bluuebunny/crossref_metadata_2025_split huggingface dataset:
Is the Forward Problem of Ground Water Hydrology Always Well Posed?
Authors: Valota Giorgio , Giudici Mauro , Parravicini Guido , Ponzini Giansilvio , Romano Emanuele 
Published: December 2005

AbstractComplex aquifer systems are often modeled with quasi‐three‐dimensional models, which consider two‐dimensional horizontal flow in the aquifers and one‐dimensional vertical flow through aquitards. When the aquifer system consists of a phreatic aquifer and one or more semiconfined aquifers connected by aquitards, the discrete model consists of a nonlinear system of algebraic equations, because the transmissivity of the phreatic aquifer depends on the phreatic head. If the water extraction is very high, the phreatic aquifer can be depleted and the equations of the model must be modified accordingly. There are not simple and general criteria to state if the phreatic aquifer is depleted before solving the system of equations. Therefore, the iterative procedures (e.g., relaxation methods), used to find the solution to the forward problem, must handle these particular conditions and can suffer several problems of convergence. These problems can be caused by the choice of the initial head values or of the relaxation coefficient of the iterative algorithms; however, they can also be caused by the nonexistence or nonuniqueness of the solution to the system of nonlinear equations. The study of existence and uniqueness of the general problem is very difficult and, therefore, we consider a simplified problem, for which the discrete model can be handled analytically. The results of the numerical experiments show that the solution to the forward problem can be nonunique. Only for some cases it is possible to invoke physical arguments to eliminate tentative solutions.
Spatial and Temporal Variability of Ground Water Recharge in Central Australia: A Tracer Approach
Authors: Harrington Glenn A. , Cook Peter G. , Herczeg Andrew L 
Published: December 2005

AbstractTwo environmental tracer methods are applied to the Ti‐Tree Basin in central Australia to shed light on the importance of recharge from floodouts of ephemeral rivers in this arid environment. Ground water carbon‐14 concentrations from boreholes are used to estimate the average recharge rate over the interval between where the ground water sample first entered the saturated zone and the bore. Environmental chloride concentrations in ground water samples provide estimates of the recharge rate at the exact point in the landscape where the sample entered the saturated zone. The results of the two tracer approaches indicate that recharge rates around one of the rivers and an extensive flood‐plain are generally higher than rates of diffuse recharge that occurs in areas of lower topographic relief. Ground water 2H/1H and 18O/16O compositions are all depleted in the heavier isotopes (δ2H = ‐67%0 to ‐50%0; 518O = ‐9.2%0 to ‐5.7%0) compared with the long‐term, amount‐weighted mean isotopic composition of rainfall in the area (δ2H = ‐33.8%0; δ18O = ‐6.3%0). This indicates that recharge throughout the basin occurs only after intense rainfall events of at least 150 to 200 mm/month. Finally, a recharge map is developed to highlight the spatial extent of the two recharge mechanisms. Floodout recharge to the freshest ground water (TDS <1000 mg/L) is ∼1.9 mm/year compared with a mean recharge rate of ∼0.2 mm/year to the remainder of the basin. These findings have important implications for management of the ground water resource.
A Global Experimental System Approach of Karst Springs1 Hydrographs and Chemographs
Authors: Grasso D. Alessandro , Jeannin Pierre‐Yves 
Published: December 2005

AbstractInvestigation techniques for karst flow systems are based mainly on the study of different signals leaving the system caused by natural or induced external influences. Each signal represents one of the systems outputs (e.g., hydraulic, chemical, physical, or isotopic responses) that reflect the characteristics of the entire system.In this paper, we present a method to infer information about the structure of karst systems. It is based on a simultaneous analysis of chemical and hydraulic responses. Beside the classical piston flow at the beginning of a flood pulse, we define a chemically based recession flow phase. During this phase, field data show that the concentration of total dissolved solids can be considered as an exponential function of the logarithm of flow. This relationship allows two parameters to be defined, one of which is dependent on the structure and degree of development of the karst conduit network, the other is dependent mainly on bioclimatic factors. Data collected from seven karst springs are used to support ideas introduced in the paper.
Errors with Small Volume Elastic Seepage Meter Bags
Authors: Schincariol Robert A. , McNeil John D. 
Published: December 2005

AbstractThe use of small volume elastic collection bags (condoms) has become popular in seepage meter studies in recent years, despite minimal field or laboratory validation of their use and, specifically, the impact of their elasticity on seepage measurements. A laboratory study was initiated after field results using small elastic collection bags produced seepage data that did not correlate with hydrometric data. The laboratory data demonstrate that condoms undergo significant mechanical relaxation during seepage measurement times typically observed in field settings. Unlike conventional nonelastic collection bags, which mechanically relax over several minutes, the condoms suffered from a slow mechanical relaxation or equilibration. Over nine hours, condoms gained 43 mL of water, ‐50% of maximum workable volume (between mechanical relaxation effect and elastic limit), under stagnant flow conditions. This long‐term equilibration invalidates simple subtraction of equilibration volumes from collection volumes as a correction technique. Previously published studies using flexible small‐volume elastic measurement bags (condoms) have not reported a mechanical relaxation effect. Overall, because the condom's small workable volume and inherent variability, we would not recommend any small‐volume elastic measurement bags for quantitative seepage measurements.
Influence of Fracture Anisotropy on Ground Water Ages and Chemistry, Valley and Ridge Province, Pennsylvania
Authors: Burton William C. , Plummer L. Niel , Busenberg Eurybiades , Lindsey Bruce D. , Gburek William J. 
Published: December 2005

AbstractModel ground water ages based on chlorofluorocarbons (CFCs) and tritium/helium‐3 (3H/3He) data were obtained from two arrays of nested piezometers located on the north limb of an anticline in fractured sedimentary rocks in the Valley and Ridge geologic province of Pennsylvania. The fracture geometry of the gently east plunging fold is very regular and consists predominately of south dipping to subhorizontal to north dipping bedding‐plane parting and east striking, steeply dipping axial‐plane spaced cleavage. In the area of the piezometer arrays, which trend north‐south on the north limb of the fold, north dipping bedding‐plane parting is a more dominant fracture set than is steeply south dipping axial‐plane cleavage. The dating of ground water from the piezometer arrays reveals that ground water traveling along paths parallel to the dip direction of bedding‐plane parting has younger 3H/3He and CFC model ages, or a greater component of young water, than does ground water traveling along paths opposite to the dip direction. In predominantly unmixed samples there is a strong positive correlation between age of the young fraction of water and dissolved sodium concentration. The travel times inferred from the model ages are significantly longer than those previously calculated by a ground water flow model, which assumed isotropically fractured layers parallel to topography. A revised model factors in the directional anisotropy to produce longer travel times. Ground water travel times in the watershed therefore appear to be more influenced by anisotropic fracture geometry than previously realized. This could have significant implications for ground water models in other areas underlain by similarly tilted or folded sedimentary rock, such as elsewhere in the Valley and Ridge or the early Mesozoic basins.
A Field Demonstration of the Simulation Optimization Approach for Remediation System Design
Authors: Zheng Chunmiao , Wang P. Patrick 
Published: December 2005

AbstractWhile significant progress has been made in the theoretical development of the simulation/optimization (S/O) approach for ground water remediation design, its application to large, field‐scale problems has remained limited. To demonstrate the applicability and usefulness of the S/O approach under real field conditions, an optimization demonstration project was conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts, involving the design of a pump‐and‐treat system for the containment and cleanup of a large trichloroethylene (TCE) plume. The optimization techniques used in this study are based on evolutionary algorithms coupled with a response function approach for greater computational efficiency. The S/O analysis was performed parallel to a conventional trial‐and‐error analysis based on simulation alone. The results of this study demonstrate that not only would it be possible to remove more TCE mass under the same amount of pumping assumed in the trial‐and‐error design, but also substantial cost savings could be achieved by reducing the number of wells needed and adapting dynamic pumping. In spite of the large model size of more than 500,000 nodes and a long planning horizon of 30 years, the optimization modeling was carried out successfully on desktop PCs. This field demonstration project clearly illustrates the potential benefits of applying optimization techniques in remediation system design.
A Simple Correction for Slug Tests in Small‐Diameter Wells
Authors: Butler James J. 
Published: December 2005

AbstractA simple procedure is presented for correcting hydraulic conductivity (K) estimates obtained from slug tests performed in small‐diameter installations screened in highly permeable aquifers. Previously reported discrepancies between results from slug tests in small‐diameter installations and those from tests in nearby larger‐diameter wells are primarily a product of frictional losses within the small‐diameter pipe. These frictional losses are readily incorporated into existing models for slug tests in high‐K aquifers, which then serve as the basis of a straightforward procedure for correcting previously obtained K estimates. A demonstration of the proposed procedure using data from a series of slug tests performed in a controlled field setting confirms the validity of the approach. The results of this demonstration also reveal the detailed view of spatial variations in K that can be obtained using slug tests in small‐diameter installations.
Measurement System for Systematic Hydrological Characterization of Unsaturated Fractured Welded Tuff in a Mined Underground Tunnel
Authors: Cook P.J. , Salve R. , Freifeld B.M. , Tsang Y.T. 
Published: December 2005

AbstractA field investigation of unsaturated flow through a lithophysal unit of fractured welded tuff containing litho‐physal cavities has been initiated. To characterize flow in this spatially heterogeneous medium, a systematic approach has been developed to perform tests in boreholes drilled at regular intervals in an underground tunnel (drift). The purpose of the testing is to quantify the amounts of water seeping into the drift versus the amount of water moving around the drift when released into boreholes at many equidistant locations along the drift. In this paper, we describe the test equipment system that has been built for this purpose. Because the field‐scale measurements—of liquid flow in the unsaturated, fractured rocks—require continuous testing for periods of days to weeks, the control of test equipment has been fully automated, allowing operation with no human presence at the field site. Preliminary results from the first set of tests indicate that, while the effects of evaporation on characterization of hydrological properties of the rock can be significant, these effects can be controlled and quantified. These tests give insight into the role of the cavities as potential storage during the initial transient flow prior to the breakthrough of water at the drift crown, as well as the role of connected fractures that provide the subsequent quasi‐steady flow. In addition to the stated purpose of realizing the flow partitioning, the results yield values for the effective porosity in the pathways for liquid flow in the regions tested thus far.
Geochemical Effects on Metals Following Permanganate Oxidation of DNAPLs
Authors: Crimi Michelle L , Siegrist Robert L. 
Published: December 2005

AbstractThe application of in situ chemical oxidation for dense, nonaqueous phase liquid (DNAPL) remediation requires delivery of substantial levels of oxidant chemicals into the subsurface to degrade target DNAPLs and to satisfy natural oxidant demand. This practice can raise questions regarding changes in subsurface conditions, yet information regarding potential effects, especially at the field scale, has been lacking. This paper describes an evaluation of the effects on metals associated with in situ chemical oxidation using potassium permanganate at Launch Complex 34 (LC34), Cape Canaveral Air Station, Florida. At LC34, high concentrations of permanganate (1 to 2 wt%) were injected into the subsurface as part of a demonstration of DNAPL remediation technologies. In a companion experimental effort at the Colorado School of Mines, field samples were characterized and laboratory batch and mini‐column studies were completed to assess effects of permanganate oxidation on metals in the subsurface one year after completion of the field demonstration. Results indicated there was potential for long‐term immobilization of a portion of introduced manganese and no treatment‐induced loss in subsurface permeability due to deposition of manganese oxides particles, which are a product of the oxidation reactions. Permanganate treatment did cause elevated manganese, chromium, and nickel concentrations in site ground water within the treated region. Some of these metals effects can be attenuated during downgradient flow through uncontaminated and untreated aquifer sediments.
A Critique of the Internal Tracer Method for Estimating Contaminant Degradation Rates
Authors: Martian Peter , Sorenson Kent S. , Peterson Lance N. 
Published: December 2005

AbstractThe internal tracer method for estimating contaminant degradation rates separates the attenuation effects not associated with degradation by using a codisposed recalcitrant internal tracer to normalize the degrading contaminant concentration. The remaining attenuation between the internal tracer and degrading contaminant is attributed to degradation and the degradation rate half‐life is estimated from the first‐order decay equation. An analytical solution of the advection‐ dispersion equation was used to evaluate flow‐and‐transport conditions that could result in incorrect estimates of contaminant degradation rate constants. Flow‐and‐transport characteristics that result in overestimating degradation rates were of particular interest because the internal tracer method often used to demonstrate natural attenuation can achieve remedial objectives. The analytical solution was also used to estimate the magnitude of error associated with using the internal tracer method at an example site and to explain different degradation rates estimated using tracers with different decay rate constants.
Comparison of Heat and Bromide as Ground Water Tracers Near Streams
Authors: Constantz Jim , Cox Marisa H. , Su Grace W. 
Published: December 2005

AbstractHeat and bromide were compared as tracers for examining stream/ground water exchanges along the middle reaches of the Santa Clara River, California, during a 10‐hour surface water sodium bromide injection test. Three cross sections that comprise six shallow (<1 m) piezometers were installed at the upper, middle, and lower sections of a 17 km long study reach, to monitor temperatures and bromide concentrations in the shallow ground water beneath the stream. A heat and ground water transport simulation model and a closely related solute and ground water transport simulation model were matched up for comparison of simulated and observed temperatures and bromide concentrations in the streambed. Vertical, one‐dimensional simulations of sediment temperature were fitted to observed temperature results, to yield apparent streambed hydraulic conductivities in each cross section. The temperature‐based hydraulic conductivities were assigned to a solute and ground water transport model to predict sediment bromide concentrations, during the sodium bromide injection test. Vertical, one‐dimensional simulations of bromide concentrations in the sediments yielded a good match to the observed bromide concentrations, without adjustment of any model parameters except solute dispersivities. This indicates that, for the spatial and temporal scales examined on the Santa Clara River, the use of heat and bromide as tracers provide comparable information with respect to apparent hydraulic conductivities and fluxes for sediments near streams. In other settings, caution should be used due to differences in the nature of conservative (bromide) versus nonconservative (heat) tracers, particularly when preferential flowpaths are present.
Laboratory and Numerical Investigation of Flow and Transport Near a Seepage‐Face Boundary
Authors: Simpson MJ. , Clement T.P. , Gallop T.A. 
Published: December 2005

AbstractLaboratory and numerical modeling investigations were completed to study the unconfined ground water flow and transport processes near a seepage‐face boundary. The laboratory observations were made in a radial sand tank and included measurements of the height of the seepage face, flow velocity near the seepage face, travel time distribution of multiple tracer slugs, and streamlines. All the observations were reliably reproduced with a three‐dimensional, axi‐symmetric, variably saturated ground water flow model. Physical data presented in this work demonstrate and quantify the importance of three‐dimensional transport patterns within a seepage‐face zone. The results imply that vertically averaged flow models that employ Dupuit approximations might introduce error in the analysis of localized solute transport near a seepage‐face boundary. The experimental dataset reported in this work will also be of interest for those who are attempting to validate a numerical algorithm for solving ground water and contaminant discharge patterns near a surface‐water boundary.
Analysis of Solute Transport in Flow Fields Influenced by Preferential Flowpaths at the Decimeter Scale
Authors: Zheng Chunmiao , Gorelick Steven M. 
Published: December 2005

AbstractSeveral recent studies at the Macrodispersion Experiment (MADE) site in Columbus, Mississippi, have indicated that the relative preferential flowpaths and flow barriers resulting from decimeter‐scale aquifer heterogeneities appear to have a dominant effect on plume‐scale solute transport. Numerical experiments are thus conducted in this study to explore the key characteristics of solute transport in two‐dimensional flow fields influenced by decimeter‐scale preferential flowpaths. A hypothetical but geologically plausible network of 10 cm wide channels of high hydraulic conductivity is used to represent the relative preferential flowpaths embedded in an otherwise homogeneous aquifer. When the hydraulic conductivity in the channels is 100 times greater than that in the remaining portion of the aquifer, the calculated concentration distributions under three source configurations all exhibit highly asymmetrical, non‐Gaussian patterns. These patterns, with peak concentrations close to the source and extensive spreading downgradi‐ent, resemble that observed at the MADE site tracer tests. When the contrast between the channel and nonchannel hydraulic conductivities is reduced to 30:1 from 100:1, the calculated mass distribution curve starts to approach a Gaussian one with the peak concentration near the central portion of the plume. Additional analysis based on a fieldscale model demonstrates that the existence of decimeter‐scale preferential flowpaths can have potentially far‐reaching implications for ground water remediation. Failure to account for them in numerical simulation could lead to over‐estimation of the effectiveness of the remedial measure under consideration.
A Combinatorial Optimization Scheme for Parameter Structure Identification in Ground Water Modeling
Authors: Tsai Frank T‐C. , Sun Ne‐Zheng , Yen William W‐G. 
Published: December 2005

AbstractThis research develops a methodology for parameter structure identification in ground water modeling. For a given set of observations, parameter structure identification seeks to identify the parameter dimension, its corresponding parameter pattern and values. Voronoi tessellation is used to parameterize the unknown distributed parameter into a number of zones. Accordingly, the parameter structure identification problem is equivalent to finding the number and locations as well as the values of the basis points associated with the Voronoi tessellation. A genetic algorithm (GA) is allied with a grid search method and a quasi‐Newton algorithm to solve the inverse problem. GA is first used to search for the near‐optimal parameter pattern and values. Next, a grid search method and a quasi‐Newton algorithm iteratively improve the GA's estimates. Sensitivities of state variables to parameters are calculated by the sensitivity‐equation method. MODFLOW and MT3DMS are employed to solve the coupled flow and transport model as well as the derived sensitivity equations. The optimal parameter dimension is determined using criteria based on parameter uncertainty and parameter structure discrimination. Numerical experiments are conducted to demonstrate the proposed methodology, in which the true transmissivity field is characterized by either a continuous distribution or a distribution that can be characterized by zones. We conclude that the optimized transmissivity zones capture the trend and distribution of the true transmissivity field.
MODFLOW/MT3DMS‐Based Reactive Multicomponent Transport Modeling
Authors: Prommer H. , Barry D.A. , Zheng C. 
Published: December 2005

AbstractThis paper presents a three‐dimensional, MODFLOW/MT3DMS‐based reactive multicomponent transport model for saturated porous media. Based on a split‐operator technique, the model, referred to as PHT3D, couples the transport simulator MT3DMS and the geochemical modeling code PHREEQC‐2. Through the flexible, generic nature of PHREEQC‐2, PHT3D can handle a broad range of equilibrium and kinetically controlled reactive processes, including aqueous complexation, mineral precipitation/dissolution, and ion exchange. The diversity of potential applications is demonstrated through simulation of five existing literature benchmarks and a new three‐dimensional sample problem. The model might be applied to simulate the geochemical evolution of pristine and contaminated aquifers as well as their cleanup. The latter problem class includes the natural and enhanced attenuation/remediation schemes of a wide range of organic and inorganic contaminants. Processes/reactions not included in the standard PHREEQC‐2 database but typical for this type of application (e.g., NAPL dissolution, microbial growth/decay) can be defined and included via the extensible PHREEQC‐2 database file.