One of the common methods for determining DOC concentrations in water samples used wet oxidation with persulphate. The effects of chloride ions on the determination of DOC by this method were investigated in a gas-tight reaction chamber at a temperature of 100C for 5 minutes. The presence of chloride in concentrations greater than 0.02 M interfered with the analysis of aqueous DOC concentrations by the wet oxidation method when a reaction time of 5 minutes was used. Chloride competed with DOC for persulphate, lowering the overall oxidation efficiency. The oxidation of chloride produced hypochlorous acid which reacted with DOC to produce chlorinated intermediate compounds. The interference could be removed by increasing the reaction time or by diluting samples so that the chloride concentration was less than 0.02 M. A method that used persulphate oxidation combined with UV irradiation was unaffected by the presence of chloride.
Aiken, G.R., McKnight, D.M., Thorn, K.A., and Thurman, E.M., 1992, Isolation of hydrophilic organic acids from water using nonionic macroporous resins: Organic Geochemistry, v. 18, no. 4, p. 567-573.
A method was developed for the isolation of hydrophilic organic acids from aquatic environments using Amberlite XAD-4 resin. The method used a 2 column array of XAD-8 and XAD-4 resins in series. The hydrophobic organic acids, composed primarily of aquatic fulvic acid, were removed from the sample on XAD-8, followed by the isolation of the more hydrophilic organic acids on XAD-4. For samples from several diverse environments, more of the dissolved organic carbon was isolated on the XAD-8 resin (23-58 per cent) than on the XAD-4 resin (7-25 per cent). For these samples, the hydrophilic acids have lower carbon and hydrogen contents, higher oxygen and nitrogen contents, and are lower in molecular weight than the corresponding fulvic acids. Carbon-13 NMR analyses indicated that the hydrophilic acids have a lower concentration of aromatic carbon and greater heteroaliphatic, ketone and carboxyl content than the fulvic acids.
Alpers, C.N., Nordstrom, D.K., and Burchard, J.M., 1992, Compilation and interpretation of water quality and discharge data for acid mine waters at Iron Mountain, Shasta County, California, 1940-91: U.S. Geological Survey Water-Resources Report, 91-4160, 173 p.
This report contains a compilation and interpretation of the historical records of water quality and discharge for the period 1940-91 from the two most significant discharge points for acid mine drainage at Iron Mountain, Shasta County, California--the Richmond and Lawson portals. The primary objective is to formulate a conceptual model of subsurface processes that accounts for trends with time of water quality and discharge volume from the two mine portals. It is proposed that Zn/Cu ratios in the effluent waters are controlled by alternating periods of precipitation and dissolution of Fe-sulfate minerals such as melanterite ((Fe(II), Zn, Cu) SO4*7H2O)). Copper is concentrated relative to zinc in the solid phases formed. Flushing of the efflorescent salts during periods of rapid infiltration causes a rapid decrease in the Zn/Cu ratio of the mine waters. Differences in the Zn/Cu ratio at low flow conditions between the Richmond and Lawson portal effluents indicate the independent generation of acid drainage from the Richmond and Lawson/Hornet mine workings. Remediation efforts at Iron Mountain need to account for the continued generation of acid mine drainage from the Lawson portal after any plugging or remediation is attempted in the Richmond mine workings.
Baedecker, P.A., Reddy, M.M., Reimann, K.J., and Sciammarella, C.A., 1992, Effects of acidic deposition on the erosion of carbonate stone--Experimental results from the U.S. National Acid Precipitation Assessment Program (NAPAP): Atmospheric Environment, v. 26B, no. 2, p. 147-158.
One of the goals of NAPAP-sponsored (National Acid Precipitation Assessment Program) research on the effects of acidic deposition on carbonate stone has been to quantify the incremental effects of wet and dry deposition of hydrogen ion, sulfur dioxide and nitrogen oxides on stone erosion. Test briquettes and slabs of freshly quarried Indiana limestone and Vermont marble were exposed to ambient environmental conditions in a long-term exposure program. Physical measurements of the recession of test stones exposed to ambient environmental conditions at an angle of 30 degrees to horizontal at the five NAPAP materials exposure sites ranged from approximately 15 to 30 micrometer/year for marble, and approximately 25 to 45 micrometers/year for limestone. These values were roughly double the recession estimates based on the observed calcium of runoff solutions from test slabs. The difference between the physical and chemical recession measurements is attributed to the loss of mineral grains from the stone surfaces that are not measured in the runoff experiments. The erosion due to grain loss does not appear to be influenced by rainfall acidity, although preliminary evidence suggests that grain loss may be influenced by the dry deposition of sulfur dioxide between rainfall events. Chemical analysis of the run-off solutions suggests that approximately 30% of erosion by dissolution can be attributed to wet deposition of hydrogen ion and the dry deposition of sulfur dioxide and nitric acid between rain events. The remaining 70% of erosion by dissolution is accounted for by the solubility of carbonate stone in rain that is in equilibrium with atmospheric carbon dioxide ('clean rain').
The effect of particle size, mineralogy and sediment organic carbon (SOC) on sorption of tetrachlorobenzene and pentachlorobenzene was evaluated using batch-isotherm experiments on sediment particle-size and mineralogical fractions from a sand and gravel aquifer, Cape Cod, Massachusetts. Concentration of SOC and sorption of chlorobenzenes increase with decreasing particle size. For a given particle size, the magnetic fraction has a higher SOC content and sorption capacity than the bulk or non-magnetic fractions. Sorption appears to be controlled by the magnetic minerals, which comprise only 5-25% of the bulk sediment. Although SOC content of the bulk sediment is less than 0.1%, the observed sorption of chlorobenzenes is consistent with a partition mechanism and is adequately predicted by models relating sorption to the octanol/water partition coefficient of the solute and SOC content. A conceptual model based on preferential association of dissolved organic matter with positively-charged mineral surfaces was developed that describes micro-scale, intergranular variability in sorption properties of the aquifer sediments.
A dissimilatory Fe(III)- and Mn(IV)-reducing bacterium was isolated from bottom sediments of the Great Bay estuary, New Hampshire. The isolate was a facultatively anaerobic gram-negative rod which did not appear to fit into any previously described genus. It was temporarily designated strain BrY. BrY grew anaerobically in a defined medium with hydrogen or lactate as the electron donor and Fe(III) as the electron acceptor. BrY required citrate, fumarate, or malate as a carbon source for growth on H2 and Fe(III). With Fe(III) as the sole electron acceptor, BrY metabolized hydrogen to a minimum threshold at least 60-fold lower than the threshold reported for pure cultures of sulfate reducers. This finding supports the hypothesis that when Fe(III) is available, Fe(III) reducers can outcompete sulfate reducers for electron donors. Lactate was incompletely oxidized to acetate and carbon dioxide with Fe(III) as the electron acceptor. Lactate oxidation was also coupled to the reduction of Mn(IV), U(VI), fumarate, thiosulfate, or trimethylamine n-oxide under anaerobic conditions. BrY provides a model for how enzymatic metal reduction by respiratory metal-reducing microorganisms has the potential to contribute to the mobilization of iron and trace metals and to the immobilization of uranium in sediments of Great Bay Estuary.
Trace element bioaccumulation was studied in immature benthic insects from two contaminated river systems to develop these animals as bioindicators. In one river, Cu, Cd, Pb, and Zn were analyzed in insects and in fine bed sediments over a 381-km reach downstream of a large copper mining complex. In the other river, As contamination from a gold mine was assessed in insect and bed sediments over a 40-km reach. All insect taxa collected in contaminated river reaches had elevated whole-body trace element concentrations. However, direct comparisons of contamination using a single, common species among stations were limited because few species were distributed throughout the study reaches. Comparisons of contamination at taxomic levels higher than species were complicated by element-specific differences in bioaccumulation among taxa. These differences appeared to be governed by biological and hydrogeochemical factors. The variation in element concentrations among species of the caddisfly Hydropsyche was slightly greater than within individual species. If this genus is representative of others, comparisons of contamination within genera may be a practical alternative for biomonitoring studies when single species are not available.
Sections of vegetated saltmarsh (Spartina alterniflora, short form) were isolated in 6 tidally-regulated experimental chambers and water samples were collected at half-hourly intervals from different depths during 4 complete tidal cycles during the growing season. Spectrophotometric analyses showed that under conditions of low initial nutrient concentration, both ammonium and phosphate were released from marsh soil to the water column. Diffusion and advection could not account for the total nutrient flux, and it was proposed that organic matter mineralization could increase nutrient concentrations in surface sediments during low tide, with subsequent fluxes into the flooding water column due to bioturbation or physical mixing. In short-term fixed-level water column incubation experiments, phosphate assimilation was similar to phosphate release from marsh sediments, while ammonium release exceeded water column uptake. Average calculated molecular diffusion rates from soil to water column were 2.2 and 0.85 mmoles/m2 for phosphate and ammonium, respectively, and water balance calculations showed no advective flux of soil porewater to the overlying tidal water. Dissolved ammonium concentrations in the adjacent creek varied from 8.8 to 14.1 during 1 tidal cycle, suggesting a net export from the mainland marshes, while the tidal pattern for phosphates was less distinct.
Field measurements and bioassay experiments were coupled to investigate the interdependent processes affecting phytoplankton biomass at Lake Tahoe using a trace metal protocol. Water samples were analyzed for suspended particulate matter, dissolved organic carbon, major ions and macronutrients, adenosine triphosphate, and phytoplankton abundance. Concentrations of total Cd (less than or equal 18 picomolar), Cu (2.25-8.85 nanomolar), and Fe (22-49 nanomolar) were similar to or lower than those reported for other oligotrophic lakes. Bioassays were carried out to assess the response of inoculated, single-species diatom populations (Cyclotella meneghiniana and Aulocosiera italica) to additions of synthetic chelators and phosphate. A chemical speciation model along with the field data also was used to predict how trace metal speciation, and hence bioavailability, was affected by the chelator additions. Results suggest that phosphate was limiting to phytoplankton biomass. Other solutes, Fe in particular, also may exert controls on biomass. Nitrate limitation seems less likely, although Fe-limiting conditions, as suggested by the bioassays, may have led to an effective N limitation because algae require Fe to carry out nitrate reduction. Small perturbations in water chemistry may have pronounced effects on phytoplankton biomass in oligotrophic systems where essential nutrients are at low concentrations.
The Middendorf aquifer of South Carolina exhibits a 40-kilometer-wide zone where dissolved ferrous iron concentrations commonly exceed 1 mg/L. Downgradient of this zone , dissolved iron concentrations decrease to less than 0.05 mg/L. Geochemical and microbiologic evidence indicates that this zonation reflects the competitive exclusion of sulfate-reducing activity by Fe(III)-reducing bacteria in the high-iron zone and the emergence of sulfate reduction as the predominant process in the low-iron zone. Viable Fe(III)-reducing and sulfate-reducing bacteria coexist throughout the aquifer. However, the observed linear relationship between dissolved iron and dissolved inorganic carbon as well as the lack of sulfate consumption indicates that sulfate-reducing bacteria are much less active than Fe(III)-reducing bacteria in the high-iron zone. Fe(III)-reducing bacteria appear to exclude sulfate-reducing activity by maintaining dissolved hydrogen, formate, and acetate concentrations at levels lower than thresholds required by sulfate-reducing bacteria. Downgradient of the high-iron zone, Fe(III)-reducing activity becomes limited by a lack of Fe(III) oxyhydroxides as Middendorf sediments become progressively more marine in origin. Hydrogen, formate, and acetate concentrations then increase to levels that allow sulfate-reducing bacteria to become active. Increased sulfide production strips ferrous iron from solution by precipitating ferrous sulfides, and dissolved iron concentrations decrease. The observed high-iron zonation is thus one manifestation of microbial competition for scarce substrates. The wide occurrence of similar water-chemistry patterns implies that microbial competition mechanisms are important to the groundwater geochemistry of many hydrologic systems.
The theoretical momentum (Boussinesq) coefficient and energy (Coreolis) coefficient for turbulent shear flow in circular pipes and wide channels were derived from the power law, then compared with their counterparts on the basis of the logarithmic law. For such unidirectional flows, the exponent of the power-law velocity distribution was the sole parameter that determined the values of the momentum and energy coefficients. A comparison of the corresponding expressions for these coefficients derived using both laws showed that their values differed only slightly, provided the value of the exponent was much less than one. Their differences increased significantly as the exponent value approached one. Use of a power-law-based momentum coefficient expression along with a power-law resistance formula in the cross-section-averaged equation of momentum could not only eliminate the theoretical incompatibility that resulted from mixing logarithmic and power laws in the equation, but also removed the inconsistent assumption of a momentum coefficient equal to one that was made in the equation. Improvement in the accuracy of the flow computation by incorporating a power-law-based momentum coefficient expression in the equation of momentum was demonstrated through an example in which a new stability limit for the Froude number for turbulent shear flow in wide channels was accurately derived without assuming that the momentum coefficient was equal to one. Two consistent expressions for the Froude number as functions of the exponent were developed using the power-law-based expression for the momentum coefficient: one for hydraulically-smooth flows and the other for fully rough flows. It was concluded that incorporating such Froude number vs. exponent relations in the flow resistance term along with the use of the power law-based expression for the momentum coefficient in the equation of momentum could improve the accuracy of flow computations for turbulent shear flow in open channels.
A dynamic coupling has been observed between the input of turbulent kinetic energy from wind and the pattern of phytoplankton spatial distribution, operating at time scales of weeks or less. It has been suggested that this generality may be extended to longer time scales. For example, if phytoplankton spatial patterns respond to hourly-daily fluctuations in wind stress, very large changes in the distribution of phytoplankton might be expected when temperate lakes freeze over and become isolated for months from the input of wind energy. To test this hypothesis, phytoplankton biomass (as chlorophyll a concentration) was mapped seasonally within 2 small lakes in the glacial terrain of northern Minnesota. The degree of phytoplankton patchiness during winter, under ice cover, was found to be significantly different from the degree of patchiness observed during the open water season for both lakes. Although measurements represented coarse-resolution (in time and space) sampling, they are consistent with the hypothesis that spatial pattern, just like biomass and community composition, can be a seasonally dynamic property of phytoplankton populations. It was concluded that the seasonal isolation of temperate lakes from the input of wind energy by ice cover can produce a physical environment that promotes spatial heterogeneity of plankton, and can provide an example of how scales of biological variability can be related to scales of physical variability.
Three methods of solving nonlinear least-squares problems were compared for robustness and efficiency using a series of hypothetical and field problems. A modified Gauss-Newton/full Newton hybrid method (MGN/FN) and an analogous method for which part of the Hessian matrix was replaced by a quasi-Newton approximation (MGN/QN) solved some of the problems with appreciably fewer iterations than required using only a modified Gauss-Newton (MGN) method. In these problems, model nonlinearity and a large variance for the observed data apparently caused MGN to converge more slowly than MGN/FN or MGN/QN after the sum of squared errors had almost stabilized. Other problems were solved as efficiently with MGN as with MGN/FN or MGN/QN. Because MGN/FN can require significantly more computer time per iteration and more computer storage for transient problems, it is less attractive for a general purpose algorithm than MGN/QN.
Uncertainty in the mass flux for advection-dominated solute movement in heterogeneous porous media was investigated using a previously developed Lagrangian framework. Expressions for the covariance of the mass flux and cumulative mass flux were derived as functions of the injection volume and sampling area size relative to the scale of heterogeneity. The result is illustrated for solute advection in three types of heterogeneous porous media: stratified formations and two-dimensional and three-dimensional porous media. Small perturbation approximation was used for the two-dimensional and three-dimensional cases. Variances of the mass flux and cumulative mass flux were evaluated as functions of the injection volume (area) scale versus log-hydraulic conductivity integral scale. The greatest decrease in coefficient of variation (CV) of the mass flux is for the source scale 1-5 times the hydraulic conductivity integral scale. Further increase in the source size decreases CV comparatively less. The variance of the cumulative mass flux (or total discharge) indicates that for the source size of 20 hydraulic conductivity integral scales, the transport conditions are almost ergodic. The present results also indicate that the cumulative mass flux is a relatively robust quantity for describing field-scale solute transport.
It is common to represent solute transport in heterogenous formations in terms of the resident concentration C(x, t), regarded as a random space function. An alternative representation by q, the solute mass flux at a point of a control plain normal to the mean flow, was investigated. This representation is appropriate for many field applications in which the variable of interest is the mass of solute discharged through a control surface. A general framework to compute the statistical moments of q and of the associated total solute discharge Q and mass M was established. With x the direction of the mean flow, a solute particle is crossing the control plane at h(eta), z (zeta) and at the travel (arrival) time t (tau). The associated expected solute flux is proportional to the joint probability density function (pdf) g1 of h, z, and t, whereas the variance of depends on the joint pdf g2 of the same variables for two particles. In turn, the statistical moments of h, z, and t depend on those of the velocity components through a system of stochastic ordinary differential equations. For a steady velocity field and neglecting the effect of pore-scale dispersion, a major simplification of the problem results in the independence of the random variables h, z, and t. As a consequence, the pdf of h and z can be derived independently of t. A few approximate approaches to derive the statistical moments of h, z, and t were obtained.
A one-dimensional prototype geochemical transport model was developed in order to handle simultaneous precipitation-dissolution and oxidation-reduction reactions governed by chemical equilibria. Total aqueous component concentrations are the primary dependent variables, and a sequential iterative approach is used for the calculation. The model was verified by analytical and numerical comparisons and is able to simulate sharp mineral fronts. At a site in Denmark, denitrification has been observed by oxidation of pyrite. Simulation of nitrate movement at this site showed a redox front movement rate of 0.58 m/yr, which agreed with calculations of others. It appears that the sequential iterative approach is the most practical for extension to multidimensional simulation and for handling large numbers of components and reactions. However, slow convergence may limit the size of redox systems that can be handled.
A quasi-three-dimensional, finite difference model that simulates freshwater and saltwater flow separated by a sharp interface has been used to study the layered coastal aquifer system of the Purisima Formation in the Soquel-Aptos basin, Santa Cruz County, California. The model has been used to evaluate the potential for seawater intrusion in the Soquel-Aptos basin. Groundwater extracted from the system has come mainly from capture of stream baseflow, rather than from reduction of offshore freshwater discharge. 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. The rate of movement of the interface in response to increased groundwater pumpage that has occurred over the last 50 years is probably of the same order of magnitude as the longer term responses. These results have implications for understanding the long-term development of the Soquel-Aptos system.
Preparative isolation and fractionation procedures coupled with spectrometric analyses were used to identify surfactant-derived contaminants in sewage effluent and sewage-contaminated groundwater from a site located on Cape Cod, Massachusetts. Anionic surfactants and their biodegradation intermediates were isolated from field samples by ion exchange and fractionated by solvent extraction and adsorption chromatography. Fractions were analyzed by 13C nuclear magnetic resonance spectrometry and gas chromatography-mass spectrometry. Carboxylated residues of alkylphenol polyethoxylate surfactants were detected in sewage effluent and contaminated groundwater. Linear alkylbenzenesulfonates (LAS) were identified in sewage effluent and groundwater. Groundwater LAS composition suggested preferential removal of select isomers and homologs due to processes of biodegradation and partitioning. Tetralin and indane sulfonates (DATS), alicyclic analogs of LAS, were also identified in field of LAS and DATS in groundwater suggested persistence of alicyclic contaminant structures over those of linear structure. Sulfophenyl-carboxylated (SPC) LAS biodegradation intermediates were determined in sewage effluent and groundwater. Homolog distribution suggested that SPC containing 3-10 alkyl-chain carbons persist during infiltration and groundwater transport. Surfactant-derived residues detected in well F300-50 groundwater have a minimum residence time in the range of 2.7-4.6 yr. LAS detected in groundwater at 500 m from infiltration has been stable over an estimated 50-500 half lives.
Terpenes are produced abundantly by environmental processes but are found in very low concentrations in natural waters. Aqueous photolysis of solutions containing alpha-pinene, a representative terpene, in the presence of humic acid resulted in degradation of the pinene. Comparison of this reaction to photolysis of alpha-pinene in the presence of methylene blue leads to the conclusion that the reactive pathway for the abiotic degradation of alpha-pinene is due to reaction with singlet oxygen produced by irradiation of the humic material. The initial product of singlet oxygen and alpha-pinene is a hydroperoxide. Since humic materials are prevalent in most natural waters, this mechanism of photodecomposition for alpha-pinene probably also applies to other terpenes in surface waters and may be reasonably considered to contribute to their low environmental concentration.
During unsteady or transient groundwater flow, the fluid mass per unit volume of aquifer changes as the potentiometric head changes, and solute transport is affected by this change in fluid storage. Three widely applied numerical models of two-dimension al transport partially account for the effects of transient flow by removing terms corresponding to the fluid continuity equation from the transport equation, resulting in a simpler governing equation. However, fluid- storage terms remaining in the transport equation that change during transient flow are, in certain cases, held constant in time in these models. For the case of increasing heads, this approximation, which is unacknowledged in these models' documentation, leads to transport velocities that are too high, and increased concentration at fluid and solute sources. If heads are dropping in time, computed transport velocities are too low. Using parameters that somewhat exaggerate the effects of this approximation, an example numerical simulation indicates a solute travel time error of about 14%, but only minor errors due to incorrect dilution volume. For horizontal flow and transport models that assume fluid density is constant, the product of porosity and aquifer thickness chang es in time: initial porosity times initial thickness plus the change in head times the storage coefficient. This formula reduces to the saturated thickness in unconfined aquifers if porosity is assumed to be constant and equal to specific yield. The computational cost of this more accurate representation is insignificant and is easily incorporated in numerical models of solute transport.
The reduction of uranium(VI) to insoluble uranium(IV) in the presence of acetate by the organism GS-15 was investigated. It was cultured under nitrogen-carbon dioxide in an acetate and ferric citrate medium. Washed cells were anaerobically injected into uranyl carbonate complex solutions of up to 0.5 mM and incubated at 30C. The uranium(VI) concentrations were measured by pulsed nitrogen dye laser in samples taken at various times before and after oxidation. X-ray diffraction showed the precipitated uranium(IV) to be uraninite; it was entirely extracellular. As the reaction proceeded, some uranium(VI) was adsorbed on to the precipitate and was not reduced. The method was promising as a novel means of removing uranium from contaminated waters.
Lake Fryxell, one of a series of closed-basin perennially ice-covered lakes located in the Taylor Valley of southern Victoria Land, is the site of continuing studies of the biogeochemistry of trace metals, organic matter, and nutrients in lake ecosystems. Approximately 7 square kilometers in area and with a maximum depth of 18.7 meters, Lake Fryxell has a center ice thickness of 5 meters. It is fed by 10 glacial meltwater streams that flow intermittently for a period of about 2 months during the austral summer. These inflows, along with a contribution from wind-blown particles that reach the water column through fissures in the ice cover, are the major source of solutes to the water column. The water column is extremely stable, and the dominance of diffusion-controlled transport of solutes has been documented. To illustrate the predominant hydrologic and chemical factors that affect cycling of metals and nutrient elements in the lake, the authors examined concentration-depth profiles for sodium, a nonreactive element. Sodium increases monotonically from 196 milligrams per liter at the surface layers just beneath the ice to 2,844 milligrams per liter at 18 meters, about 0.5 meters above the sediment surface. Such a profile shape indicates a source of sodium from the sediments, a stable water column, and diffusion-dominated transport as dominant features.
Associations of free-living bacteria (FLB) and dissolved organic contaminants in a 4-km-long plume of sewage-contaminated groundwater were investigated. Abundance of FLB in the core of the plume (as delineated by maximum specific conductance) steadily decreased in the direction of flow from a point 0.25 km downgradient from the source to the toe of the plume. At 0.25 km downgradient, FLB comprised up to 31% of the total bacterial population, but constituted less than 7% of the population at 2 km downgradient. Abundance of FLB correlated strongly (r=0.80, n =23) with total dissolved organic carbon (DOC) in contaminated groundwater between 0.64 and 2.1 km downgradient, although distributions of individual contaminants such as di-, tri-, and tetrachloroethene were highly variable, and their association with FLB less clear. Numbers of FLB in the downgradient portion of the plume which is contaminated with branched-chain alkylbenzenesulfonate (ABS) surfactants were low ( less than 500,000, 000/L) in spite of relatively high levels of DOC along vertical transects through the plume. The ratio of FLB to DOC and the ratio of FLB to attached bacteria generally decreased in the direction of flow and, consequently, with the age of the organic contaminants.
There have been many recent advances in the understanding of the structure, activities, and movement of microorganisms that inhabit the terrestrial subsurface. As can be inferred from this literature review, much of this information has only been made available within the last ten years. However, there is much that is still unknown about these organisms, their in situ activity, their ecology, their metabolic potential, and their ability to migrate through porous media. Of particular interest is the role of this community in the fate of subsurface contaminants. Although much work has already been done on the biodegradability of a variety of organic contaminants under a number of chemical conditions, much is unknown about in situ biotransformations of these compounds in aquifers. The indirect role of protozoa in the fate of subsurface contaminants has yet to be examined. The application of analytical biochemical assays to whole microbial populations in aseptically collected subsurface samples promises to yield important information about groundwater microbial communities. In situ tracer experiments have resulted in important new information about microbial transport behavior in aquifers, but many of the biological and environmental controls are still poorly understood.
A finite-volume Eulerian-Lagrangian local adjoint method was developed for solution of the advection-dispersion equation. The method is mass conservative and can solve advection-dominated groundwater solute transport problems accurately and efficiently. An integrated finite difference approach was used in the method. A key component of the method is that the integral representing the mass-storage is evaluated numerically at the current time level. Integration points, and the mass associated with these points, are then forward tracked up to the next time level. The number of integration points required to reach a specified level of accuracy is problem dependent and increases as the sharpness of the simulated solute front increases. Integration points are generally equally spaced within each grid cell. For problems involving variable coefficients it was found to be advantageous to include additional integration points at strategic locations in each cell. These locations are determined by backtracking. Forward tracking of boundary fluxes by the method alleviates problems that are encountered in the backtracking approaches of most characteristic methods. Future research will focus on developing an adaptive routine to automatically determine the number if integration points needed for each node and an extension of the method to allow solution of multi-dimensional problems.
Measurement of the fluid-content distribution at sites contaminated by immiscible fluids, including crude oil, is needed to better understand the movement of these fluids in the subsurface, and to provide data to calibrate and verify numerical models and geophysical methods. A laboratory method was used to quantify the fluid contents of 146 core sections retrieved from boreholes aligned along a 120-m longitudinal transect at a crude oil spill site near Bemidji, Minnesota. The 47-mm-diameter, minimally disturbed cores spanned a vertical interval contaminated by oil. Cores were frozen on-site to prevent redistribution and loss of fluids while sectioning the cores. Oil and water contents were gravimetrically determined using a two-step method: (1) samples were slurried and the oil was removed by absorption onto strips of hydrophobic porous polyethylene (PPE); and (2) the samples were oven-dried to remove the water. The resulting data show sharp vertical gradients in the oil and water contents and a clearly defined oil body. The subsurface distribution is complex and appears to be influenced by sediment heterogeneities and water table fluctuations. The center of the oil body had depressed the water-saturated zone boundary, and the oil was found to migrate laterally within the capillary fringe. Oil contents were as high as 0.3 cu cm/cu cm, indicating that oil is probably still mobile 10 years after the spill. The thickness of oil measured in the wells suggests that accumulated thickness in wells is a poor indicator of the actual distribution of oil in the subsurface. An error analysis indicates that adsorption of water and sediment into the PPE adds as much as 4% to the measured oil masses, and that uncertainties in the calculated sample volume and the assumed oil density introduce an additional +/-3% error when the masses are converted to fluid contents.
The Ramapo River valley is a narrow valley bordered by bedrock highlands in northeastern New Jersey and southeastern New York. The valley-fill deposits consist mostly of sand and gravel and are as much as 200 ft thick along the center of the valley.Water table conditions prevail except in the northern part of the study area, where a silt and clay layer about 2 mi long and 0.5 mi wide confines a basal sand and gravel layer. Near the center of this area, the vertical hydraulic conductivity of the confining unit was 0.0003 ft/d, based on results of permeameter tests; the calculated average transmissivity of the confined aquifer was 15,700 sq ft/d, based on results of two aquifer tests, and the average storage coefficient was 0.00013. Aquifer test data indicated that recharge to the confined aquifer through the confining unit was less important than recharge around its edges. Results of three comprehensive base-flow seepage run indicated that the Ramapo River is hydraulically connected to the underlying aquifer, and that gaining and losing reaches are present under natural conditions. Streambed-conductivity measurements, based on data from local seepage runs, range from 25 to 35 ft/d. Results of a calibrated three-dimensional numerical model ofthe groundwater system in the northern part of the study area indicated that measured streamflow gains and losses caused by groundwater withdrawals from the valley-fill deposits also are affected by variations in hydrogeologic characteristics of the groundwater system.
In April 1988, approximately 1500 cubic m of a San Joaquin Valley crude oil were accidently released from a Shell Oil Co. refinery near Martinez, California. The oil flowed into Carquinez Strait and Suisun Bay in northern San Francisco Bay. Sediment and oil samples were collected from six different sites within a week and analyzed for geochemical marker compounds using gas chromatography/mass spectroscopy to track the molecular signature of the oil spill in the bottom sediment. Identification of the spilled oil in the sediment was complicated by the degraded nature of the oil and the similarity of the remaining, chromatographically resolvable constituents to those already present in the sediments from anthropogenic petroleum contamination, pyrogenic sources, and urban drainage. Ratios of hopane and sterane biomarkers, and of polycyclic aromatic hydrocarbons and the alkylated derivatives best identified the oil impingement. The ratios showed the oil impact at this early stage to be surficial only, and to be patchy even within the area of heavy oil exposure. Assignment of sources frequently requires a broad spectrum of molecular parameters within a complex sediment system of hydrocarbons.
As an addition to the U.S. Geological Survey's Modular Three-Dimensional Finite-Difference Ground-Water Flow Model, the Horizontal-Flow-Barrier (HFB) Package simulates thin, vertical low-permeability geologic features that impede the horizontal flow of groundwater. These geological features are approximated as a series of horizontal-flow barriers conceptually situated on the boundaries between pairs of adjacent cells in the finite difference grid. The key assumption underlying the HFB Package is that the width of the barrier is negligibly small in comparison with the horizontal dimensions of the cells in the grid. Barrier width is not explicitly considered in the Package, but is included implicitly in a hydraulic characteristic defined as either: (1) barrier transmissivity divided by barrier width, if it is in a constant transmissivity layer; or (2) barrier hydraulic conductivity divided by barrier width, if it is in a variable-transmissivity layer. Furthermore, the barrier is assumed to have zero storage capacity. Its sole function is to lower the horizontal branch conductance between the two cells that it separates. Documentation of the HFB Package includes data-input instructions, narratives, flowcharts, program listings, and variables lists for the three primary modules and one submodule in the Package.
An alternative quantification of the scaling properties of river channel networks was explored using a spatial network model. Whereas scaling descriptions of drainage networks previously have been presented using a fractal analysis primarily of the channel lengths, the scaling of the surface area of the channels defining the network pattern was illustrated with an exponent which is independent of the fractal dimension but not of the fractal nature of the network. The methodology is a fat fractal analysis in which the drainage basin minus the channel area was considered the fat fractal. Random channel networks within a fixed basin area were generated on grids of different scales. The sample channel networks generated by the model had a common outlet of fixed width and a rule of upstream channel narrowing specified by a diameter branching exponent using hydraulic and geomorphologic principles. Scaling exponents were computed for each sample network on a given grid size and were regressed against network magnitude. Results indicated that the size of the exponents were related to the magnitude of the networks, and generally decreased as network magnitude increased. Cases showing differences in scaling exponents with like magnitudes suggest a direction of future work regarding other topologic basin characteristics as potential explanatory variables.
A transient-storage submodel and a biotic-uptake submodel based on Michaelis/Menten kinetics were coupled to a convection/dispersion hydrologic transport model. The purpose was threefold: (1) to simulate nitrate retention in response to change in load in a third-order stream, (2) to differentiate biotic versus hydrologic factors in nitrate retention, and (3) to produce a research tool whose properties are consistent with laboratory and field observations. Hydrodynamic parameters were fitted from chloride concentration during a 20-day chloride-nitrate coinjection, and biotic-uptake kinetics were based on flume studies. Nitrate concentration from the coinjection experiment served as a base for model validation. The complete transport retention model reasonably predicted the observed nitrate concentration. However, simulations which lacked either the transient-storage submodel or the biotic-uptake submodel poorly predicted the observed nitrate concentration. Model simulations indicated that transient storage in channel and hyporrheic interstices dominated nitrate retention within the first 24 hours, whereas biotic uptake dominated thereafter. A sawtooth function of Vmax ranging from 0.10 to 0.17 microg NO3-N/s/g ash-free dry mass slightly underpredicted nitrate retention in simulations of 2-7 days. This result was reasonable since uptake by other nitrate-demanding processes were not included. The model demonstrated how ecosystem retention is an interaction between physical and biotic processes and supports the validity of coupling separate hydrodynamic and reactive submodels to established solute transport models in biological studies of fluvial ecosystems.
The numerical simulation of groundwater flow and associated solute transport under conditions of variable fluid density and viscosity results in sets of coupled partial differential equations. When these equations are discretized in space and time by a finite difference technique, sets of coupled algebraic equations result. Often they can be linearized and solved sequentially for the flow-dependent and transport-dependent variables using a direct or an iterative matrix-equation solution algorithm. Nodal renumbering is frequently used to form a reduced matrix from the original matrix equation. This renumbering reduces the computer workload and storage requirements. D4 zig-zag (D4Z) is a new renumbering scheme for producing a reduced matrix to be solved by an incomplete LU preconditioned, restarted conjugate-gradient (RCG) iterative solver. By renumbering alternate diagonals in a zig-zag fashion, a very low sensitivity of convergence rate to renumbering direction is obtained. For two demonstration problems involving groundwater flow and solute transport, iteration counts are related to condition numbers and spectra of reduced matrices. The spectra substantiate the behavior of the RCG solver under each renumbering scheme. The results were for a homogeneous, anisotropic flow system. The sensitivities are anticipated to diminish with increasing heterogeneity of the permeability. Although this work was done in two dimensions, the D4Z-renumbering scheme can be extended to matrices resulting from three-dimensional regions.
Diel relationships between physical and chemical parameters and biomass were examined along a 57-km reach of Whitewood Creek, South Dakota, between August 29 and September 2, 1988. A time lag of approximately 3 to 6 hours for fluctuations in soluble reactive phosphorus (SRP) concentrations (ranging from 0.1 to 0.5 mM at the downstream sites) relative to dissolved arsenic (ranging from 0.3 to 1.2 mM as arsenate (pentavalent arsenic)) was consistent with results of preferential cell sorption of orthophosphate over arsenate by creek periphyton. The potential biological effects on SRP diel fluctuations contrast with abiotic sorption controls for irradiance cycles by 1 to 3 hours. Like pH, the amplitude of dissolved arsenic diel cycles was greatest at the site with most abundant biomass. Diel fluctuations in specific conductance (an indicator of groundwater inputs at elevated conductivity relative to the water column) were out of phase with both SRP and dissolved arsenic concentrations suggesting that groundwater was not the direct source of these solutes. These observed relationships provided further evidence for the need for a process-interactive approach to describe stream systems and for consideration of diel variations when designing sampling protocol and interpreting water quality data.
Enzymatic uranium reduction by Desulfovibrio desulfuricans was found to readily remove uranium from solution in a batch system or when D. desulfuricans was separated from the bulk of the uranium-containing water by a semipermeable membrane. Uranium reduction continued at concentrations as high as 24 mM. Of a variety of potentially inhibiting anions and metals evaluated, only high concentrations of copper inhibited uranium reduction. Freeze-dried cells, stored aerobically, reduced uranium as fast as fresh cells. D. desulfuricans reduced uranium in pH 4 and pH 7.4 mine drainage waters and in uranium-containing groundwaters from a contaminated Department of Energy site. The enzymatic uranium reduction technique has several potential advantages over other bioprocessing techniques for uranium removal. The most important advantages are: the ability to precipitate uranium that is in the form of a uranyl carbonate complex; high capacity for uranium removal per cell; and the formation of a compact relatively pure, uranium precipitate.
Most assessments of selenium toxicity have employed selenite which is the most bioreactive form of the element in solution. Equilibrium thermodynamic calculations, however, do not accurately predict the speciation of selenium in oxidized natural waters because of the influences of biological processes. Particulate organo-Se was assimilated with 86% efficiency by the deposit feeding bivalve Macoma balthica, when the clam was fed Se-75 labeled diatoms. Absorption efficiencies of particulate elemental Se were 22%, when the animals were fed Se-75 labeled sediments in which elemental Se was precipitated by microbial activity. Precipitation of elemental Se did not eliminate its biological availability. Selenite was taken up from solution slowly by M. balthica (mean concentration factor was 712). Concentrations of selenite high enough to influence Se bioaccumulation by M. balthica did not occur in the oxidized water column of San Francisco Bay. However, 98-99% of observed Se could be explained by ingestion of concentrations of Se particulates found in the bay. The potential for adverse environmental effects occurred at much lower concentrations of environmental Se when food web transfer was considered than when predictions of effects were based on bioassays with solute forms of the element. Selenium clearly requires a protective criterion based on particulate concentrations or food web transfer.
An irrigation model based on a modified Thornthwaite water balance was used to simulate the effects of various hypothetical climatic changes on annual irrigation demand in a humid-temperate climate. The climatic-change scenarios consisted of combinations of changes in temperature, precipitation, and stomatal resistance of plants to transpiration. The objectives were to: (1) examine the effects of long-term changes in these components of climatic change on annual irrigation demand; and (2) identify which of these factors would cause the largest changes in annual irrigation demand. Hypothetical climatic changes that only included increases in temperature and changes in precipitation resulted in increased annual irrigation demand, even with a 20% increase in precipitation. The model results showed that, for the ranges of changes in temperature and precipitation used in this study, changes in irrigation demand were more sensitive to changes in temperature than to changes in precipitation. Model results also indicated that increased stomatal resistance to transpiration counteracted the effects of increases in temperature and decreases in precipitation on irrigation demand. Changes in irrigation demand were even more sensitive to changes in stomatal resistance than to changes in temperature. A large amount of uncertainty is associated with predictions of future climatic conditions; however, uncertainty associated with natural climatic variability may be larger and may mask the effects of climatic change on irrigation demand.
Deuterium and oxygen-18 were used as natural tracers to investigate the hydraulic relationship between the Columbia river and the Blue lake grave aquifer near Portland, Oregon. A time series of stable-isotope data collected from surface and ground waters during a March 1990 aquifer test confirmed that the river and aquifer are hydraulically connected. Calculations based on simple mixing showed that the river contributed 40-50 per cent of the yield of 3 wells after 5-6 d of pumping. Data collected during August 1990, showed that the river contributed 65-80 per cent of the yield of 1 after 22 d of pumping and indicated that the contribution of the river was continuing to increase.
Organic solute sorption by hydrous iron and aluminum oxides was studied in an acidic, metal-enriched stream (the Snake River) at its confluence with a pristine stream (Deer Creek). From 1979 to 1986, typically 40% of the dissolved organic carbon (DOC) was removed from solution by sorption onto aluminum and iron oxides, which precipitate as the two streamwaters mix. Upstream DOC concentrations, which increase during snowmelt, were identified as the most significant variables in a multiple regression for determining the DOC concentration below the confluence, and the extent of Al and Fe precipitation was much less significant. On hourly timescales, removal of Al and Fe varied erratically but DOC removal was steady, indicating that 'sorbable' organic solutes are sorbed either by precipitating oxides or by oxides on the streambed. Characterization of two reactive DOC fractions (fulvic and hydrophilic acids) showed that sorption results in chemical fractionation. Molecules with greater contents of aromatic moieties, carboxylic acid groups, and amino acid residues were preferentially sorbed, which is consistent with the ligand exchange-surface complexation model. It is concluded that sorption of dissolved organic material by hydrous metal oxides can control the transport and chemical characteristics of organic carbon in a stream system with abundant oxides.
The US Geological Survey has maintained a network of stations to collect samples for the measurement of tritium concentrations in precipitation and streamflow since the early 1960s. Tritium data from outflow waters of river basins draining 4500-75000 sq/km used to determine average residence times of water within the basins. The basins were modeled with the assumption that the outflow in the river comes from two sources--prompt (within-year) runoff from precipitation, and flow from the long-term reservoirs of the basin. Predicted tritium concentrations for the outflow water in the river basins were calculated for different residence times and for different relative contributions from the two reservoirs. A box model was used to calculate tritium concentrations in the long-term reservoir. Calculated values of outflow tritium concentrations for the basin were regressed against the measured data to obtain a slope as close as possible to 1. These regressions assumed an intercept of zero and were carried out for different values of residence time and reservoir contribution to maximize the fit of modeled versus actual data for all the rivers. The final slopes of the fitted regression lines ranged from 0.95 to 1.01 (correlation coefficient >0.96) for the basins studied. Values for river basin residence times ranged from 2 years to 20 years. The residence time indicate the time scale in which the basin responds to anthropogenic inputs. The modeled tritium concentrations for the basins also furnish input data for urban and agricultural settings where these river waters are used.
There is a conceptual inconsistency between the parameterization of evaporation from the continents in several climate models and the empirical evidence upon which those parameterizations are based. The climate models concerned are atmospheric general circulation models (GCMs) that describe evaporation rate and a moisture-availability function. When there is a restriction on moisture availability in the affected models, the method of calculation of the potential evaporation rate yields a value that exceeds the value consistent with the moisture availability function used, leading to artificially accelerated drying of the soil and distortion of the modeled surface water and energy balances. Simple theoretical analysis and direct computations, all ignoring atmospheric feedbacks, indicate that whenever the soil moisture is limited, GCM-based climatic models produce rates of potential evaporation that exceed, by a factor of two or more, the rates that would be yielded by use of the consistent temperature. However, further analyses and numerical simulations indicate that the moisture-availability function can be approximated by a unique function of soil moisture--the expected value of dry-season soil moisture has a short memory relative to the annual cycle, thus dry-season evaporation is nearly equal to dry-season precipitation. When potential evaporation is overestimated, soil moisture is artificially reduced, and actual evaporation may or may not be overestimated. These arguments explain the substantial differences between GCM-generated and observation-based values of potential evaporation in studies of the effects of climatic change on continental hydrology and water resources. They also provide a partial explanation of the excessively low values of summer soil moisture in GCMs and raise questions concerning soil-moisture changes induced by an increase of greenhouse gases.
Water, suspended sediment, and bed sediment samples were collected for physical (particle size and mineralogy) and chemical analysis (radioactive elements, trace metals, nutrients, petrochemical hydrocarbons, organic volatiles, pesticides, detergents, organic carbon, and humic substances) from 21 sites on the Mississippi River and its main tributaries. Three cruises were made at low water during a 1-yr period from July 18, 1987, to June 7, 1988. The maximum measured discharge was abut 10,400 cu m/sec on December 15, 1987, at Vicksburg, MS, and the maximum measured suspended sediment discharge was 354,000 metric tons/d in the Missouri River at Hermann, Missouri, on July 20, 1987. The equal-width-increment (equal-transit-rate), depth-integration method was used at 10-40 verticals across the river to collect between 70 and 137 L of river water with an isokinetic samplers (made of Teflon to prevent chemical contamination). Hydrologic data associated with the suspended sediment samples, tabulated in this report, include: cross-sectional area of the river; mean depth; mean velocity; water discharge; particle sizes; concentrations of the suspended sand, silt, and colloid fractions; and, surface temperature and conductivity at 10-40 locations across the river. These data provide the framework for subsequent interpretive chemical analyses of the samples collected during the three cruises.
The depth-integration method of measuring water discharge makes a continuous measurement of the water velocity from the water surface to the bottom at 20 to 40 locations or verticals across a river. Field calibration measurements showed that: (1) the mean velocity measured on the upcast (bottom to surface) was within 1% of the standard mean velocity determined by 9-11 point measurements; (2) if the transit velocity was less than 25% of the mean velocity, then average error in the mean velocity was 4% or less. The major source of bias error was a result of mounting the current meter above a sounding weight and sometimes above a suspended-sediment sampling bottle, which prevented measurement of the velocity all the way to the bottom. The measured mean velocity was slightly larger than the true mean velocity. This bias error in the discharge was largest in shallow water (approximately 8% for the Missouri River at Hermann, MO, where the mean depth was 4.3 m) and smallest in deeper water (approximately 3% for the Mississippi River at Vicksburg, MS, where the mean depth was 14.5 m). The major source of random error in the discharge was the natural variability of river velocities, which was assumed to be independent and random at each vertical. The standard error of the estimated mean velocity, at an individual vertical sampling location, may be as large as 9%, for large sand-bed alluvial rivers. The computed discharge, however, is a weighted mean of these random velocities. Consequently the standard error of computed discharge was divided by the square root of the number of verticals, producing typical values between 1 and 2%. The discharges measured by the depth-integrated method agreed within 5% of those measured simultaneously by the standard two and eight-tenths, six-tenths, and moving boat methods.
Using concepts developed in an earlier study, a solution in Laplace transform space was obtained for transport of resident concentration in an imperfectly but yet highly stratified porous medium. The flow field, into which an instantaneous pulse of tracer is injected, is taken to be steady and mean uniform parallel to the direction of stratification. From this transform-space solution either temporal moments can be derived by taking derivatives with respect to the Laplace parameter, or the transform-space solution can be inverted numerically to obtain breakthrough curves for the mean concentration. When compared to an equivalent solution with a Fickian dispersive flux, these temporal moments indicate the extent to which transport in heterogeneous porous media deviates from classical Fickian behavior. The numerical inversion of the Laplace transform solution gives partial breakthrough curves for the mean concentration which have the appearance of conflicting with the derived moment information. A hypothesis is put forth which resolves this apparent conflict; this hypothesis is verified by adding a component of local dispersion to the governing transport equation. On the basis of the flux-averaged concentration a form for the expected probability density function for the arrival time of a tracer particle is derived; arrival time moments and an arrival time cumulative distribution function are available as a consequence. Arrival time moments, as derived from the flux-averaged concentration, do not differ significantly from the resident moments, as derived from the resident concentration.
A new model was proposed to represent hysteritic soil water retention using as few measurements as possible. Its concepts of soil properties had a definite physical interpretation; 2 parameters were employed. One parameter represented the fraction of the pore space not subject to Haines jump hysteresis. A single value of this parameter characterized a given medium. The second parameter was the effective body-to-neck size ratio of the medium's largest pore, with a particular relation postulated between the size distribution of pore bodies and of pore necks. The model predicted accurately measured water content and matric pressure wetting curves for many media. The optimized parameters correlated with uniformity of particles, complexity of structure, and degree of compaction. A complete simulated wetting curve for a medium could be predicted where only a drying curve and 2 points on the wetting curve had been measured.
An improved steady state centrifuge apparatus, with better range and adjustability, for measuring unsaturated hydraulic conductivity (K) is described. Mechanical adjustments permitted the measured K to be varied by a factor of 360. Different flow regulating ceramic materials gave a total K range covering 6 orders of magnitude. The increment of K adjustment was a factor of about 1.6. This rendered the apparatus potentially useful for measured targeted values of K or of water content, the latter by a trial and error procedure.
A suite of geophysical logs designed to characterize fractures and water production from fractures was run in five boreholes ranging from <100 ft to >300 ft in depth in the vicinity of a suspected gasoline contamination site near Ashford, CT. The geophysical logs used in this study are: conventional caliper log; borehole televiewer log, which produces a photograph-like image of the borehole wall; and the heat-pulse flowmeter, which produces a high resolution profile of vertical flow in the well bore under ambient conditions and during injection. Downward flow was measured in three of the five boreholes under ambient conditions, while no flow was measured in one of the boreholes; a very weak upward flow in another borehole was attributed to recovery from pump removal. Steady injection tests at about 1 gpm indicated that a limited number of the fractures intersecting the boreholes are capable of accepting water under injection conditions or of producing water under pumping conditions. Of the eight producing fractures identified on the televiewer logs in these boreholes, five appear approximately parallel in orientation to the NE strike and 30 degree NW dip of foliation bedrock; two others appear as steeply dipping fractures cutting across the foliation, and the other could possibly be assigned to either of these classes. Two other zones accept flow during injection tests, but are so close to the bottom of the borehole that they could not be characterized with geophysical logs.
A study of the Central Oklahoma aquifer (also known as the Garber-Wellington aquifer) was undertaken in the pilot phase of the National Water-Quality Assessment Program of the U.S. Geological Survey. The aquifer is used extensively for municipal, industrial, commercial, and domestic water supplies. This report examines chemical and isotopic composi- tions of groundwater, abundances and textures of minerals in cores, and hydraulic measurements to identify geochemical reactions occurring within the aquifer and to determine rates and directions of groundwater flow. Local flow systems in the uncon- fined aquifer discharge to nearby streams; transit times generally are less than 500 years. For the regional flow system in the unconfined aquifer, the Deep Fork and Little River are important discharge areas; transit times generally are less than 5,0000 years. The dominant chemical reaction in the uncon- fined aquifer is uptake of carbon dioxide from the unsaturated zone and dissolution of dolomite, which results in calcium magnesium bicarbonate water compositions. Recharge to the regional flow system in the confined aquifer occurs in a small area of the Garber Sandstone. Discharge from the confined aquifer is to the Deep Fork and Little River, and to parts of the Cimarron River drainage basin. Transit times through the confined aquifer range from 10,000 to 30,000 year. The dominant chemical reaction in the confined part of the aquifer is cation exchange of calcium and magnesium for sodium on clays, which results in sodium bicarbonate water compositions.
Research on the hydrology of William Lake, north-central Minnesota includes study of evaporation. Presented here are those climatic data needed for mass-transfer studies, including: air temperature, wind speed, humidity, and precipitation. Some calculated values necessary for this study, such as vapor pressure and vapor pressure deficit also are presented. Data are collected at raft and land stations.
Data are presented on the concentrations and distribution of hydrocarbons in sediments in San Francisco bay estuary, Calif., and in the Asian clam (Potamocorbula amurensis) which was introduced into the area in 1986 in ballast water from cargo vessels and which had spread rapidly throughout the bay. Both sediment and clams were contaminated with a chronic background of hydrocarbons which were present in Suisun bay and the distribution of hopane and sterane biomarkers in both showed the petroleum hydrocarbons in the sediment were bio-available to the clams. Polycyclic aromatic hydrocarbons in the sediment and clams were derived chiefly from sources of combustion. P. amurensis could be a useful biological monitor of hydrocarbon pollution in San Francisco bay.
Pereira, W.E. , Rostad, C.E., and Leiker, T.J., 1992, Synthetic Organic Agrochemicals in the lower Mississippi River and its major tributaries: Distributions, Transport and Fate: International Journal of Contaminant Hydrology, V. 9, p. 175-188.
The Mississippi River and its major tributaries transport herbicides and their degradation products from agricultural areas in the mid-western U.S. These compounds include atrazine and its degradation products (desethylatrazine and desisopropylatrazine), simazine, cyanazine, metolachlor, and alachlor and its degradation products (2-chloro-2 ,6-diethylacetanilide, 2-hydroxy-2,6-diethylacetanilide and 2 ,6-diethylaniline). These compounds were identified and confirmed by gas chromatography-ion trap mass spectrometry. Loads of these compounds were determined during five sampling trips in 1987-1989. Stream loads of these compounds indicated that atrazine and metolachlor were relatively conservative in downstream transport. Alachlor and its degradation products were generated from point and non-point sources. Seasonal variations and hydrologic conditions controlled the loads of these compounds in the Mississippi River. Cross-channel mixing was slow downstream from major river confluences, possibly requiring several hundred kilometers of downstream transit for completion. The annual transport of these compounds into the Gulf of Mexico was estimated to be <2% of the annual application of each herbicide in the Midwest.
A 1982-84 field study defined the chemistry of water collected from the unsaturated zone at a low-level radioactive-waste disposal site near Sheffield, Illinois. Chemical data were interpreted to determine the principal geochemical reactions that occurred naturally in the unsaturated zone and to evaluate waste-induced effects on pore-water chemistry. Samples of precipitation, unsaturated-zone pore water, and saturated zone water were analyzed for specific conductance, pH, alkalinity, major cations and anions, dissolved organic carbon, gross alpha and beta radiation, and tritium. There was little change in concentrations of the major inorganic constituents in the unsaturated zone water with respect to depth or distance from water-disposal trenches. The concentrations of tritium and dissolved organic carbon were, however, dependent on proximity to trenches. The primary chemical reactions, both on-site and off-site, were carbonate and clay dissolution, cation exchange, and the oxidation of pyrite. The primary difference between on-site and off-site inorganic water chemistry was the result of removal of the Roxana Silt and the Radnor Till Member of the Glasford Formation from on-site. Off-site, the Roxana Silt contributed substantial quantities of sodium to solution from montmorillonite dissolution and associated cation-exchange reactions. The Radnor Till Member provided exchange surfaces for magnesium. Lysimeter sampling, installation, and construction methods were evaluated to ensure data reliability. These evaluations indicate that, with respect to most constituents, the samples retrieved from the lysimeters accurately represent pore-water chemistry.
The factors controlling the chemistry of 69 low-order streams in the Blue Ridge and Valley and Ridge physiographic provinces of Virginia and Maryland were studied over a 13-month period. Principal component analysis was used to examine regional patterns in stream chemistry and to examine the degree to which the chemistry of low-order streams is controlled by the bedrock upon which they flow. Streams clustered into regionally isolated groups, strongly related to bedrock type, with SO42- and HCO3- the chemical variables of most importance. Sulfate concentrations appear to be strongly controlled by climate and hydrology, and sorption in the soils within the watershed. Much of the atmospherically derived SO42- accumulates in watersheds during the growing season and is later flushed out. Weathering reactions were found to be particularly important in the production of HCO3-, accounting for 91 percent on an annual basis, and export of divalent cations from these watersheds, accounting for 48-50 percent on an annual basis. About half of non-anthropogenic Na+ was derived from weathering of silicates , whereas nearly all K+ was identified with leaching by SO42-. Water chemistry was strongly related to the rock type in the watershed and the weatherability of the component minerals. Rock type is not a randomly distributed function; instead, it is controlled by geologic factors that result in clusters of similar rock types in a given region. When planning large synoptic studies, it is extremely important to consider that a sampling scheme based on random sampling of a non-randomly distributed function may not provide the most accurate representation of the variables of interest. Instead, a hierarchical sampling scheme may be indicated. The results also suggest that, although one sample in time may be sufficient to characterize the primary geochemical factors controlling stream chemistry throughout the year, it may not be sufficient to detect subtle, flow-related alterations in chemistry.
Acid deposition is an environmental problem in the Eastern United States. The Mid-Atlantic region of the United States receives some of the most acidic precipitation in the Nation. The U.S. Geological Survey has been monitoring precipitation pH and streamwater quality on Catoctin Mountain, Maryland, since 1982. A slight upward trend in precipitation pH (+0.02 pH unit) at that location has been detected for the 10 years of record. The volume-weighted average precipitation pH on Catoctin Mountain is approximately 4.2. While the precipitation in the area is quite acidic, streamwater pH is variable because of the seasonality of biological processes, flow conditions, topography, type of bedrock, and soil cover. The most important of these factors in determining streamwater pH is the type of underlying bedrock. On Catoctin Mountain, the two dominant bedrock types are metabasalt, a good buffer, and quartzite, a poor buffer.
The hydrologic and solute budgets of a lake can be strongly influenced by transient groundwater flow. Several shallow interdunal lakes in southwest Spain are in close hydraulic connection with the shallow groundwater. Two permanent lakes and one intermittent lake have chloride concentrations that differ by almost an order of magnitude. A two-dimensional solute-transport model, modified to simulate transient groundwater-lake interaction, suggests that the rising water table during the wet season leads to local flow reversals toward the lakes. Response of the individual lakes, however, varies depending on the lake's position in the regional flow system. The most dilute lake is a flow-through lake during the entire year; the throughflow is driven by regional groundwater flow. The other permanent lake, which has a higher solute concentration, undergoes seasonal groundwater flow reversals at its downgradient end, resulting in complex seepage patterns and higher solute concentrations in the groundwater near the lake. The solute concentration of the intermittent lake is influenced more strongly by the seasonal wetting and drying cycle than by the regional flow system. Although evaporation is the major process affecting the concentration of conservative solutes in the lakes, geochemical and biochemical reactions influence the concentration of nonconservative solutes. Probable reactions in the lakes include biological uptake of solutes and calcite precipitation; probable reactions as lake water seeps into the aquifer are sulfate reduction and calcite dissolution. Seepage reversals can result in water composition that appears inconsistent with predictions based on head measurements because, under transient flow conditions, the flow direction at any instant may not satisfactorily depict the source of the water.
Chemical analyses of water samples from several observation wells around a saline lake on the Southern High Plains, USA, have indicated that ion exchange plays a significant role in the creation of a shallow calcium chloride brine. The solute chemistry, exchangeable cations, isotope ratios, and ground-water flow in the Kiamichi Formation beneath the lake all support an ion-exchange model for the origin of this brine. Relatively high concentrations of calcium and low concentrations of sodium and potassium in the shale pore water suggest ion exchange is the dominant reaction. A numerical model was constructed to analyze the simultaneous advection, diffusion, and exchange of the four major cations. Results from the model indicate that high concentrations of calcium can be generated by ion exchange under the conditions present in the shale.
The transport of bacteria through porous media is a subject of current interest, having application in the fields of bioremediation and public health. The effects of pH and sediment surface characteristics on sorption of indigenous groundwater bacteria were determined using contaminated and uncontaminated aquifer material from Cape Cod, Massachusetts. Over the pH rang of the aquifer (5-7), the extent of bacterial sorption onto sediment in uncontaminated groundwater was strongly pH-dependent, but relatively pH-insensitive in contaminated groundwater from the site. Bacterial sorption was also affected by the presence of oxyhydroxide coatings (iron, aluminum, and manganese). Surface coating effects were most pronounced in uncontaminated groundwater (pH 6.4 at 10oC). Desorption of attached bacteria (up to 14% of the total number of labelled cells added) occurred in both field and laboratory experiments upon adjustment of groundwater to pH 8. The dependence of bacterial sorption upon environmental conditions suggests that bacterial immobilization could change substantially over relatively short distances in contaminated, sandy aquifers and that effects caused by changes in groundwater geochemistry can be significant.
Pore waters extracted from sediment cores were analyzed for their oxygen and hydrogen isotopic compositions and major ion chemistry to determine the source of water from a vent area for diffuse lake-bottom thermal springs or seeps in Frolikha Bay, northeastern Lake Baikal. The d18O values of pore waters range from -15.2 ppt to -16.7 ppt, and dD values range from -119 ppt to -126 ppt (both isotopes determined relative to standard ocean water (SMOW)). Bottom water in Lake Baikal has a d18O value of -5.6 ppt and a dD value of -120 ppt. Pore waters in the vent are significantly enriched in Mg, K, Ca, and especially Na, and have the lowest dD and d18O values. These pore waters are isotopically and chemically distinct from pore waters in other, more typical parts of the lake. The pore water isotopic data fall on a local meteoric water line, and covariation in water isotopes and chemistry are not consistent with evaporation or hydrothermal water-rock interaction. The thermal springs represent discharging meteoric waters that have been gently heated during subsurface circulation and are largely unaltered isotopically. Chemical variations are most likely due to dissolution of subsurface evaporites.
Hundreds of miles of streams in West Tennessee have been channelized or otherwise modified since the turn of the century. After all or parts of a stream are straightened, dredged, or cleared, systematic hydrologic, geomorphic, and ecologic processes begin to act collectively to reduce energy conditions towards the premodified state. This paper presents the interdisciplinary analyses and interpretations of geomorphic and vegetative recovery processes after channel modifications. The paper is a comprehensive summary of four previous studies of modified streams in West Tennessee and builds upon this earlier work. One hundred and five sites along 15 streams were studied in the Obion, Forked Deer, Hatchie, and Wolf River basins. Approaches and analyses from geomorphology, soil mechanics, slope stability, and dendrogeomorphology were used at each site. Bank material shear-strength properties were determined through borehole-shear testing and used to interpret critical bank conditions, and factors of safety. Mean values of cohesion and angle of internal friction are 1.26 pounds/sq in and 30.1 degrees, respectively. Dendrogeomorphic analyses were made using botanical evidence of bank failures to determine rates of channel widening; buried riparian stems were analyzed to determine rates of bank accretion. Bed-level changes through time and space were determined with a power function. Plant ecological analyses were made to infer relative bank stability, to identify indicator species of the stage of bank recovery, and to determine patterns of vegetation development through the course of channel evolution. Quantitative data on morphologic changes were used with previously developed six-stage models of channel evolution and bank-slope development to estimate trends of geomorphic and ecologic processes and forms through time and space.
Records of streamflow can provide an account of climatic variation over a hydrologic basin. The ability to do so is conditioned on the absence of confounding factors that diminish the climate signal. A national data set of streamflow records that are relatively free of confounding anthropogenic influences has been developed for the purpose of studying the variation in surface water conditions throughout the United States. Records in the U.S. Geological Survey (USGS) National Water Storage and Retrieval System (WATSTORE) database for active and discontinued streamflow gaging stations through water year 1988 (that is, through September 30, 1988) were reviewed jointly with data specialists in each USGS District office. The resulting collection of stations, each with its respective period of record satisfying the qualifying criteria, is called the Hydro-Climatic Data Network, or HCDN. The HCDN consists of 1,659 sites throughout the United States and its territories, totaling 73,231 water years of daily mean discharge values. For each station in the HCDN, information necessary for its identification, along with any qualifying comments about the available record and a set of descriptive watershed characteristics are provided in tabular format in this report, both on paper and on computer disk (enclosed). For each station in the HCDN, the appropriate daily mean discharge values were compiled, and statistical characteristics, including monthly mean discharges and annual mean, minimum and maximum discharges, were derived. The discharge data values are provided in a companion report.
Evaporation from Williams Lake, computed by the energy budget method for the five open-water seasons of 1982-1986, varied from a maximum seasonal rate of 0.282 cm/d in 1983 to a minimum seasonal rate of 0.219 cm/d in 1982. Monthly values of evaporation are not consistent from year to year. The normally expected pattern of low evaporation values in May, followed by increasing values in June to maximum values in July, is true for only 3 of the 5 years. A comparison of the annual evaporation, calculated by the energy budget and mass transfer methods, indicates that energy budget values varied from 13% greater to 11% less than mass transfer values. Furthermore, there was no seasonal bias in the pattern. Large differences were found to exist in the magnitude of energy fluxes to and from Williams Lake. By far, the greatest energy fluxes, having magnitudes of hundreds of watts per sq meter, are incoming solar radiation, incoming atmospheric radiation, and outgoing long-wave radiation emitted by the lake water. The least energy fluxes are related to advection, which generally have magnitudes less than 5 W/sq m.
Hydroxylamine has been used previously to determine the carbonyl content of humic substances. However, it has not been known to what extent the hydroxylamine reacts with groups other than ketones, quinones, and aldehydes to form derivatives other than oximes, or how completely these functionalities are derivatized. Five fulvic and humic acid samples of diverse origins were derivatized with 15N-labeled hydroxylamine and analyzed by liquid-phase 15N-NMR spectrometry. The 15N-NMR spectra indicated that hydroxylamine reacted similarly with all samples and could discriminate among carbonyl functional groups. Oximes were the major derivatives; resonances attributable to hydroxamic acids, the reaction products of hydroxylamine with esters, and resonances attributable to the tautomeric equilibrium position between the nitrosophenol and monoxime derivatives of quinones, the first direct spectroscopic evidence for quinones, also were evident. The 15N-NMR spectra also suggested the presence of nitriles, oxazoles, oxazolines, isocyanides, amides, and lactams , which may all be explained in terms of Beckmann reactions of the initial oxime derivatives. INEPT and ACOUSTIC 15N-NMR spectra provided complementary information on the derivatized samples. 13C-NMR spectra of derivatized samples indicated that the ketone/quinone functionality is incompletely derivatized with hydroxylamine.
Various methods have been proposed for the extraction of large, undisturbed soil cores and for subsequent analysis of fluid movement within the cores. The major problems associated with these methods are expense, cumbersome field extraction, and inadequate simulation of unsaturated flow conditions. A field and laboratory procedure was developed that is economical and convenient, that simulates unsaturated and saturated flow without interface flow problems, and that can be used on a variety of soil types. In the field, a stainless steel core barrel (30 cm diam. and 38 cm high) is hydraulically pressed into the soil. The barrel and core are then extracted from the soil, and after the barrel is removed from the core, the core is wrapped securely with flexible sheet metal and a stainless mesh screen is attached to the bottom of the core for support. In the laboratory, the soil core is set atop a porous ceramic plate over which a soil-diatomaceous earth slurry has been poured to assure good contact between plate and core. A cardboard cylinder (mold) is fastened around the core and the empty space is filed with paraffin wax. Soil cores were tested under saturated and unsaturated conditions using a hanging water column for potentials less than or equal to zero. Breakthrough curves indicated that no interface flow occurred along the edge of the core. The procedure proved to be reliable for field extraction of large, intact soil cores and for laboratory analysis of solute transport.
A spatial stochastic model has been proposed for drainage networks defined on a square lattice of points. The probability of a particular spanning tree s draining a given basin represented by a set of lattice points is proportional to exp (-beta H(s)), where beta is a parameter to be estimated and H(s) is defined to be the difference between total flow distance to the outlet through the drainage tree and total distance along shortest paths to the outlet. Thus H(s) is a global measure of sinuosity of the channels constituting the drainage tree. This probability distribution on trees is known as Gibbs' distribution and is well known in statistical mechanical contexts. The distribution with beta=0 (a uniform distribution) has been previously studied and was found to yield networks that are too sinuous. However, if beta is allowed to be greater than zero, more realistic networks are produced that are not as sinuous. The question of spatial variability of beta is addressed from an equilibrium point of view using ideas from statistical mechanics. To examine the question of spatial variability, the parameter beta is defined in a Bayesian sense and allowed to vary randomly from basin to basin. In a case study beta is expressed in terms of a log linear model with three independent variables: pixel size (grid spacing), drainage area, and average channel slope. The parameters of this model were estimated with a set of 50 drainage areas obtained using digital elevation data for Willow Creek in Montana. Parameter estimates indicate that beta tends to vary directly with slope and inversely with drainage area, which suggests heterogeneities within larger basins that are not adequately accounted for by Gibbs' distribution with a constant parameter value.
Beneath an atoll island, fresh groundwater may occur as a thin lenticular shaped body referred to as a lens, that is separated from the underlying seawater by a zone of mixed water commonly called the transition zone. The size and shape of a fresh groundwater lens beneath an atoll island are controlled by the geologic framework and hydrodynamic processes. A variable density groundwater model was used to analyze the effects of various controls on the size of the freshwater lens, the structure of the transition zone, and the propagation of tidal fluctuations in a two-layer atoll island groundwater system. Modeling results indicate that mixing of freshwater and saltwater occurs primarily as a result of oscillating vertical flow due to tidal fluctuations and depends to a lesser extent on transverse dispersion along the dominantly horizontal recharge-discharge path of flow. The controls on the amount of mixing are (1) the accumulated vertical distance, which increases with tidal range and is restricted by vertical permeabilities; (2) vertical longitudinal dispersion. Comparison of cross-sectional simulations of atoll islands using nontidal and tidal models shows that the nontidal model must use artificially high values of transverse dispersivity to compensate for the lack of tidally driven, vertical mixing processes. Although the tidal model has high computational requirements, it can be used to calibrate vertical permeabilities and is best suited for problems dealing with groundwater resource evaluations, hydrologic events, and hydrologic processes. The limitations of the nontidal model are that it cannot be used for calibration of vertical permeabilities and will not realistically simulate those cases in which transition zones are thick or recharge low.
The wood of tulip trees (Liriodendron tulipifera L.) growing above groundwater contamination from a hazardous-waste landfill in Maryland contained elevated concentrations of potassium (K). The groundwater contamination also contained elevated concentrations of dissolved K, as well as arsenic (As), cadmium (Cd), chloride (Cl), iron (Fe), manganese (Mn), zinc (Zn), and organic solvents. The dissolved K is derived from disposed smoke munitions. The excess K in the tulip trees is concentrated in the heartwood, the part of the xylem most depleted in K in trees growing outside of the contamination. These data show that the uptake and translocation of K by tulip trees can be strongly influenced by the availability of K in groundwater contamination and suggest the utility of this species as an areal indicator of groundwater contamination.
Parameter estimation and contaminant source characterization are key steps in the development of a coupled groundwater flow and contaminant transport simulation model. A methodology for simultaneous model parameter estimation and source characterization was developed. The parameter estimation/source characterization inverse model combines groundwater flow and contaminant transport simulation with non-linear maximum likelihood estimation to determine optimal estimates of the unknown model parameters and source characteristics based on measurements of hydraulic head and contaminant concentration. First-order uncertainty analysis provides a means for assessing the reliability of the maximum likelihood estimates and evaluating the accuracy and reliability of the flow and transport model predictions. A series of hypothetical examples demonstrated the ability of the inverse model to solve the combined parameter estimation/source characterization inverse problem. Hydraulic conductivities, effective porosity, longitudinal and transverse dispersivities, boundary flux, and contaminant flux at the source were estimated for a two-dimensional groundwater system. In addition, characterization of the history of contaminant disposal or location of the contaminant source was demonstrated. The problem of estimating the statistical parameters that describe the errors associated with the head and concentration data was studied. A stage-wise estimation procedure was used to jointly estimate these statistical parameters along with the unknown model parameters and source characteristics.
Past estimates of the 100-yr flood for the Santa Cruz River at Tucson, Arizona, range from 572 to 2,780 m3/s. An apparent increase in flood magnitude during the past two decades raises concerns that the annual flood series is nonstationary in time. The apparent increase is accompanied by more annual floods occurring in the fall and winter, and fewer in summer. This greater mixture of storm types that produce annual flood peaks is caused by a higher frequency of meridional flow in the upper-air circulation and increased variance of ocean-atmosphere conditions in the tropical Pacific Ocean. Estimation of flood frequency on the Santa Cruz River is complicated because climate affects the magnitude and frequency of storms that cause floods. Mean discharge does not change significantly, but the variance and skew coefficient of the distribution of annual floods change with time. The 100-yr flood during El Niño-Southern Oscillation conditions is 1,300 m3/s, more than double the value for other years. The increase is caused mostly by an increase in recurvature of dissipating tropical cyclones into the Southwestern United States during El Niño-Southern Oscillation conditions. Flood frequency based on hydroclimatology was determined by combining populations of floods caused by monsoonal storms, frontal systems, and dissipating tropical cyclones. For 1930-59, annual flood frequency is dominated by monsoonal floods, and the estimated 100-yr flood is 323 m3/s. For 1960-86, annual flood frequency at recurrence intervals of > 10 yrs is dominated by floods caused by dissipating tropical cyclones, and the estimated 100-yr flood is 1,660 m3/s. For design purposes, 1,660 m3/s might be an appropriate value for the 100-yr flood at Tucson, assuming that climatic conditions during 1960-86 are representative of conditions expected in the immediate future.
Humus results from the partial degradation of the molecules that make up plants and, to a much lesser extent, animals. The degradation reactions that produce humus consist mainly of enzymatic depolymerization reactions and enzymatic oxidation reactions. These reactions give rise to molecules that are amphiphiles (an amphiphile is a molecule that has a hydrophobic part and a hydrophilic, polar part). Exudates from plants and microorganisms--especially proteins and lipids derived from microorganisms--also are included in the pool of molecules that constitute humus. These amphiphilic molecules are stabilized in soils, sediments, peats, and composts by incorporation into membrane structures that coat mineral grains, or by incorporation into micelles and vesicles that are dispersed in solution or are present as free organic aggregates in peats and composts of low mineral content. Although humus generally constitutes only a small percentage of the total mass of soils and sediments, the physical and chemical properties of soils and sediments are, to a large extent, controlled by the humus that coats the more reactive mineral surfaces. Other organic and inorganic components in natural water system will interact with humus. Ionic species will interact with the hydrophilic surfaces of humic membranes and micelles, whereas hydrophobic species will partition into the hydrophobic interiors of the humic membranes and micelles. Humus, therefore, may be modeled as consisting of separate hydrophilic and hydrophobic phases.
The surface chemistry of fresh and weathered historical basalt flows was characterized using surface-sensitive X-ray photoelectron spectroscopy (XPS). Surfaces of unweathered 1987-1990 flows from the Kilauea Volcano, Hawaii, exhibited variable enrichment in Al, Mg, Ca, and F due to the formation of refractory fluoride compounds and pronounced depletion in Si and Fe from the volatilization of SiF4 and FeF3 during cooling. These reactions, as predicted from shifts in thermodynamic equilibrium with temperature, are induced by diffusion of HF from the flow interiors to the cooling surface. The lack of Si loss and solid fluoride formation for recent basalts from the Krafla Volcano, Iceland, suggest HF degassing at higher temperatures. Subsequent short-term subaerial weathering reactions are strongly influenced by the initial surface composition of the flow and therefore its cooling history. Successive samples collected from the 1987 Kilauea flow demonstrated that the fluoridated flow surfaces leached to a predominantly SiO2 composition by natural weathering within one year. These chemically depleted surfaces were also observed on Hawaiian basalt flows dating back to 1801 AD. Solubility and kinetic models, based on thermodynamic and kinetic data for crystalline AlF23, MgF2, and CaF2, support observed elemental depletion rates due to chemical weathering. Additional loss of alkalis from the Hawaiian basalt occurs from incongruent dissolution of the basalt glass substrate during weathering.
Water samples were collected in the San Francisco Bay estuary during 22 cruises from January through December 1991. 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 ridge in the southern part of the bay to Rio Vista on the Sacramento River. Discrete water samples were analyzed for chlorophyll a and phaeophytin. Water density was calculated from values for salinity, temperature, and pressure (depth), and is included in the data summaries.
The origin of the approximately 40-50 topographically large lake basins on the southern High Plains of Texas and New Mexico has been an enigma. Previous workers have considered deflation or evaporite dissolution at depth and subsequent collapse as the most probable mechanisms of formation. However, the eolian hypotheses have been unable to provide convincing arguments as to how the wind selectively erodes the thick, deflation-resistant calcrete 'caprock' that is persistent over much of the southern High Plains. The deflation hypothesis has been modified by proposing that the calcrete caprock may never have been deposited in the areas now occupied by the basins. The absence of calcrete deposition is proposed to have resulted from high water tables caused by an increase in hydraulic gradient where aquifers thinned above bedrock highs. A high water table close to and/or intersecting the surface prevents deposition of calcrete and, thus, the uncemented surface would be more susceptible to deflation than the surrounding calcrete-covered areas after decline of the water table. The rise in water table associated with bedrock highs is documented by numerical simulation using boundary conditions and hydrologic parameters representative of the southern High Plains.
Increment cores were collected from trees growing at two sites where groundwater is contaminated by nickel. Proton-induced X ray emission spectroscopy was used to determine the nickel concentrations in selected individual rings and in parts of individual rings. Ring nickel concentrations were interpreted on the basis of recent concentrations of nickel in aquifers, historical information about site use activities, and model simulations of groundwater flow. Nickel concentrations in rings increased during years of site use but not in trees outside the contaminated aquifers. Consequently, it was concluded that trees may preserve in their rings an annual record of nickel contamination in groundwater. Tulip trees and oak trees were found to contain higher concentrations of nickel than did sassafras, sweet gum, or black cherry. No evidence was found that nickel accumulates consistently within parts of individual rings or that nickel is translocated across ring boundaries.
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