bluuebunny/crossref_metadata_2025 huggingface dataset:
Noble Gas and Major Element Constraints on the Water Dynamics in an Alpine Floodplain
Authors: Holocher Johannes , Matta Vijay , Aeschbach‐Hertig Werner , Beyerle Urs , Hofer Markus , Peeters Frank , Kipfer Rolf
Published: December 2005

The hydrogeological system of an ecologically sensitive alpine floodplain in the Valle di Blenio, Switzerland, was investigated using hydrochemical and 3H‐3He dating methods. Water samples from six wells and from different surface locations were analyzed. The analysis of the concentrations of major ions in conjunction with age determination by the 3H‐3He‐method allowed the main hydrological properties of the system to be consistently characterized. Two geochemically distinct water zones can be distinguished: Ca‐SO4‐dominated water from the main river and Ca‐HCO3‐dominated floodplain water. The floodplain water component characterizes the whole floodplain including the surficial hillslope drainage system. Within the ground water samples, two spatially and temporally different types of water can be determined. A younger (age < 1.5 years), less mineralized water is found in the upper part of the aquifer during the summer season. The underlying aquifer zone contains older and more highly mineralized water. However, the general hydrochemical characterization of both types of ground water is similar. In winter, the water ages increase with decreasing ground water levels. Because precipitation is stored temporarily in the snow cover, the contribution of the younger near‐surface ground water decreases, resulting in higher apparent water ages and higher mineralization in the upper zone of the aquifer. Water exchange between the main river and the ground water system is limited to ground water exfiltration from the shallow aquifer zone, whereas the hydrochemical separation of the deeper aquifer zone indicates the isolation of the deeper ground water from the main river.
Improved Calculations for Dewatered Cells in MODFLOW
Authors: Doherty John
Published: December 2005

Use of the United States Geological Survey ground water flow model MODFLOW is often hampered by the occurrence of “dry cells.” While MODFLOW allows such cells to “rewet” in the course of a simulation, stability of the heads solution process is often problematical with rewetting functionality operative. In many cases of practical interest (particularly in mining applications), MOD‐FLOW simply fails to converge. However by making a number of adjustments to the MODFLOW Block‐Centered Flow package, it is possible to overcome this problem in many instances of MODFLOW deployment. These adjustments are such as to allow a layer to transmit water, albeit with a vastly reduced transmissivity, even if the water level in that layer is below its base. With these alterations MODFLOW cells can remain active even if they lie within the unsaturated zone.

Testing of the code has demonstrated its ability to perform well in situations where performance of the unmodified MODFLOW is degraded by the necessity to dry and rewet cells. Comparison of heads calculated using the modified MODFLOW with those calculated using MODFLOW‐SURFACT (a MODFLOW‐based code developed by HydroGeoLogic Inc. that prevents the occurrence of dry cells through use of pseudo soil functions) reveals near‐identical results between the two codes. Comparison with analytical solutions of water table location also reveals near coincidence. An example of one such application is presented herein.
Analytical Studies on the Impact of Land Reclamation on Ground Water Flow
Authors: Jiao Jiu J. , Nandy Subhas , Li Hailong
Published: December 2005

Land reclamation has been a common practice to produce valuable land in coastal areas. The impact of land reclamation on coastal environment and marine ecology is well recognized and widely studied. It has not been recognized yet that reclamation may change the regional ground water regime, which may in turn modify the coastal environment, flooding pattern, and stability of slopes and foundations. This paper represents the first attempt to examine quantitatively the effect of reclamation on ground water levels. Analytical solutions are developed to study the ground water change in response to reclamation based on two hypothetical models. In the first model, the ground water flow regime changes only in the hillside around the reclamation areas. In the second model, the ground water regime changes in the entire hill. Both models assume that the ground water flow is in a steady state and satisfies the Dupuit assumptions. Hypothetical examples are used to demonstrate how the ground water level, ground water divide and ground water submarine discharge will change with the scale and hydraulic conductivity of the reclamation materials. The results show that the change of ground water regime depends mainly on the length of the reclaimed area and the values of hydraulic conductivity of the reclaimed materials. It is also seen that the reclamation may impact not only the ground water regime near the coast areas around the reclamation site, but also that in the coast areas opposite the reclamation area. A reclamation site near Tseung Kwan O in the New Territories in Hong Kong, China, is used as a case study to discuss the possible modification of the ground water system caused by reclamation.
Characterization of Iron and Manganese Precipitates from an In Situ Ground Water Treatment Plant
Authors: Mettler S. , Abdelmoula M. , Hoehn E. , Schoenenberger R. , Weidler P. , von Gunten U.
Published: December 2005

Aquifer samples from the precipitation zone of an in situ iron and manganese removal plant that was operated for 10 years were analyzed for iron and manganese minerals. Measurements were performed by various chemical extraction techniques (5 M HC1,0.008 M Ti(III)‐EDTA, 0.114 M ascorbic acid), X‐ray diffraction and Mössbauer spectroscopy. Chemical extractions showed that iron was precipitated as ferric oxides, whereas manganese was not oxidized but deposited as Mn(II) probably within carbonates. The ferric oxides in particular accumulate preferentially in the smaller grain‐ size fractions. This tendency was observed to a lesser extent for manganese. X‐ray diffraction and Mössbauer spectroscopy showed that the ferric oxides were mainly crystalline (goethite, 50% to 100% of the iron). Ferrihydrite was found as well, but only as a minor fraction (≤ 12%). Pure manganese minerals were not found by X‐ray diffraction. The precipitated amounts of iron (5 to 27 μmol/g Fe as ferric oxide) and manganese (1 to 4 μmol/g Mn) during 10 years operation of the treatment plant agree with values that were estimated from operational parameters (9 to 31 μmol/g Fe and 3 to 6 μmol/g Mn). Considering the small amounts of precipitated iron and manganese, no long‐term risks of clogging of the aquifer are expected.
Dispersion in VLEACH and Similar Models
Authors: Sukop Michael C.
Published: December 2005

VLEACH is a simple one‐dimensional vadose zone leaching model capable of simulating volatile chemical transport. There is no explicit dispersion coefficient in VLEACH. Solutions computed with it show dispersion due to diffusion in the gaseous phase and its interaction with the liquid phase and also due to the discretization of the problem domain. By turning off gaseous diffusion and comparing VLEACH results to the standard convection dispersion equation, this Computer Note illustrates that the effective dispersivity due to the discretization is equal to half of the model cell size.
Natural Attenuation Reactions at a Uranium Mill Tailings Site, Western U.S.A.
Authors: Zhu Chen , Anderson Greg M. , Burden David S.
Published: December 2005

This paper presents a modeling analysis of the geochemical evolution of a contaminated sandy aquifer at a uranium mill tailings site in the western United States. The tailings pond contains fluids having a pH of 1.5 to 3.5 and high levels of As, Be, Cd, Cr, Pb, Mo, Ni, Se, 226Ra, 228Ra, 230Th, 238U, and 234U. Seepage of tailings fluids into the aquifer has formed a low‐pH ground water plume. The reclamation plan is to install a low‐permeability cover on the tailings pond to stop the seepage and allow the plume to be attenuated by reactions with the aquifer matrix and flushed by uncontaminated upgradient ground water. To evaluate this reclamation scenario, ground water and sediment core samples were analyzed along one flowpath. Speciation‐solubility and mass‐transfer modeling revealed two sets of chemical reactions for acid seepage and flushing, respectively. The current concentrations and distribution of ground water constituents can be interpreted as being controlled by stepwise pH‐buffer reactions with calcite, amorphous aluminum hydroxide, and amorphous iron hydroxides. These buffer reactions divide the aquifer into zones of near‐constant pH, separated by interface zones. For the flushing stage, it is predicted that reactions with surface‐bound species will dominate the reaction paths, and more pore volumes are required to neutralize the plume than predicted by models that do not consider surface reactions. Direct mineralogical and surface analysis is needed to substantiate this assertion.
A Reassessment of Ground Water Flow Conditions and Specific Yield at Borden and Cape Cod
Authors: Grimestad Garry
Published: December 2005

Recent widely accepted findings respecting the origin and nature of specific yield in unconfined aquifers rely heavily on water level changes observed during two pumping tests, one conducted at Borden, Ontario, Canada, and the other at Cape Cod, Massachusetts. The drawdown patterns observed during those tests have been taken as proof that unconfined specific yield estimates obtained from long‐duration pumping tests should approach the laboratory‐estimated effective porosity of representative aquifer formation samples. However, both of the original test reports included direct or referential descriptions of potential supplemental sources of pumped water that would have introduced intractable complications and errors into straightforward interpretations of the drawdown observations if actually present. Searches for evidence of previously neglected sources were performed by screening the original drawdown observations from both locations for signs of diagnostic skewing that should be present only if some of the extracted water was derived from sources other than main aquifer storage. The data screening was performed using error‐guided computer assisted fitting techniques, capable of accurately sensing and simulating the effects of a wide range of non‐traditional and external sources. The drawdown curves from both tests proved to be inconsistent with traditional single‐source pumped aquifer models but consistent with site‐specific alternatives that included significant contributions of water from external sources. The corrected pumping responses shared several important features. Unsaturated drainage appears to have ceased effectively at both locations within the first day of pumping, and estimates of specific yield stabilized at levels considerably smaller than the corresponding laboratory‐measured or probable effective porosity. Separate sequential analyses of progressively later field observations gave stable and nearly constant specific yield estimates for each location, with no evidence from either test that more prolonged pumping would have induced substantially greater levels of unconfined specific yield.
Correlation of Lineaments to Ground Water Inflows in a Bedrock Tunnel
Authors: Mabee Stephen B. , Curry Patrick J. , Hardcastle Kenneth C.
Published: December 2005

Lineaments derived from three image types (1:80,000 black and white, 1:58,000 color infrared, and 1:250,000 side‐looking airborne radar) were compared to water‐bearing features within a 9.6 km section of tunnel being constructed through foliated crystalline metamorphic bedrock in a glaciated region of eastern Massachusetts. Lineaments drawn by three observers during two independent trials (N = 9137) were reduced to three sets (one per image type) of coincident lineaments (N = 794). Thirty‐five coincident lineaments crossed the tunnel. Nineteen discrete flow zones, each producing ≥ 19 L/min, were identified in the tunnel and used to quantify the reliability of lineament analysis as a method of predicting water‐bearing features in glaciated metamorphic rocks.

Thirteen (68%) of the flow zones correlate with coincident lineaments, six zones correlate with more than one image type, and one zone correlates with all three image types. Overall, without additional corroborating evidence, it is difficult to interpret in advance which lineaments will result in a successful correlation with water‐producing zones in the subsurface and which ones will not. Most of the observed flow (80%) correlates with northwest‐trending coincident lineaments; however, the majority of the flow (67%) associated with these lineaments is produced from structures that strike to the north or northeast. In addition, only 15 of the 35 coincident lineaments correlate with the flow zones, indicating that 20 lineaments are not associated with any appreciable flow. Six flow zones are undetected by the lineament analysis.
Redox Patterns and Trace‐Element Behavior in the East Midlands Triassic Sandstone Aquifer, U.K.
Authors: Smedley Pauline L. , Edmunds W. Mike
Published: December 2005

Redox conditions exercise important controls on water chemistry in the red‐bed Sherwood Sandstone Aquifer of the English East Midlands. A distinct redox boundary exists some 3 to 5 km downgradient of the onset of confined conditions, defined by a 300 mV drop in Eh and complete reaction of dissolved oxygen. The aerobic aquifer contains polluted water with high nitrate concentrations and organic carbon significantly above background concentrations (> 0.2 mg/L). Concentrations of Fe, Mn, and Mo are highest in reducing ground water. As, Sb, Se, and U show a residence‐time‐dependent increase in aerobic ground water, but are much lower under reducing conditions. Iron oxides are believed to play a key role in determining the spatial patterns in many of these trace elements as a result of Eh‐ and pH‐controlled sorption/desorption reactions, as well as some reductive dissolution in the confined aquifer. Fresh ground water persists in the confined aquifer to approximately 30 km downgradient of the redox boundary. However, SO4 concentrations increase progressively along the flowline as a result of the dissolution of gypsum or anhydrite. Concentrations of available organic carbon are low in ground water (1 mg/L or less) and are also likely to be limited in the sediments; conditions are insufficiently reducing for significant sulphate reduction to have taken place. Only in the extreme down‐gradient (eastern) part of the aquifer do conditions become sufficiently reducing with some evidence of sulphate reduction. In this part of the aquifer, ground water is more saline (TDS values up to 10 g/L) and is believed to be composed substantially of older formation water. This has distinctive concentrations of several redox‐influenced trace elements, with relatively high Fe, Mn, As, and Sb, occasional high Cr, and low Mo relative to the confined fresh ground water upgradient.
A Permeable Reactive Barrier for Treatment of Heavy Metals
Authors: Ludwig Ralph D. , McGregor Rick G. , Blowes David W. , Benner Shawn G. , Mountjoy Keith
Published: December 2005

Historical storage of ore concentrate containing sulfide minerals at an industrial site in British Columbia, Canada, has resulted in widespread contamination of the underlying soil and ground water. The oxidation of sulfide minerals has released significant quantities of heavy metals, including Cu, Cd, Co, Ni, and Zn, into the ground water. A pilot‐scale, compost‐based, sulfate‐reducing permeable reactive barrier was installed in the path of the dissolved heavy‐metal plume. The permeable reactive barrier uses sulfate‐reducing bacteria to promote precipitation of heavy metals as insoluble metal sulfides. Monitoring over a 21‐month period indicated significant removal of heavy metals within the barrier. Copper concentrations declined from a mean concentration of 3630 μg/L in the influent to a mean concentration within the barrier of 10.5 μg/L, Cd from 15.3 μg/L to 0.2 μg/L, Co from 5.3 μg/L to 1.1 μg/L, Ni from 131 μg/L to 33.0 μg/L, and Zn from 2410 μg/L to 136 μg/L. Within the lower half of the barrier where tidal influences were more limited and sulfate‐reducing conditions were better maintained, mean treatment levels of 2.9 μg/L (Cu), 0.1 μg/L (Cd), 0.4 μg/L (Co), 2.7 μg/L (Ni), and 6.3 μg/L (Zn) were observed.
Fluid Flow and Solute Migration Within the Capillary Fringe
Authors: Silliman Stephen E. , Berkowitz Brian , Simunek Jirka , van Genuchten M. Th.
Published: December 2005

Laboratory experiments involving both homogeneous and heterogeneous porous media are used to demonstrate that fluid flow and solute transport will occur regularly in the capillary fringe (CF), including both vertical (upward as well as downward) and horizontal flow velocities. Horizontal flow above the water table appears to be limited primarily to the region of high water saturation (i.e., the CF), an observation supported by numerical modeling and consistent with the literature. Beyond observations presented in prior literature, it was observed that exchange of water within the CF with water below the water table is active, with flux both from the CF downward across the water table and from the region below the water table, upward into the CF. This flux is enhanced by the presence of physical heterogeneity. These findings strongly contrast the common conceptualization of predominantly downward vertical fluid flow through the unsaturated zone, with transition to fully three‐dimensional flow only below the water table. Based on these observations, it is suggested that the CF may affect, far more significantly than is usually assumed, the natural geochemical and microbial conditions present in the region of transition from unsaturated to saturated ground water flow.
Generating MODFLOW Grids from Boundary Representation Solid Models
Authors: Jones Norman L. , Budge Trevor J. , Lemon Alan M. , Zundel Alan K.
Published: December 2005

Complex stratigraphy can be difficult to simulate in MODFLOW models. MODFLOW uses a structured grid that requires that each grid layer be continuous throughout the model domain. This makes it difficult to explicitly represent common features such as pinchouts and embedded seams in a MODFLOW model. In this paper, we describe a method for automatically generating MODFLOW‐compatible grids from boundary‐representation solid models. Solid models are data structures developed originally for computer‐aided design applications that define the geometry of three‐dimensional objects. Solid models can be used to represent arbitrarily complex stratigraphy. The elevations defined by the solids are then extracted from the solids in a manner that preserves the continuous‐layer requirement imposed by MODFLOW. Two basic approaches are described: The first method adjusts the MODFLOW grid dimensions (layer elevations) to fit the solid model boundaries, and the second method creates a regular MODFLOW grid and adjusts the material properties to match the changes in stratigraphy. One of the main benefits of using solid models to define stratigraphy for MODFLOW models is that it provides a grid‐independent definition of the layer elevations that can be used to immediately re‐create the MODFLOW grid geometry after any change to the grid resolution.
Nonparametric Method for Transmissivity Distributions Along Boreholes
Authors: Fransson Åsa
Published: December 2005

The transmissivities of individual fractures along a borehole are difficult to obtain unless each fracture is tested. To estimate a fracture‐transmissivity distribution from section transmissivities, a method was developed based on fixed‐interval‐length transmissivities and the corresponding number of fractures for each interval. The method is nonparametric and iterative, and the fractures are viewed as two‐dimensional features, in which the total transmissivity of a borehole is equal to the sum of individual fracture transmissivities. Initially, a linear a priori assumption of the transmissivity distribution is made, and from this a so‐called mean transmissivity function is derived. Subsequently, the mean transmissivity of the Nj fractures within a section, j, of the borehole is estimated, and the same value of the mean transmissivity function represents Nj‐ possible fracture transmissivities from the initial distribution. This is repeated for each borehole section, and, eventually, all fracture transmissivities are sorted to give the next iteration's transmissivity distribution and the corresponding mean transmissivity function. Finally, the distributions converge, yielding a possible fracture‐transmissivity distribution. The method was verified for a synthetic data sample and then tested on a sample from a borehole at the Aspo Hard Rock Laboratory, Sweden. For the synthetic data, the method gave a distribution that was fairly close to the original one; for the Aspo data, 15% of the fractures had a transmissivity larger than the measurement limit (1×10−9 m2/sec), and these transmissivities follow a log‐normal distribution.
Prediction of Diffusion Coefficients in Porous Media Using Tortuosity Factors Based on Interfacial Areas
Authors: Saripalli K. Prasad , Serne R. Jeffery , Meyer Philip D. , McGrail B. Peter
Published: December 2005

Determination of aqueous phase diffusion coefficients of solutes through porous media is essential for understanding and modeling contaminant transport. Prediction of diffusion coefficients in both saturated and unsaturated zones requires knowledge of tortuosity and constrictivity factors. No methods are available for the direct measurement of these factors, which are empirical in their definition. In this paper, a new definition for the tortuosity factor is proposed, as the real to ideal interfacial area ratio. We define the tortuosity factor for saturated porous media (τs) as the ratio S/So (specific surface of real porous medium to that of an idealized capillary bundle). For unsaturated media, tortuosity factor (τa) is defined as 2awla (ratio of the specific air‐water interfacial area of real and the corresponding idealized porous medium). This tortuosity factor is suitably measured using sorptive tracers (e.g., nitrogen adsorption method) for saturated media and interfacial tracers for unsaturated media. A model based on this new definition of tortuosity factors, termed the interfacial area ratio (IAR) model, is presented for the prediction of diffusion coefficients as a function of the degree of water saturation. Diffusion coefficients and diffusive resistances measured in a number of saturated and unsaturated granular porous media, for solutes in dilute aqueous solutions, agree well with the predictions of the IAR model. A comparison of permeability of saturated sands estimated based on τs and the same based on the Kozeny‐Carman equation confirm the usefulness of the τs parameter as a measure of tortuosity.
Analytical Model for Aquifer Response Incorporating Distributed Stream Leakage
Authors: Fox Garey A. , DuChateau Paul , Dumford Deanna S.
Published: December 2005

An analytical model of stream/aquifer interaction is proposed that predicts drawdown in an aquifer with leakage from a finite‐width stream induced by pumping from a well. The model is formulated based on the assumptions of stream partial penetration, a semipervious streambed, and distributed recharge across a finite‐width stream. Advantages of the analytical solution include its simple structure, consisting of the Theis well function with integral modifications. The solution is derived for the semi‐infinite domain between the stream and pumping well, which is of primary interest to hydrogeologists. Previous stream/aquifer analytical models are compared to the analytical solution based on dimensionless drawdown profiles. Drawdown in the aquifer near a wide stream was found to be less than that predicted by a solution that ignored stream width. Deviations between the proposed analytical solutions and previous solutions increase as stream width increases. For a hypothetical stream/aquifer system, the proposed analytical solution was equivalent to prior solutions when the ratio of the distance between the stream and aquifer to the stream width was greater than 25. This analytical solution may provide improved estimates of aquifer and streambed leakage parameters by curve fitting experimental field drawdown data.
The Role of Capillary Forces in Steady‐State and Transient Seepage Flows
Authors: Naba Bassem , Boufadel Michel C. , Weaver James
Published: December 2005

Steady and transient seepage problems were investigated numerically using a dimensionless formulation for water flow in variably saturated, two‐dimensional anisotropic and homogenous porous media. The dimensionless formulation combines the aspect ratio and the anisotropy ratio in one dimensionless parameter, M, and accounts for capillarity effects using the ratio of the domain height to the height of capillary fringe as another dimensionless parameter, a. Two domain geometries were considered: rectangular and trapezoidal. It was found that M and a play major roles in the development and the height of the seepage face. The height of the steady‐state seepage face increased with the decreasing value of a.

The effects of capillarity on the development of the transient seepage face were investigated by lowering the open water level on one side of the domain at various uniform velocities while the other side was kept at a constant value. It was found for most scenarios that decoupling between the water table and the open water level first occurred for domains with large a. In other words, the seepage face heights of these systems were larger than those with a small a. These results are the opposite of the steady‐state results and are due to the fact that drainage of the pores was the major mechanism controlling the drop of the transient water table, especially at earlier times.

A criterion for the decoupling of the water table from the falling open water level (i.e., the formation of the transient seepage face) was developed by Dracos (1965). It states that decoupling does not occur as long as the falling speed, VF, of the open water level is less than Vd= Ko sin2 B/φ where Ko is the saturated hydraulic conductivity of the soil, φ is the porosity, and β is the angle of the exit face with the horizontal (i.e., β=π/2 implies a vertical face). Our investigation revealed that decoupling occurs for smaller falling velocities than Vd, and is caused by, among others, the fact that the water table is not tangent to the exit face but rather intersects it at a nonzero angle.
Asymmetric Dipole‐Flow Test in a Fractured Carbonate Aquifer
Authors: Halihan Todd , Zlotnik Vitaly A.
Published: December 2005

In this study, a new method‐the asymmetric dipole‐flow test—is proposed and tested for characterization of conductive properties and structure of fractured aquifers. Analytical solutions were developed and then used for interpretation of a modification of the dipole‐flow test with a single packer at the Bissen Quarry test site (Wisconsin, USA). The asymmetric dipole‐flow tests were conducted by packing a well at various elevations, and fluids were pumped from the upper section (chamber) of the well to the lower section (chamber). The head was then monitored at 11 observation points and in both sections of the well, and the conductivities of the well segments were determined. The tests at seven packer elevations in the well were rapid (less than one hour to reach steady state). The asymmetric dipole‐flow test demonstrates the potential to quantify heterogeneities of a fractured aquifer and delineate the applicability of the continuum and discrete approaches for conceptualization of ground water flow.
Analysis of a Vertical Dipole Tracer Test in Highly Fractured Rock
Authors: Sanford William E. , Cook Peter G. , Dighton John C.
Published: December 2005

The results of a vertical dipole tracer experiment performed in highly fractured rocks of the Clare Valley, South Australia, are presented. The injection and withdrawal piezometers were both screened over 3 m and were separated by 6 m (midpoint to midpoint). Due to the long screen length, several fracture sets were intersected, some of which do not connect the two piezometers. Dissolved helium and bromide were injected into the dipole flow field for 75 minutes, followed by an additional 510 minutes of flushing. The breakthrough of helium was retarded relative to bromide, as was expected due to the greater aqueous diffusion coefficient of helium. Also, only 25% of the total mass injected of both tracers was recovered. Modeling of the tracer transport was accomplished using an analytical one‐dimensional flow and transport model for flow through a fracture with diffusion into the matrix. The assumptions made include: streamlines connecting the injection and withdrawal point can be modeled as a dipole of equal strength, flow along each streamline is one dimensional, and there is a constant Peclet number for each streamline. In contrast to many other field tracer studies performed in fractured rock, the actual travel length between piezometers was not known. Modeling was accomplished by fitting the characteristics of the tracer breakthrough curves (BTCs), such as arrival times of the peak concentration and the center of mass. The important steps were to determine the fracture aperture (240 μm) based on the parameters that influence the rate of matrix diffusion (this controls the arrival time of the peak concentration); estimating the travel distance (11 m) by fitting the time of arrival of the centers of mass of the tracers; and estimating fracture dispersivity (0.5 m) by fitting the times that the inflection points occurred on the front and back limbs of the BTCs. This method works even though there was dilution in the withdrawal well, the amount of which can be estimated by determining the value that the modeled concentrations need to be reduced to fit the data (∼50%). The use of two tracers with different diffusion coefficients was not necessary, but it provides important checks in the modeling process because the apparent retardation between the two tracers is evidence of matrix diffusion and the BTCs of both tracers need to be accurately modeled by the best fit parameters.
The Capture Efficiency Map: The Capture Zone Under Time‐Varying Flow
Authors: Festger Adam D. , Walter Gary R.
Published: December 2005

The capture zone or contributing area of a ground water extraction well can be defined as that portion of the aquifer from which the well draws its water. Accurate delineation of capture zones is important in many ground water remediation applications and in the definition of wellhead protection areas. Their mathematical delineation is often simplified by using quasi‐steady‐state models based on time‐weighted average pumping rates and background hydraulic gradients. We present a new semianalytic approach for the definition of capture zones under transient‐flow conditions. We then use this approach to evaluate the effects of time variations in the direction of the background hydraulic gradient on capture. Results are presented in the form of capture efficiency maps (CEMs). Although the area contributing to a given well is found to generally expand relative to the steady‐state average capture zone when the gradient direction varies, the zone of 100% capture may expand or contract depending on site‐specific conditions. We illustrate our CEM approach by applying it to the design of a plume containment system.
Recharge Estimation for Transient Ground Water Modeling
Authors: Jyrkama Mikko I. , Sykes Jon F. , Normani Stefano D.
Published: December 2005

Reliable ground water models require both an accurate physical representation of the system and appropriate boundary conditions. While physical attributes are generally considered static, boundary conditions, such as ground water recharge rates, can be highly variable in both space and time.

A practical methodology incorporating the hydrologic model HELP3 in conjunction with a geographic information system was developed to generate a physically based and highly detailed recharge boundary condition for ground water modeling. The approach uses daily precipitation and temperature records in addition to land use/land cover and soils data. The importance of the method in transient ground water modeling is demonstrated by applying it to a MODFLOW modeling study in New Jersey.

In addition to improved model calibration, the results from the study clearly indicate the importance of using a physically based and highly detailed recharge boundary condition in ground water quality modeling, where the detailed knowledge of the evolution of the ground water flowpaths is imperative. The simulated water table is within 0.5 m of the observed values using the method, while the water levels can differ by as much as 2 m using uniform recharge conditions. The results also show that the combination of temperature and precipitation plays an important role in the amount and timing of recharge in cooler climates. A sensitivity analysis further reveals that increasing the leaf area index, the evaporative zone depth, or the curve number in the model will result in decreased recharge rates over time, with the curve number having the greatest impact.
Remediation of the Wells G & H Superfund Site, Woburn, Massachusetts
Authors: Bair E. Scott , Metheny Maura A.
Published: December 2005

Remediation of ground water and soil contamination at the Wells G & H Superfund Site, Woburn, Massachusetts, uses technologies that reflect differences in hydrogeologic settings, concentrations of volatile organic compounds (VOCs), and costs of treatment. The poorly permeable glacial materials that overlie fractured bedrock at the W.R. Grace property necessitate use of closely spaced recovery wells. Contaminated ground water is treated with hydrogen peroxide and ultraviolet (UV) oxidation. At UniFirst, a deep well completed in fractured bedrock removes contaminated ground water, which is treated by hydrogen peroxide, UV oxidation, and granular activated carbon (GAC). The remediation system at Wildwood integrates air sparging, soil‐vapor extraction, and ground water pumping. Air stripping and GAC are used to treat contaminated water; GAC is used to treat contaminated air. New England Plastics (NEP) uses air sparging and soil‐vapor extraction to remove VOCs from the unsaturated zone and shallow ground water. Contaminated air and water are treated using separate GAC systems. After nine years of operation at W.R. Grace and UniFirst, 30 and 786 kg, respectively, of VOCs have been removed. In three years of operation, 866 kg of VOCs have been removed at Wildwood. In 15 months of operation, 36 kg of VOCs were removed at NEP. Characterization work continues at the Olympia Nominee Trust, Whitney Barrel, Murphy Waste Oil, and Aberjona Auto Parts properties. Risk assessments are being finalized that address heavy metals in the floodplain sediments along the Aberjona River that are mobilized from the Industri–Plex Superfund Site located a few miles upstream.
Geostatistics and Bayesian Updating for Transmissivity Estimation in a Multiaquifer System in Manitoba, Canada
Authors: Kennedy Paula L. , Woodbury Allan D.
Published: December 2005

In ground water flow and transport modeling, the heterogeneous nature of porous media has a considerable effect on the resulting flow and solute transport. Some method of generating the heterogeneous field from a limited dataset of uncertain measurements is required. Bayesian updating is one method that interpolates from an uncertain dataset using the statistics of the underlying probability distribution function. In this paper, Bayesian updating was used to determine the heterogeneous natural log transmissivity field for a carbonate and a sandstone aquifer in southern Manitoba. It was determined that the transmissivity in m2/sec followed a natural log normal distribution for both aquifers with a mean of ‐7.2 and ‐ 8.0 for the carbonate and sandstone aquifers, respectively. The variograms were calculated using an estimator developed by Li and Lake (1994). Fractal nature was not evident in the variogram from either aquifer. The Bayesian updating heterogeneous field provided good results even in cases where little data was available. A large transmissivity zone in the sandstone aquifer was created by the Bayesian procedure, which is not a reflection of any deterministic consideration, but is a natural outcome of updating a prior probability distribution function with observations. The statistical model returns a result that is very reasonable; that is homogeneous in regions where little or no information is available to alter an initial state. No long range correlation trends or fractal behavior of the log‐transmissivity field was observed in either aquifer over a distance of about 300 km.
Stream‐Aquifer Interactions: Evaluation of Depletion Volume and Residual Effects from Ground Water Pumping
Authors: Chen Xunhong , Shu Longcang
Published: December 2005

Numerical modeling techniques were used to simulate stream‐aquifer interactions from seasonal ground water pumping. We used stream‐aquifer models in which a shallow stream penetrates the top of an aquifer that discharges ground water to the stream as base flow. Because of the pumping, the volume of base flow discharged to the stream was reduced, and as the pumping continued, infiltration from the stream to the aquifer was induced. Both base‐flow reduction and stream infiltration contributed to total stream depletion. We analyzed the depletion rates and volumes of the reduced base flow and induced stream infiltration during pumping and postpumping periods. Our results suggested that for a shallow penetrating stream with a low streambed conductance, base‐flow reduction accounts for a significant percentage of the total stream depletion. Its residual effects in postpumping can last very long and may continue into the next pumping season for areas where recharge is nominal. In contrast, the contribution of the induced stream infiltration to the total stream depletion is much smaller, and its effects often become negligible shortly after pumping was stopped. For areas where surface recharge replenishes the aquifer, the residual effects of base‐flow reduction and thus its depletion volume will be significantly reduced. A stream of large conductance has a high hydraulic connection to the aquifer, but the relationship between stream conductance and stream depletion is not linear.
Retardation of Organic Contaminants in Natural Fractures in Chalk
Authors: Wefer‐Roehl A. , Graber E.R. , Adar E. , Yakirevich A. , Ronen Z.
Published: December 2005

Transport of a conservative compound and two sorbing compounds through fractured chalk was studied using flow‐through columns consisting of chalk cores with a single subvertical fracture. Two types of chalk matrix were compared, an oxidized white chalk with low organic carbon content (0.2%), and a gray chalk with a higher organic carbon content (1.3%). Initial rapid breakthrough followed by a delayed approach to a relative concentration of unity for the conservative compound (2,6–difluorobenzoic acid [DFBA]) was clear evidence for diffusion into the porous chalk matrix. Matrix diffusion of DFBA was apparently much greater in the gray chalk columns than in the white chalk columns. Breakthrough curves (BTCs) of the sorbing compounds (2,4,6–tri‐bromophenol [TBP] and ametryn [AME]) were retarded in all cases as compared to the conservative compound. Sorption retardation was far greater in the gray chalk as compared with the white chalk, in good agreement with results from batch sorption experiments. BTCs for the conservative compound were relatively nonhysteretic for both white and gray chalk columns. In contrast, BTCs for the sorbing compounds were hysteretic in all cases, demonstrating that sorption was not at equilibrium before desorp‐tion began. These experiments suggest that on a field scale, transport of contaminants through fractures in chalk and other fractured porous media will be attenuated by diffusion and sorption into the matrix.
Static and Push‐Pull Methods Using Radon‐222 to Characterize Dense Nonaqueous Phase Liquid Saturations
Authors: Davis B.M. , Istok J.D. , Semprini L
Published: December 2005

Naturally occurring radon in ground water can potentially be used as an in situ partitioning tracer to characterize dense nonaqueous phase liquid (DNAPL) saturations. The static method involves comparing radon concentrations in water samples from DNAPL‐contaminated and noncontaminated portions of an aquifer, while the push‐pull method involves the injection (push) and extraction (pull) of a radon‐free test solution from a single well. In the presence of DNAPL, radon concentrations during the pull phase are retarded, with retardation manifested in greater dispersion of radon concentrations relative to a conservative tracer. The utility of these methods was investigated in the laboratory using a physical aquifer model (PAM). Static and push‐pull tests were performed before and after contamination of the PAM sediment pack with trichloroethene (TCE), and after alcohol cosolvent flushing and pump‐and‐treat remediation. Numerical simulations were used to estimate the retardation factor for radon in push‐pull tests. Radon partitioning was observed in static and push‐pull tests conducted after TCE contamination. Calculated TCE saturations ranged up to 1.4% (static test) and 14.1% (push‐pull test). Post‐remediation tests showed decreases in TCE saturations. The results show that radon is sensitive to changes in DNAPL saturation in space and time. However, the methods are sensitive to DNAPL saturation heterogeneity, test location, sample size, and test design. The influence of these factors on test results, as well as the apparent overestimation of the retardation factor in push‐pull tests, warrant further investigation.
Regional Estimation of Base Recharge to Ground Water Using Water Balance and a Base‐Flow Index
Authors: Szilagyi Jozsef , Harvey F. Edwin , Ayers Jerry F.
Published: December 2005

Naturally occurring long‐term mean annual base recharge to ground water in Nebraska was estimated with the help of a water‐balance approach and an objective automated technique for base‐flow separation involving minimal parameter‐optimization requirements. Base recharge is equal to total recharge minus the amount of evapotranspiration coming directly from ground water. The estimation of evapotranspiration in the water‐balance equation avoids the need to specify a contributing drainage area for ground water, which in certain cases may be considerably different from the drainage area for surface runoff. Evapotranspiration was calculated by the WREVAP model at the Solar and Meteorological Surface Observation Network (SAMSON) sites. Long‐term mean annual base recharge was derived by determining the product of estimated long‐term mean annual runoff (the difference between precipitation and evapotranspiration) and the base‐flow index (BFI). The BFI was calculated from discharge data obtained from the U.S. Geological Survey's gauging stations in Nebraska. Mapping was achieved by using geographic information systems (GIS) and geostatistics. This approach is best suited for regional‐scale applications. It does not require complex hydrogeologic modeling nor detailed knowledge of soil characteristics, vegetation cover, or land‐use practices. Long‐term mean annual base recharge rates in excess of 110 mm/year resulted in the extreme eastern part of Nebraska. The western portion of the state expressed rates of only 15 to 20 mm annually, while the Sandhills region of north‐central Nebraska was estimated to receive twice as much base recharge (40 to 50 mm/year) as areas south of it.
Application of Harmonic Analysis of Water Levels to Determine Vertical Hydraulic Conductivities in Clay‐Rich Aquitards
Authors: Boldt‐Leppin Brigitte E.J. , Jim Hendry M.
Published: December 2005

A harmonic analysis method was used to determine vertical hydraulic conductivities (Kv) in geologic media between vertically separated piezometers using water level measurements. In this method, each water level time series was filtered and then decomposed using harmonic analysis into a sum of trigonometric components. The phase and amplitude of each harmonic function were calculated. These data were used to estimate Kv values between vertically separated data sets assuming one‐dimensional transient flow. The method was applied to water level data collected from nested piezometers at two thick clay‐rich till aquitards in Saskatchewan, Canada. At one site, routine water levels were measured in 12 piezometers (installed between 1 and 29 m below ground surface) since installation (1995). At the other site, water levels were measured in seven piezometers (installed between 4 and 53 m below ground surface) since installation (1998–1999). The Kv calculated using harmonic analysis decreased with depth below the water table at both sites, approaching matrix estimates of hydraulic conductivity between 10 and 11 m and between 21 and 43 m below ground surface. These depths reflected the depth of extensive vertical fracturing at the sites and showed that the depth of fracturing may be site specific.
Characterization of the Hydraulic Properties of Fractures in Chalk
Authors: Nativ Ronit , Adar Eilon , Assaf Lior , Nygaard Erik
Published: December 2005

The fracture systems intersecting Eocene chalk formations in the Negev desert, Israel, and their hydraulic properties were characterized using a variety of geologic and hydrologic techniques. These included identification of the prevailing directions of fracture systems in outcrops, in cores retrieved from inclined coreholes, in coreholes using video logs, and in trenches. The orientation and inclination of these fracture systems were determined, and evidence of ground water flow on the fracture surfaces was described and ranked. Their hydraulic conductivity was determined through slug and pumping tests performed at discrete intervals. Temperature, electrical conductivity, caliper, gamma and heat‐pulse logs were conducted in the same coreholes. The results from the logs, tests, and core descriptions were compared to identify reliable and cost‐effective tools for investigating the hydraulic characteristics of fracture systems. We concluded that in the study area: (1) fracture mapping in outcrops and coreholes (including downhole video and caliper logs) must be supplemented by hydraulic testing of the mapped fracture sets in the coreholes; (2) inclined coreholes provide information regarding the orientation of the hydraulically active fracture systems that cannot be obtained from vertical boreholes; (3) hydraulic testing of unpacked holes provides a reasonable estimate of the maximum hydraulic conductivity; and (4) the hydraulic conductivity distribution with depth is log normal and all significant ground water flow takes place within the upper 25 m.