National Research Program

Partial List of Abstracts, 1990

Bencala, K.E., McKnight, D.M., and Zellweger, G.W., 1990, Characterization of transport in an acidic and metal- rich mountain stream based on a lithium tracer injection and simulations of transient storage: Water Resources Research, v. 26, p. 989-1000.

Physical parameters characterizing solute transport in the Snake River (an acidic and metal-rich mountain stream near Montezuma, Colorado) were variable along a 5.2-km study reach. Stream cross-sectional area and volumetric inflow each varied by a factor of 3. Because of transient storage, the residence time of injected tracers in the Snake River was longer than would be calculated by consideration of convective travel time alone. Distributed inflows along the stream were a significant source of in-stream chemical variations. These transport characteristics of the Snake River were established on the basis of the assumption of lithium as an ideally conservative tracer and use of simulations of advection, dispersion, and transient storage.

McKnight, D.M., Aiken, G.R. and Smith, R.L., 1991, Aquatic fulvic acids in microbially based ecosystems: Results from two desert lakes in Antarctica: Limnology and Oceanography, v. 36, no. 5, p. 998-1006.

The possible microbial contribution to dissolved marine fulvic acids is particularly significant because marine fulvic acids are a major pool of organic carbon on a global scale. Fulvic acids from two Antarctic lakes were characterized to determine the chemical characteristics of dissolved fulvic acid derived from organic material of microbial origin. These lakes receive very limited inputs of organic material from the surrounding barren desert, but they sustain algal and bacterial populations under permanent ice cover. One lake has an extensive anoxic zone and high salinities; the other is oxic and has low salinities. Despite these differences, fulvic acids from both lakes had similar elemental compositions, carbon distributions, and amino acid contents, indicating that the chemistry of microbially derived fulvic acids is not strongly influenced by chemical conditions in the water column. Compared to fulvic acids from other natural waters, these fulvic acids have low carbon:nitrogen atomic ratios (19-25) and low contents of aromatic carbons (5-7% of total carbon atoms); they are most similar to marine fulvic acids.

McKnight, D.M., and Bencala, K.E., 1990, The chemistry of iron, aluminum, and dissolved organic material in three acidic, metal-enriched, mountain streams, as controlled by watershed and in-stream processes: Water Resources Research, v.26, p. 3087-3100.

Several studies were conducted in three acidic, mountain streams , and the results provide a synthesis of watershed and in-stream processes controlling iron, aluminum and dissolved organic carbon (DOC) concentrations. One of the streams, the Snake River , is naturally acidic; the other two, Peru Creek and St. Kevin Gulch, receive acid mine drainage. Analysis of stream water chemistry data for the acidic headwaters of the Snake River shows that some trace metal solutes (aluminum, manganese, zinc) are correlated with major ions, indicating that watershed processes control their concentrations. Once in the stream, biogeochemical processes can control transport is they occur over time scales comparable to those for hydrologic transport. In-stream reactions include photoreduction and dissolution of hydrous iron oxides in response to an experimental decrease in stream pH, precipitation of aluminum at three stream confluences , and sorption of dissolved organic material by hydrous iron and aluminum oxides in a stream confluence. The extent of these reactions is evaluated using conservative tracers and a transport model that includes storage in the substream zone. (Author 's abstract)

Carter, Virginia, 1990, Importance of hydrologic data for interpreting wetland maps and assessing wetland loss and mitigation: in Kiraly, S.J., Cross, F.A.,and Buffington, J.D., eds., Federal Coastal Wetland Mapping Programs: U.S. Fish and Wildlife Biological Report 90, no. 18, p. 79-85.

The US Geological Survey collects and disseminates, in written and digital formats, groundwater and surface water information related to the tidal and non-tidal wetlands of the United States. This information includes quantity, quality, and availability of groundwater and surface water; groundwater and surface water interactions (recharge-discharge); groundwater flow; and the basic surface water characteristics of streams, rivers, lakes, and wetlands. Water resources information in digital format can be used in geographic information systems (GIS's) for many purposes related to wetlands. US Geological Survey wetland-related activities include collection of information important for assessing and mitigating coastal wetland loss and modification, hydrologic data collection and interpretation, GIS activities, identification of national trends in water quality and quantity, and process-oriented wetland research.

Carter, Virginia, 1990, The Great Dismal Swamp: an illustrated case study: chapter 8 in Lugo, A.E., Brinson, M., and Brown, S., eds., Ecosystems of the World l5, Forested Wetlands: Amsterdam, Elseviers Publishing Company, p. 201-211.

The Great Dismal Swamp is an 84,000 ha forested wetland on the Virginia-North Carolina border in the southern Atlantic Coastal Plain of the United States. The organic soils of the swamp range in depth from 4 m in ancient drainage channels to less than 0.3 m along the outer edges. Lake Drummond, approximately 4 km in diameter, is almost centrally located within the swamp. The flora includes individual species and plant assemblages otherwise scattered widely to the north and south along the Coastal Plain. Anthropogenic disturbance of the natural vegetation has resulted in a wide diversity of wildlife habitats. Present studies include the dynamics of the wetland-to-upland transition zone, wetland hydrology, litter production and nutrient studies in individual communities, water quality and phytoplankton populations in the lake and ditches, vegetation trends and regeneration strategies, organic soil development and wildlife habitat requirements. Remotely-sensed data provide an overview and level of detail needed to put small areas into perspective with the total ecosystem represented by this swamp.

Carter, Virginia, and Rybicki, N.B., 1990, Light attenuation and submersed macrophyte distribution in the tidal Potomac River and Estuary: Estuaries, v. 13, no. 4. p. 441-442.

Changing light availability may be responsible for the discontinuous distribution of submerged aquatic macrophytes in the freshwater tidal Potomac River. During the 1985-1986 growing seasons, light attenuation and chlorophyll a and suspended particulate material concentrations were measured in an unvegetated reach (B) and in two vegetated reaches (A and C). Light attenuation in reach B ( the lower, fresh to oligohaline tidal river) was greater than that in reach A (the recently revegetated, upper, freshwater tidal river) in both years. Reach B attenuation was greater than that in reach C (the vegetated, oligohaline to mesohaline transition zone of the Potomac Estuary) in 1985 and similar to that in reach C in 1986. In reach B, 5% of the total below-surface light penetrated only an average of 1.3 m in 1985 and 1.0 m in 1986, compared with 1.9 m and 1.4 m in reach A and B in 1985 and 1986 respectively. Water column chlorophyll a concentration controlled light availability in reaches A and B in 1985, whereas both chlorophyll a and suspended particulate material concentrations were highly correlated with attenuation in both reaches in 1986. Reach C light attenuation was correlated with suspended particulate material in 1986. The relationship between attenuation coefficient and Secchi depth was K par = 1.38/Secchi depth (where K par is the photosynthetically active radiation). The spectral distribution of light at 1 m was shifted toward the red portion of the visible spectrum compared to surface light. Blue light was virtually absent at 1.0 m in reach B during July and August 1986. Tidal range is probably an important factor in determining light availability for submersed macrophyte propagule survival at the sediment-water interface in this shallow turbid system.

Huzzey, L.M., Cloern, J.E., and Powell, T.M., 1990, Episodic changes in lateral transport and phytoplankton distribution in South San Francisco Bay: Limnology and Oceanography, v. 35, p. 472-478.

bservations in South San Francisco Bay during 1982 showed that substantial cross-channel, nontidal flows accompanied episodic increases in the longitudinal, nontidal flows. Along the channel the nontidal circulation was enhanced during the monthly minima in tidal energy or as a result of wind forcing, producing up-estuary flows 2-3 times greater than normal. These longitudinal pulses modified the horizontal and vertical salinity distributions and generated cross-channel flows of up to 0.07 m/s that persisted for several days. The increased lateral flows were directed to the west and may explain the large fluctuations in phytoplankton biomass observed over the broad eastern shoal during spring. These findings apply to other constituents, such as planktonic larvae, nutrients, or suspended sediments, which also can have large horizontal gradients in estuaries.

Wienke, S.M., Cloern, J.E., and Cole, B.E., 1990, Plankton studies in San Francisco Bay. XI. Chlorophyll distributions and hydrographic properties in San Francisco Bay, 1988-1989: U.S. Geological Survey Open-File Report 90-562, 212 p.

Water samples were collected in the San Francisco Bay estuary during 42 cruises from February, 1988, through December, 1989. Conductivity, temperature, light attenuation (turbidity), and in-vivo fluorescence were measured longitudinally and vertically in the main channel of the estuary from near the Dumbarton Bridge in the extreme southern part to Rio Vista on the Sacramento River. Discrete water samples were analyzed for chlorophyll a. Water density was calculated from values for salinity, temperature, and pressure (depth), and is included in the data summaries.

Cooley, R.L., and Naff, R.L., 1990, Regression modeling of ground-water flow: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. B4, 232 p.

Scientists and engineers have been using groundwater flow models to study groundwater flow systems for more than 20 years. The basic modeling process seems to be relatively straightforward. Initially, a sound conceptual model is formed and is translated into a tractable, mathematical model. Contributing to (and following) this conceptualization process is the collection of field information, such as: (1) location and extent of hydrostratigraphic units, recharge areas, discharge areas, and system boundaries; (2) hydraulic head measurements; and (3) pumping discharges. These data form the basis for input to the flow model. Finally, the model is run, and the desired information such as head distribution or flux rates is extracted. However, people engaged in modeling usually observe that two pervasive problems considerably complicate the situation. One problem is that good, general methods of measuring (or computing) some of the variables that characterize the flow system and its geologic framework do not exist. The second problem relates to errors in the measurements and their propagation into model results. As a consequence of these two problems, measurement (or computation) of the necessary input variables, application of them to an adequate model, and calculation of the desired results to an acceptable accuracy generally are not possible. Other methods that recognize and deal with the problems of incompleteness and/or inaccuracy of data must also be applied. A basic methodology of multiple nonlinear regression has been developed in which the regression model is some type of groundwater flow model. The main body of the text is organized into six major sections: (1) an introduction that discusses the general topic of modeling groundwater flow; (2) since the specific optimization approach employed is regression this section is included to provide the student with the necessary statistical background material; (3) presents detailed material on linear and nonlinear regression; (4) applies the nonlinear regression method to the specific problem of developing a general finite difference model of steady-state groundwater flow; (5) statistical procedures are given to analyze and use general linear and nonlinear regression models; and (6) supplements to the preceding sections.

Cozzarelli, I.M., Eganhouse, R.P., and Baedecker, M.J., 1990, Transformation of monoaromatic hydrocarbons to organic acids in an anoxic ground-water environment: Environmental Geology and Water Sciences, v. 16, p. 135-141.

The transformation of benzene and a series of alkylbenzenes was studied in anoxic groundwater of a shallow glacial-outwash aquifer near Bemidji, Minnesota. Monoaromatic hydrocarbons, the most water-soluble components of crude oil, were transported downgradient of an oil spill, forming a plume of contaminated groundwater. Organic acids that were not original components of the oil were identified in the anoxic groundwater. The highest concentrations of these oxidized organic compounds were found in the anoxic plume where a decrease in concentrations of structurally related alkylbenzenes was observed. These results suggest that biological transformation of benzene and alkylbenzenes to organic acid intermediates may be an important attenuation process in anoxic environments. The transformation of a complex mixture of hydrocarbons to a series of corresponding oxidation products in an anoxic subsurface environment provides new insight into in situ anaerobic degradation processes.

Smith, J.A., Chiou, C.T., Kammer, J.A., and Kile, D.E., l990, Effect of soil moisture on the sorption of trichloroethene vapor to vadose-zone soil at Picatinny Arsenal, New Jersey: Environmental Science and Technology, v. 24, p. 676-683.

The impact of moisture on the sorption of tricholorethene (TCE) vapor to vadose-zone soil above a contaminated water-table aquifer at Picatinny Arsenal in Morris County, NJ was assessed. To accomplish this, batch experiments on the sorption of TCE vapor by field soil were carried out as a function of relative humidity. The TCE sorption decreases as soil moisture content increases from zero to saturation soil moisture content, the soil moisture content in equilibrium with 100% relative humidity. The moisture content of soil samples collected from the vadose zone was found to be greater than the saturation soil-moisture content, suggesting that adsorption of TCE by the mineral fraction of the vadose zone soil should be minimal relative to the partition uptake by soil organic matter. Analyses of soil and soil-gas samples collected from the field indicate that the ratio of the concentration in the soil gas is 1 to 3 orders of magnitude greater than the ratio predicted by using an assumption of equilibrium conditions. This apparent disequilibrium presumably results from the slow desorption of TCE from the organic matter of the vadose-zone soil relative to the dissipation of TCE vapor from the soil gas.

Constantz, J. and F. Murphy, 1990, Monitoring moisture storage in trees using time domain reflectometry: J. Hydrol., v. 119, p. 31-42.

Laboratory and field tests were performed to examine the feasibility of using time domain reflectrometry (TDR) to monitor changes in the moisture storage of the woody parts of trees. To serve as wave guides for the TDR signal, pairs of stainless steel rods (13 cm long, 0.32 cm in diameter, and 2.5 cm separation) were driven into parallel pilot holes drilled into the woody parts of trees, and a cable testing oscilloscope was used to determine the apparent dielectric constant. A calibration curve was developed and used to estimate the volumetric water content to a dept of 13 cm in living trees. One field experiment was conducted on an English walnut tree (Juglans regia) with a diameter of 40 cm, growing in a flood- irrigated orchard on a Hanford sandy loam near Modesto, California. The moisture content determined by TDR showed a gradual decrease from 0.44 to 0.42 cu cm/cu cm over a 2-wk period prior to flood irrigation, followed by a rapid rise to 0.47 cu cm/cu cm over a 4-day period after irrigation, then again a gradual decline approaching the next irrigation. A second field experiment was made on 10 evergreen and deciduous trees with diameters ranging from 30 to 120 cm, growing in the foothills of the Coast Range of central California. Most trees showed an early spring maximum in moisture content determined by TDR associated with leaf growth, and a late summer minimum in moisture content associated with the end of the dry season. Moisture contents ranged from 0.20 to 0.70 cu cm/cu cm, with an annual percentage change in moisture of 15-70% depending on species and environmental conditions. A final field test was performed in northern New Mexico to examine the effect of trunk freezing on TDR measurements. This test confirmed that freezing conditions were recorded as a total loss of liquid water by the TDR method. These results suggest that further TDR calibration of wood, plus some understanding of the relation between tree moisture and physiological stress, could be useful to several disciplines, ranging from irrigation scheduling to watershed management to forest ecology.

Duff, J.H., and Triska, F.J., 1990, Denitrification in sediments from the hyporheic zone adjacent to a small forested stream: Canadian Journal of Fisheries and Aquatic Sciences v. 47, no. 6, p. 1140-1147.

Denitrification was assayed by the acetylene blockage technique in hyporheic sediments. Samples were obtained along transects perpendicular to the stream at two sites: (1) the base of a slope dominated by old-growth redwood and (2) the base of a slope dominated by alder regenerating from a clearcut in 1965. Denitrification was evident at in situ nitrate concentrations from all locations tested. Activity was stimulated by nitrate, but nitrate plus glucose had no additional effect. Denitrifying potentials increased with increasing distance from the stream channel. Dissolved oxygen was 100% of the concentration expected in equilibrium with the atmosphere in water obtained from monitoring wells immediately adjacent to the stream, but was as low as 7% of the expected value in water 11.4 m inland. Both nitrate and dissolved organic carbon decreased over the summer in wells at the base of the alder-forested slope. A 48-h injection of nitrate-amended stream water into hyporheic water 8.4 m inland stimulated nitrous oxide production in the presence of acetylene. Nitrous oxide, generated as nitrate, and acetylene were co-transported to a well 13 m down-gradient. The acetylene-block experiments coupled with the chemistry data suggest that denitrification can modify the chemistry of water during passage through the hyporheic zone.

Essaid, H. I., 1990, A multilayered sharp interface model of coupled freshwater and saltwater flow in coastal systems: Model development and application: Water Resources Research, v. 26, no. 7. p. 1431-1454.

Coastal aquifers are an important water resource in areas bordering seas. A quasi three-dimensional, finite difference model, that simulates freshwater and salt water flow separated by a sharp interface, has been developed to study layered coastal aquifer systems. The model allows for regional simulation of coastal groundwater conditions, including effects of salt water dynamics on the freshwater system. Vertically integrated freshwater and salt water flow equations incorporating the interface boundary condition are solved within each aquifer. Leakage through confining layers is calculated by Darcy 's law, accounting for density differences across the layer. The locations of the interface tip and toe, within grid blocks, are tracked by linearly extrapolating the position of the interface. The model has been verified using available analytical solutions and experimental results. Application of the model to the Soquel-Aptos basin, Santa Cruz County, California, illustrates the use of the quasi three-dimensional, sharp interface approach for the examination of freshwater-salt water dynamics in regional systems. Simulation suggests that the interface today is still responding to long-term Pleistocene sea level fluctuations and has not achieved equilibrium with present day sea level conditions.

Fuller, C.C., Davis, J.A., Cain, D.J., Lamothe, P.J., Fries, T.L., Fernandez, G., Vargas, J.A., and Murillo, M.M., 1990, Distribution and transport of sediment-bound metal contaminants in the Rio Grande de Tarcoles, Costa Rica (Central America): Water Research, v. 24, p. 805-812.

A reconnaissance survey of the extent of metal contamination in the Rio Grande de Tarcoles river system of Costa Rica indicated high levels of chromium (Cr) in the fine-grained bed sediments (< 60 micro m) of tributaries downstream from leather tanneries (50-83 times Cr background or 3000-5000 micro g/g). In the main channel of the river downstream of the Sane Jose urban area Cr contamination in sediments was 4-6 times background and remained relatively constant over 50 km to the mouth of the river. Sediments from a mangrove swamp at the river mouth had Cr levels 2-3 times above background. Similar patterns of dilution were observed for lead (Pb) and Zinc (Zn) sediment contamination although the contamination levels were lower. The high affinity of Cr towards particulate phases, probably as Cr(III), allows the use of Cr contamination levels for delineating regions of deposition of fine- grained sediments and dilution of particle associated contaminants during transport and deposition.

Glynn, P.D., 1990, Modeling solid-solution reactions in low-temperature aqueous systems, in Melchoir, D.C., and Bassett, R.L., eds., Chemical modeling of aqueous systems II: American Chemical Society Symposium Series 416, Washington, D.C., American Chemical Society, p. 74-86.

The effect of substitutional impurities on the stability and aqueous solubility of a variety of solids was investigated. Stoichiometric saturation, primary saturation and thermodynamic equilibrium solubilities were compared to pure phase solubilities. Contour plots of pure phase saturation indices (SI) were drawn at minimum stoichiometric saturation, as a function of the amount of substitution and of the excess-free-energy of the substitution. SI plots drawn for the major component of a binary solid-solution generally show little deviation from pure phase solubility except at trace component fractions greater than 1%. In contrast, trace component SI plots reveal that aqueous solutions at minimum stoichiometric saturation can achieve considerable supersaturation with respect to the pure trace-component end-member solid, in cases where the major component is more soluble than the trace. Field or laboratory observations of miscibility gaps, spinodal gaps, critical mixing points or distribution coefficients can be used to estimate solid-solution excess-free-energies, when experimental measurements of thermodynamic equilibrium or stoichiometric saturation states are not available. As an example, a database of excess-free-energy parameters is presented for the calcite, aragonite, barite, anhydrite, melanterite and epsomite mineral groups, based on their reported compositions in natural environments.

Hill, M.C., 1990, Solving groundwater flow problems by conjugate-gradient methods and the strongly implicit procedure: Water Resources Research, v. 26, no. 9, p. 1961-1969.

The performance of the preconditioned conjugate-gradient method with three preconditioners for solving groundwater flow problems was compared with the strongly implicit procedure (SIP) using a scalar computer. The preconditioners considered are the incomplete Cholesky (ICCG) and the modified incomplete Cholesky (MICCG), which require the same computer storage as SIP as programmed for a problem with a symmetric matrix, and a polynomial preconditioner (POLCG), which requires less computer storage than SIP. Although POLCG is usually used on vector computers, it was included because of its small storage requirements. Published comparisons of the solvers were evaluated, all four solvers were compared for the first time, and new test cases were examined to provide a more complete basis by which the solvers can be judged for typical groundwater flow problems. Based on nine test cases, the following conclusions were reached: (1) SIP is actually as efficient as ICCG for some of the published, linear, two-dimensional test cases that were reportedly solved much more efficiently by ICCG; (2) SIP is more efficient than other published comparisons would indicate when common convergence criteria are used; and (3) for problems that are three-dimensional, nonlinear, or both, and for which common convergence criteria are used, SIP is often more efficient than ICCG, and is sometimes more efficient than MICCG.

Jarrett, R.D., 1990, Hydrologic and hydraulic research in mountain rivers: Water Resources Bulletin, v. 26, no. 3, p. 419-429.

Although current (1990) knowledge of hydrologic and hydraulic processes is based on many years of study, there are river environments where these processes are complex and poorly understood. One of these environments is in mountainous areas, which cover about 25% of the United States. Use of conventional hydrologic and hydraulic techniques in mountain-river environments may produce erroneous results and interpretations in a wide spectrum of water resources investigations. An ongoing U.S. Geological Survey research project is being conducted to improve the understanding of hydrologic and hydraulic processes of mountainous areas and to improve paleohydrologic techniques used in mountain rivers. Much of this research has been conducted in Colorado but rivers in other Rocky Mountain states are also being studied. To understand runoff from mountainous areas, the causative hydrologic processes, particularly precipitation, need to be understood. Floods of interest are produced by short-duration and high- duration intensity rainstorms. An elevation limit (2,450 m) has been observed for significant rainfall which supports the elevation limit of significant flood runoff from studies of streamflow data and paleoflood investigations. Knowledge of data errors, problems with application of measurement techniques, and overestimated peak discharge is essential because the values of the largest floods commonly are the most erroneous.

Lichty, R.W., and Karlinger, M.R., 1990, Climate factor for small-basin flood frequency: Water Resources Bulletin, v. 26, no. 4, p. 577-586.

A climate factor, CT, (T = 2-year, 25-year and 100-year recurrence intervals) that delineates regional trends in small-basin flood frequency was derived using data from 71 long-term rainfall record sites. Values of CT at these sites were developed by a regression analysis that related rainfall-runoff model estimates of T-year floods to a sample set of 50 model calibrations. CT was regionalized via kriging to develop maps depicting its geographic variation for a large part of the United States east of the 105th meridian. Kriged estimates of CT and basin-runoff characteristics were used to compute regionalized T-year floods for 200 small drainage basins. Observed T-year flood estimates also were developed for these sites. Regionalized floods are shown to account for a large percentage of the variability in observed flood estimates with coefficients of determination ranging from 0.89 for 2-year floods to 0.82 for 100-year floods. The relative importance of the factors comprising regionalized flood estimates is evaluated in terms of scale (size of drainage area), basin-runoff characteristics (rainfall-runoff model parameters), and climate.

Goode, D. J., and Konikow, L. F., 1990, Apparent dispersion in transient groundwater flow: Water Resources Research, v. 26, no. 10, p. 2339-2351.

Effects of large-scale temporal velocity fluctuations, particularly changes in the direction of flow, on solute spreading in a two- dimensional aquifer are generally believed to result from spatial variability in aquifer properties. Relations for apparent longitudinal and transverse dispersivity are developed through an analytical solution for dispersion in a fluctuating, quasi-steady uniform flow field, in which storativity is zero. For transient flow, spatial moments are evaluated from numerical solutions. Ignored or unknown transients in the direction of flow primarily act to increase the apparent transverse dispersivity because the longitudinal dispersivity is acting in a direction that is not the assumed flow direction. This increase is a function of the angle between the transient flow vector and the assumed steady state flow direction and the ratio of transverse to longitudinal dispersivity. The maximum effect on transverse dispersivity occurs if storativity is assumed to be zero, such that the flow field responds instantly to boundary conditions. Theoretical and field analyses indicate that longitudinal dispersivity is scale-dependent in porous media and that transverse dispersivity is generally one or more orders of magnitude smaller, causing solute plumes to be long and thin.

Kuwabara, J.S. and Harvey, R.W., 1990, Application of a hollow-fiber tangential-flow device for sampling suspended bacteria and particles from natural waters: Journal of Environmental Quality, v. 19, p. 625-629.

Increasing interest in suspended particles that control transport, partitioning, and bioavailability of a variety of contaminants in natural waters has necessitated the development of efficient dewatering/particle- concentration devices. The design and application of a hollow-fiber tangential-flow filtration device for concentration of bacteria and suspended particles from large volumes of surface water and groundwater samples (i.e., hundreds of liters) was described. The device is composed of a filtration module, containing two all polypropylene, hollow-fiber tangential-flow filtration cartridges (0.2 microm pore size), and a pump module. Filtrate flux rates (4-8 L/min) are equal to or faster than those of other devices that are based on continuous flow centrifugation and plate and frame filtration. Particle recovery efficiencies for inorganic particles (approximately 90%) were similar to other dewatering devices, but microbial cell recoveries (30-90%) were greatly improved by this technique relative to other currently available methods. Although requirements for operation and maintenance of the device are minimal, its size, as with other dewatering devices, limits its applicability at remote sites. Nevertheless, it has proven useful for sample collection in studies involving microbial transport and analysis of particle-associated trace inorganic solutes.

Kuwabara, J.S., Chang, C.C.Y. and Pasilis, S.P., 1990, Effects of benthic flora on arsenic transport: Journal of Environmental Engineering, v. 116, p. 394-409.

Chemical and biological interactions involving arsenic (As) and phosphorus (P) appear to affect significantly As transport and distribution in Whitewood Creek, South Dakota. Data (first-order uptake rate constants, standing crop, and accumulation factors) that can be used to predict As transport have been determined using algae collected in the creek along a transect from upstream of mine discharge downgradient through a 57-km impacted reach. Cultures of Achnanthes minutissima (Bacillariophycae) were isolated from four sites along a longitudinal gradient of dissolved As within the study reach and were maintained at ambient dissolved-As concentrations. As sorption rate constants for cell surfaces of these isolates were estimated as a function of dissolved arsenate and orthophosphate. All isolates sorbed orthophosphate preferentially over arsenate. Initial sorption of both arsenate and orthophosphate appeared to follow a first-order equation within media formulations but did not adequately describe other observed effects among formulations or between isolates. Although estimated sorption-rate constants increased slightly with increased dissolved arsenate concentration, algae isolated from a site with elevated dissolved As had a significantly slower rate of As uptake compared with the same species isolated from an uncontaminated site upstream. Field and laboratory results indicate that the benthic flora represent a significant As pool, which may episodically affect water-column concentrations.

Leland, H.V. and Scudder, B.C., 1990, Trace elements in Corbicula fluminea from the San Joaquin River, California: Science of the Total Environment, v. 97/98, p. 641-672.

Trace element concentrations in soft tissue of the benthic bivalve, Corbicula fluminea, from the San Joaquin River and its major tributaries were examined during the primary irrigation season in relation to the spatial variation in concentrations of major, minor and trace constituents in riverwater and sediments. Selenium concentrations in Corbicula from perennial flow reaches of the San Joaquin River and its major tributaries varied directly with the solute Se concentrations of riverwater. Elevated concentrations occurred in clams from sites with substantial discharge originating as subsurface drainage and irrigation return flows. Both tissue and solute Se concentrations declined from June through the end of the primary irrigation season. Arsenic concentrations in Corbicula from perennial flow reaches of the San Joaquin River varied directly with the HNO3-extractable (pH 2) As:Fe ratio of suspended matter , providing evidence that sorption to oxyhydroxide surfaces is an important control on the biological availability of As. Concentrations of As in clams from downstream perennial flow reaches of the San Joaquin River increased from June through the end of the primary irrigation season. Mercury concentrations in Corbicula were elevated in upstream reaches of the San Joaquin River, in the Merced and Tuolumne Rivers, and in tributaries draining the Coast Ranges. Mean Cd and Cu concentrations in Corbicula were elevated in the Merced and Tuolumne Rivers, Orestimba Creek and a perennial flow reach of the San Joaquin River which receives water directly from Delta Mendota Canal. Concentrations of Ni and clams from the San Joaquin River decreased downstream of the Delta Mendota Pool. Boron and Mo were not accumulated by Corbicula despite high solute concentrations in riverwater during the primary irrigation season. Concentrations of Cr, Pb, Ag, V and Zn in Corbicula exhibited little geographic variability in the drainage.

Johns, C. and Luoma, S. N., 1990, Arsenic in benthic bivalves of San Francisco Bay and Sacramento/San Joaquin River Delta: Science of Total Environment, v. 97/98, p. 673-684

The spatial distribution of arsenic in two species of benthic bivalves and in fine, oxidized surface sediments of San Francisco Bay was examined. The major goal was to determine if arsenic contamination was common in bivalves and sediments in the estuary, with a particular focus on the northern reach, which shows regional enrichment with selenium. The species studied included Corbicula sp. and Macoma balthica. Areas sampled included the San Francisco Bay, the Sacramento/San Joaquin River Delta, and selected rivers not influenced by urban or industrial activity. Arsenic concentrations in all samples were characteristic of values reported for uncontaminated estuaries. Small temporal fluctuations and low arsenic concentrations in bivalves and sediments suggest that the most inputs of arsenic are likely to be minor and arsenic contamination is not widespread in the Bay. (Mertz-PTT)

Luoma, S. N., Dagovitz, R., Axtmann, E., 1990, Temporally intensive study of trace metals in sediments and bivalves from a large river-estuarine system: Suisun Bay/Delta in San Francisco Bay: Science of the Total Environment, v. 97/98, p. 675 - 712.

Distributions in time and space of Ag, Cd, Cr, Cu, Pb and Zn were determined in fine-grained sediments and in the filter-feeding bivalve Corbicula sp. of Suisun Bay/delta at the mouth of the Sacramento and San Joaquin Rivers in North San Francisco Bay. Samples were collected from seven stations at near-monthly intervals for 3 years. Aggregated data showed little chronic contamination with Ag, Zn and Pb in the river and estuary. Substantial chronic contamination with Cd, Cu and Cr in Suisun Bay/delta occurred, especially in Corbicula, compared with the lower San Joaquin River. Salinity appeared to have secondary effects, if any, on metal concentrations in sediments and metal bioavailability to bivalves. Space/time distributions of Cr were controlled by releases from a local industry. Analyses of time series suggested substantial inputs of Cu might originate from the Sacramento River during high inflows to the Bay, and Cd contamination had both riverine and local sources. Concentrations of metals in sediments correlated with concentrations in Corbicula only in annually or 3-year aggregated data. Condition index for Corbicula was reduced where metal contamination was most severe. The biological availability of Cu and Cd to benthos was greater in Suisun Bay than in many other estuaries. Thus small inputs into this system could have greater impacts than might occur elsewhere; and organisms were generally more sensitive indicators of enrichment than sediments in this system.

Cain, D. J., and Luoma, S. N., 1990, Influence of seasonal growth, age and environmental exposure on Cu and Ag in a bivalve indicator, Macoma balthica, in San Francisco Bay: Marine Ecology Progress Series, v. 60, p. 45-55.

emporal and spatial variations in copper and silver in the deposit-feeding clam Macoma balthica and in surficial sediments were analyzed at 8 stations in San Francisco Bay at near-monthly intervals for periods ranging from 3 to 10 years during 1977 to 1986. Strong seasonal variations in metal concentrations of Macoma balthica were associated with seasonal variations in soft tissue weight. Aperiodic fluctuations in metal concentrations appeared to be driven by changes in metal contents of the soft tissues. Metal content of clams of standard shell length was less variable than tissue metal concentration, and generally followed changes in the concentrations of copper and silver in the sediments. Correlations between metal content and sediment concentrations were improved when content was standardized to age rather than shell length. Metal content of Macoma balthica displayed few consistent temporal trends among stations, evidently reflecting different sources of input and complex hydrologic and geochemical processes affecting metal availability in San Francisco Bay. Increases in copper and silver were noted at several stations in South Bay during 1977 to 1980. A continuous 10 year record at one of these stations showed that the 1977 to 1980 increase and the subsequent decline beginning in 1981 coincided with fluctuations in metal inputs from a nearby source.

McCabe, G.J., Wolock, D.M., Hay, L.E., and Ayers, M.A., 1990, Effects of climatic change on the Thornthwaite moisture index: Water Resources Bulletin, v. 26, no. 4, p. 633-643.

The Thornthwaite moisture index is a useful indicator of the supply of water (precipitation) in an area relative to the demand for water under prevailing climatic conditions (potential evapotranspiration). The study examines the effects of changes in climate (temperature and precipitation) on the Thornthwaite moisture index in the conterminous United States. Estimates of changes in mean annual temperature and precipitation for double-atmospheric CO2 conditions derived from three general circulation models (GCMs) are used to study the response of the moisture index under steady-state doubled-CO2 conditions. Results indicate that temperature and precipitation changes under doubled-CO2 conditions generally will cause the Thornthwaite moisture index to decrease, implying a drier climate for most of the United States. The pattern of expected decrease is consistent among the three GCMs, although the amount of decrease depends on which GCM climatic-change scenario is used. Results also suggest that changes in the moisture index are related mainly to changes in the mean annual potential evapotranspiration as a result of changes in the mean annual temperature, rather than to changes in the mean annual precipitation.

Nimmo, J. R., 1990, Experimental testing of transient unsaturated flow theory at low water content in a centrifugal field: Water Resources Research, v. 26, p. 1951-1960.

Experimental measurements and theoretical predictions of transient moisture conditions have been compared for a sandy soil approaching hydrostatic equilibrium in a centrifugal field. Starting near saturation, samples were centrifuged at constant speed with a constant suction at the outflow boundary. Water flowed freely out of the sample through a porous plate. Step increases in centrifuge speed produced transient moisture conditions suitable for comparison between experiment and theory. Measurements of electrical conductivity by a direct contact four- electrode technique indicated the water content according to a calibration based on known moisture conditions at various equilibrium states. A specially modified centrifuge permitted electrical measurements during centrifugation. For comparison, the transient water contents were computed by a finite-difference solution of Richards ' equation (modified by replacing gravitational with centrifugal potential), using soil characteristics measured previously by steady state techniques. The time dependence of water content changes, used as the basis for comparison between experiment and theory, shows agreement which is reasonable given the degree of uncertainty of the measurements. The experiment confirms, within a factor of 4, the validity of Richards ' equation for moisture conditions as dry as 25% of saturation, over a hydraulic conductivity range of 5 x 10 to the minus 11th power to 1 x 10 to the minus 8th power m/s, and in a centrifugal field up to about 200 g.

Nichols, F.H., Thompson, J.K., and Schemel, L.E., 1990, The remarkable invasion of San Francisco Bay (California, U.S.A) by the Asian clam Potamocorbula amurensis, II. Displacement of a former community: Marine Ecology Progress Series, v. 66, p. 95-101.

Long-term macrobenthic sampling at a site in northern San Francisco Bay has provided an unusual opportunity for documenting the time course of an invasion by a recently introduced Asian clam Potamocorbula amurensis. Between 1977, when sampling began, and 1986, when the new clam was first discovered, the benthic community varied predictably in response to river inflow. During years of normal or high river inflow, the community consisted of a few brackish or freshwater species. During prolonged periods of low river inflow, the number of species doubled as estuarine species (e.g. Mya arenaria) migrated up the estuary. In June 1987, at the beginning of the longest dry period in recent decades, large numbers (>12, 000/sq m) of juvenile P. amurensis were discovered at the site. By mid- summer 1988 the new clam predominated (>95%) in both total number of individuals and biomass, and the expected dry-period estuarine species did not become re-established. The rapid rise of P. amurensis to numerical dominance throughout the region of the original introduction was probably facilitated by the fact that this region of the bay had been rendered nearly depauperate by a major flood in early 1986. Once introduced, the clam had sufficient time (>1 yr) to become well established before the salinity regime was appropriate for the return of the estuarine species. Subsequently, the new clam was apparently able to prevent the return of the dry-period community. Its ability to live in low salinity water (1 ppt) suggests that p. amurensis may not be displaced with the return of normal winter river flow and, therefore, may have permanently changed benthic community dynamics in this region of san francisco bay.

Nordstrom, D.K., Plummer, L.N., Langmuir, D., Busenberg, E., May, H.M., Jones, B.F., and Parkhurst, D.L., 1990, revised chemical equilibrium data for major water-mineral reactions and their limitations, in Melchoir, D.C., and Bassett, R.L., eds., Chemical modeling of aqueous systems ii: American Chemical Society Symposium Series 416, Washington, D.C., American Chemical Society, p. 398-413.

A revised summary of equilibrium constants and reaction enthalpies for aqueous ion association reactions and mineral solubilities has been compiled from the literature for common equilibria occurring in natural waters at 0-100 C and 1 bar pressure. The species have been limited to those containing the elements Na, K, Li, Ca, Mg, Ba, Sr, Ra, Fe(II/III), Al, Mn(II, III, IV), Si, C, Cl, S(VI) and F. The necessary criteria for obtaining reliable and consistent thermodynamic data for water chemistry modeling are achieved when the following criteria are met: the fundamental thermodynamic relationships and their consequences are obeyed; common scales are used for temperature, energy atomic mass and the fundamental physical constants; conflicts and inconsistencies among measurements are resolved; an appropriate mathematical model is chosen to fit all the temperature and pressure dependent data; an appropriate aqueous chemical model is chosen to fit all aqueous solution data; and an appropriate choice of standard states is made and applied to all similar substances. An important limitation on the application of equilibrium computations is that minerals that do not show reversible solubility behavior should not be assumed to attain chemical equilibrium in natural aquatic systems. (See also W90-09846) (Geiger-PTT)

0remland, R.S., Steinberg, N.A., Maest, A.S., Miller, L.G.,and Holilbaugh, J.T., 1990, Measurement of in situ rates of selenate removal by dissimilatory bacterial reduction in sediments: Environmental Science and Technology, v. 24, p. 1157-1164.

Oxyanions of selenium have been identified as toxic constituents in drainage waters from irrigated, seleniferous agricultural soils. Recently a novel process was reported by which anaerobic bacteria respire selenate, which in turn biochemically reduces this oxyanion to selenite and ultimately to elemental selenium. An understanding of how this process of dissimilatory selenate reduction operates should ultimately aid in the design of treatment schemes and the rehabilitation of selenate contaminated regions. Accordingly, a radioisotope method for measurement of bacterial respiratory reduction of selenate to elemental selenium in aquatic sediments was devised. Sediments were labeled with Se75 selenate, incubated, and washed, and Se75(s) was determined as counts remaining in the sediment. Core profiles of selenate reduction, sulfate reduction, and denitrification were made simultaneously in the sediments of an agricultural wastewater evaporation pond. Most of the in situ selenate reduction (85%) and all the denitrification activities were confined to the upper 4 to 8 cm of the profile whereas sulfate reduction was greatest below 8 cm (89% of total). The integrated areal rate of selenate reduction was 301 micromoles/sq m/day, which results in a turnover of water column selenate in 82.4 days. The rate of selenate reduction in sediments was enhanced in laboratory experiments by provision of the proper electron donors (e.g., acetate, lactate, hydrogen, etc.). When this is done in conjunction with removal of competitive electron acceptors like fertilizer-derived nitrate, a treatment scheme can be envisioned.

Steinberg, N.A., Switzer Blum, J., Hochstein, L., and 0remland, R.S., 1992, Nitrate is a preferred electron acceptor for growth of freshwater selenate-respiring bactena: Appl. Environ. Microbiol., v. 58, p. 426-428.

The presence of toxic selenium oxyanions in agricultural wastewaters which drain from seleniferous soils is wide-spread in the western United States and poses serious environmental problems. Dissimilatory reduction of selenate, primarily to elemental selenium, occurs in anaerobic sediments. Potential rates of bacterial dissimilatory reduction of 75- SeO4(2-) to 75-Se(0) in a diversity of sediment types were measured. Salinities ranged from freshwater (salinity = 1 g/L) to hypersaline (salinity + 320 g/L) and pH values ranged from 7.1 to 9.8. Significant biological selenate reduction occurred in all samples with salinities from 1 to 250 g/L but not in samples with a salinity of 320 g/L. Potential selenate reduction rates ranged from 0.07 to 22 micromoles of SeO4(2-) reduced/L/hour. Activity followed Michaelis-Menten kinetics in relation to SeO4(2-) concentration. There was no linear correlation between potential rates of SeO4(2-) reduction and salinity, pH, concentrations of total Se, porosity, or organic carbon in the sediments. However, potential selenate reduction was correlated with apparent kinetics reflecting enzymatic affinity for selenate and with potential rates of denitrification. NO3(-), NO2(-), MoO4(2-), and WO4(2-) inhibited selenate reduction activity to different extents in sediments from both Hunter Drain and Massie Slough, Nevada. Sulfate partially inhibited activity in sediment from freshwater Massie Slough samples but not from the saline Hunter Drain samples. Dissimilatory selenate reduction in sediments appears widespread in nature. In addition , in situ selenate reduction is a first-order reaction, because the ambient concentrations of selenium oxyanions in the sediments were orders of magnitude less than their kinetic parameter reflecting enzymatic affinity for selenate. (Mertz-PTT).

0remland, R.S., 1990, Nitrogen fixation dynamics of two diaztrophic communities in Mono Lake, California: Applied and Environmental Microbiology, v. 56, no. 3, p. 614-622.

Two types of diazotrophic microbial communities were found in the littoral zone of alkaline hypersaline Mono Lake, California. One consisted of anaerobic bacteria inhabiting the flocculent surface layers of sediments. Nitrogen fixation (acetylene reduction) in these layers occurred under anaerobic conditions, was not stimulated by light or by additions of organic substrates, and was inhibited by oxygen, nitrate, and ammonia. The second community consisted of a ball-shaped association of a filamentous chlorophyte (Ctenocladus circinnatus) with diazotrophic, nonheterocystous cyanobacteria, as well as anaerobic bacteria (Ctenocladus balls). Nitrogen fixation by Ctenocladus balls was usually, but not always, stimulated by light. Rates of anaerobic dark fixation equaled those in the light under air. Fixation in the light was stimulated by 3-(3 ,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and by propanil (N-(3 ,4-dichlorophenyl)propanamide). DCMU-elicited nitrogenase activity was inhibited by ammonia (96%) and nitrate (65%). Fixation was greatest when Ctenocladus balls were incubated anaerobically in the light with sulfide. Dark anaerobic fixation was not stimulated by organic substrates in short-term (4-h) incubations, but was in long-term (67-h) ones. Areal estimates of benthic nitrogen fixation were measured seasonally, using chambers. Highest rates (about 29.3 micromoles of acetylene/sq m/h) occurred under normal diel regimens of light and dark. These estimates indicate that benthic nitrogen fixation has the potential to be a signifiant nitrogen source in Mono Lake.

Dubrovsky, N.M., J.M. Neil, R. Fujii, R. S. 0remland, and J.T. Hoilibsugh, 1990, Influence of redox potential on selenium distribution in ground water, Mendota, Western San Joaquin Valley, California: U.S. Geological Survey Open-File Report 90-138, 24 p.

Stratigraphy at a site in Mendota, in the western San Joaquin Valley, California consists of about 50 ft of complexly interbedded sediments from Sierra Nevada and Coast Range sources , overlying a thick aquifer of Sierra Nevada origin. Under natural conditions, the site was a groundwater discharge area; however, the water table has declined to its present position of 17 to 27 ft below land surface as a result of extensive pumping from the underlying aquifer. Detailed profiles of groundwater chemistry obtained from wells and from pore water extracted from undisturbed cores showed saline water at shallow depths. Specific conductance peaked at > 20,000 microsiemens/cm at a depth of 36 to 48 ft, and decreased to 3,270 microsiemens/cm at a depth of 60 ft. A sharply bounded zone containing water with selenium concentrations > 100 microg/L was present at a depth between 18 and 28 ft, with a peak of 1,520 microg/L at a depth of 22.5 ft. Selenium and specific conductance were not correlated in groundwater at the site in contrast to the high correlation for groundwater from oxidized Coast Range sediments to the west. Redox indicators suggest that the lack of correlation is due to the removal of selenium from solution by reduction to a less mobile species. This is inferred by the coincidence of the lower boundary of the high selenium zone with: (1) decreasing platinum electrode redox potentials; (2) the disappearance of dissolved oxygen and nitrate; and (3) a rapid increase in iron and manganese concentrations. Laboratory analyses of undisturbed cores indicate that the mechanism of selenium removal is likely microbial reduction to insoluble elemental selenium. The conceptual model for the site consists of the evolution of saline soil with high selenium concentrations in a regional groundwater discharge area prior to agricultural development; downward migration of saline, seleniferous groundwater due to water table drawdown; and selenium removal from the downward-migrating groundwater by microbially mediated redox reactions.

Steinberg, N.A. and 0remland, R.S., 1990, Dissimilatory selenate reduction potentials in a diversity of sediment types: Appi. Environ. Microbial., v. 56, p. 3550-3557.

The presence of toxic selenium oxyanions in agricultural wastewaters which drain from seleniferous soils is wide-spread in the western United States and poses serious environmental problems. Dissimilatory reduction of selenate, primarily to elemental selenium, occurs in anaerobic sediments. Potential rates of bacterial dissimilatory reduction of 75- SeO4(2-) to 75-Se(0) in a diversity of sediment types were measured. Salinities ranged from freshwater (salinity = 1 g/L) to hypersaline (salinity + 320 g/L) and pH values ranged from 7.1 to 9.8. Significant biological selenate reduction occurred in all samples with salinities from 1 to 250 g/L but not in samples with a salinity of 320 g/L. Potential selenate reduction rates ranged from 0.07 to 22 micromoles of SeO4(2-) reduced/L/hour. Activity followed Michaelis-Menten kinetics in relation to SeO4(2-) concentration. There was no linear correlation between potential rates of SeO4(2-) reduction and salinity, pH, concentrations of total Se, porosity, or organic carbon in the sediments. However, potential selenate reduction was correlated with apparent kinetics reflecting enzymatic affinity for selenate and with potential rates of denitrification. NO3(-), NO2(-), MoO4(2-), and WO4(2-) inhibited selenate reduction activity to different extents in sediments from both Hunter Drain and Massie Slough, Nevada. Sulfate partially inhibited activity in sediment from freshwater Massie Slough samples but not from the saline Hunter Drain samples. Dissimilatory selenate reduction in sediments appears widespread in nature. In addition , in situ selenate reduction is a first-order reaction, because the ambient concentrations of selenium oxyanions in the sediments were orders of magnitude less than their kinetic parameter reflecting enzymatic affinity for selenate.

Parkhurst, D.L., 1990, Ion-association models and mean-activity coefficients of various salts, in Bassett, R.L. and Melchior, D., eds., Chemical modeling in aqueous systems II: Washington, D.C., American Chemical Society Symposium Series 4l6, p. 30-43.

Calculations using the aqueous model from WATEQ and an aqueous model modified from WATEQ were compared to experimental mean activity coefficients for various salts to determine the range of applicability and the sources of errors in the models. An ion-association aqueous model was derived by least-squares fitting of ion-association stability constants and individual-ion, activity-coefficient parameters to experimental mean activity coefficients for various salts at 25 C. Salts of the following cations and anions were considered: Al(+3), Ba(+2) , Ca(+2), Cd(+2), Co(+2), Cs(+), Cu(+2), Fe(+2), H(+), K(+), Li( +), Mg(+2), Mn(+2), Na(+), Ni(+2), Pb(+2), Sr(+2), Zn(+2), Cl(-) , ClO4(-), F(-), OH(-), and SO4(-2). The stability constants of the derived model and the WATEQ model were in agreement for most two-ion complexes but were not in agreement for most complexes containing three or more ions. The largest discrepancies in stability constants were for complexes Cu(+2), Mn(+2), Ni(+2), and Zn(+2) with Cl(-). The derived-model calculations matched the experimental data for all salts to a concentration of about 2 molal, but the parameters of the model could not be defined uniquely by the fitting process. Alternative choices for the complexes included in the model and for the individual-ion, activity-coefficient parameters could fit the experimental data equally well. (See also W90-09846)

Plummer, L.N., Busby, J.F., Lee, R.W., and Hanshaw, B.B., 1990, Geochemical modeling of the Madison aquifer in parts of Montana, Wyoming, and South Dakota: Water Resources Research, v. 26, no. 9, p. 1981-2014.

Stable isotope data for dissolved carbonate, sulfate, and sulfide were combined with water composition data to construct geochemical reaction models along eight flow paths in the Madison aquifer in parts of Wyoming, Montana, and South Dakota. All reaction models reproduce the observed chemical and carbon and sulfur isotopic composition of the final waters and are partially validated by predicting the observed carbon and sulfur isotopic compositions of dolomite and anhydrite from the Madison Limestone. The geochemical reaction models indicate that the dominant groundwater reaction in the Madison aquifer is dedolomitization (calcite precipitation and dolomite dissolution driven by anhydrite dissolution). Sulfate reduction, (Ca(2+) + Mg(2+))/Na(+) cation exchange, and halite dissolution are locally important, particularly in central Montana. The groundwater system was treated as closed to CO2 gas from external sources such as the soil zone or cross-formational leakage but open to CO2 from oxidation of organic matter coupled with sulfate reduction and other redox processes occurring within the aquifer. Carbon 14 groundwater ages, adjusted for the modeled carbon mass transfer, range from modern to about 23,000 years B.P. and indicate flow velocities of 2.1-26.5 m/yr. Most horizontal hydraulic conductivities calculated from Darcy 's Law using the average 14C flow velocities are within a factor of 5 of those based on digital simulation. The calculated mineral mass transfer and adjusted 14C groundwater ages permit determination of apparent rates of reaction in the aquifer. The apparent rate of organic matter oxidation is typically 0.12 micromol/L/yr. Sulfate and, to a lesser extent, ferric iron are the predominant electron acceptors. The (kinetic) biochemical fractionation of 34S between sulfate and hydrogen sulfide is approximately -44 ppt at 25 C, with a temperature variation of -0.4 ppt/C. The rates of precipitation of calcite and dissolution of dolomite and anhydrite typically are 0.59, 0.24, and 0.95 micromol/L/yr, respectively.

Plummer, L.N. and Parkhurst, D.L., 1990, Application of the Pitzer Equations to the PHREEQE geochemical model, in Melchoir, D.C., and Bassett, R.L., eds., Chemical modeling of aqueous systems II: American Chemical Society Symposium Series 416, Washington, D.C., American Chemical Society, p. 128-137.

A computer program that simulates geochemical reactions in brines and other concentration electrolyte solutions use the ion-interaction virial-coefficient approach for activity-coefficient corrections developed by Pitzer. Reaction-modeling capabilities include calculation of (1) aqueous speciation and mineral saturation indices, (2) mineral solubilities, (3) mixing or titration of aqueous solutions, (4) irreversible reactions, and (5) reaction paths, including evaporation. The program 's data base of Pitzer interaction parameters includes a partially validated data base at 25 C for the system Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O, and largely untested literature data for Fe(II), Mn(II), Sr, Ba, Li, and Br, with provision for calculations at temperatures other than 25 C. The need to maintain an internally consistent data base of interaction parameters and equilibrium constants is emphasized through an example of the calculated solubility of nahcolite in Na2CO3 solutions. The choice of activity-coefficient scale has particular significance to thermodynamic calculations in brines if the measured pH is introduced. The code was applied to modeling of brine sample speciation on different activity-coefficient scales, calculating invariant point in the evaporation of seawater, and defining the possibilities and limitations in modeling the initial stages of the evaporation of seawater. (See also W90-09846)

Pereira, W.E. and Rostad, C.E., 1990, Occurrence, Distributions and Transport of Herbicides and their degradation products in the lower Mississippi River and its tributaries. Journal of Environmental Science & Technology, v. 24, #9, p. 1400-1406.

The Mississippi River and its tributaries drain extensive agricultural regions of the mid-continental US where large amounts of herbicides are applied as weed control agents on crops such as corn and soybeans. Studies being conducted by the US Geological Survey (USGS) along the lower Mississippi River and its major tributaries, representing a 1930-km river reach, have confirmed that several triazine and chloroacetanalide herbicides and their degradation products are present in this riverine system. These herbicides include atrazine, and its degradation products, desethl-, and desisopropylatrazine; cyanazine; simazine; metolachlor; and alachlor and its degradation products, 2-chloro-2 ',6 ' - diethylacetanalide, and 2-hydroxy-2 ',6 ' - diethylacetanilide. Loads of these compounds were determined at 17 different sampling stations under various seasonal and hydrologic conditions, during five sampling trips from July 1987 to June 1989. Stream loads of herbicides were relatively small during the drought of 1987 and 1988. Stream loads were much greater during the relatively wet year of 1989. Trace levels of atrazine, cyanazine, and metolachlor also were associated with suspended sediments. Distribution coefficients of these compounds varied considerably between sites and were much larger than values reported in the literature. The annual transport of atrazine into the Gulf of Mexico was estimated to be less than 2% of the amount of atrazine applied each year in the midwest.

Pereira, W.E., Rostad, C.E., Leiker, T.J., 1990, Determination of trace levels of herbicides and their degradation products in surface and ground waters by gas chromatographic-ion trap mass spectrometry: Analytica Chimica Acta, v. 228, p. 69-75.

A rapid, specific and highly sensitive method is described for the determination of several commonly used herbicides and their degradation products in the surface and ground waters by using gas chromatography/ion trap mass spectrometry. The compounds included atrazine, and its degradation products, desethylatrazine and deisopropylatrazine; simazine; cyanazine; metolachlor; and alachlor and its degradation products, 2-chloro-2 ',6 ' - diethylacetanilide, 2-hydroxy-2 ',6 ' - diethylacetanilide and 2,6-diethylaniline. The method was applied to surface-water samples collected from 16 different stations along the Mississippi River and its major tributaries, and ground water samples beneath a cornfield in central Nebraska. Average recovery of a surrogate herbicide, terbuthylazine, was greater than 99%. Recoveries of the compounds of interest from river water spiked at environmental levels are also presented. Full-scan mass spectra of these compounds were obtained on 1 ng or less of analyte. Data were collected in the full-scan acquisition mode. Quantitation was based on a signal-to-noise ratio of greater than 10:1.

Johnsson, Patricia A., and Reddy, Michael M., 1990, A monitor for continuous measurement of temperature, pH, and conductance of wet precipitation, Preliminary results from the Adirondack Mountains, New York: Atmospheric Environment, v. 24A, no. 1, p. 233-236.

A continuous wet-only precipitation monitor is presented which was designed by the USGS to record variations in rainfall, temperature, pH and specific conductance at 1 minute intervals over the course of storms. The initial study site was located at Huntington Wildlife Forest at the Adirondack Ecological Center in the central Adirondack Mountains of New York. After six storm events, the preliminary sampling with the monitor showed that rainfall acidity varied over the course of summer storms, with low initial pH values increasing as storm intensity increased.

Revesz, K., and Woods, P., 1990, A method to extract soil water for stable isotope analysis: Journal of Hydrology, v. 115, p. 397- 406.

A method has been developed to extract soil water for determination of deuterium (D) and O18 content. The principle of this method is based on the observation that water and toluene form an azeotropic mixture at 84.1 degrees C but are completely immiscible at ambient temperature. In a specially designed distillation apparatus, the soil water is distilled at 84.1 degrees with toluene and is separated quantitatively in the collecting funnel at ambient temperature. Traces of toluene are removed and the sample can be analyzed by mass spectrometry. Kerosene may be substituted for toluene. The accuracy of this technique is +/-2 and +/-0. 2%, respectively, for delta D and delta O18. Reduced accuracy is obtained at low water contents.

Rosenberry, D.O., 1990, Effect of sensor error on interpretation of long-term water-level data: Ground Water, v. 28, no. 6, p. 927-936.

Submersible strain-gage pressure transducers and potentiometer-float systems were used to monitor water-level fluctuations in wells. Use of uncorrected data from six pressure transducers at a site in New Hampshire would have caused an incorrect interpretation of the vertical hydraulic- head distribution. Seven potentiometer-float systems at a site in Nebraska provided data that indicated a slight attenuation of water-level fluctuations, which likely caused some small fluctuations to go unobserved. Digital data loggers were used at both sites to scan the sensors and calculate hourly and daily average values of water levels in the wells. Check measurements using a chalked steel tape were made weekly at the New Hampshire site and monthly at the Nebraska site. Comparisons of recorded data with check measurements indicated values measured by pressure transducers over periods of weeks to months were irregularly larger or smaller than the check measurement. This error was attributed to a drift of the zero reference point of the sensor, and was unpredictable. Values obtained by potentiometer-float systems lagged the water-level change indicated by the check measurements. The lag was attributed to the sum of the mechanical friction of the moving parts of the monitoring system. This error was somewhat constant and predictable. The average absolute error of six submersible pressure transducers that were in operation for 15 months was 0.098 foot. The average absolute error of seven potentiometer-float systems that were operated for 17 months was 0.027 foot.

McKnight, D.M., Smith, R.L., Bradbury, J.P., Baron, J.J., and Spaulding, S., 1990, Phytoplankton dynamics in Three Rocky Mountain lakes, Colorado, U.S.A.: Arctic and Alpine Research, v. 22, no. 3, p. 264-274.

In 1984 and 1985 seasonal changes in phytoplankton were studied in a system of three lakes in Loch Vale, Rocky Mountain National Park, Colorado, to determine the effects of urban atmospheric deposition. Three periods were evident: (1) A spring bloom, during snowmelt, of the planktonic diatom Asterionella formosa, (2) a midsummer period of minimal algal abundance, and (3) a fall bloom of the blue-green alga Oscillatoria limnetica. Seasonal phytoplankton dynamics in these lakes are controlled partially by the rapid flushing rate during snowmelt and the transport of phytoplankton from the highest lakes to the lower lakes by the stream, Icy Brook. During snowmelt, the A. formosa population in the most downstream lake has a net rate of increase of 0.34/d, which is calculated from the flushing rate and from the A. formosa abundance in the inflow from the upstream lake and in the downstream lake. Measurement of photosynthetic rates at different depths during the three periods confirmed the rapid growth of A. formosa during the spring. The decline in A. formosa after snowmelt may be related to grazing by developing zooplankton populations. The possible importance of seasonal variations in nitrate concentrations were evaluated in in situ enrichment experiments. For A. formosa and O. limnetica populations, growth stimulation resulted from 8 or 16 micromolar amendments of calcium nitrate and sulfuric acid, but the reason for this stimulation could not be determined from these experiments.

Stollenwerk, K.G., and Kipp, K.L., 1990, Simulation of molybdate transport with different rate- controlled mechanisms, in Melchior, D.C., and Bassett, R.L., eds., Chemical modeling of aqueous systems II: ACS Symposium Series No. 416, Washington, D.C., American Chemi- cal Society, chap. 19, p. 243- 257.

Laboratory column experiments identified potential rate-controlling mechanisms that could affect transport of molybdate in a natural-gradient tracer test conducted at Cape Cod, Massachusetts. Column-breakthrough curves for molybdate were simulated by using a one-dimensional solute-transport model modified to include four different rate mechanisms: equilibrium sorption, rate-controlled sorption, and two side-pore diffusion models. The equilibrium sorption model failed to simulate the experimental data, which indicated the presence of a rate-controlling mechanisms. The rate-controlled sorption model simulated results from one column reasonably well, but could not be applied to five other columns that had different input concentrations of molybdate without changing the reaction-rate constant. One side-pore diffusion model was based on an average side-pore concentration of molybdate (mixed side-pore diffusion); the other on a concentration profile for the overall side-pore depth (profile side-pore diffusion). The mixed side-pore diffusion model gave a reasonable correlation with experimental data, and the parameters could be used for a variety of input concentrations. However, the profile side-pore diffusion model gave the most accurate simulations for the largest variety of input concentrations.

Striegl, R.G., and Healy, R.W., 1990, Transport of 14-carbon dioxide in unsaturated glacial and eolian sediments, in Bassett, R.L., and Melchoir, D.C., eds., Chemical Modeling in Aqueous Systems II: American Chemical Society Symposium Series, no. 416, chap. 15, p. 202- 210.

Measurements of losses of CO2 to unsaturated sediment-water mixtures indicate that diffusion of CO2(14) in the unsaturated zone may be substantially retarded by isotopic exchange of C14 to an adsorbed inorganic C phase. Two geochemical models for calculating CO2(14) retention in the unsaturated zone were compared. The first was a theoretical model based on calcite equilibrium control (calcite equilibrium model), and the second was an empirical model based on measured losses of CO2 from a surrogate unsaturated zone atmosphere to unsaturated water-sediment mixtures (CO2 retention model). The first model accounted only for CO2(14) retention caused by C14 dilution to dissolved inorganic carbon. The second accounted for additional C14 dilution to an adsorbed C phase predicted from CO2(14)-loss experiments. The geochemical models were separately coupled with a two-dimensional, finite-difference model for gas diffusion to simulate the distribution of pCO2(14) in the unsaturated zone near a disposal trench at a low-level radioactive waste-disposal site near Sheffield, Illinois. Comparison of simulated pCO2(14) distribution with onsite data supported the presence of the adsorbed C phase. (See also W90-09846)

Kim, B.K., Jackman, A.P., and Triska, F.J., 1990, Modeling transient storage and nitrate uptake kinetics in a flume containing a natural periphyton community: Water Resources Research v. 26, no. 3, p. 505-515.

An existing transport model including storage zones was combined with a submodel describing biotic retention of nutrient based on Michaelis-Menten kinetics. This transport/retention model was used to simulate the results of an experiment in which the inflows to flumes containing nitrate-limited natural stream periphyton on artificial substrates were amended with chloride and nitrate. Hydrodynamic parameters were determined by fitting the model to the chloride data. The Michaelis-Menten maximum uptake rate parameter was determined by fitting the model to the nitrate data. The transport/retention model accurately simulated the responses of the flumes to the amendments. Independent batch experiments to determine Michaelis-Menten parameters were performed on periphyton from a control flume. The maximum uptake rate parameters from the two experiments are in good agreement. Both experiments indicate possible inadequacies of the Michaelis-Menten kinetic model for describing nutrient uptake in a complex field community.

Triska, F.J., Kennedy, V.C., Avanzino, R.J., Zellweger, G.W., and Bencala, K.E., 1990, In situ retention-transport response to nitrate loading and storm discharge in a third-order stream: Journal of the North American Benthological Society, v. 9, no. 3, p. 229-239.

The retentive properties of channels are the most critical to biota, because retention sustains the lotic community. Nitrate retention was assayed in a 264-m reach of a third-order stream, Little Lost Man Creek, Humboldt County, California. Nitrate budgets (24-48 h) were calculated under background conditions, and during 4 other intervals of modified nitrate concentration caused by nutrient amendment or storm-enhanced discharge. Under background, low-flow conditions, the reach was a source of nitrate to downstream communities. Retention during the first 36 h of nitrate amendment was dominated by storage in the hyporheic zone and later by biotic uptake as storage zones became saturated (plateau concentration). The increase in net retention caused by increased ni trate concentration decreased the output/input (O/I) ratio from 1.11 before amendment to 0.61 after 36 h, and to 0.86 after transient storage zones were filled. Dilution, caused by a nearly 4-fold increase in discharge, increased biotic retention and also export as previously stored nitrate leached from the hyporheic zone into the channel. Nitrate continued to leach from the hyporheic zone 7 d after the amendment ended. This type of response may enhance biotic nutrient cycling by providing waters of higher nutrient concentration to partially scoured epilithic surfaces following reset of the benthic community by a major storm.

Triska, F.J., Duff, J.H., and Avanzino, R.J., 1990, Influence of exchange flow between the channel and hyporheic zone on nitrate production in a small mountain stream: Canadian Journal of Fisheries and Aquatic Sciences, v. 47, no. 11, p. 2099-2111.

Variation in a local exchange of flows between the channel and hyporheic zone produced temporally shifting concentration gradients of dissolved oxygen, nitrate, and ammonium in subsurface waters of a small, gravel-cobble bed stream. Channel water advected laterally supplied dissolved oxygen, and groundwater supplied ammonium to support hyporheic nitrification. Nitrate production was highest in sediment slurries from aerobic hyporheic sites, was absent at nearly anoxic sites, and was stopped by nitrification inhibitors (chlorate and nitrapyrin). Ammonium amendment to sediment slurries only slightly enhanced nitrate production indicating that sorption competed with biota for available substrate. Nitrate concentrations increased from 75 to 130 microg N/L during 9 days of ammonium amendment to a hyporheic subsurface flow. Ammonium concentrations rose slowly initially relative to a sulfate tracer and declined slowly after cutoff as ammonium desorbed. Nitrate levels remained elevated for 6 days after cutoff as desorbed ammonium became biotically available. Interactions between the channel's hydrology, lithology, and biology observed in nitrate production are probably more common than reported. However, the magnitude of the resulting nutrient flux will depend on factors which determine the depth and lateral extension of suitable hyporheic habitat.

White, A. F., Peterson, M. L. and Solbau, R. D., 1990, Measurement and interpretation of low levels of dissolved oxygen in ground water: Ground Water v. 28, p. 584-590.

A Rhodazine-D colorimetric technique was adapted to measure low-level dissolved oxygen concentrations in groundwater. Prepared samples containing between 0 and 8.0 micromoles/l dissolved oxygen in equilibrium with known gas mixtures produced linear spectrophotometric absorbance with a lower detection limit of 0.2 micromoles/l. Excellent reproducibility was found for solutions ranging in composition from deionized water to sea water with chemical interferences detected only for easily reduced metal species such as ferric ion, cupric ion and hexavalent chromium. Such effects were correctable based on parallel reaction stoichiometries relative to oxygen. The technique, coupled with a downhole wire line tool, permitted low-level monitoring of dissolved oxygen in wells at the selenium-contaminated Kesterson Reservoir in California. Results indicated a close association between low but measurable dissolved oxygen concentrations and mobility of oxidized forms of selenium in the shallow aquifer which were in general electrochemical disequilibrium.

White, A. F. and Peterson, M.L., 1990, Role of reactive-surface-area characterization in geochemical kinetic models: in Melchior, D. C. and Bassett, R. L. eds., Chemical Modeling of Aqueous Systems II: Amer. Chem. Soc. Symp. Series p. 416-475

Modeling of kinetic and sorption reactions requires estimates of the surface area of natural substrates which are often difficult to measure directly. A synthesis of data indicates that the Brunauer, Emmett, and Teller (BET) method for measurements on fresh surfaces exceed geometric estimates by a mean roughness factor of 7 over a wide range in particle sizes. Surface roughness factors for naturally weathered silicates are shown to approach 200 and are strongly dependent on mineral composition. Fractal analysis indicates a dimension of 2.0, and a self similarity comparable to a smooth spherical geometry. Estimates of reactive surface areas are commonly related through transition state theory to the surface defect density. Data indicate, however, that the actual surface dislocation density is lower than commonly assumed and the number of surface dislocations that actually represent potential reaction sites must be extremely low. A compilation of available kinetic models indicates that reactive surface areas commonly are one to three orders-of-magnitude lower than physical surface areas, with closer fits for geochemical systems having short residence times.

Winter, T. C. and Woo, M-K., 1990, Hydrology of lakes and wetlands, in Wolman, M. G., and Riggs, H. C., eds. Surface Water Hydrology: Boulder, Colorado, Geological Society of America, The Geology of North America, v. 0-1, p. 159-187.

The existence of lakes and wetlands depends on the specific geologic setting that favors the ponding of water, and on the hydrologic processes that allow the body of water to persist at a given site. Geologic settings favorable to both lakes and wetlands include topographic depressions, slope discontinuities, subsurface stratigraphy, and permafrost. Hydrologic processes related to the water balance of lakes and wetlands depend on interactions with atmospheric water, surface water, and groundwater. The hydrologic characteristics of lakes and wetlands in different parts of North America may be compared on the basis of water balance. The water balance of small Arctic lakes is characterized by a large influx of snowmelt, lack of water exchange with deep groundwater due to the presence of permafrost, and a relatively short evaporation season due to the lengthy duration of ice cover. Small lakes in the subarctic respond quickly to water inflow because of their limited storage capacity. The water budget of the Canadian Shield lakes are dominated by surface water. In the Great Lakes, most of the available runoff is derived from the northern and eastern sections of the basin and annual precipitation increases southward. Lakes and wetlands in mountainous regions outside the Arctic have significant water input from the higher elevations in the drainage basins. The principal differences in the hydrology of the eastern and western lakes of the glacial terrain are related to climate: precipitation increases and evaporation rates decrease to the east, and stream inflow and outflow predominate in the east. The water balance of lakes and wetlands located on flood plains is dominated by the river. Direct input of water in most desert lakes is nearly all from streamflow. The water balance of sinkhole lakes in karst terrain depends mainly on precipitation and evaporation. Lakes and wetlands dominated by surface water inflow and outflow largely reflect the chemistry of the river; those that gain the most water from surface water inflow and lose the most water by evapotranspiration, (such as desert lakes) are an ultimate sink for chemicals; those dominated by water gains from the atmosphere are generally chemically dilute; and, those dominated by groundwater inflow generally have chemical characteristics similar to groundwater.

Wood, W.W., Kraemer, T.F., and Hearn, Jr., P.P., 1990, Intergranular diffusion: An important mechanism influencing solute transport in clastic aquifers?: Science, v. 247, p. 1569-1572.

Intragranular porosity of sand-size material from an aquifer on Cape Cod, Massachusetts was identified and quantified by SEM, mercury injection and epifluorescence techniques. Results from laboratory and field tracer tests show solute nonequilibrium for a reacting ion consistent with a model of diffusion into, and exchange within, grain interiors. These observations have several important implications including: (1) The presence of more reaction sites than indicated by standard short-term laboratory tests; (2) The necessity of including a diffusion expression in transport codes and; (3) Actual porosity greater than that estimated by normal grain-size and packing techniques.

Wood, W.W., and Sanford, W.E., 1990, Ground-water control of evaporite deposition: Economic Geology, v. 85, p. 1226-1235.

Some topographically closed basins may be hydrologically open as a result of seepage losses to underlying or surrounding ground-water systems. In such cases, these losses can have a substantial control over the suite and thicknesses of evaporite minerals formed in the basin. The ratio of ground- water outflow-to-inflow (flux ratio) in hydrologically open basins is as important in determining mineralogy and thicknesses of evaporite deposits as the solute composition of the inflow water. Attainment of steady-state flux ratios permits large thicknesses of two or three minerals to form rather than thin veneers of many minerals. The presence or absence of glauberite, mirabilite, halite, bloedite, polyhalite and hexahydrite, caused by subtle changes in the ground-water seepage is illustrated using an example from the Southern High Plains of Texas and New Mexico. However, the model is general and is applicable with any solute composition including that of sea water and the use of surface rather than ground water. An analytical, lumped-parameter, solute mass-balance model is developed to define the concept of ground-water flux ratio as it applies to a topographically closed basins in which evaporation exceeds precipitation. Diffusion, advection, and density-driven flow are proposed as mechanisms by which solutes can escape to the ground water from these closed basins. The geochemical-reaction computer program PHRQPITZ is used to document the effects of various flux ratios on the mineralogy and thickness of deposits. Solute analyses used in conjunction with the model can be used to screen prospective basins as well as providing insight into exploratory drilling.

Vroblesky, D.A., and Yanosky, T.M., 1990, Use of tree-ring chemistry to document historical ground-water contamination events: Ground Water, v. 28, p. 677-684.

The annual growth rings of tulip trees (Liriodendron tulipifera L.) appear to preserve a chemical record of groundwater contamination at a landfill in Maryland. Zones of elevated iron and chlorine concentrations in growth rings from trees immediately downgradient from the landfill are closely correlated temporally with activities in the landfill expected to generate iron and chloride contamination in groundwater. Successively later iron peaks in trees increasingly distant from the landfill along the general direction of groundwater flow imply movement of iron contaminated groundwater away from the landfill. The historical velocity of iron movement (2 to 9m/yr) and chloride movement (at least 40 m/yr) in groundwater at the site was estimated from element concentration trends of trees at successive distances from the landfill. The tree ring derived chloride transport velocity approximates the known groundwater velocity (30-80 m/yr). A minimum horizontal hydraulic conductivity (0.01 to 0.02 cm/s) calculated from chloride velocity agrees well with values derived from aquifer tests (about 0.07 cm/s) and from groundwater modeling results (0.009 to 0.04 cm/s).

Webb, R.H., and Betancourt, J.L., 1990, Climatic effects on flood frequency: An example from southern Arizona, in Betancourt, J.L. and MacKay, A.M., eds., Proceedings of the Sixth Annual Pacific Climate (PACLIM) Workshop, Asilomar, California, March 5-8, 1989: California Department of Water Resources, Interagency Ecological Studies Program, Technical Report 23,p. 61-66.

After 1960, the Santa Cruz River at Tucson, Arizona, an ephemeral stream normally dominated by summer floods, experienced an apparent increased frequency of flooding coincident with an increased percentage of annual floods occurring in fall and winter. This shift reflects large-scale and low-frequency changes in the eastern Pacific Ocean, in part associated with El Nino-Southern Oscillation (ENSO) phenomena. Using method of moments and a log Pearson type III distribution, the 100-year flood associated with ENSO conditions is double that for non-ENSO conditions. Statistical analysis of floods caused by different storm types yielded a 100-year flood for 1960-1986 that is about five times larger than that for 1930-1959. For 1960-1986, annual flood frequency above the 10-year recurrence interval is dominated by floods caused by dissipating tropical cyclones; for 1930-1959 annual flood frequency is dominated by floods caused by monsoonal storms. One pitfall with mixed-population analysis is the inability to assign uncertainties to discharges at given recurrence intervals. Presently, there is no method available for estimating standard errors of estimate or confidence limits for mixed populations. Second, the assumption that floods are stationary for ENSO or non-ENSO conditions or during each of the periods may be incorrect. Nevertheless, divergent flood frequencies for ENSO vs. non-ENSO, different storm types, and for the three periods in the twentieth century show that climatic effects are time variant. Consequently, some of the assumptions needed for standard flood-frequency analysis may not be valid. (See also W91-09238)

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