bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytical Modeling of Aquifer Pollution Caused by Solid Waste Depositories
Authors: Rudakov Dmitry V. , Rudakov Victor C.
Published: August 2005

An analytical model to assess the risk of ground water pollution caused by leaks from surficial depositories containing water‐soluble toxic substances is presented. The proposed model includes two stages of pollutant migration: (1) predominantly vertical transport through a low‐permeability protective soil layer above the aquifer, and (2) two‐ or three‐dimensional transfer of the pollutant in ground water flow. Convection, longitudinal and transverse dispersion, sorption, and pollutant decay are taken into consideration both for the soil layer and the aquifer. Corresponding boundary problems are formulated and solved. The exact solutions allow us to estimate the possibility of toxic substance percolation into the aquifer and the expected intensity of this process, and to simulate pollution zone formation in ground water and possible material flow to a nearby surface water body. The calculations showed good numerical accuracy and the practical convenience of the model for monitoring and predicting ground water pollution near landfills and depositories of industrial solid waste.
Hydraulic and Geochemical Performance of a Permeable Reactive Barrier Containing Zero‐Valent Iron, Denver Federal Center
Authors: McMahon P.B. , Dennehy K.F. , Sandstrom M.W.
Published: August 2005

The hydraulic and geochemical performance of a 366 m long permeable reactive barrier (PRB) at the Denver Federal Center, Denver, Colorado, was evaluated. The funnel and gate system, which was installed in 1996 to intercept and remediate ground water contaminated with chlorinated aliphatic hydrocarbons (CAHs), contained four 12.2 m wide gates filled with zero‐valent iron. Ground water mounding on the upgradient side of the PRB resulted in a tenfold increase in the hydraulic gradient and ground water velocity through the gates compared to areas of the aquifer unaffected by the PRB. Water balance calculations for April 1997 indicate that about 75 % of the ground water moving toward the PRB from upgradient areas moved through the gates. The rest of the water either accumulated on the upgradient side of the PRB or bypassed the PRB. Chemical data from monitoring wells screened down‐gradient, beneath, and at the ends of the PRB indicate that contaminants had not bypassed the PRB, except in a few isolated areas. Greater than 99 % of the CAH mass entering the gates was retained by the iron. Fifty‐one percent of the CAH carbon entering one gate was accounted for in dissolved C1 and C2 hydrocarbons, primarily ethane and ethene, which indicates that CAHs may adsorb to the iron prior to being dehalogenated. Treated water exiting the gates displaced contaminated ground water at a distance of at least 3 m downgradient from the PRB by the end of 1997. Measurements of dissolved inorganic ions in one gate indicate that calcite and siderite precipitation in the gate could reduce gate porosity by about 0.35 % per year. Results from this study indicate that funnel and gate systems containing zero‐valent iron can effectively treat ground water contaminated with CAHs. However, the hydrologic impacts of the PRB on the flow system need to be fully understood to prevent contaminants from bypassing the PRB.
The Effects of Bag Type and Meter Size on Seepage Meter Measurements
Authors: Isiorho Solomon A. , Meyer John H.
Published: August 2005

Seepage meters provide a useful, inexpensive method of measuring seepage flux through unconsolidated sediments in lakes, reservoirs, and stream channels. In the literature, the bag type used is usually not indicated, and its effect on seepage measurement has not been evaluated. The effects of bag type and seepage meter size have been evaluated in a laboratory setting.

A sandbox 100.4 (L) × 76.2 (W) × 24 (H) cm size was half filled with coarse sand and water to a depth of 10 cm. Four seepage meters and two bag types (balloon and condom) were used in the seepage measurements.

There was no significant difference between the seepage measurements obtained from the two bag types. A two‐sample t‐test (df=56, Wt‐sat=2.58, alpha=0.01, p=0.0013) failed to reject the null hypothesis of similarity between measurements from the two bag types. The seepage measurements from four seepage meters were similar with no significant difference; however, the smaller diameter seepage meters had a greater variance.
Multiple Geochemical and Isotopic Approaches for Assessing Ground Water NO3− Elimination in a Riparian Zone
Authors: Mengis M. , Schif S. L. , Harris M. , English M. C. , Aravena R. , Elgood RJ. , MacLean A.
Published: August 2005

During the last 40 years, nitrate (NO3−) has become one of the most common ground water contaminants. Stream riparian zones are considered important ecological ecotones that decrease the NO3− load of ground water discharging into streams. This study uses NO3−/Cl− ratios, natural abundances of15N and18O in NO3−, and an in situ15NO3− tracer experiment to evaluate NO3− reducing processes occurring in ground water within a narrow grassed buffer strip bordering a stream in an agricultural watershed. The NO3−/Cl− ratios (1.9‐0.0 [M/M]) indicate that both NO3− consuming processes and mixing of two ground water flow regimes with different NO3− loads contribute to the drop observed in ground water NO3− concentrations within the riparian zone. δ15N and δ18O of the ground water NO3− within the riparian zone (δ15N = 5.1 to 48.8 %c; δ18O = 1.1 to 17.8 %c) were enriched compared to the ground water below the adjacent cultivated field (δ15N = 3.8 to 10.1 %0; δ18O = 0.5 to 6.2 %c). A significant linear relationship (r2=0.97) between ground water δ15N and δ18O in NO3− was found, which is consistent with NO3− consumption by microbial denitrification. The estimated enrichment factors for15N are a factor of 1.5 higher than for18O. The in situ15NO3− tracer experiment conclusively confirmed that denitrification is occurring within the ground water of the riparian zone and demonstrates that denitrification rates can be directly measured in situ.
Numerical Modeling of Perched Water Under Yucca Mountain, Nevada
Authors: Hinds Jennifer J. , Ge Shemin , Fridrich Chris J.
Published: August 2005

The presence of perched water near the potential high‐level nuclear waste repository area at Yucca Mountain, Nevada, has important implications for waste isolation. Perched water occurs because of sharp contrasts in rock properties, in particular between the strongly fractured repository host rock (the Topopah Spring welded tuff) and the immediately underlying vitrophyric (glassy) subunit, in which fractures are sealed by clays that were formed by alteration of the volcanic glass. The vitrophyre acts as a vertical barrier to unsaturated flow throughout much of the potential repository area. Geochemical analyses (Yang et al. 1996) indicate that perched water is relatively young, perhaps younger than 10,000 years. Given the low permeability of the rock matrix, fractures and perhaps fault zones must play a crucial role in unsaturated flow. The geologic setting of the major perched water bodies under Yucca Mountain suggests that faults commonly form barriers to lateral flow at the level of the repository horizon, but may also form important pathways for vertical infiltration from the repository horizon down to the water table. Using the numerical code UNSAT2, two factors believed to influence the perched water system at Yucca Mountain, climate and fault‐zone permeability, are explored. The two‐dimensional model predicts that the volume of water held within the perched water system may greatly increase under wetter climatic conditions, and that perched water bodies may drain to the water table along fault zones. Modeling results also show fault flow to be significantly attenuated in the Paintbrush Tuff non‐welded hydrogeologic unit.
Analysis of Convective Heat Transfer in Deformed and Stratified Aquifers Associated with Frasch Thermal Mining
Authors: Williams Daphne D. , Lee Ming‐Kuo , Crawford Joe E. , Tyree Philip O.
Published: August 2005

This paper presents numerical experiments designed to simulate heat transfer in deformed, stratified geologic formations during Frasch thermal mining operations in the Delaware Basin. In such operations, superheated water (163°C) is injected into permeable ore zones to melt and mobilize sulfur. The efficiency of Frasch mining depends largely on various aspects of hydrologic controls and geologic factors, such as directing heat flow toward target areas and minimizing heat dissipation through advection and conduction in ore zones. Numerical modeling techniques were used in the search of an optimum thermal mining strategy for maximum sulfur recovery in various geologic settings present at the Culberson Mine, west Texas. The sample calculations illustrate heat transfer patterns in inclined, folded, and fractured geologic formations. Important results presented include the controls of geologic structures on directions and rates of heat transfer and ground water flow, a display of field evidence for the occurrence of thermal convection in permeable ore zones, and a depiction of heat transfer during a thermal mining operation proceeding down‐dip along an inclined geologic unit. Modeling results and field data strongly support the hypothesis that thermal convection occurs and controls the heat transfer process in inclined ore zones. Simulations further suggest that the current thermal mining practice, which proceeds down‐slope along an inclined ore zone, may result in lowered ultimate sulfur recovery. In this mining approach most heat migrates up‐slope where the rock's permeability is enhanced by previous sulfur extraction, rather than down‐dip toward the target area.
Some Analytical and Approximate Darcian Means
Authors: Baker Donald L. , Arnold Mark E. , Scott H. Don
Published: August 2005

In the finite‐difference modeling of unsaturated infiltration by Richards’equation, standard interblock conductivity means like the arithmetic, geometric, and harmonic have been shown to produce flows different from true Darcian by up to orders of magnitude (Warrick 1991; Baker 1995a, 1998). This may have significant impact on models used for contaminant transport of nuclear and other hazardous waste. Previously, it has been impractical to calculate Darcian means in a running model. This paper introduces the analytic solution for Darcian means for the case of an exponential relative conductivity relation, and one possible approximation for general use, a piecewise Brooks‐Corey Darcian (PBCD) mean. It compares the PBCD mean to the arithmetic mean in a modern mass‐conservative numerical model (Celia et al. 1990) for vertical space steps from 0.8 to 105 cm, using soil parameters from Haverkamp et al. (1977). The results demonstrate that even in a model with mass balance good to one part in 108, non‐Darcian flow errors are alive and well, costing up to 50% of the total mass infiltrated with the arithmetic mean and large model space steps. The new PBCD mean reduces that figure to 0.5% for this example soil.
Estimation of Permeability of Disturbed Halite Using Inverse Modeling
Authors: Cardenas M. , LeCompte C. , Cua G.
Published: August 2005

The numerical modeling tool ITOUGH2 was examined to determine its effectiveness in parameter estimation for the small scale seals performance tests (SSSPT) at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico. Gas flow injection tests were performed at various distances inward from the wall of Room D to estimate the effects of creeping of the disturbed rock zone (DRZ) since excavation of Room D and boreholes, and to estimate the amount of healing of the DRZ that has taken place due to an emplaced concrete seal. Salt creeping into an excavation is expected to show increased porosity and absolute permeability values compared to undisturbed halite. Healed halite is expected to show porosity and absolute permeability values comparable to that of undisturbed halite. Single‐phase flow estimates of absolute permeability were made using ITOUGH2. Estimates of DRZ absolute permeability ranged from 10−11 near Room D to 10−21 m2 at the seal. Simultaneous estimates of absolute permeability and porosity were calculated for the disturbed rock zone in halite, and the effects of salt creep and healing were seen in the estimates of porosity and absolute permeability, as evidenced by the differences in absolute permeability and porosity with distance inward from the rib of Room D. Estimate of porosity were 0.04 near Room D, and the locations near the seal showed porosities of around 0.03. ITOUGH2 proved effective in interpreting test data seen in the SSSPT, especially test data with constant injection pressure.
Inferring Ground Water Flow in Fractured Rock from Dissolved Radon
Authors: Cook P. G. , Love A. J. , Dighton J.C.
Published: August 2005

Ground water radon concentrations have been measured in nested piezometers and an open borehole in fractured, metasedimentary aquifers in the Clare Valley, South Australia. The vertical radon concentration profile within the undisturbed, open borehole appears to indicate the locations of hydraulically active fractures. If radon concentrations within the aquifer are uniform, then the highest radon concentration within the borehole will indicate the location of the highest flow rate into the bore. In relatively short screened piezometers, radon concentrations have been measured both before and after purging. The ratio of radon concentration before to after purging normalizes for variations in aquifer mineralogy, and can be used to quantify the ground water flow rate.
A Numerical Study on the Relationship Between Transmissivity and Specific Capacity in Heterogeneous Aquifers
Authors: Meier Peter M. , Carrera Jesus , Sanchez‐Vila Xavier
Published: August 2005

Specific capacity (Q/s) data are usually much more abundant than transmissivity (T) data. Theories which assume uniform transmissivity predict a nearly linear relationship between T and Q/s. However, linear dependence is seldom observed in field studies. Since hydrogeologic studies usually require T data, many hydrogeologists use linear regression analysis of T versus Q/s data to estimate T values where only Q/s data are available. In this paper we use numerical models to investigate the effects of aquifer heterogeneity on the relationship between Q/s and T estimates. The simulations of hydraulic tests in heterogeneous media show that estimates of T derived using Jacob's method tend to their late‐time effective value much faster than Q/s values. The latter are found to be more dependent upon local transmissivities near the well. This explains why the regression parameters for T versus Q/s data depend on heterogeneity and the‘lateness’of the test period analyzed. Since this effect is more marked in high T zones than in low T zones, we conclude that natural aquifer heterogeneity can explain the convex deviation from linearity often observed in the field. A further result is that the geometric mean of T estimates, obtained from short and intermediate time pumping tests, seems to systematically underestimate effective T (Teff) of heterogeneous aquifers. In the studied simulation cases, the median of the T values or the arithmetic mean yield better estimates for Teff.
On the Use of Semi‐Logarithmic Plots for Baseflow Separation
Authors: Szilagyi Jozsef
Published: August 2005

Recession flow has long been considered a composite of exponential terms, where each exponential term represents a different source of water discharged from the watershed. The changing slopes in the semi‐logarithmic plot of the discharge have been considered indicative of the decreasing contribution of surface runoff and interflow to the discharge. The results of this analysis show that the changing slope in the recession plot can be the consequence of baseflow drainage. This can invalidate the semi‐logarithmic baseflow separation technique when applied to some hydrologic settings.
Predicted Retardations of Concentration Fronts Using a Mass‐Balance Approach
Authors: Stoessell Ronald K.
Published: August 2005

Regardless of the chemical controls for a pollutant on sediment sorption or release from an immiscible organic liquid, a simple mass balance can be used to predict the retardation of the ground water pollutant front moving downstream from a source of constant composition. The retardation is the ratio of the total molar difference of pollutant across the front to the molar difference in ground water. The mass‐balance approach presented here uses a flushing factor for the entire front. It is implicit in the commonly used retardation factor for pollutants undergoing linear sorption, but does not appear to have been used to predict retardation of concentration fronts involving nonlinear sorption or partitioning in hydrochemical studies. Yet the computations require only limited experimental or field data or the use of an algorithm describing sorption or partitioning of the pollutant. Close agreement was obtained between retardations predicted using the mass‐balance approach and those reported from numerical modeling and column experiments in the literature.
Natural Attenuation of BTEX Compounds: Model Development and Field‐Scale Application
Authors: Lu Guoping , Clement T. Prabhakar , Zheng Chunmiao , Wiedemeier Todd H.
Published: August 2005

Benzene, toluene, ethyl benzene and xylene (BTEX) dissolved into ground water and migrated from a light nonaqueous phase liquid (LNAPL) source in a sandy aquifer near a petroleum, oil, and lubricants (POL) facility at Hill Air Force Base (AFB), Utah. Field observations indicated that microbially mediated BTEX degradation using multiple terminal electron‐accepting processes including aerobic respiration, denitrification, Fe(III) reduction, sulfate reduction, and methanogenesis has occurred in the aquifer. To study the transport and transformation of dissolved BTEX compounds under natural conditions, a reactive flow and transport model incorporating biochemical multispecies interactions and BTEX was developed. The BTEX, oxygen, nitrate, Fe(II), sulfate, and methane plumes calculated by the model agree reasonably well with field observations. The first‐order biodegrada‐tion rate constants, estimated based on model calibration are 0.051, 0.031, 0.005, 0.004, and 0.002 day−1 for aerobic respiration, denitrification, Fe(III), sulfate reduction, and methanogenesis, respectively. The results of a sensitivity analysis show that the saturated aquifer thickness, hydraulic conductivity, and reaction rate constants are the most critical parameters controlling the natural attenuation of BTEX at this site. The hydraulic conductivity and aquifer thickness were found to be the key factors affecting the restoration of oxygen, nitrate, and sulfate after their interaction with the BTEX plume. The multispecies reactive transport modeling effort, describing BTEX degradation mediated by multiple electron‐accepting processes, represents one of the few attempts to date to quantify a complete sequence of natural attenuation processes with a detailed field data set. Because the case study is representative of many petroleum‐product contaminated sites, the results and insights obtained from this study are of general interest and relevance to other fuel‐hydrocarbon natural attenuation sites
Simple Probabilistic and Statistical Risk Calculations in an Aquifer
Authors: Şen Zekai
Published: August 2005

An objective methodology is proposed for assessing potential ground water yield on the basis of the randomness inherent in hydrogeological parameters as a result of uncertainty. Randomness is used here in the statistical sense to describe the ground water phenomenon on a regional basis. A systematic approach to selecting aquifer parameters is presented that leads to reliable ground water storage and subsurface flow estimates by incorporating a certain risk level. In addition, the averages of the specific ground water capacity and subsurface flow rate are calculated through a perturbation approach. This study demonstrates that cross‐correlations between basic hydrogeologic variables might underestimate or overestimate model parameter values.

Furthermore, the results of aquifer storage and subsurface flow estimates are obtained in the form of probability distribution functions. It is proved that the surface flow rates are presented by a log‐normal distribution. Contrary to conventional point estimate, a simple and objective methodology is proposed in order to obtain the most probable interval estimates. Accordingly, projections of likely future ground water storage and flow rates can be readily calculated.
Skin Localization at Wells Drilled in a Vertical Fault Zone
Authors: Bardenhagen Ingo
Published: August 2005

The efficiency of wells drilled into fractured zones that act as draining conduits in hard rock aquifers can be strongly reduced by skin effects. In such aquifers, skin effects (well losses) can appear in close proximity to the well and/or at the contact between fracture and formation. In this paper it is shown how to locate those skin conditions in the case of two wells that have been drilled in a single vertical fault with infinite conductivity.
Oscillatory Slug‐Test Data Sets: A Comparison of Two Methods
Authors: Weight Willis D. , Wittman Gregory P.
Published: August 2005

Slug tests performed in high transmissivity aquifers may exhibit underdamped inertial oscillatory behavior. Analytical methods for oscillatory data have been developed, but are mathematically intimidating. Spreadsheet modeling is helpful because ready adjustments to the equations required to match the well‐response data are quickly applied. A simplified presentation of the Kipp method for the practicing hydrogeologist is presented along with a spreadsheet model. A file containing the type curves is available over the Internet at http://www.mtech.edu from the Geological Engineering home page.

Examples from southwestern Montana showing a variety of damping responses and well completions are presented to show how to perform the analysis. Sixteen well responses were modeled using the van der Kamp and Kipp methods, and the outcomes were compared using the two methods with oscillatory responses. Generally, there is good agreement between the methods. Screened wells tend to show more uniform oscillatory behavior, although open‐hole completions can be analyzed with success. Other practical field applications and observations are presented.

The theory describing inertial effects has been constrained to confined aquifers, although all aquifers exhibit elastic behavior during the initial disturbance from pumping or instantaneous slug removal.
Estimating Transmissivity from the Water Level Fluctuations of a Sinusoidally Forced Well
Authors: Mehnert E. , Valocchi A.J. , Heidari M. , Kapoor S.G. , Kumar P.
Published: August 2005

The water levels in wells are known to fluctuate in response to earth tides and changes in atmospheric pressure. These water level fluctuations can be analyzed to estimate transmissivity (T). A new method to estimate transmissivity, which assumes that the atmospheric pressure varies in a sinusoidal fashion, is presented. Data analysis for this simplified method involves using a set of type curves and estimating the ratio of the amplitudes of the well response over the atmospheric pressure.

Type curves for this new method were generated based on a model for ground water flow between the well and aquifer developed by Cooper et al. (1965). Data analysis with this method confirmed these published results: (1) the amplitude ratio is a function of transmissivity, the well radius, and the frequency of the sinusoidal oscillation; and (2) the amplitude ratio is a weak function of storativity. Compared to other methods, the developed method involves simpler, more intuitive data analysis and allows shorter data sets to be analyzed.

The effect of noise on estimating the amplitude ratio was evaluated and found to be more significant at lower T. For aquifers with low T, noise was shown to mask the water level fluctuations induced by atmospheric pressure changes. In addition, reducing the length of the data series did not affect the estimate of T, but the variance of the estimate was higher for the shorter series of noisy data.
Characterizing a Sewage Plume Using the 3H‐3He Dating Technique
Authors: Shapiro Stephanie Dunkle , LeBlanc Denis , Schlosser Peter , Ludin Andrea
Published: August 2005

An extensive 3 H −3He study was performed to determine detailed characteristics of a regional flow system and a sewage plume over a distance of 4 km in a sand and gravel aquifer at Otis Air Base in Falmouth, Massachusetts. 3 H −3He ages increase with depth in individual piezometer clusters and with distance along flowpaths. However, the age gradient with depth (Δt/Δ z) is smaller in the plume than that in the regional waters, due to the intense recharge in the infiltration beds. The 1960s bomb peak of tritium in precipitation is archived longitudinally along a flowline through the main axis of the plume and vertically in individual piezometer clusters. On the eastern side of the sampling area, where water from Ashumet Pond forces plume water deeper into the flow system, 3 H −3He ages are young at depth because the 3 H −3He “clock” is reset due to outgassing of helium in the pond. A reconstruction of the tritium input functions for the regional and plume samples shows that there is no offset in the peak [3H]+[3Hetrit] concentrations for the plume and regional water, indicating that the water from supply wells for use on the base is young. The 3 H −3He ages and detergent concentrations in individual wells are consistent with the beginning of use of detergents and the time period when their concentrations in sewage would have been greatest. Ages and hydraulic properties calculated using the 3 H −3He data compare well with those from previous investigations and from particle‐tracking simulations.
Numerical Modeling of Drainage Schemes in Layered Formations: A Kuwait City Case Study
Authors: Székely Ferenc
Published: August 2005

A computer modeling technique is developed to simulate ground water drainage considering a heterogeneous, discontinuous, layered formation with pumping wells screened in several layers and drawdown‐induced recharge/discharge under complex urban hydrological conditions. A composite finite‐difference mesh is used to simulate both the more distant areas (coarse mesh) and the actual drainage site (fine mesh). The skin loss parameters of screens are involved to control the aquifer yield. A pilot drainage scheme is being operated in Kuwait City to eliminate the harmful effects of water table elevation. The scheme involves six multiscreened pumping wells and numerous observation wells. Calibration through computer simulation indicated that the transmissivity exhibits relatively little areal variation, whereas the leakance of partially cemented silty beds and lenses fluctuates significantly over the study area. This heterogeneity effect as well as the induced recharge from various man‐made sources have considerable impact on the drainage efficiency. Skin loss parameters of pumping wells affect the amplitude of drawdown in different aquifer units.
In Situ Redox Manipulation by Dithionite Injection: Intermediate‐Scale Laboratory Experiments
Authors: Istok J.D. , Amonette J.E. , Cole C.R. , Fruchter J.S. , Humphrey M.D. , Szecsody J.E. , Teel S.S. , Vermeul V.R. , Williams M.D. , Yabusaki S.B.
Published: August 2005

The goal of in situ redox manipulation (ISRM) is to create a permeable treatment zone capable of removing redox‐sensitive contaminants from ground water. The objective of this study was to evaluate the effectiveness of one promising ISRM technology: chemical reduction of aquifer sediments by sodium dithionite (Na 2S2O4) injection. The technology was evaluated in intermediate‐scale laboratory experiments designed to investigate the kinetics of Fe(III)‐reduction and dithionite‐disproportionation reactions in a radial flow field over similar transport distances (∼ 7 m) and time scales (∼ 72 hours) as those used in a field trial for remediation of chromate contaminated ground water at the Department of Energy Hanford site in Washington state. Four hundred liters (∼ 1 pore volume) of 0.1 M Na2S2O4 in a 0.4 M K2CO3/0.04 M KHCO3 buffer were injected at a rate scaled to field values. Dithionite breakthrough curves at sampling ports were approximately described by the advection‐dispersion equation with a two‐part reaction model containing first‐order rate coefficients for dithionite reaction with sediment Fe(III) (k1= 0.13 hr −1) and dithionite disproportionation (k2= 0.05 hr −1). Analyses on sediment cores collected from the physical model indicated that substantial Fe(III) was reduced to Fe(II) and that the dithionite‐treated sediment was capable of removing 2 mg/L chromate from ∼ 100 column pore volumes of synthetic ground water. These results indicate that the ISRM technology is a potentially feasible method for removing chromate from Hanford ground water.
TEM Investigations of Aquifers in the Southwest Coast of Taiwan
Authors: Chen Chow‐Son
Published: August 2005

A large data set of more than 330 transient electromagnetic (TEM) soundings was collected in the southwest coast of Taiwan. The aims of the study were (1) to define the shallow geoelectrical structure, and (2) locate large bodies of ground water by integrating electrical anomalies and data from more than 140 wells. A one‐dimensional, smooth‐model inversion of TEM data was used, and trend surface analysis of those results was employed to refine the results for interpretation. A resistive zone (15 to 160 ohm‐m resistivity) at elevations 20 to 120 m below sea level in the northern part of the survey area is interpreted as fresh water saturated sand or gravel. The southern part has extremely low resistivity, less than 1 ohm‐m, indicating a widespread sea water intrusion. The divide between these two parts is located at about the middle of the study area. This study demonstrates that TEM investigation can help resolve ambiguities, even in an area with many wells, and thereby increase the likelihood of success in ground water exploration.
Scale Dependency of Hydraulic Conductivity in Heterogeneous Media
Authors: Schulze‐Makuch Dirk , Carlson Douglas A. , Cherkauer Douglas S. , Malik Peter
Published: August 2005

Various types of sediments and rocks were analyzed for the relationship between hydraulic conductivity (K) and scale of measurement No variations of K with scale were observed for homogeneous media such as quartz‐arenites (quartz sandstones). However, hydraulic conductivity increased with scale of measurement in heterogeneous media. The scaling behavior can be described with the equation K = c (V)m, where c is a parameter characteristic of the geological medium that relates to geological variables such as average pore size and pore interconnectivity in porous media, and probably fracture opening and fracture interconnectivity in fractured media. V is the volume of tested material (used as scale measure), and m is the exponent of the relationship (slope of the line on a log‐log plot). The value of the exponent depends on the type or types of flow present. Porous flow media have an exponent of 0.5, multiple flow media an exponent between 0.5 and 1.0, and fracture and conduit flow controlled media an exponent of about 1.0. The more dominant fracture/conduit flow is relative to porous flow, the closer the exponent is to 1.0. K increases with scale up to a rock volume after which the aquifer approaches the properties of an equivalent homogeneous medium and K remains constant with scale. This volume (upper bound of the relationship) is related to the degree of heterogeneity in a medium. It is at a much larger scale in karstic media (if encountered at all) than in nonkarstic and more homogeneous media. Both confined and unconfined aquifers exhibit a similar scale dependence.
Sensitivity Analysis of Three‐Dimensional Steady‐State and Transient Spray Irrigation Models
Authors: Heebner David , Toran Laura
Published: August 2005

We conducted a sensitivity analysis of ground water flow on a spray irrigation site to evaluate key parameters affecting use of spray irrigation for sewage disposal. Spray irrigation is considered successful when water recharges the aquifer (low runoff) and mounding of the water table is below set limits. It can be difficult to predict success and allowable irrigation rates given uncertainty in parameters.

The U.S. Geological Survey (USGS) code MODFLOW was used to compare two‐ and three‐dimensional steady‐state simulations, and three‐dimensional transient simulations. We compared Latin hypercube sampling and systematic adjustment of parameters as methods of sensitivity analysis, transient versus steady‐state sensitivity, and variations in layer configurations. These models were not calibrated because they were created during planning stages before field data became available.

The Latin hypercube sampling effectively tested a large number of parameters simultaneously, and can be used as a screening tool for systematic adjustment of parameters. Transient and steady‐state simulations predicted similar sensitivity to hydraulic conductivity, but steady‐state modeling produced higher peak heads and discharges. Again, we recommend that the steady‐state simulations could be used as a screening tool to evaluate trends. Transient modeling is likely to provide more realistic predictions of heads and discharges. When different layer configurations were compared, we found that two‐dimensional modeling overpredicted mounding and drain discharge because flow in a permeable soil layer is neglected. Two‐dimensional modeling would not be effective for predicting allowable irrigation rates. These results provide modelers an indication of the effects of model simplification, which can be used to plan modeling strategies.
Analytic Determination of Hydrocarbon Transmissivity from Baildown Tests
Authors: Huntley David
Published: August 2005

Hydrocarbon baildown tests involve the rapid removal of floating hydrocarbon from an observation or production well, followed by monitoring the rate of recovery of both the oil/air and oil/water interfaces. This test has been used erroneously for several years to calculate the “true thickness” of hydrocarbon in the adjacent formation. More recent analysis of hydrocarbon distribution by Farr et al. (1990), Lenhard and Parker (1990), Huntley et al. (1994), and others have shown that, under vertical equilibrium conditions, there is no thickness exaggeration of hydrocarbon in a monitoring well, though there is a significant volume exaggeration. This body of work can be used to demonstrate that the calculation of a “true hydrocarbon thickness” using a baildown test has no basis in theory. The same body of work, however, also demonstrates that hydrocarbon saturations are typically much less than one, and are often below 0.5. Because the relative permeability decreases as hydrocarbon saturation decreases, the effective conductivity and mobility of the hydrocarbon is much less than that of water, even ignoring the effects of increased viscosity and decreased density.

It is important to evaluate this decreased mobility of hydrocarbon due to partial pore saturation, as it has substantial impacts on both risk and remediation. This paper presents two analytic approaches to the analysis of hydrocarbon baildown test results to determine hydrocarbon transmissivity. The first approach is based on a modification of the Bouwer and Rice (1976) analysis of slug withdrawal test data. The second approach is based on a modification of Jacob and Lohman's (1952) constant drawdown—variable discharge aquifer test approach. The first approach can be applied only when the effective water transmissivity across the screened interval to water is much greater than the effective hydrocarbon transmissivity. When this condition is met, the two approaches give effectively identical results.
Modeling of Ground Water Flow to Adits
Authors: Zhang Beiyan , Lerner David N.
Published: August 2005

Many of the large ground water sources in the Chalk Aquifer have adits, horizontal tunnels below the ground water table, which are connected to pumped wells. The flow in an adit may be pipe or open channel flow. Adits in the United Kingdom are normally full of water, so that the adit flow is pressurized. Darcy's formula is not applicable to the adit, and conventional ground water models are inappropriate to model the aquifer‐adit system. The U.S. Geological Survey (USGS) model BRANCH simulates one‐dimensional unsteady, nonuniform, multiple‐branch interconnected open channel flow. MODBRNCH incorporates BRANCH into MODFLOW simulating open channel and aquifer interaction using deterministic responses of both systems. An aquifer‐adit model can be created by two steps. First, an integrated surface‐ground water model (MODBRNCH) enables open channel flow to be simulated. Second, introducing a fictitious narrow slot (Preissmann slot) above the adit allows pipe flow to be simulated by open channel flow equations. The slot does not affect the adit cross‐section area, and the water level in the slot represents the pressurized adit head, which can be used by MODBRNCH to calculate the water exchange between aquifer and adit according to their head difference. The approach has been tested on the Wilmington public supply source in southeast England.
Mechanisms Controlling the Distribution and Transport of 14C in a Clay‐Rich Till Aquitard
Authors: Wassenaar L.I. , Hendry M.J.
Published: August 2005

The distribution and transport of radiocarbon in dissolved inorganic carbon (DIC) in a thick clay‐rich till aquitard was investigated. Radiocarbon activities decreased with depth from 109 percent modern carbon (pmC) in the surficial oxidized and fractured zone (at 1.5 m BG) through the thick underlying unoxidized and nonfractured aquitard, reaching minimum values of between 3 and 5 pmC at between 29 and 37 m depth. Numerical simulations of diffusive transport of radiocarbon from the oxidized zone into the unoxidized zone during the Holocene closely approximated the measured values and showed that diffusive transport and radioactive decay fully described the transport of radiocarbon in the aquitard. The lack of geochemical reactions involving DIC permitted the age of DIC in the pore water deposited with the till (between 29 and 37 m BG) to be estimated at 25 to 31 ka BP. Results of this study showed that the use of radiocarbon in the study of solute transport in aquitards provides age information that cannot be obtained from natural conservative tracers such δ2H and δ18. Our results also showed that the application of conventional radiocarbon age‐dating models based on piston‐flow transport assumptions, is inappropriate in diffusion‐dominated environments and considerably overestimates the age of the ground water.
Transmissivity Estimates from Well Hydrographs in Karst and Fractured Aquifers
Authors: Powers Jefferey G. , Shevenell Lisa
Published: August 2005

Hydrograph recessions from rainfall events have been previously analyzed for discharge at springs and streams; however, relatively little quantitative research has been conducted with regard to hydrograph analysis of recessions from monitoring wells screened in karst aquifers. In previous work, a quantitative hydrograph analysis technique has been proposed from which matrix transmissivity (i.e., transmissivity of intergranular porosity) and specific yields of matrix, fracture, and conduit components of the aquifer may be determined from well hydrographs. The technique has yielded realistic results at three sites tested by the authors (Y‐12, Oak Ridge, Tennessee; Crane, Indiana; and Fort Campbell, Kentucky). Observed field data, as well as theoretical considerations, show that karst well hydrographs are valid indicators of hydraulic properties of the associated karst aquifers. Results show matrix transmissivity (T) values to be in good agreement with values calculated using more traditional parameter estimation techniques, such as aquifer pumping tests and slug tests in matrix dominated wells. While the hydrograph analysis technique shows promise for obtaining reliable estimates of karst aquifer T with a simple, relatively inexpensive and passive method, the utility of the technique is limited in its application depending on site‐specific hydrologic conditions, which include shallow, submerged conduit systems located in areas with sufficient rainfall for water levels to respond to precipitation events.
Relation of Pathways and Transit Times of Recharge Water to Nitrate Concentrations Using Stable Isotopes.
Authors: Landon Matthew K. , Delin Geoffrey N. , Komor Stephen C. , Regan Charles P.
Published: August 2005

Oxygen and hydrogen stable isotope values of precipitation, irrigation water, soil water, and ground water were used with soil‐moisture contents and water levels to estimate transit times and pathways of recharge water in the unsaturated zone of a sand and gravel aquifer. Nitrate‐nitrogen (nitrate) concentrations in ground water were also measured to assess their relation to seasonal recharge. Stable isotope values indicated that recharge water usually had a transit time through the unsaturated zone of several weeks to months. However, wetting fronts usually moved through the unsaturated zone in hours to weeks. The much slower transit of isotopic signals than that of wetting fronts indicates that recharge was predominantly composed of older soil water that was displaced downward by more recent infiltrating water. Comparison of observed and simulated isotopic values from pure‐piston flow and mixing‐cell water and isotope mass balance models indicates that soil water isotopic values were usually highly mixed. Thus, movement of recharge water did not occur following a pure piston‐flow displacement model but rather follows a hydrid model involving displacement of mixed older soil water with new infiltration water. An exception to this model occurred in a topographic depression, where movement of water along prelerential flowpaths to the water table occurred within hours to days following spring thaw as result of depression‐focused infiltration of snow melt. In an adjacent upland area, recharge of snow melt occurred one to two months later. Increases in nitrate concentrations at the water table during April‐May 1993 and 1994 in a topographic low‐land within a cora field were related to recharge of water that had infiltrated the previous summer and was displaced from the unsaturated zone by spring infiltration. Increases in nitrate concentrations also occurred during July‐August 1994 in response to recharge of water that infiltrated during May‐August 1994. These results indicate that the largest ground water nitrate concentrations were associated with recharge of water that infiltrated into the soil during May‐August, when most nitrogen fertilizer was applied.
On Fracture Structure and Preferential Flow in Unsaturated Chalk
Authors: Dahan Ofer , Nativ Ronit , Adar Eilon M. , Berkowitz Brian , WeisbrodB Noam
Published: August 2005

The mechanisms controlling fluid flow through fractures intersecting chalk in the vadose zone were studied through water percolation experiments in natural discrete fractures and by close examination of the inner Structure of fracture voids. The percolation experiments showed that the flow is focused in dissolution channels along the fracture plane, and that fluxes and flow trajectories within that net vary in both time and space. The locations of the dissolution channels, the main potential flow paths within the fracture plane, were generally associated with fracture Intersections. The flow through these channels was governed primarily by the mineralogical composition of the filling material and the inner structure of the fracture voids. Salt dissolution, solid‐particle migration, and clay swelling were found to be the predominant processes controlling flow through the dissolution channels. These physical changes in the structure of the filling material in the dissolution channels accounted for the observed unstable flow behavior. Our results suggest that models aimed at simulating water percolation through fractures in unsaturated chalk should consider the mapping of fracture intersections in addition to the commonly used mapping of fracture lineaments. Moreover, the detailed characterization of fracture apertures may not be the key parameter determining fracture flow, because in such formations the flow is controlled primarily by filling material. These materials undergo significant physical variations during wetting and drying cycles.
Salt Accumulation and Flushing in Unsaturated Fractures in an Arid Environment
Authors: Weisbrod Noam , Nativh Ronit , Adar Kilon M. , Ronend Daniel
Published: August 2005

Salt precipitation and dissolution in fractured chalk was investigated through flow experiments in both the laboratory and the field (in the Negev Desert, Israel). In the laboratory, a flow‐cell experimental setup was used to simulate intermittent Infiltration and drying periods along coated and uncoated fracture surfaces. Three Infiltration events, lasting 24, 8, and 8 hours, were carried out with long drying periods of 82 and 44 days between them. In the field, two flow experiments were conducted through a single fracture. Water was percolated from land surface through a discrete fracture into a compartmental sampler. The duration of the two field experiments was 5 and 119 hours, respectively, with a drying period of seven months between them. The percolating outflows in both the laboratory and field experiments were collected and analyzed for electrical conductivity.

The electrical conductivity of the outflows and its temporal variations during the experiments suggest that evaporation triggered capillary forces that mobilized water and solutes from the bulk matrix toward the fracture surface. As the water evaporated, the solutes precipitated on the fracture surface. The precipitated soluble salts were dissolved during the first few hours of the sub‐sequent flow event that followed the drying period. This mechanism, enhanced in arid environments, may result in the transport of salts that accumulate in the upper few meters of the unsaturated zone to the ground water, bypassing the low‐permeability matrix. The calculated amount of solutes transported by back‐diffusion from the chalk matrix (190 g) could not account for the large (1200 g) amount of salts released from the fracture void during the experiments.
A Tracer Method to Determine Hydraulic Conductivity and Effective Porosity of Saturated Clays Under Low Gradients
Authors: Yeh Yi‐Jang , Lee Cheng‐Haw , Chen Shih‐Tsu
Published: August 2005

Based on the column tracer test, we developed a research method to determine the hydraulic conductivity and effective porosity of saturated clays under low hydraulic gradients or small flow rates. Derived from Darcy's law and the solute transport equation, this method evaluates the hydraulic conductivity through measuring solute concentrations rather than by measuring flow rates. And the effective porosity is determined by applying an analytical solution of the one‐dimensional uniform flow equation.

Two types of experimental data drawn from a review of the literature and four sorts of accuracy test data carried out in the laboratory are used to examine the proposed method. The reproducibilities of accuracy tests for hydraulic conductivity determination indicate a consistency within a 5.5% error margin. The experimental results further indicate that hydraulic conductivities determined using the tracer method are more precise than those from the conventional flowmeter method. In addition, with the assistance of the proposed tracer method, we argue that the effective porosities may be overestimated as shown in the drawn example cases due to mistaking total pore volumes as the effective pores. The results of the accuracy tests further indicate that the effective porosities of saturated clay specimens are significantly smaller than the total porosities, at least when the samples are allowed to swell freely.