Showing papers in "Ground Water in 1998"

Uses of Chloride/Bromide Ratios in Studies of Potable Water

Abstract: In natural ground water systems, both chlorine and bromine occur primarily as monovalent anions, chloride and bromide Although dissolution or precipitation of halite, biological activity in the root zone, anion sorption, and exchange can affect chloride/bromide ratios in some settings, movement of the ions in potable ground water is most often conservative Atmospheric precipitation will generally have mass ratios between 50 and 150; shallow ground water, between 100 and 200; domestic sewage, between 300 and 600; water affected by dissolution of halite, between 1,000 and 10,000; and summer runoff from urban streets, between 10 and 100 These, and other distinctive elemental ratios, are useful in the reconstruction of the origin and movement of ground water, as illustrated by case studies investigating sources of salinity in ground water from Alberta, Kansas, and Arizona, and infiltration rates and pathways at Yucca Mountain, Nevada

Use of Multiple Isotope Tracers to Evaluate Denitrification in Ground Water: Study of Nitrate from a Large-Flux Septic System Plume

Abstract: This study explores the use of multiple isotopic tracers to evaluate the processes involved in nitrate attenuation in ground water. δ15N and δ18O are used to provide information about the role of denitrification on nitrate attenuation, and δ34S, δ18O, and δ13C are used to evaluate the role of reduced sulfur and carbon as electron donors for nitrate reduction. The focus of this study is a zone of significant NO3−1 attenuation occurring in a sand aquifer impacted by septic system contamination. The NO3−1 pattern, the ground water flow system, and changes in other chemical parameters suggest that the NO3−1 depletion is caused by denitrification. This is supported by the nitrate δ15N and δ18O data which both show significant isotopic enrichment as NO3−1 depletion proceeds along the flow path. The increase of sulfate and dissolved inorganic carbon observed in the zone of nitrate attenuation suggests that reduced sulfur in addition to carbon is also involved in denitrification. This is supported by a trend toward depleted sulfate δ34S and δ18O values in the zone of sulfate increase, which reflects the input of sulfate formed by the oxidation of biogenic pyrite present in the aquifer sediments. The trend toward depleted δ13 values in the zone of increasing dissolved inorganic carbon reflects the input of organic carbon into this carbon pool. Chemical mass balance indicates that carbon is the dominant electron donor; however, this study demonstrates the effectiveness of using multiple isotopic tracers for providing insight into the processes affecting nitrate attenuation in ground water.

Long‐Term Performance of an In Situ “Iron Wall” for Remediation of VOCs

Abstract: The use of granular iron for in situ degradation of dissolved chlorinated organic compounds is rapidly gaining acceptance as a cost-effective technology for ground water remediation. This paper describes the first field demonstration of the technology, and is of particular importance since it provides the longest available record of performance (five years). A mixture of 22% granular iron and 78% sand was installed as a permeable “wall” across the path of a contaminant plume at Canadian Forces Base, Borden, Ontario. The major contaminants were trichloroethene (TCE, 268 mg/L) and tetrachloroethene (PCE, 58 mg/L). Approximately 90% of the TCE and 86% of the PCE were removed by reductive dechlorination within the wall, with no measurable decrease in performance over the five year duration of the test. Though about 1% of the influent TCE and PCE appeared as dichloroethene isomers as a consequence of the dechlorination of TCE and PCE, these also degraded within the iron-sand mixture. Performance of the field installation was reasonably consistent with the results of laboratory column studies conducted to simulate the field behavior. However, if a more reactive iron material, or a higher percentage of iron had been used, complete removal of the chlorinated compounds might have been achieved. Changes in water chemistry indicated that calcium carbonate was precipitating within the reactive material; however, the trace amount of precipitate detected in core samples collected four years after installation of the wall suggest that the observed performance should persist for at least another five years. The study provides strong evidence that in situ use of granular iron could provide a long-term, low-maintenance cost solution for many ground water contamination problems.

A comparison of zero-order, first-order, and monod biotransformation models

Abstract: Under some conditions, a first-order kinetic model is a poor representation of biodegradation in contaminated aquifers. Although it is well known that the assumption of first-order kinetics is valid only when substrate concentration, S, is much less than the half-saturation constant, K{sub S}, this assumption is often made without verification of this condition. The authors present a formal error analysis showing that the relative error in the first-order approximation is S/K{sub S} and in the zero-order approximation the error is K{sub S}/S. They then examine the problems that arise when the first-order approximation is used outside the range for which it is valid. A series of numerical simulations comparing results of first- and zero-order rate approximations to Monod kinetics for a real data set illustrates that if concentrations observed in the field are higher than K{sub S}, it may be better to model degradation using a zero-order rate expression. Compared with Monod kinetics, extrapolation of a first-order rate to lower concentrations under-predicts the biotransformation potential, while extrapolation to higher concentrations may grossly over-predict the transformation rate. A summary of solubilities and Monod parameters for aerobic benzene, toluene, and xylene (BTX) degradation shows that the a priori assumption of first-order degradationmore » kinetics at sites contaminated with these compounds is not valid. In particular, out of six published values of K{sub S} for toluene, only one is greater than 2 mg/L, indicating that when toluene is present in concentrations greater than about a part per million, the assumption of first-order kinetics may be invalid. Finally, the authors apply an existing analytical solution for steady-state one-dimensional advective transport with Monod degradation kinetics to a field data set.« less

Chloride/Bromide and Chloride/Fluoride Ratios of Domestic Sewage Effluents and Associated Contaminated Ground Water

Abstract: To establish geochemical tools for tracing the origin of ground water contamination, we examined the variations of Cl/Br and Cl/F (weight) ratios in (1) domestic waste water from the Dan Region Sewage Reclamation Project and from reservoirs in the central coast of Israel; (2) associated contaminated ground water; and (3) pristine ground water from the Mediterranean coastal aquifer of Israel. Our data show that supply water, anthropogenic NaCl and fluoridation control the Cl/Br (410 to 873) and Cl/F (468 to 1070) ratios of domestic waste water, and conventional sewage treatment does not affect the anthropogenic inorganic signals. The Cl/Br ratios of ground water contaminated with sewage effluent reflect conservative mixing proportions of sewage and regional ground water components. Sensitivity tests demonstrate that it is possible to detect and distinguish sewage contamination from marine ratios after a sewage contribution of 5 to 15% is mixed with regional ground water. Mixing with Br-enriched fresh water (e.g., Sea of Galilee; Cl/Br=145), however, would reduce this sensitivity. Since the high Cl/Br signal of sewage effluents is distinguishable from other anthropogenic sources with low Cl/Br ratios (e.g., street runoff, agriculture return flows) and from natural contamination sources (e.g., salt water intrusion; Cl/Br=293), Cl/Br ratios can therefore be a useful inorganic tracer for identification of the origin of contaminated ground water. The Cl/F ratios of sewage-contaminated ground water (284 to 5186) were higher than those in the original sewage effluent, which suggests retention of fluoride into the aquifer solid phase.

Simplified Method of “Push‐Pull” Test Data Analysis for Determining In Situ Reaction Rate Coefficients

Abstract: The single-well, ``push-pull`` test method is useful for obtaining information on a wide variety of aquifer physical, chemical, and microbiological characteristics. A push-pull test consists of the pulse-type injection of a prepared test solution into a single monitoring well followed by the extraction of the test solution/ground water mixture from the same well. The test solution contains a conservative tracer and one or more reactants selected to investigate a particular process. During the extraction phase, the concentrations of tracer, reactants, and possible reaction products are measured to obtain breakthrough curves for all solutes. This paper presents a simplified method of data analysis that can be used to estimate a first-order reaction rate coefficient from these breakthrough curves. Rate coefficients are obtained by fitting a regression line to a plot of normalized concentrations versus elapsed time, requiring no knowledge of aquifer porosity, dispersivity, or hydraulic conductivity. A semi-analytical solution to the advective-dispersion equation is derived and used in a sensitivity analysis to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push-pull tests in a homogeneous, confined aquifer with a fully-penetrating injection/extraction well and varying porosity, dispersivity, test duration, and reaction rate. A numerical flow andmore » transport code (SUTRA) is used to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push-pull tests in a heterogeneous, unconfined aquifer with a partially penetrating well. In all cases the simplified method provides accurate estimates of reaction rate coefficients; estimation errors ranged from 0.1 to 8.9% with most errors less than 5%.« less

Spatial Variation in Nitrogen Isotope Values Beneath Nitrate Contamination Sources

Abstract: Nitrogen isotope ratios of soil water and ground water nitrate have been used to identify or implicate sources; however, lack of data on delta 15N in thick (> 12 m) vadose zones beneath the source and potential effects of denitrification on delta 15N signatures have raised questions about the meaning of delta 15N values measured in ground water. In this study, nitrogen isotope ratios (delta 15N) were measured on nitrate extracted from 218 core samples removed from the surface to the water table below natural (soil organic matter) fertilizer, onsite sewage disposal systems (septic tank effluent), and animal sources located in Salinas and Sacramento Valleys, California. Additionally, spatial variability of delta 15N in the horizontal plane was measured beneath an agricultural field, and delta 15N values of native geologic materials (organic-rich shales) were determined. In general, delta 15N values throughout the vadose zone were consistent with the literature and remained fairly constant with depth, indicating little denitrification during transport. The delta 15N values from soil organic matter sources varied from about 0 to +4 per thousand, and the mean was not significantly different from that of fertilizer sources. The delta 15N values of animal sources varied from about +8 to +20 per thousand and were dependent on site and animal source. The delta 15N of onsite sewage disposal sources varied from about +2 to +12 per thousand, and the mean was significantly different from that of animal sources at a 90% confidence level. delta 15N of organic-rich shales of the Panoche and Moreno Formations (Cretaceous; San Joaquin Valley) and soils derived from these rocks were generally between +4 and +8 per thousand. Thus, it may be difficult to distinguish a geologic-N source from septic tank or commercial fertilizer sources using delta 15N. Little spatial variation was found in delta 15N beneath the agricultural field in the horizontal plane.

Review of Phosphate Mobility and Persistence in 10 Septic System Plumes

Abstract: Phosphate distribution was reviewed in 10 mature, highly monitored septic system ground water plumes in central Canada It was shown that six plumes (primarily those on calcareous sands) of enriched P concentrations (05 to 5 mg/L P) exceeding 10 m in length are present In each case, phosphate migration velocity is highly retarded (retardation factor, 20 to 100) compared to the ground water velocity, hut migration rates remain sufficiently fast (−1 m/a) to the of concern when considering long-term operation and the normal setback distance of septic systems from adjacent surface water bodies (∼ 15 m) Much smaller scale phosphate plumes (< 3 m in length) are present at the acidic sites on noncalcareous sands and on silt- and clay-rich sediments At all of the sites, ground water concentrations are lower than effluent values by amounts ranging from 23 to 99%, suggesting that P accumulation has occurred in the vadose zone This was confirmed by sediment analyses at four of the sites which, in each case, showed that zones of Penrichment were present within 1 m of the infiltration pipes (Wood 1993; Zanini et al 1998) Also, observed phosphate concentrations are generally consistent with values expected based on the solubility constraints of the minerals vivianite in reducing zones (including the septic tank), and strengite and variscite in oxidizing zones, providing further evidence that min-eral precipitation reactions play a role in limiting P concentrations Strengite and variscite have the potential to limit P to low con-centrations (co1 mg/L) under acidic conditions, but oxidation of sewage efiluent leads to acidic conditions only in noncalcareous terrain or beneath old septic systems where calcium carbonate has been depleted Overall, phosphate plume migration velocities in ground water appear to be controlled by sorption processes, but the phosphate concentrations that are present in the plumes appear to be strongly controlled by mineral precipitation reactions that occur in close proximity to the infiltration pipes

Relationship Between Pumping-Test and Slug-Test Parameters: Scale Effect or Artifact?

Abstract: In most field investigations, information about hydraulic conductivity (K) is obtained through pumping or slug tests. A considerable body of data has been amassed that indicates that the K estimate from a pumping test is, on average, considerably larger than the estimate obtained from a series of slug tests in the same formation. Although these data could be interpreted as indicating a natural underlying scale dependence in K, an alternate explanation is that the slug-test K is artificially low as a result of incomplete well development and, to a much lesser extent, failure to account for vertical anisotropy. Incomplete well development will often result in only the most permeable zones being cleared of drilling debris, with much of the screened interval remaining undeveloped. More cursory development can leave a low-K skin along the entire screened interval. Failure to recognize such conditions can result in a K estimate from a slug test that is much lower than the average K of the formation in the vicinity of the well. By contrast, neither a skin nor vertical anisotropy will have a significant impact on K estimates from pumping tests when semi-log analyses and/or observation wells are used. However, a reasonable estimate of aquifer thickness is required to convert the transmissivity calculated from a pumping test into an average K for the aquifer. Prior to invoking a natural scale dependence to explain the results of different types of hydraulic tests, head data should be closely examined and serious consideration given to alternate explanations.

Modeling a Complex Multi-Aquifer System: The Waterloo Moraine

Abstract: The Regional Municipality of Waterloo in Ontario, Canada (population 250,000) depends on ground water for most of its water supply. The ground water is extracted from the Waterloo Moraine, an extensive and complex glacial aquifer system extending over a 400 km2 area. A methodology is being developed to inventory the ground water resource, to define its susceptibility to contamination, and to create the basis for optimal management and protection strategies. A key component of this methodology is a three-dimensional conceptual hydrogeologic model based on the geologic characteristics of the multiple-aquifer Moraine system. The steps in the development of the model include screening of the large database, interpretation and interpolation of the data to define the variable hydrostatigraphy and to generate consistent hydraulic conductivity functions, and model calibration. The numerical basis is a fully three-dimensional finite element model that provides flexibility and adaptability in representing the natural boundaries, the highly irregular stratigraphy, and the numerous wellfields. The model has the capability to automatically find the location of the water table consistent with given recharge and pumping conditions, and to direct recharge from low-permeability areas to higher-permeability areas. Capture zones generated by the model are found to be highly sensitive with respect to the geologic structure, in particular the presence or absence of windows in the aquitard units. Professional judgment is found to be an essential component of the modeling process.

A controlled experiment in ground water flow model calibration

Abstract: Nonlinear regression was introduced to ground water modeling in the 1970s, but has been used very little to calibrate numerical models of complicated ground water systems. Apparently, nonlinear regression is thought by many to be incapable of addressing such complex problems. With what we believe to be the most complicated synthetic test case used for such a study, this work investigates using nonlinear regression in ground water model calibration. Results of the study fall into two categories. First, the study demonstrates how systematic use of a well designed nonlinear regression method can indicate the importance of different types of data and can lead to successive improvement of models and their parameterizations. Our method differs from previous methods presented in the ground water literature in that (1) weighting is more closely related to expected data errors than is usually the case; (2) defined diagnostic statistics allow for more effective evaluation of the available data, the model, and their interaction; and (3) prior information is used more cautiously. Second, our results challenge some commonly held beliefs about model calibration. For the test case considered, we show that (1) field measured values of hydraulic conductivity are not as directly applicable to models as their use in some geostatistical methods imply; (2) a unique model does not necessarily need to be identified to obtain accurate predictions; and (3) in the absence of obvious model bias, model error was normally distributed. The complexity of the test case involved implies that the methods used and conclusions drawn are likely to be powerful in practice.

Virus Occurrence and Transport in a School Septic System and Unconfined Aquifer

Abstract: Federal efforts to establish reliable natural disinfection criteria for ground water supplies require the identification of appropriate indicator viruses to represent pathogenic viruses and an understanding of parameters affecting virus survival and transport in a variety of hydrogeologic settings. A high school septic system and the associated fecal waste-impacted unconfined sand and gravel aquifer were instrumented to: (1) evaluate if the concentrations of enterovirus and coliphage in this system were sufficient to allow their use as indicator viruses; (2) establish viral transport rates, transport distances, and concentrations in a highly conductive cold water aquifer. Enteroviruses were found in only two of eight assays of the septic tank effluent (0.26 and 4.4 virus/L) and were below detection in eight ground water samples. Male-specific and somatic coliphage were detectable in both the septic tank effluent (averaging 674,000 and 466,000 coliphage/L, respectively) and in the impacted underlying ground water, decreasing to detection limits beyond 38 m of the drainfield. Virus transport parameters in this aquifer were measured by seeding high numbers of MS2 and OX174 coliphage into the ground water and documenting their transport over 17.4 m. A portion of the seeded virus traveled at least as fast as the bromide tracer (1 to 2.9 m/d). Proposed natural disinfection criteria would not be met in this aquifer using standard 30.5 m setback distances. In addition, the virus sorption processes and long survival times resulted in presence of viable seed virus for more than nine months.

Rhodamine WT and Bacillus subtilis Transport through an Alluvial Gravel Aquifer

Abstract: A field study was conducted to characterize the transport of rhodamine WT dye and endospores of the bacterium Bacillus subtilis through preferential flowpaths in an alluvial gravel aquifer using natural gradient tracer experiments. Preliminary tracer experiments were conducted with rhodamine WT to determine preferential flowpaths using a resin bag method, because of spatial heterogeneity at the study site. Rhodamine WT, Cl, and B. subtilis endospores were then injected 0.1 to 1.6 m below the water table, and downgradient ground water was monitored. Rhodamine WT behaved like Cl in the field and was used as a nonreactive tracer in this study. The method of time moments (MTM) and a curve fitting method (CFM) using the AT123D model were applied to estimate transport parameters of rhodamine WT and B. subtilis endospores along preferential flowpaths within the aquifer, assuming each individual flowpath to be uniform. The results from CFM fit the observed data better than did MTM. Ground water velocities through preferential flowpaths ranged from 30 to 85 m/day with a median of 63 m/day, and longitudinal dispersivities ranged from 0.71 to 5.24 m with a median of 2.70 m. These values are within the range reported in the literature for similar hydrogeological conditions. B. subtilis endospores exhibited slightly faster velocities (median retardation factor 0.86) and lower longitudinal dispersivities (0.23 to 2.4 m, median 0.79 m) compared to rhodamine WT. When multiple concentration peaks were observed in the rhodamine WT breakthrough curves (BTCs), the bacteria always arrived with the first rhodamine WT peak, regardless of whether the first peak was small or large. For those wells with a single peak for both the dye and the bacteria, the bacteria tended to coincide with the front portion of the dye BTC. This suggests that convection is relatively more important for B. subtilis endospores than for the dye, probably due to pore size exclusion of the spores. For this reason, the use of non-reactive solute tracers to reflect the movement of microorganisms may not be appropriate. The total removal rates of B. subtilis varied from 2.4 to 9.36 day−1 (median 3.36 day−1, probably resulting from filtration, sedimentation, and irreversible adsorption, as die-off was not observed in the field during the 45 days of the study. The longevity of B. subtilis endospores in ground water makes it a good choice to use as a bacterial tracer in the study of transport processes in the absence of die-off.

Improving a Complex Finite-Difference Ground Water Flow Model Through the Use of an Analytic Element Screening Model

Abstract: This paper demonstrates that analytic element models have potential as powerful screening tools that can facilitate or improve calibration of more complicated finite-difference and finite-element models. We demonstrate how a two-dimensional analytic element model was used to identify errors in a complex three-dimensional finite-difference model caused by incorrect specification of boundary conditions. An improved finite-difference model was developed using boundary conditions developed from a far-field analytic element model. Calibration of a revised finite-difference model was achieved using fewer zones of hydraulic conductivity and lake bed conductance than the original finite-difference model. Calibration statistics were also improved in that simulated base-flows were much closer to measured values. The improved calibration is due mainly to improved specification of the boundary conditions made possible by first solving the far-field problem with an analytic element model.

Spatial Variability in In Situ Aerobic Respiration and Denitrification Rates in a Petroleum-Contaminated Aquifer

Abstract: An extensive series of single-well, push-pull tests was performed to quantify horizontal and vertical spatial variability in aerobic respiration and denitrification rates in a petroleum-contaminated aquifer. The results indicated rapid consumption of injected O2 or NO3− in shallow and deep test intervals across a large portion of the site. Computed first-order rate coefficients for aerobic respiration ranged from 0.15 to 1.69 h−1 in the shallow test interval, and from 0.08 to 0.83 h−1 in the deep test interval. The largest aerobic respiration rates occurred on the upgradient edge of the contaminant plume where concentrations of petroleum hydrocarbons and dissolved O2 were relatively high. Computed first-order rate coefficients for denitrification ranged from 0.09 to 0.42 h−1 in the shallow test interval, and from 0.11 to 0.28 h−1 in the deep test interval. The largest denitrification rates occurred on the downgradient edge of the plume where hydrocarbon concentrations were relatively high but dissolved oxygen concentrations were small. The rates reported here represent maximal rates of aerobic respiration and denitrification, as supported by high concentrations of electron acceptors in the injected test solutions. Production of dissolved CO2 during aerobic respiration and denitrification tests provided evidence that O2 and NO3− consumption was largely due to microbial activity. Additional evidence for microbial NO3− consumption was provided by reduced rates of NO3−consumption when dissolved O2 was injected with NO3−, and by increased N2O production when C2H2 was injected with NO3−.

A Wetland Simulation Module for the MODFLOW Ground Water Model

Abstract: The alteration of wetland habitats by natural and anthropogenic processes is an issue of worldwide concern. Understanding the changes that occur in wetlands often requires knowledge of how surface water levels relate to adjacent aquifer systems. The ability to simulate surface water movement and its interaction with ground water and wetland slough channels is a desirable step in the design of many projects constructed in or near wetlands. Currently, most ground water flow models incorporate wetland systems as general head boundary nodes. The purpose of this research was to develop a computer package for the widely used MOD-FLOW code that would simulate three-dimensional wetland flow hydroperiods and wetland interactions with aquifers and slough channels. The ground water flow model was used to reproduce the surface water flow process through wetlands, and then to estimate new flow rates and values using a Manning-type equation. This package represents flow routing, export and import of water, and evapotranspiration from wetlands for different hydroperiods. A basic verification procedure for the numerical solution of the diffusion equation was applied, based on a test case that was solved using a two-dimensional surface water model. This example is a transient solution to the diffusion equation, in which the initial conditions were depicted by a sinusoidal water surface profile and a flat bottom.

A 3H/3He Study of Ground Water Flow in a Fractured Bedrock Aquifer

Abstract: The concentrations of tritium, helium isotopes, and neon have been measured in ground water from a fractured bedrock aquifer in a densely populated suburban area near New York City. Samples were obtained from heavily pumped production wells of a regional water supply company. Helium and neon concentrations exceed the values for air-saturated water, which is explained by the addition of unfractionated atmospheric air, radiogenic helium, and tritiogenic 3He. The identification of the composition of these excess components allows reliable separation of the tritiogenic 3He concentration, and hence the calculation of the 3H/3He ages of the ground water. Comparison of the combined tritium plus tritiogenic 3He concentrations with the historical record of tritium input from precipitation confirms that the 3H/3He data are self-consistent and provides constraints on the degree of mixing or helium loss. The distribution of 3H/3He ages is related to the large-scale topography of the study area and the depth of the wells. Furthermore, correlations between the 3H/3He ages and concentrations of total dissolved solids and total CO2 show that the ages contain meaningful information related to the temporal changes of the ground water chemistry.

Ground Water Age and Nitrate Distribution Within a Glacial Aquifer Beneath a Thick Unsaturated Zone

Abstract: The impact on ground water quality from increasing fertilizer application rates over the past 40 years is evaluated within a glacial aquifer system beneath a thick unsaturated zone. Ground water ages within the aquifer could not be accurately determined from the measured distribution of 3H and as a result, chlorofluorocarbon (CFC) and 3H/3He dating techniques were applied. Beneath a 25 m thick unsaturated zone, ground water ages based on CFC-11 concentrations were greater than 3H/3He ground water ages by 6 to 10 years, due to the time lag associated with the diffusion of CFCs through the unsaturated zone. Using the corrected CFC-11 and 3H/3He ground water ages and the estimated travel time of 3H within the unsaturated zone, the approximate position of ground water recharged since the mid-1960s was determined. Nitrate concentrations within post mid-1960s recharge were generally elevated and near or above the drinking water limit of 10 mg-N/L. In comparison, pre mid-1960s recharge had nitrate concentrations <2.5 mg-N/L. The elevated NO3− concentrations in post mid-1960s recharge are attributed mainly to increasing fertilizer application rates between 1970 and the mid- to late 1980s. Anaerobic conditions suitable for denitrifkation are present within pre mid-1960s recharge indicating that removal of DO is a slow process taking tens of years. Over the next 10 to 20 years, nitrate concentrations at municipal well fields that are currently capturing aerobic ground water recharged near the mid-1960s are expected to increase because of the higher fertilizer application rates beginning in the 1970s and 1980s.

Using Stable Isotopes of Water and Strontium to Investigate the Hydrology of a Natural and a Constructed Wetland

Abstract: Wetlands cannot exist without water, but wetland hydrology is difficult to characterize. As a result, compensatory wetland mitigation often only assumes the proper hydrology has been created. In this study, water sources and mass transfer processes in a natural and constructed wetland complex were investigated using isotopes of water and strontium. Water isotope profiles in the saturated zone revealed that the natural wetland and one site in the constructed wetland were primarily fed by ground water; profiles in another constructed wetland site showed recent rain was the predominant source of water in the root zone. Water isotopes in the capillary fringe indicated that the residence time for rain is less in the natural wetland than in the constructed wetland, thus transpiration (an important water sink) was greater in the natural wetland. Strontium isotopes showed a systematic difference between the natural and constructed wetlands that we attribute to the presence or absence of peat. In the peat-rich natural wetland, δ87Sr in the pore water increased along the flowline due to preferential weathering of minerals containing radiogenic Sr in response to elevated Fe concentrations in the water. In the constructed wetland, where peat thickness was thin and Fe concentrations in water were negligible, δ87Sr did not increase along the flowline. The source of the peat (on-site or off-site derived) applied in the constructed wetland controlled the δ87Sr at the top of the profile, but the effects were restricted by strong cation exchange in the underlying fluvial sediments. Based on the results of this study, neither constructed wetland site duplicated the water source and weathering environment of the adjoining natural wetland. Moreover, stable isotopes were shown to be effective tools for investigating wetlands and gaining insight not easily obtained using non-isotopic techniques. These tools have potential widespread application to wetlands that have distinct isotopic endmember sources.

A Method to Infer In Situ Reaction Rates from Push‐Pull Experiments

Abstract: A method to evaluate first-order and zero-order in situ reaction rates from a push-pull test is presented. A single-well push-pull test starts with the rapid injection of a well-mixed slug containing a known quantity of a conservative tracer and a reactive solute into the saturated zone. The slug is then periodically extracted and sampled from the same well. For zero- or first-order reactions, in the absence of sorption and assuming negligible background concentrations, these measurements can be used to evaluate reaction rate coefficients directly. The method does not involve computer-based solute transport models and requires no knowledge of regional ground water flow or hydraulic parameters. The method performs well when the dominate processes are advection, dispersion, and zero- or first-order irreversible reactions. Regional flow velocities must be sufficiently low such that the slug stays within the area of the well during the sampling phase. In the case of zero-order reactions, results using the method proposed here are compared with those obtained through the traditional method of calibrating a computer-based transport model. The two methods give similar estimates of the reaction rate coefficient. The method is general enough to work with a broad range of push-pull experiment designs and sampling techniques.

Field Evaluation of Interfacial and Partitioning Tracers for Characterization of Effective NAPL‐Water Contact Areas

Abstract: The use of surface-active tracers for measuring the interfacial area between nonaqueous phase liquids (NAPLs) and water was evaluated in a hydraulically isolated test cell installed in a surficial aquifer located at Hill Air Force Base (AFB), Utah. Interfacial tracers were developed at the University of Florida for quantifying immiscible fluid-fluid interfaces (air-water or NAPL-water) in porous media. Sodium dodecyl benzene sulfonate (SDBS) was used as the interfacial tracer to measure the effective NAPL-water interfacial area (a{sub Nw}), while 2,2-dimethyl-3-pentanol (DMP) was used as the partitioning tracer to estimate the NAPL saturation (S{sub N}). The observed retardation of SDBS and DMP when compared to a nonreactive tracer (bromide or methanol) at eight multi-level sampling locations and one extraction well, was used to quantify the a{sub Nw} and S{sub N} values averaged over the interval between the injection wells and each monitoring point. The NAPL morphology index, defined here as H{sub N} = a{sub Nw}/{psi}S{sub N} ({psi} = porosity), varied significantly within the test cell. In locations where the magnitude of H{sub N} was large, the NAPL was assumed to be more or less uniformly spread, providing good contact with the mobile fluid. In contrast, regions with low H{sub N} valuesmore » were assumed to have NAPL that was more heterogeneously distributed as isolated patches providing poor contact with the mobile fluid. The index H{sub N}, provided by the combined use of interfacial and partitioning tracers, has important implications for NAPL source region remediation employing in situ flushing technologies.« less

Utilization of intrinsic boron isotopes as co-migrating tracers for identifying potential nitrate contamination sources

Abstract: The stable isotopes of the conservative element boron, {sup 11}B and {sup 10}B, have been employed as co-migrating isotopic tracers to trace potential sources of nitrate observed in ground water pumped from a large capacity 0.167 m{sup 3}/s irrigation well in the Avra Valley of southeastern Arizona. The isotopic ratios provided an identifying signature for two nitrogen carrying source waters: municipal waste water and agricultural return flow. Additional chemical parameters were also examined to corroborate the isotopic indications. Boron isotopes provided a superior delineation of mixing processes in the system compared to the general inorganic chemical parameters. Findings of this investigation indicate that the water pumped by the study well at the beginning of the 1993 irrigation season was composed of a mixture of approximately 25% municipal waste water and 75% background ground water. As the irrigation season progressed, an increasing proportion of water was contributed by irrigation return flow from neighboring agricultural fields.

Pumping Test Analysis for a Tidally Forced Aquifer

Abstract: The migration of pollutant plumes in ground water systems in the vicinity of surface water bodies is a subject of continuing interest. Modeling the behavior and fate of such plumes is complicated by the dynamics of the surface water bodies. The analysis of pumping test data for a shallow riverbank aquifer afforded the opportunity to compare standard methods for determining hydraulic conductivity (K) and storativity (S) with techniques based on the propagation of periodic fluctuations of head in the aquifer. Tidal fluctuations in water level at the riverbank were found to propagate approximately nondispersively for 22 m in the aquifer, allowing tidal phase lags and amplitude attenuations in the aquifer to be estimated from river levels by least-squares techniques. The least-squares attenuations and phases were used to correct measured pumping test drawdowns for tidal influences. Standard transient analyses of the corrected drawdowns were performed, yielding values of approximately 8 to 10 m/d for K and 0.002 for S. The low S value was taken to indicate semi-confined conditions. The tidal attenuation coefficients were used to determine the hydraulic diffusivity T/S, providing values that were integrated over the distance from the riverbank to the relevant monitoring wells. S values from themore » drawdown analyses then yielded K estimates ranging from 3 to 7 m/d. A three-dimensional finite-element simulation of the pumping tests showed that the estimated K and S values yielded simulated drawdowns in reasonable agreement with field observations.« less

Ground Water Contaminant Source and Transport Parameter Identification by Correlation Coefficient Optimization

Abstract: The initial step in a ground water contamination remediation process is to identify the extent of the plume. One way to optimize well deployment is to solve an inverse contaminant transport problem. Inverse procedures bas4ed on correlation coefficient optimization are developed to locate ground water contaminant sources and to identify transport parameters. For cases involving two-dimensional instantaneous and continuous sources, the inverse formulas are explicit. These procedures allow not only for the delineation of the sampled contaminant plume, but also the tracing and the projection of the plume history.

Watershed Delineation and Ground Water Discharge to a Coastal Embayment

Abstract: Nonpoint source management strategies for watersheds to eutrophic coastal waters require an adequate characterization of subsurface hydrogeology to understand the sources and fate of ground water and contaminant flow. An investigation was conducted to define the watershed to Waquoit Bay, a shallow eutrophic coastal embayment at the margin of the sand and gravel aquifer of Cape Cod, Massachusetts. This investigation included: (1) the development of a subregional water table map over 56 square kilometers; (2) delineation of a watershed and subwatersheds by plotting reverse ground water flowpaths; (3) a water budget based on a mass balance between recharge rates over riverine watershed segments and measured stream flows; and (4) a series of well points driven beneath the bed of the embayment and a transect of deep geologic borings perpendicular to the shore to characterize submarine ground water and hydrogeologic conditions at depth. The principal aquifer thins from the apex of the watershed, where it is approximately 46 m thick, to approximately 11 m thick as it nears Waquoit Bay. The detailed water table map shows extremely high hydraulic gradients to the rivers and comparably smaller gradients near the bay, indicating the thinning aquifer forces ground water to discharge into the rivers and causes them to function as significant surface drains for the aquifer. A classic sharp salt water interface was found in the upper 11.3 m thick High K unit, while the interface in the lower 34 m thick Low K unit was estimated to extend 700 m from the shoreline beneath the saline water of the bay. Eighty-nine percent (89%) of the bay water budget is from ground water flow (55% channeled into rivers and 34% direct ground water discharge), while 11% is from atmospheric deposition. The Low K unit is an insignificant source of fresh water to the embayment. The conceptual hydro-geologic model of the watershed is consistent with the water table configuration and the mass balance water budget and was confirmed by subsurface characterization of the near shore hydrogeology. This type of watershed characterization provides a firm basis for the development of nonpoint source management options for coastal embayments.

Air Permeability of Porous Materials Under Controlled Laboratory Conditions

Abstract: A series of air permeability tests were conducted on four hand-packed samples of alluvial sands and glass beads using a newly developed air permeameter. The permeameter was tested and found capable of precisely controlling soil-water matric potential (in the range 0 to 1 bar) while simultaneously facilitating the direct measurement of air permeability in porous media. Permeameter results indicate that air permeability increases with a corresponding decrease in water content over a monotonic drainage cycle. It was observed that the rate of change in air permeability with respect to changes in water content is highest at high water content and lowest at low water content. In several soil samples, the air permeability approached a constant value at low water content. Air permeability variations with water content were observed to differ among soils of different texture. For example, the intrinsic permeability of water was 11 to 86% of the maximum air permeability. The new permeameter allowed rapid and accurate measurements of air permeability in fine-textured materials over a wide range of matric potentials and water content.

Development of a Preliminary Ground Water Flow Model for Water Resources Management in the Pingtung Plain, Taiwan

Abstract: Pingtung Plain is formed by Quaternary alluvial fan material from the three main rivers: Kaoping, Tungkang and Linpien. Ground water is the major water supply source on the plain. This is principally extracted from two aquifers. The natural ground water source is derived mainly from direct rainfall percolation and infiltration from the three rivers, with their catchments lying partly outside the plain. Rainfall characteristics are therefore the main factors controlling water resources availability. Pingtung Plain is an important primary production area for southern Taiwan, the comparatively warm climate allowing a long growing season, diversified cropping and the rearing of aquacultural produces. Approximately 75 percent of irrigation and domestic water supplies are derived from ground water. A water balance for the entire plain indicates that ground water resources, under optimized management, are sufficient to meet the existing multi-purpose uses. Development of a hydrogeological conceptual model is the first phase of a numerical ground water flow simulation. Preliminary results are encouraging, with the final simulations affording better insight to the hydraulic behavior of the aquifer system. Data input requirements for model operation fall into three categories: hydrological stresses, hydrogeological parameters and boundary conditions. After the model is built, the normal numerical modeling process requires significant calibration and sensitivity analyses for the hydrogeological parameters and stresses which are the most sensitive, but the least well defined. A well-calibrated simulation model can lead to a reliable and realistic management model. With this in mind, the calibration processes detailed are presented, and these data are introduced as initial values in the calibration process.

Characterizing a ground water basin in a New England mountain and valley terrain

Abstract: A ground water basin is defined as the volume of subsurface through which ground water flows from the water table to a specified discharge location. Delineating the topographically defined surface water basin and extending it vertically downward does not always define the ground water basin. Instead, a ground water basin is more appropriately delineated by tracking ground water flowpaths with a calibrated, three-dimensional ground water flow model. To determine hydrologic and chemical budgets of the basin, it is also necessary to quantify flow through each hydrogeologic unit in the basin. In particular, partitioning ground water flow through unconsolidated deposits versus bedrock is of significant interest to hillslope hydrologic studies. To address these issues, a model is developed and calibrated to simulate ground water flow through glacial deposits and fractured crystalline bedrock in the vicinity of Mirror Lake, New Hampshire. Tracking of ground water flowpaths suggests that Mirror Lake and its inlet streams drain a ground water recharge area that is about 1.5 times the area of the surface water basin. Calculation of the ground water budget suggests that, of the recharge that enters the Mirror Lake ground water basin, about 40% travels through the basin along flowpaths that stay exclusively in the glacial deposits, and about 60% travels along flowpaths that involve movement in bedrock.