the hypothesis that plants were scoured from the channel by the flood. The diel amplitudes of oxygen concentration and pH change were decreased after the flood as the biomass was scoured. Patterns of oxygen production and carbon dioxide removal varied along the 25-km reach.

Marzolf, G.R., Jackson, W., and Randle, R., 1999, Flood releases from dams as management tools: interactions between science and management, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., the 1996 controlled flood in Grand Canyon: American Geophysical Union Monograph no. 110.

The linkage between management and science is sometimes strained. Management must be broadly attentive to the perceived desires of various resource users, while scientists are usually forced on natural phenomena, how they are controlled, and how perturbation effects change. These different perspectives commonly result in mismatching objectives and misunderstandings. The 1996 controlled flood represents a productive convergence of river science and dam management stimulated by the process of writing an environmental impact statement. The EIS task focused on effects of the dam operations on the river. The understanding and prediction of the effects required by the EIS was underpinned by scientific inquiry coordinated by the Bureau of Reclamation's Glen Canyon Environmental Studies (GCES) program. The controlled flood itself was a demonstration of a management tool while serving also as a manipulative experiment to test theoretical ideas about how the river works. It became clear that management's role involves the definition of the problems and goals and science's role in development of objective knowledge about natural phenomena.

Changing patterns of correlations between the historical average June-November Southern Oscillation Index (SOI) and October-March precipitation totals for 84 climate divisions in the western US indicate a large amount of variability in SOI/precipitation relations on decadal time scales. Correlations of western US precipitation with SOI and other indices of tropical El Niño-Southern Oscillation (ENSO) processes were much weaker from 1920 to 1950 than during recent decades. This variability in teleconnections is associated with the character of tropical air-sea interactions as indexed by the number of out-of-phase SOI/tropical sea surface temperature (SST) episodes, and with decadal variability in the North Pacific Ocean as indexed by the Pacific Decadal Oscillation (PDO). Decadal modes of tropical and North Pacific Ocean climate variability are important indicators of periods when ENSO indices, like SOI, can be used as reliable predictors of winter precipitation in the US.

April 1 snowpack accumulations measured at 311 snow courses in the western United States (U.S.) are grouped using a correlation-based cluster analysis. A conceptual snow accumulation and melt model and monthly temperature and precipitation for each cluster are used to estimate cluster-average April 1 snowpack. The conceptual snow model is subsequently used to estimate future snowpack by using changes in monthly temperature and precipitation simulated by the Canadian Centre for Climate Modeling and Analysis (CCC) and the Hadley Centre for Climate Prediction and Research (HADLEY) general circulation models (GCMs). Results for the CCC model indicate that although winter precipitation is estimated to increase in the future, increases in temperatures will result in large decreases in April 1 snowpack for the entire western US. Results for the HADLEY model also indicate large decreases in April 1 snowpack for most of the western US, but the decreases are not as severe as those estimated using the CCC simulations. Although snowpack conditions are estimated to decrease for most areas of the western US, both GCMs estimate a general increase in winter precipitation toward the latter half of the next century. Thus, water quantity may be increased in the western US; however, the timing of runoff will be altered because precipitation will more frequently occur as rain rather than as snow.

McGlynn, B., McDonnell, J.J., Shanley, J., Kendall, C., 1999, Riparian zone flowpath dynamics: Journal of Hydrology, v. 222, p. 75-82.

The hydrology of the near-stream riparian zone in upland humid catchments is poorly understood. We examined the spatial and temporal aspects of riparian flowpaths during snowmelt in a headwater catchment within the Sleepers River catchment in northern Vermont. A transect of 15 piezometers was sampled for Ca, Si, DOC, other major cations, and d¹⁸O. Daily piezometric head values reflected variations in the stream hydrograph induced by melt and rainfall. The riparian zone exhibited strong upward discharge gradients. An impeding layer was identified between the till and surficial organic soil. Water solute concentrations increased toward the stream throughout the melt. Ca concentrations increased with depth and DOC concentrations decreased with depth. The concentrations of Ca in all piezometers were lower during active snowmelt than during post-melt low flow. Ca data suggest snowmelt infiltration to depth; however, only upslope piezometers exhibited snowmelt infiltration and consequent low d¹⁸O values, while d¹⁸O values varied less than 0.5ppt in the deep riparian piezometers throughout the study period. Ca and d¹⁸O values in upslope piezometers during low streamflow were comparable to Ca and d¹⁸O in riparian piezometers during high streamflow. The upland water Ca and d¹⁸O may explain the deep riparian Ca dilution and consistent d¹⁸O composition. The temporal pattern in Ca and d¹⁸O indicate that upland water moves to the stream via a lateral displacement mechanism that is enhanced by the presence of distinct soil/textural layers. Snowmelt thus initiates the flux of pre-melt, low Ca upland water to depth in the riparian zone, but itself does not appear at depth in the riparian zone during spring melt. This is despite the coincident response of upland groundwater and stream discharge.

Vertical profiles of streamwise velocity measured over bed forms are commonly used to deduce boundary shear stress for the purpose of estimating sediment transport. These profiles may be derived locally or from some sort of spatial average. Arguments for using the latter procedure are based on the assumption that spatial