bluuebunny/crossref_metadata_2025_split huggingface dataset:
Characterization of the Hydraulic Properties of Fractures in Chalk
Authors: Nativ Ronit , Adar Eilon , Assaf Lior , Nygaard Erik 
Published: December 2005

AbstractThe 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.
From Models to Performance Assessment: The Conceptualization Problem
Authors: Bredehoeft John D. 
Published: December 2005

AbstractToday, models are ubiquitous tools for ground water analyses. The intent of this paper is to explore philosophically the role of the conceptual model in analysis. Selection of the appropriate conceptual model is an a priori decision by the analyst. Calibration is an integral part of the modeling process. Unfortunately a wrong or incomplete conceptual model can often be adequately calibrated; good calibration of a model does not ensure a correct conceptual model. Petroleum engineers have another term for calibration; they refer to it as history matching. A caveat to the idea of history matching is that we can make a prediction with some confidence equal to the period of the history match. In other words, if we have matched a 10‐year history, we can predict for 10 years with reasonable confidence; beyond 10 years the confidence in the prediction diminishes rapidly. The same rule of thumb applies to ground water model analyses. Nuclear waste disposal poses a difficult problem because the time horizon, 1000 years or longer, is well beyond the possibility of the history match (or period of calibration) in the traditional analysis. Nonetheless, numerical models appear to be the tool of choice for analyzing the safety of waste facilities. Models have a well‐recognized inherent uncertainty. Performance assessment, the technique for assessing the safety of nuclear waste facilities, involves an ensemble of cascading models. Performance assessment with its ensemble of models multiplies the inherent uncertainty of the single model. The closer we can approach the idea of a long history with which to match the models, even models of nuclear waste facilities, the more confidence we will have in the analysis (and the models, including performance assessment). This thesis argues for prolonged periods of observation (perhaps as long as 300 to 1000 years) before a nuclear waste facility is finally closed.
Ground Water/Surface Water Interaction in a Fractured Rock Aquifer
Authors: Oxtobee Jaime P.A. , Novakowski Kent S. 
Published: December 2005

AbstractIn a recent field study of ground water/surface water interaction between a bedrock stream and an underlying fractured rock aquifer, it was determined that the majority of ground water discharge occurred through sparsely located vertical fractures. In this paper, the dominant mechanisms governing ground water/surface water exchange in such an environment are investigated using a numerical model. The study was conducted using several conceptual models based on the field study results. Although the field results provided the motivation for the modeling study, it was not intended to match modeling and field results directly. In addition, the extent of capture zones for discharging or recharging fractures was explored. The results of this study are intended to provide a better understanding of contaminant migration in the vicinity of bedrock streams. Based on the numerical results, the rate of ground water discharge (or recharge) was found to depend on the aperture size of the discharging feature, and on the distribution of hydraulic head with depth within the fracture network. It was determined that the extent of both the capture zone and reverse capture zone for an individual fracture can be extremely large, and will be determined by the height of the stream stage, the fracture apertures of the network, and the hydraulic‐head distribution within the network. Because both the stream stage and the hydraulic‐head distribution are transient, the size of the capture zone and/or the reverse capture zone for an individual fracture may change significantly over time. As a result, the migration path for contaminants within the fracture network and between the surface and subsurface will also vary significantly with time.
Monitoring Well Interception with Fractures in Clayey Till
Authors: Jørgensen Peter R. , Klint Knud Erik S. , Kistrup Jens P. 
Published: December 2005

AbstractWhen using monitoring wells for investigation of contaminant sources in clayey till, there is a high risk that fractures may cause mobile contaminants to bypass the monitoring wells. This paper indicates that the probability of interception between monitoring wells and hydraulic conductive fractures is often significantly less than 50%. Based on a field experiment and application of a calibrated discrete fracture matrix diffusion numerical model (FRAC3Dvs), the paper also evaluates pesticide‐monitoring results for different positions of monitoring well screen relative to fractures. For well screens situated 0.25 and 2 m from a conductive fracture, the first concentrations of the pesticide metabolite (2,6 dichlorobenzamide, “BAM”)would be measured two years and 18 years, respectively, after the contaminant had been transported into an underlying aquifer. In this way, underlying aquifers may be subjected to contamination by downward moving contamination without being observed in monitoring wells in the till.
Long‐Term Disturbance of Ground Water Chemistry Following Well Installation
Authors: Kim Kangjoo 
Published: December 2005

AbstractGround water samples collected from a multilevel sampler shortly after its construction showed significantly higher alkalinity and concentrations of calcium and magnesium than those from nearby wells installed 10 years earlier. The sampler was drilled using a conventional hollow‐stem power auger in a sandy, silicate aquifer lying beneath an isthmus between two lakes in northern Wisconsin. Ground water in the study area is of low ionic strength and its chemistry is dominated by silicate mineral weathering. Periodic sampling over two years following installation of the sampler showed that the higher solute concentrations had subsequently decreased. Oxygen isotope signature and other solute species, such as sulfate and chloride, were comparable to those of older wells and did not show any notable trends over time. Independent variation of other chemical species that cannot be derived from aquifer minerals, and the similarly high concentrations in older wells shortly after their installation, suggest that rapid dissolution of fresh mineral surfaces and hyperfine particles generated during drilling has induced the enhanced concentrations. This observation is consistent with the field equivalent of laboratory mineral dissolution experiments that show initially increased dissolution rates that decay over time. Well installations for geochemical sampling in dominantly silicate material may require longer times to reach an equilibrium state than has been previously thought.
Steady Flow to a Well Near a Stream with a Leaky Bed
Authors: Bakker Mark , Anderson Erik I. 
Published: December 2005

AbstractWe present an explicit analytic solution for steady, two‐dimensional ground water flow to a well near a leaky streambed that penetrates the aquifer partially. Leakage from the stream is approximated as occurring along the centerline of the stream. The problem domain is infinite and pumping on one side of the stream induces flow on the other side. The solution includes the effects of uniform flow in the far field and a sloping hydraulic head in the stream. We use the solution to investigate the interaction between ground water and surface water in the stream, the effects of pumping on the opposite side of the stream, and the effects of the leaky streambed on the capture zone envelope of the well. We develop a relationship between parameters such that the pumping well will not capture water from the stream, or from the opposite side of the stream. When the discharge of the well is large enough to capture water from the stream, the shape of the capture zone envelope depends on flow conditions on the side of the stream opposite the well.
A Simple Constant‐Head Injection Test for Streambed Hydraulic Conductivity Estimation
Authors: Cardenas M. Bayani , Zlotnik Vitaly A. 
Published: December 2005

AbstractA fast, efficient constant‐head injection test (CHIT) for in situ estimation of hydraulic conductivity (K) of sandy streambeds is presented. This test uses constant‐head hydraulic injection through a manually driven piezometer. Results from CHIT compare favorably to estimates from slug testing and grain‐size analysis. The CHTT combines simplicity of field performance, data interpretation, and accuracy of K estimation in flowing streams.
A Comparison of Two Stochastic Inverse Methods in a Field‐Scale Application
Authors: Larocque Marie , Delay Fred , Banton Olivier 
Published: December 2005

AbstractInverse modeling is a useful tool in ground water flow modeling studies. The most frequent difficulties encountered when using this technique are the lack of conditioning information (e.g., heads and transmissivities), the uncertainty in available data, and the nonuniqueness of the solution. These problems can be addressed and quantified through a stochastic Monte Carlo approach. The aim of this work was to compare the applicability of two stochastic inverse modeling approaches in a field‐scale application. The multi‐scaling (MS) approach uses a downscaling parameterization procedure that is not based on geostatistics. The pilot point (PP) approach uses geostatistical random fields as initial transmissivity values and an experimental variogram to condition the calibration. The studied area (375 km2) is part of a regional aquifer, northwest of Montréal in the St. Lawrence lowlands (southern Québec). It is located in limestone, dolomite, and sandstone formations, and is mostly a fractured porous medium. The MS approach generated small errors on heads, but the calibrated transmissivity fields did not reproduce the variogram of observed transmissivities. The PP approach generated larger errors on heads but better reproduced the spatial structure of observed transmissivities. The PP approach was also less sensitive to uncertainty in head measurements. If reliable heads are available but no transmissivities are measured, the MS approach provides useful results. If reliable transmissivities with a well inferred spatial structure are available, then the PP approach is a better alternative. This approach however must be used with caution if measured transmissivities are not reliable.
Enhanced Attenuation of Septic System Phosphate in Noncalcareous Sediments
Authors: Robertson W.D. 
Published: December 2005

AbstractReview of phosphate behavior in four mature septic system plumes on similar textured sand has revealed a strong correlation between carbonate mineral content and phosphate concentrations. A plume on calcareous sand (Cambridge site, 27 wt % CaCO3 equiv.) has proximal zone PO4 concentrations (4.8 mg/L P average) that are about 75% of the septic tank effluent value, whereas three plumes on noncalcareous sand (Muskoka, L. Joseph, and Nobel sites, <1 wt % CaCO3 equiv.) have proximal zone phosphate concentrations (<0.1 mg/L P) that are consistently less than 2% of the effluent values. Phosphate attenuation at the noncalcareous sites appears to be an indirect result of the development of acidic conditions (site average pH 3.5 to 5.9) and elevated Al concentrations (up to 24 mg/L), which subsequently causes the precipitation of Al‐P minerals such as variscite (AlPO4. 2H2O). This is supported by scanning electron microscope analyses, which show the widespread occurrence of (Al+P)—rich secondary mineral coatings on sand grains below the infiltration beds. All of these septic systems are more than 10 years old, indicating that these attenuation reactions have substantial longevity.A field lysimeter experiment demonstrated that this reaction sequence can be readily incorporated into engineered waste water treatment systems. We feel this important P removal mechanism has not been adequately recognized, particularly for its potential significance in reducing P loading from septic systems in lakeshore environments.
Hydrostratigraphic Modeling of a Complex, Glacial‐Drift Aquifer System for Importation into MODFLOW
Authors: Herzog Beverly L. , Larson David R. , Abert Curtis C. , Wilson Steven D. , Roadcap George S. 
Published: December 2005

AbstractDeposition from at least three episodes of glaciation left a complex glacial‐drift aquifer system in central Illinois. The deepest and largest of these aquifers, the Sankoty‐Mahomet Aquifer, occupies the lower part of a buried bedrock valley and supplies water to communities throughout central Illinois. Thin, discontinuous aquifers are present within glacial drift overlying the Sankoty‐Mahomet Aquifer. This study was commissioned by local governments to identify possible areas where a regional water supply could be obtained from the aquifer with minimal adverse impacts on existing users.Geologic information from more than 2200 existing water well logs was supplemented with new data from 28 test borings, water level measurements in 430 wells, and 35 km of surface geophysical profiles. A three‐dimensional (3‐D) hydrostratigraphic model was developed using a contouring software package, a geographic information system (GIS), and the 3‐D geologic modeling package, Earth Vision®. The hydrostratigraphy of the glacial‐drift sequence was depicted as seven uneven and discontinuous layers, which could be viewed from an infinite number of horizontal and vertical slices and as solid models of any layer. Several iterations were required before the 3‐D model presented a reasonable depiction of the aquifer system. Layers from the resultant hydrostratigraphic model were imported into MODFLOW, where they were modified into continuous layers. This approach of developing a 3‐D hydrostratigraphic model can be applied to other areas where complex aquifer systems are to be modeled and is also useful in helping lay audiences visualize aquifer systems.
An Evaluation of Conditioning Data for Solute Transport Prediction
Authors: Scheibe Timothy D. , Chien Yi‐Ju 
Published: December 2005

AbstractThe large and diverse body of subsurface characterization data generated at a field research site near Oyster, Virginia, provides a unique opportunity to test the impact of conditioning data of various types on predictions of flow and transport. Bromide breakthrough curves (BTCs) were measured during a forced‐gradient local‐scale injection experiment conducted in 1999. Observed BTCs are available at 140 sampling points in a three‐dimensional array within the transport domain. A detailed three‐dimensional numerical model is used to simulate breakthrough curves at the same locations as the observed BTCs under varying assumptions regarding the character of hydraulic conductivity spatial distributions, and variable amounts and types of conditioning data. We present comparative results of six cases ranging from simple (deterministic homogeneous models) to complex (stochastic indicator simulation conditioned to cross‐borehole geophysical observations). Quantitative measures of model goodness‐of‐fit are presented. The results show that conditioning to a large number of small‐scale measurements does not significantly improve model predictions, and may lead to biased or overly confident predictions. However, conditioning to geophysical interpretations with larger spatial support significantly improves the accuracy and precision of model predictions. In all cases, the effects of model error appear to be significant in relation to parameter uncertainty.
Including Geophysical Data in Ground Water Model Inverse Calibration
Authors: Dam Dorte , Christensen Steen 
Published: December 2005

AbstractA nonlinear regression method is developed that can be used to estimate parameters of a ground water flow model from a combination of observations of hydrological variables and observations of geophysical properties that are functionally related with the hydraulic conductivity. The procedure estimates: parameters characterizing the hydraulic conductivity field (e.g., zonal or pilot point values); geophysical properties that have been observed and that are functionally related with the hydraulic conductivity parameters; and a few parameters of the function that relates the hydraulic conductivity parameters with the geophysical properties (the type of function is assumed known). A fidelity factor, σr2, of a term of the minimized objective function reflects the faith one has in the validity of this functional relationship. The estimation methodology has been tested by means of synthetic models. The experimental results demonstrate that the number of estimated hydraulic conductivity parameters can be increased by adding geophysical observations to the set of hydrological observations that are traditionally used for model calibration. The improvement of the estimated hydraulic conductivity field and the simulated hydraulic head field can be significant but is dependent on the number, the locations, and the uncertainty of geophysical observations. The sensitivity of the estimation results to the value of σr is small for the studied problems except when the uncertainty of geophysical observations is high. In the latter case, a large σr value was found to be optimal to avoid that hydraulic conductivity estimates are closely tied to corresponding but highly uncertain geophysical observations.
Stepwise Use of GFLOW and MODFLOW to Determine Relative Importance of Shallow and Deep Receptors
Authors: Feinstein Daniel , Dunning Charles , Hunt Randall J. , Krohelski Jim 
Published: December 2005

AbstractA stepwise modeling approach is implemented in which a regional one‐layer analytic element model is used to simulate the flow system and to furnish boundary conditions for an extracted local three‐dimensional model. In this case study the stepwise approach is used to evaluate the fate of recharge in the Menomonee Valley adjacent to Lake Michigan. Two major receptors exist for recharge that flows through contaminated valley fill: the surface water estuary and a tunnel system constructed ∼75 to 110 m below land surface to store storm runoff. The primary objective of the modeling is to delineate the contributing areas of recharge to each receptor. Of interest is the ability of the one‐layer regional model to furnish flux boundary conditions to the local three‐dimensional model despite the presence of vertical flow conditions at the boundaries of the local model. Sensitivity analysis suggests that the local model was insensitive to the vertical distribution of the flux. Each step of the modeling approach demonstrates that both receptors play an important role in capturing valley recharge. The pattern of capture of the one‐layer model differed in shape from that delineated by the multi‐layer local model in the presence of a flow system with pronounced vertical anisotropy and with sinks drawing water from different elevations.
Coupling Basin‐ and Site‐Scale Inverse Models of the Espanola Aquifer
Authors: Keating Elizabeth H. , Vesselinov Velimir V. , Kwicklis Edward , Lu Zhiming 
Published: December 2005

AbstractLarge‐scale models are frequently used to estimate fluxes to small‐scale models. The uncertainty associated with these flux estimates, however, is rarely addressed. We present a case study from the Española Basin, northern New Mexico, where we use a basin‐scale model coupled with a high‐resolution, nested site‐scale model. Both models are three‐dimensional and are analyzed by codes FEHM and PEST. Using constrained nonlinear optimization, we examine the effect of parameter uncertainty in the basin‐scale model on the nonlinear confidence limits of predicted fluxes to the site‐scale model. We find that some of the fluxes are very well constrained, while for others there is fairly large uncertainty. Site‐scale transport simulation results, however, are relatively insensitive to the estimated uncertainty in the fluxes. We also compare parameter estimates obtained by the basin‐ and site‐scale inverse models. Differences in the model grid resolution (scale of parameter estimation) result in differing delineation of hydrostratigraphic units, so the two models produce different estimates for some units. The effect is similar to the observed scale effect in medium properties owing to differences in tested volume. More important, estimation uncertainty of model parameters is quite different at the two scales. Overall, the basin inverse model resulted in significantly lower estimates of uncertainty, because of the larger calibration dataset available. This suggests that the basin‐scale model contributes not only important boundary condition information but also improved parameter identification for some units. Our results demonstrate that caution is warranted when applying parameter estimates inferred from a large‐scale model to small‐scale simulations, and vice versa.
Ground Water Flow in a Desert Basin: Challenges of Simulating Transport of Dissolved Chromium
Authors: Andrews Charles B. , Neville Christopher J. 
Published: December 2005

AbstractA large chromium plume that evolved from chromium releases in a valley near the Mojave River was studied to understand the processes controlling fate and migration of chromium in ground water and used as a tracer to study the dynamics of a basin and range ground water system. The valley that was studied is naturally arid with high evapotranspiration such that essentially no precipitation infiltrates to the water table. The dominant natural hydrogeologic processes are recharge to the ground water system from the Mojave River during the infrequent episodes when there is flow in the river, and ground water flow toward a playa lake where the ground water evaporates. Agricultural pumping in the valley from the mid‐1930s to the 1970s significantly altered ground water flow conditions by decreasing water levels in the valley by more than 20 m. This pumping declined significantly as a result of dewatering of the aquifer, and water levels have since recovered modestly.The ground water system was modeled using MODFLOW, and chromium transport was simulated using MT3D. Several innovative modifications were made to these modeling programs to simulate important processes in this ground water system. Modifications to MODFLOW include developing a new well package that estimates pumping rates from irrigation wells at each time step based on available drawdown. MT3D was modified to account for mass trapped above the water table when the water table declines beneath nonirrigated areas and to redistribute mass to the system when water levels rise.