bluuebunny/crossref_metadata_2025 huggingface dataset:
A Nonlinear Time Series Model for Spring Flow: An Example from Shanxi Province, China
Authors: Jian Wen‐Bin , Yao Huan , Wen Xian‐Huan , Chen Bao‐Ren
Published: August 2005

A nonlinear time series model is presented to describe the dynamics of ground water flow. The procedures for model‐establishing and parameter estimation are discussed. The proposed nonlinear model uses a threshold parameter that relates different precipitation processes, and a lag parameter that relates the time lag between precipitation events and observed increasing spring flow. This model is then applied to the prediction of time‐dependent flow of the Longzichi Spring, Shanxi Province, northwest China. The results in this analysis show that the proposed approach can accurately describe the complicated nonlinear time series and the discharge regime of the Longzichi Spring.
Efficient and Responsible Use of Prior Information in Inverse Methods
Authors: Weiss Richard , Smith Leslie
Published: August 2005

Prior information on parameters such as hydraulic conductivity or ground water recharge rates is often used to stabilize the inverse problem in parameter estimation. A cautious use of prior information is advisable, however, because it may be biased or unrepresentative of the model parameters and/or it may not significantly stabilize the parameter estimates. Procedures are given: (1) to identify the model parameters for which prior information may best stabilize the parameter set; and (2) to identify the model parameters for which errors in the prior information lead to the smallest possible errors in the final set of parameter estimates. The first case is referred to as the efficient use of prior information; the second case is referred to as the responsible use of prior information. The procedures are based on an analysis of the model parameter space using response surfaces, multiparameter confidence regions, and eigenspace analysis. The guidelines lead to the selection of prior information on those parameters whose axes are most closely aligned with the longest axis of the parameter confidence region. Simple synthetic examples are used to explain the concepts. The advantages gained in screening the model parameters to identify those parameters for which prior information will be most efficient and responsible in determining the final values of the parameter estimates are demonstrated in the calibration of a ground water flow and solute transport model for the San Juan Basin, New Mexico.
A Numerical Model for Simulation of Bioremediation of Hydrocarbons in Aquifers
Authors: Muňoz José F. , Irarrázaval Manuel J.
Published: August 2005

A numerical model was developed to describe the bioremediation of hydrocarbons in ground water aquifers considering aerobic degradation. The model solves the independent transport of three solutes (oxygen, hydrocarbons, and microorganisms) in ground water flow using the method of characteristics. Interactions between the three solutes, in which oxygen and hydrocarbons are consumed by microorganisms, are represented by Monod kinetics, solved using a Runge‐Kutta method. Model simulations showed good correlation as compared with results of soil column experiments. The model was used to estimate the time needed to remediate the columns, which varied from one to two years.
Anomalously High Nitrate Concentrations in Ground Water
Authors: Guimerà Jordi
Published: August 2005

Anomalously high nitrate concentrations are found in aquifers with agricultural lands in their recharge area, with restricted water outflows and with partially penetrating wells which induce ground water recirculation. A comprehensive methodology is proposed, which characterizes the source term ‐ nitrate leaching ‐ in time and space and couples it to the hydrodynamic characterization of the aquifer. The methodology is applied to a heavily polluted aquifer. Results show that poor management of irrigation and fertilization practices cannot be separated from unfavorable hydrodynamic conditions to explain concentrations as high as 160 mg/L NO3−N in ground water.
Prediction of Regional Ground Water Flow to Streams
Authors: Christensen S. , Rasmussen K.R. , Moller K.
Published: August 2005

Database information on geology, hydrology, and hydrometeorology may form an excellent basis for studying ground water flow and seepage to surface water in a catchment. In a field case study of a 114 km2catchment, geological database information was used to determine layer thicknesses and boundary conditions as well as to parameterize a ground water flow model. Hydraulic head and stream flow data were used to estimate the model parameters by nonlinear regression. The uncertainty of the estimated parameters and of the predicted stream flow gains was quantified by individual likelihood method confidence intervals. During four stages of calibration, ranging from using only head data to also using an extensive set of measured stream flow gains, no parameter estimates changed significantly, but the number of parameters was increased from 12 to 14 in order to fit local stream flow gains. This indicates that the geology‐based parameterization is firm.

Adding stream flows to the calibration data reduced the uncertainty of the estimated parameters significantly. However, the uncertainty of some of the parameters was significant even when an extensive set of measured stream flow gains and hydraulic heads was used to calibrate the model. Parameter uncertainty is reflected in the uncertainty of the predicted stream flow gains. When an extensive set of stream flow data was used during calibration, the prediction uncertainty is up to ±25% in large streams, and up to ±60% in smaller streams. The confidence intervals in general are skewed, and they are very skewed in the case where no stream flow measurements were used to calibrate the model.

The case study shows that even when relatively extensive geological information and calibration data are available, there may be significant uncertainty connected with the prediction of local discharge of ground water to streams. Reducing uncertainty in such cases will require extensive field investigations in order to improve the definition of recharge areas and to describe the local fluxes and flow patterns in the aquifers.
Use of Solute and Isotopic Composition of Ground Water to Constrain the Ground Water Flow System of the Azraq Area, Jordan
Authors: Abu‐Jaber Nizar S. , Ali Ali Jawad , Al Qudah Khaldoon
Published: August 2005

This study shows that hydrological systems in arid areas where water level data are incomplete can be better understood and constrained by using chemical and isotopic data. These constraints significantly alter the conceptual hydrological model of the Azraq area.

Previous studies, based on water level data, suggest that water in the Azraq oasis is derived from two major piezometric highs; the western basalt aquifer to the northwest and the Tulul el Ashaquif highlands to the northeast of the oasis. Water from the Tulul el Ashaquif aquifer is also considered to drain to a piezometric low at Ruwaishid to the east, and then into deeper aquifers.

Chemical data from this study show that the northwest basalt aquifer provides most of the water (80‐85 %) to the Azraq oasis. Water from the Tulul el Ashaquif supplies a relatively small proportion, but due to its high solute content, imparts its chemical signature on the oasis water. This study has shown that water from Tulul el Ashaquif are not supplied to the Ruwaishid area, as previously thought.

Stable isotopic data show that each piezometric high contains water which originated in a distinct climatic setting. Both water types are poor in tritium. Traces of tritium at the Azraq oasis and at Ruwaishid indicate some recent local recharge.
Solution of Ground Water Flow Problems with General Purpose and Special Purpose Computer Codes
Authors: Gribb Molly M. , Sewell Granville
Published: August 2005

Increasingly, ground water hydrologists and engineers must solve saturated/unsaturated flow and transport problems. The unsaturated component of flow is especially important when considering transport of contaminants that originate from surface or near‐surface sources such as waste spill sites, landfills, and hazardous waste storage sites. Numerical solution of such problems has been accomplished with special purpose computer programs developed for solution of unsaturated flow problems. However, general purpose partial differential equation (PDE) solvers also exist, which often provide greater model flexibility. In this paper, the performance of PDE2D, a general PDE solver, and SWMS_2D, a special purpose program, are compared for solution of unsaturated flow and transport problems. Both programs accurately calculated changes in water volume in the flow domain for four examples, but PDE2D did not calculate boundary fluxes as well as SWMS_2D, resulting in high mass balance errors. However, PDE2D yielded a smoother solution of the example solute transport problem, despite a large mass balance error. SWMS_2D offers ease of use and excellent mass balances while PDE2D offers the user greater flexibility in terms of the types of problems solved, and a grid generator that ensures that the element size and distribution of elements are appropriate for a given problem.
Dipole Probe: Design and Field Applications of a Single‐Borehole Device for Measurements of Vertical Variations of Hydraulic Conductivity
Authors: Zlotnik Vitaly A. , Zurbuchen Brian R.
Published: August 2005

A dipole probe (DP) for measuring vertical variations of hydraulic conductivity has been designed, constructed, and tested in field conditions. The variable‐length device designed for the dipole flow test (DFT) consists of three packers separated by changeable spacer rods. The assembled DP is lowered into a well with a long screened interval. The packers are inflated, isolating two screened sections (chambers) between them. A small submersible pump, mounted on the central packer, transfers water from the upper (extraction) chamber into the lower (injection) chamber. This simultaneous injection and extraction creates a recirculatory flow within the aquifer. The chamber head responses are measured by two pressure transducers in the DP chambers. The DP can be used at discrete intervals along the entire well screen length. The DP has been successfully tested in a specially constructed well with a continuous screen in a highly conductive sand and gravel aquifer. During drilling, disturbed samples were collected using the split spoon method and the vertical profile of K was estimated from grain‐size analysis. A total of 153 DFTs were performed with various DP geometries. Results indicate that the chamber head changes are sensitive to aquifer properties, reach steady state rapidly, vary linearly with redrculation pumping rate, and can be controlled by changes in DP dimensions. Vertical profiles of K compare well with grain‐size analysis estimates.
Evaluation of Multi‐Well Test Data in a Faulted Aquifer Using Linear and Radial Flow Models
Authors: Allen Diana M. , Michel Frederick A.
Published: August 2005

An extensive drilling and testing program was undertaken at Carleton University, Ottawa, Canada, to characterize the aquifer for the design of an aquifer thermal energy storage (ATES) system. The substantial data base provides an excellent record of well yields and hydraulic responses to a series of aquifer tests in a faulted carbonate aquifer. Well yield variability is reflected in the wide range in specific capacity (1.4 to 75.8 L/s/m) and is dependent upon the proximity of a well to a major fault. Constant discharge data, available for nine pumping wells and several observation wells, reflect both the excellent hydraulic connection along major faults and the limited hydraulic connection between fault blocks. Three different flow models, including vertical fracture and vertical dyke (linear), and Theis (radial), are used to interpret constant discharge test data from nine pumping wells and several observation wells. In all tests, the duration of pumping is sufficient to identify early‐time linear flow along faults and late‐time pseudo‐radial flow between fault blocks. This paper demonstrates the influence of fault‐induced heterogeneity on the hydraulic response of a carbonate aquifer and demonstrates the application and limitation of simple radial continuum models to aquifers of this type.
Water and Radioactive Tracer Flow in a Heterogeneous Field‐Scale System
Authors: Dunnivant Frank M. , Newman Meredith E. , Bishop Carolyn W. , Burgess Dave , Giles John R. , Higgs Brian D. , Hubbell Joel M. , Neher Erick , Norrell Greg T. , Pfiefer M. Cathy , Porro Indrek , Starr Robert C. , Wylie Allan H.
Published: August 2005

A coupled field‐scale aquifer pumping and water infiltration test was conducted at the Idaho National Engineering and Environmental Laboratory in order to evaluate subsurface water and contaminant transport processes in a heterogeneous flow system. The test included an aquifer pumping test to determine the storage properties of the aquifer and the state of confinement of the aquifer (∼190 m below land surface), and a vadose zone infiltration test to determine vertical moisture and radioactive tracer migration rates. Pump test results indicated that the Snake River Plain Aquifer was locally unconfined with a transmissivity ranging from 5.57 × 105 to 9.29 × 104 m2day. Moisture monitoring with neutron probes indicated that infiltrating water was initially transported vertically through the upper basalt layer of the vadose zone, primarily through fractures and rubble zones, at an average rate of 5 m/day (based on vertical distance traveled and first arrival of water at the monitoring points). Analysis of breakthrough curves for a conservative tracer allowed estimation of the arrival of the peak concentration and yielded an average velocity of 1 m/day. The migration velocities from the neutron probe and tracer tests are in good agreement given the scale of the test and difference in analysis techniques. None of the data sets showed a correlation between migration velocity (arrival time) and distance from the point source, but they strongly indicate preferential flow through discrete fractures. Upon reaching the first continuous sedimentary interbed layer in the basalt formation, water flow was diverted laterally along the interbed surface where it spread outward in primarily three areas corresponding to topographic lows on the interbed surface, and slowly infiltrated into the interbed. The nonpredictable movement of water and tracer through specific fractures underlying the site suggests that a priori prediction of trans‐missive fractures in this media is not possible. Results do suggest that the continuous sedimentary interbed layers, in general, impede vertical water flow and contaminant migration.
A Critique of a Steady‐State Analytical Method for Estimating Contaminant Degradation Rates
Authors: McNab Walt W. , Dooher Brendan P.
Published: August 2005

Contaminant transformation rates are routinely estimated from monitoring well data using a regression method based on a steady‐state analytical model. This approach assumes one‐dimensional solute transport in the presence of a continuous boundary concentration and a uniform first‐order degradation coefficient. It must be recognized that this type of analysis is easily subject to misinterpretation, particularly when few monitoring points are used. This is because the dispersive nature of solute transport can produce concentration profiles which closely resemble those associated with a degrading contaminant even in the absence of any transformation processes. Monte Carlo analyses using simulated plumes suggest that this method may lead to significant overestimates of apparent transformation rates.
Improving a Complex Finite‐Difference Ground Water Flow Model Through the Use of an Analytic Element Screening Model
Authors: Hunt Randall J. , Anderson Mary P. , Kelson Victor A.
Published: August 2005

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.
Arsenic Hydrogeochemistry in an Irrigated River Valley—A Reevaluation
Authors: Nimick David A.
Published: August 2005

Arsenic concentrations in ground water of the lower Madison River valley, Montana, are high (16 to 176 μg/L). Previous studies hypothesized that arsenic‐rich river water, applied as irrigation, was evapoconcentrated during recharge and contaminated the thin alluvial aquifer. Based on additional data collection and a reevaluation of the hydrology and geochemistry of the valley, the high arsenic concentrations in ground water are caused by a unique combination of natural hydrologic and geochemical factors, and irrigation appears to play a secondary role. The high arsenic concentrations in ground water have several causes: direct aquifer recharge by Madison River water having arsenic concentrations as high as 100 μg/L, leaching of arsenic from Tertiary volcano‐clastic sediment, and release of sorbed arsenic where redox conditions in ground water are reduced. The findings are consistent with related studies that demonstrate that arsenic is sorbed by irrigated soils in the valley. Although evaporation of applied irrigation water does not significantly increase arsenic concentrations in ground water, irrigation with arsenic‐rich water raises other environmental concerns.
How Observations and Structure Affect the Geostatistical Solution to the Steady‐State Inverse Problem
Authors: Kitanidis Peter K.
Published: August 2005

The solution to the steady‐state inverse problem can be expanded into a series of spline functions with weights adjusted to reproduce the observations within the observation error. The splines depend on the model spatial structure, the ground water flow model, and the location of the observations. This representation of the solution, which is a rigorous and exact expansion, provides insight into the form of the best estimate and explicitly shows how observations and the conceptual model may affect the solution.
A Wetland Simulation Module for the MODFLOW Ground Water Model
Authors: Restrepo Jorge I. , Montoya Angela M. , Obeysekera Jayantha
Published: August 2005

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.
Isotopic Constraints on Off‐Site Migration of Landfill CH4
Authors: Desrocher S. , Lollar B. Sherwood
Published: August 2005

Occurrences of CH4 in residential areas in the vicinity of the Beare Road landfill, Toronto, Canada, have raised public concern about potential off‐site migration of CH4 from the landfill site. Carbon isotopic analysis of dissolved and gas phase CH4 at the Beare Road site, however, indicates that CH4 in the ground water systems in the vicinity of the landfill is related to naturally occurring microbial methanogenesis within these geologic units, rather than to contamination by landfill CH4. CH4 gas in the landfill and landfill cover has δ13C values (−60.2 ± 0.9%e) typical of microbially produced gas. Concentrations of CH4 (508 to 693 uM) found in deep ground water in the Scarborough, Don, and Whitby Formations underlying the landfill are isotopically distinct from the landfill gases. They are isotopically and compositionally similar, however, to naturally occurring microbial CH4 identified in organic‐rich glacial deposits throughout Ontario. The lack of any significant CH4 concentrations or concentration gradients in the upper till zone between the landfill and the deep ground water aquifer is further evidence that no transport between the landfill and deep ground water is occurring.
Practical Approximations of the Well Function
Authors: Srivastava Rajesh , Guzman‐Guzman Amado
Published: August 2005

Computation of the well function is required in the analysis of pumping test data for leaky or confined aquifers. Existing electronic dataloggers facilitate collection of hydraulic test data, which is commonly interpreted using computer programs. Approximate expressions have recently been proposed to compute the well function over the entire range of argument values. We propose approximations that, while providing reasonable accuracy, are less computationally intensive than previous approximations. A detailed comparative analysis of the execution times and the errors of these approximations is performed. The proposed expressions are generally accurate to within 1%, and result in a savings of computation time on the order of 15 to 87% for confined aquifers, and 29 to 76% for leaky aquifers.
Stochastic Determination of the Wierden (Netherlands) Capture Zones
Authors: van Leeuwen Matthijs , te Stroet Chris B. M. , Butler Adrian P. , Tompkins Jacob A.
Published: August 2005

A spatial stochastic approach was adopted to estimate the capture zones of a welifield in eastern Netherlands near the town of Wierden. The main sources of uncertainty were found to be the hydraulic conductivity of the production aquifer and the spatial distribution and thickness of an impeding clay layer situated above the production aquifer. Both were modeled as random space functions and a Monte Carlo analysis was performed to assess the probability distributions of the capture zones for t = 25 years and t →∞. The distributions were compared with two deterministic capture zones to show how the stochastic approach preserves the discontinuous character of the impeding clay layer while the deterministic approach smooths out the discontinuities. Furthermore, it is shown how the stochastic approach reveals the uncertainty about the location of the capture zones while the deterministic approach obscures it.
An Analytical Solution for Steady Flow into a Ttonnel
Authors: Lei Shizhong
Published: August 2005

An analytical solution is derived for two‐dimensional, steady ground water flow into a horizontal tunnel in a fully saturated, homogeneous, isotropic, and semi‐infinite aquifer. The hydraulic head and streamline distributions in the flow field, as well as the inflow rate, are obtained for a constant hydraulic head boundary condition at the tunnel perimeter. The inflow rate formula derived in this paper is an extension to the Polubarinova‐Kochina (1962) formula, which can be considered a special case of the present solution.
Designing a Monitoring Network for Contaminated Ground Water in Fractured Chalk
Authors: Nativ R. , Adar E.M. , Becker A.
Published: August 2005

One of the challenges of monitoring network design in a fractured rock setting is the heterogeneity of the rocks. This paper summarizes the activities and problems associated with the monitoring of contaminated groundwater in porous, low‐permeability fractured chalk in the Negev Desert, Israel. Preferential flow documented in the study area required siting the monitoring boreholes in the predominant fracture systems. Lineaments traced from aerial photographs were examined in the field to sort out the large‐extension, through‐going, multilayer fracture systems crossing the study area. At each proposed drilling site, these fractures were exposed below the sediment cover using trenches. Slanted boreholes were drilled at a distance from the fracture systems so that each borehole would intersect the targeted fracture plane below the water table. Based on their short recovery period and contaminated ground water, these newly drilled, fracture‐oriented boreholes appeared to be better connected to preferential flowpaths crossing the industrial site than the old boreholes existing on site. Other considerations concerning the drilling and logging of monitoring boreholes in a fractured media were: (1) coring provides better documentation of the vertical fracture distribution, but dry augering is less costly and enables immediate ground water sampling and the sampling of vadose rock for contaminant analysis; (2) caliper and TV camera logs appear to provide only partial information regarding the vertical fracture distribution; and (3) the information gained by deepening the monitoring boreholes and testing fractures crossing their uncased walls has to be carefully weighed against the risk of potential cross‐contamination through the monitoring boreholes, which is enhanced in fractured media.
Multicell Model for Ground Water Recharge Analysis: The Ibi River Basin, Japan
Authors: Sato Takeshi , Kojima Junichi , Loaiciga Hugo A.
Published: August 2005

A coupled water quantity/quality model was developed and applied to estimate ground water recharge and simulate tracer concentrations in the Ibi River Basin of Japan. The ground water basin was divided into five sub‐basins, each of which contains a surface reservoir and up to three subsurface reservoirs. The reservoirs exchange water and solute mass among themselves and contribute direct runoff and base flow to adjacent streams. The simulation model is parameterized in terms of 14 water quantity and three water quality parameters. A parameter calibration procedure was implemented wherein streamflow and tracer concentration data were used in parameter estimation. Prior information on the parameters can be used to initialize and constrain calibrated parameters.

The calibrated model was validated successfully and used to simulate Na+ and Ca2+ and to estimate ground water recharge in the sub‐basins and the Ibi River Basin as a whole. Ground water recharge varied substantially among sub‐drainage basins, reflecting the predominant role of precipitation variability in determining deep percolation to the first confined aquifer. It was determined that about 10% of the gross precipitation becomes ground water recharge over the entire Ibi River Basin. Analysis of ground water recharge to a large pulse of precipitation demonstrated the important effect of impermeable clay layers in controlling the magnitude and timing of deep ground water recharge. The validation results indicate that the equilibrium concentrations of Na− and Ca2+ in the first confined aquifer have negligible sensitivity to their initial concentrations in the simulation model. Similar behavior was observed for the equilibrium concentrations of these two tracers in reservoir 3, and to a lesser extent in reservoir 2. The top reservoir exhibited large fluctuations in tracer concentrations due to the effect of direct rainfall input.
A Model of Ground Water Discharge from an Unconfined Coastal Aquifer
Authors: Robinson Michael A. , Gallagher Daniel L.
Published: August 2005

A two‐dimensional, field scale, finite‐element ground water model was developed to simulate the ground water discharge process within an unconfined coastal aquifer. Model development was based on the density dependent fluid flow approach with water table and dynamic tidal boundary conditions. The performance of the model was compared against data collected from the Columbia Aquifer on the eastern shore of Virginia. The dynamic tidal boundary conditions were necessary to reproduce the surficial mixing zone observed within the ground water intertidal zone of the aquifer. The model was able to reproduce field data on the movement of the near‐shore water table, ground water salt concentrations, and ground water discharge rates and patterns. The model was not able to replicate the short‐term fluctuation in salt concentration observed within the surficial mixing zone. The inability to account for the wave action of the tides within the intertidal zone is believed to be responsible for this difficulty. The development of the model provided a new method to investigate ground water discharge to coastal systems.
Unsteady Stream Depletion from Ground Water Pumping
Authors: Hunt Bruce
Published: August 2005

A solution is obtained for stream flow depletion created by pumping from a well beside a stream. This solution assumes that streambed penetration of the aquifer and dimensions of the streambed cross section are all relatively small. It also assumes that the streambed is clogged and that a linear relationship exists between the outflow seepage through the streambed and the change in piezometric head across the semipervious clogging layer. The solution is general enough to include the earlier solutions of Theis, Glover and Balmer, and Hantush. A solution is also obtained for the drawdown at any point within the aquifer, and it is suggested that this solution might be matched with experimental field data to obtain estimates for aquifer and streambed leakage parameters.
Tracing Ground Water Recharge in an Agricultural Watershed with Isotopes
Authors: Cane Greg , Clark Ian D.
Published: August 2005

Nitrate and pesticide use in the 800 km2 Raisin River agricultural watershed in eastern Ontario, Canada, threatens the quality of ground water in the highly exploited regional carbonate aquifer overlain by sandy till. To assess local recharge contributions through the cultivated fields, monthly monitoring of ground water levels, geochemistry, and environmental isotopes (δ2H H2O, (δ13CDIC) was carried out for 12 wells over a 14‐month period.

Seasonal water level variations suggest that recharge is constrained to spring and late fall when transpiration is minimized and the ground is not frozen. However,2H monitoring shows that early summer precipitation also contributes to recharge. Variations in (δ2H values between monitoring sites suggest a local component to recharge.

δ13CDIC was used to distinguish between dissolved inorganic carbon (DIC) originating from natural (C3) vegetation and DIC from the cultivated corridors where corn is grown (C 4 vegetation). Seasonal variations in δ13CDIC are remarkably coherent for all wells, with uniform trends to positive values during periods of low water table elevation. During periods of high water table (spring and late fall), δ13CDIC values are between ‐13 and ‐ 16%c VPDB (Vienna Peedee Belemnite), reflecting DIC originating from a dominantly natural (C3) vegetation. When ground water levels are low (summer and mid‐winter), δ13CDIC values shift to between ‐11 and ‐l%c. The seasonal enrichments in δ13CDIC are clear evidence for a local contribution to recharge by direct infiltration though the fields. This contribution is enhanced during periods of low water level, likely due to drainage from the phreatic aquifer. High DOC (dissolved organic carbon) concentrations (> 10 to 30 mg‐C/L) correlate with periods of high water levels indicating infiltration of labile organics to the carbonate aquifer.

The work carried out for this paper shows that the conjunctive use of environmental isotope geochemistry and physical parameters are fundamental to assessing the risk of ground water contamination in agricultural watersheds.
Dynamics of DNAPL Penetration into Fractured Porous Media
Authors: Ross Benjamin , Lu Ning
Published: August 2005

Dense nonaqueous phase liquids (DNAPLs) that pool above stratigraphic contacts as a result of capillary forces can migrate downward through fractures in the underlying fine‐grained layer. Although the DNAPL in the fracture is excluded from the surrounding porous matrix by capillary forces, the DNAPL constituent will dissolve and migrate by diffusion into the matrix. As a front of moderately soluble DNAPL advances into a fracture, the flux of dissolved material into the matrix increases until it becomes comparable to the rate of flow into the fracture, reducing the pressure in the DNAPL phase and slowing its advance. If the DNAPL front encounters a constriction with sufficiently large entry pressure, its advance will halt temporarily. But as the concentration gradients driving diffusion into the matrix decrease, there will be less DNAPL lost by diffusion and the DNAPL pressure at the constriction will increase until it exceeds the entry pressure, causing the downward advance of the front to suddenly resume. Because the time scales of diffusion are much slower than those of density flow, this analysis suggests that under certain circumstances a DNAPL can suddenly resume its downward advance after a long period of apparent immobility. In one plausible example, a dichloromethane front passes through 5 m of fractured clay in 16 days, is immobile for more than four years, and then suddenly moves again. Where this phenomenon is possible, removal of DNAPL to protect underlying aquifers becomes more important as a remediation goal.
Flow to a Well in a Water Table Aquifer: Speeding Up the Computation of Type Curves for Low Beta Values
Authors: Gieske A.
Published: August 2005

A study is made of the computer implementation of a recently developed Laplace transform solution (Moench 1996) for flow to a well in a water table aquifer. Some improvements are suggested to increase the speed of computation, especially for low beta values. The first is to calculate the type curves at points which are spaced a factor of two apart, thus allowing more efficient use of the Stehfest inversion algorithm. An improved numerical method to determine the roots of the equation xtanx =α is suggested. Finally, more efficient determination of the number of terms in the required series summation may lead to significant reduction in computing time.
Joint Management of Surface and Ground Water Supplies
Authors: Başaǧaoǧlu Hakan , Mariňo Miguel A.
Published: August 2005

A management model is developed to determine optimal water allocation policies for hydraulically connected time‐variant surface and ground water supplies in a hypothetical system. The system involves a multipurpose reservoir, a hydraulically connected stream and aquifer, agricultural plots, water supply and observation wells, and an artificial recharge zone. The model minimizes deviations from a set of rule curves defined for storage in the reservoir and along the stream course, so as to consider possibilities for storage of excess water in wet periods and its distribution in subsequent dry periods. The hypothetical system is divided into components which are subsequently integrated using numerically generated response functions that relate the systemis behavior to various system excitations. The management model is formulated and solved for monthly time steps, which include both dry and wet conditions, to determine reservoir release, pumpage rate from supply wells, artificial recharge rate, water diversion from reservoir and stream to the demand area, and storage both in the reservoir and along the stream course at an optimal level in each time step over a six‐month planning horizon. Furthermore, sensitivity analysis of the management plan with respect to potential use of ground water supplies is performed to analyze the latter's impact on operating policies.
Determining the Hydraulic Conductivity Spatial Structure at the Twin Lake Site by Grain‐Size Distribution
Authors: Indelman Peter , Moltyaner Greg , Dagan Gedeon
Published: August 2005

Concentrations of a radiotracer were measured continuously in observation wells in a Held test conducted in the sandy aquifer at the Chalk River site. These measurements were used recently (Dagan et al. 1997) to assess the values of the parameters characterizing the spatial distribution of logconductivity (mean, variance, and horizontal and vertical integral scales).

Soil samples were taken independently from a few wells and the grain‐size distributions were determined by sieve analysis. The aim of the present study is to infer the aforementioned parameters’values of logconductivity with the aid of grain‐size distributions. A Carman‐Kozeny‐type formula was used to derive the sample hydraulic conductivity and a geostatistical analysis to infer the parameters characterizing the logconductivity spatial distribution. The agreement for the results based on the two methods was quite satisfactory in view of the various approximations adopted by the two methodologies.
Phosphate Plume Persistence at Two Decommissioned Septic System Sites
Authors: Robertson W.D. , Harman J.
Published: August 2005

Long‐term monitoring of two well‐characterized, oxidizing septic system plumes (Langton and Long Point 2 sites) over periods extending two to four years after decommissioning, has revealed that ground water PO 43− concentrations (0.4 to 5 mg/L P) have persisted at levels virtually unchanged from those observed during active sewage loading. In addition, the frontal part of the PO43− plume at the Long Point 2 site can be observed to continue to advance during the decommissioned period. At the Langton site, where an active regional ground water flow system is present, all major plume solutes (Na+, Ca2+, Cl−, NO 3−) returned to background values within one year of decommissioning, with the exception of PO43−.

This evidence suggests that phosphate behavior in the ground water zone at these sites is dominated by sorption reactions that are both rapid and reversible. Thus, if septic system phosphorus is not retained in the vadose zone, but is transported into the ground water zone, it has the potential to be persistent and to be mobile enough to constitute a threat to downgradient surface water environments. This evidence also shows that when a septic system is decommissioned, if an oxidizing PO43− plume is present, downgradient P loading is not likely to diminish for many years thereafter.
Importance of Rock Matrix Entry Pressure on DNAPL Migration in Fractured Geologic Materials
Authors: Slough KJ. , Sudicky E.A. , Forsyth P.A.
Published: August 2005

The capillary pressure versus saturation relationship for various carbonate rocks exhibits a wide range of entry pressures depending on the depositional and diagenetic environment of the rock. The results of a numerical study are presented that focus on the effect of the DNAPL entry pressure of the porous rock matrix on the migration of DNAPL within the fracture network of carbonate rocks. Representative numerical experiments involving the release of DNAPL into a single vertical fracture terminating within a porous carbonate rock that was assigned a variety of measured capillary pressure versus saturation relationships showed that the volume of DNAPL that entered the rock matrix increased dramatically as the entry pressure of the rock matrix decreased. Simulations of a DNAPL‐contaminated site located on fractured carbonate bedrock in southern Ontario are also presented which highlight the sensitivity of the extent of the zone of DNAPL contamination within the bedrock to the capillary pressure curves assigned to the rock matrix. One scenario presented demonstrates that the penetration depth and horizontal extent of the zone of DNAPL contamination within the bedrock can be large if the carbonate rock has a high entry pressure, because much of the DNAPL flow is restricted to the fracture network. Another case involving a lower entry pressure for the rock matrix produced a smaller zone of DNAPL contamination, but the bulk of the DNAPL was contained in the rock matrix. The results for both cases suggest that attempts at DNAPL‐zone restoration in fractured rock using technologies that are available today would be problematic.
Post Audit of a Numerical Prediction of Wellfield Drawdown in a Semiconfined Aquifer System
Authors: Stewart Mark , Langevina Christian
Published: August 2005

A numerical ground water flow model was created in 1978 and revised in 1981 to predict the drawdown effects of a proposed municipal wellfield permitted to withdraw 30 million gallons per day (mgd; 1.1 × 105 m3/day) of water from the semiconfined Floridan Aquifer system. The predictions are based on the assumption that water levels in the semiconfined Floridan Aquifer reach a long‐term, steady‐state condition within a few days of initiation of pumping. Using this assumption, a 75 day simulation without water table recharge, pumping at the maximum permitted rates, was considered to represent a worst‐case condition and the greatest drawdowns that could be experienced during wellfield operation. This method of predicting wellfield effects was accepted by the permitting agency.

For this post audit, observed drawdowns were derived by taking the difference between pre‐pumping and post‐pumping potentiometric surface levels. Comparison of predicted and observed drawdowns suggests that actual drawdown over a 12 year period exceeds predicted drawdown by a factor of two or more. Analysis of the source of error in the 1981 predictions suggests that the values used for transmissivity, storativity. specific yield, and leakance are reasonable at the wellfield scale. Simulation using actual 1980–1992 pumping rates improves the agreement between predicted and observed drawdowns. The principal source of error is the assumption that water levels in a semiconfined aquifer achieve a steady‐state condition after a few days or weeks of pumping. Simulations using a version of the 1981 model modified to include recharge and evapotranspiration suggest that it can take hundreds of days or several years for water levels in the linked Surficial and Floridan Aquifers to reach an apparent steady‐state condition, and that slow declines in levels continue for years after the initiation of pumping. While the 1981 “impact” model can be used for reasonably predicting short‐term, wellfield‐scale effects of pumping, using a 75 day long simulation without recharge to predict the long‐term behavior of the wellfield was an inappropriate application, resulting in significant underprediction of wellfield effects.
Time‐Related Capture Zones for Contaminants in Randomly Heterogeneous Formations
Authors: Guadagnini Alberto , Franzetti Silvio
Published: August 2005

Prediction of solute travel distance based on homogeneous and isotropic hydraulic conductivity may lead to substantial differences in the actual travel distance in a heterogeneous system. The effect of randomly varying hydraulic conductivity on the spatial location of time‐related well capture zones for a nonreactive tracer in a confined aquifer with uniform base flow is considered. A numerical Monte Carlo procedure is used in conjunction with fast Fourier transform‐based spectral methods. The log hydraulic conductivity field is assumed to be Gaussian and stationary, with isotropic exponential autocovariance. Various degrees of domain heterogeneity are considered, and stability and accuracy of the numerical procedure are examined. The total probability that a particle injected at a point in the aquifer is extracted by the well within a given time is identified. The concept of probabilistic isochrone, which is the boundary of a given time‐related capture zone, is introduced. A simple analytical model that extends the deterministic capture‐zone model for uniform media is derived, by taking into account the effect of random variations of hydraulic conductivity. Knowledge of the spatial pattern of probabilistic isochrones allows reconstructing the breakthrough curve at the well for solute injections at selected locations in the aquifer.
Ground Water Salinization Caused by Residual Neogene and Pliocene Sea Water‐An Example from the Judea Group Aquifer, Southern Israel
Authors: Rosenthal E. , Weinberger G. , Kronfeld J.
Published: August 2005

The Judea Group Aquifer of late Albian‐Turonian age is mainly composed of karstic limestone and dolomite. In the western part of Israel it is known as the Yarkon‐Taninim ground water basin. The study area is located in the southern part of the basin. Fresh ground water (46 to 200 mg/L Cl) flows northward from the south and southeast. Saline ground water (1200 to 8350 mg/L Cl), which occurs irregularly in the midst of this flowpath, severely jeopardizes the development and exploitation of this aquifer. By defining the hydrochemical typology and applying mass balance considerations, it was found that the saline water is derived from two endmembers designated as the “Lahish” and “Hazerim” water types. The Lahish water type is generally responsible for salinization of the lower portion of the Judea Group Aquifer. It apparently evolved from transgressing Messinian sea water which penetrated inland through prior incised Neogene erosional channels. Upon inundation, it dissolved halite and gypsum from the Mavqi'im Formation. This was followed by massive bacterial reduction of sulfates in the presence of oil. The Hazerim water type infiltrated into the Judea Group Aquifer through the overlying Avedat and Mt. Scopus rock successions. It appears that the Hazerim water developed from the transgressing Pliocene sea which percolated through these low‐permeability rock units. Subsequent rain percolated through these formations, dissolved evaporites that are mainly concentrated in the shallow layers and mixed with residual sea water of Pliocene age, which became more diluted over time. Geological studies revealed that during the Neogene age, the back‐cut incision of channels into the Coastal Plain facilitated inland ingression of the sea and dissolution of evaporites. The Pliocene marine transgression deposited a thick sequence of clay that sealed the underlying beds. This process resulted in trapping and pressurizing the saline water landward along pre‐existing fault lines. These findings contribute to the formulation of an updated ground water exploitation scheme that avoids subsurface fault systems, which act as conduits of brines. The geological processes occurred during the Neogene and Pliocene. This case study could help solve similar problems in the eastern Mediterranean region which experienced similar geological events.
Model Calibration with Multiple Targets: A Case Study
Authors: Kim Kangjoo , Anderson Mary P. , Bowser Carl J.
Published: August 2005

Multiple calibration targets were used to calibrate a two‐dimensional finite‐difference model of a ground water lake system. The calibration targets included (1) steady‐state head data, (2) transient head data, (3) head gradients, and (4) flowpath information. Because calibration was sensitive to the ratio of horizontal to vertical hydraulic conductivity, four models, each with different assumptions about anisotropy, were developed. All four models produced acceptable calibration to either heads or flowpath, but only one model was well calibrated to all targets. In that model, stratification of the upper aquifer was represented by introducing several dipping layers of low permeability. This allowed the use of a small ratio of horizontal to vertical anisotropy for individual layers but produced a large effective anisotropy for the upper aquifer as a whole.