bluuebunny/crossref_metadata_2025_split huggingface dataset:
Wellbore Skin Effect in Slug‐Test Data Analysis for Low‐Permeability Geologic Materials
Authors: Yang Y. Jeffrey , Gates Todd M. 
Published: August 2005

AbstractThe wellbore skin effect on slug‐test results was analyzed using numerical simulation and field tests for a well at progressive stages of development. The numerical simulation is based on a composite flow model that incorporates a zone of disturbed formation surrounding the wellbore. Field tests were performed on a water‐bearing clayey silt formation at a ground‐water remediation site in Wisconsin. Based on the numerical simulation, the radius of investigation was examined. The results show that the early‐time and late‐time data reflect ground‐water flow in the wellbore skin and undisturbed formation, respectively.Both the numerically simulated and the field slug‐test data define a downward concave curve on a semilog plot of time versus the logarithm of dimensionless head. For the Hvorslev (1951) and Bouwer and Rice (1976) methods, the late‐time segment of the simulated data yields estimates of hydraulic conductivity close to the value defined in the flow model. When a wellbore skin exists, the data curve in a plot of the logarithm of time versus the dimensionless head is shifted horizontally along the time axis. This shift leads to an inaccurate determination of hydraulic conductivity based on the Cooper et al. (1967) method. In the plots of time versus dimensionless head derivatives, the data curve geometry depends on the hydraulic properties of the wellbore skin. Consequently, the wellbore skin effect can be identified and eliminated using derivative‐based type curve methods.For low‐permeability materials, the effect of wellbore skin on estimates of hydraulic conductivity can be minimized through use of the late‐time data. However, proper well installation and development appears to be the most effective and practical solution.
Delineation of Preferred‐Flow Paths by Response to Transient Lagoon Exfiltration
Authors: Donovan Joseph J. , Frysinger Kevin W. 
Published: August 2005

AbstractObservations of hydraulic head response to lagoon leakage into an extremely heterogeneous aquifer allow qualitative identification of spatial patterns in heterogeneity. In a 150–hour leakage experiment over such an aquifer, the fluid mass balance was estimated using a stage‐fluctuation model while transient response was observed in downgradient wells and springs. Based on timing with respect to pond leakage, water‐level response at different locations within the aquifer may be identified as either within or outside the influence of preferred‐flow paths (PFPs). Flows at springs 150–200 m from the pond increased by 20–60% above baseline flows, showing initial increase within ca. 40 hours. Synoptic water‐level rise patterns observed during the test describe enlargement of a recharge mound from beneath the pond and suggest the spatial pattern of PFPs to > 150 m distant. An analytical model assuming homogeneity (Hantush, 1967), applied to water‐level response for wells > 30 m from the pond, overestimated K by > 1 order of magnitude in comparison to slug‐test estimates. This discrepancy is attributed to the inferred presence of linearly continuous heterogeneities causing preferential flow. Large‐scale hydraulic tests such as this sample the spatial structure for conductive pathways and may be of utility in parameterization of numerical flow models. In contrast, slug‐test results give local parameter estimates, but more limited information on PFP distribution.
Multivariate Statistical Analysis of Environmental Monitoring Data
Authors: Ross D. Lauren 
Published: August 2005

AbstractEPA requires statistical procedures to determine whether soil or ground water adjacent to or below waste units is contaminated. These statistical procedures are often based on comparisons between two sets of data: one representing background conditions, and one representing site conditions. Since statistical requirements were originally promulgated in the 1980s, EPA has made several improvements and modifications. There are, however, problems which remain. One problem is that the regulations do not require a minimum probability that contaminated sites will be correctly identified. Another problem is that the effect of testing several correlated constituents on the probable outcome of the statistical tests has not been quantified.Results from computer simulations to determine power functions for realistic monitoring situations are presented here. Power functions for two different statistical procedures: the Student's t‐test, and the multivariate Hotelling's T2test, are compared. The comparisons indicate that the multivariate test is often more powerful when the tests are applied with significance levels to control the probability of falsely identifying clean sites as contaminated. This program could also be used to verify that statistical procedures achieve some minimum power standard at a regulated waste unit.
Cost‐Effective Monitoring Strategies to Estimate Mean Water Table Depth
Authors: Mew H.E. , Jr M.A. Medina , Heath R.C. , Reckhow K.H. , Jacobs T.L. 
Published: August 2005

AbstractCost‐effective monitoring strategies exist for estimating mean depth to the water table and for characterizing fluctuations about that mean. These strategies can also be used to evaluate the utility of sparse water level data sets for making such estimates. This type of information is needed to design or reduce costs of observation well networks, to characterize regional water‐table fluctuations for use in maps or models, and to calibrate ground‐water models using hydraulic head estimates.The study is based on a statistical analysis of hourly water levels measured at observation wells by the U. S. Geological Survey (USGS) over a five‐year period. Using Monte Carlo simulation techniques, sampling results for different monitoring strategies were compared with parameters of the underlying distribution of hourly water levels. Monthly, bimonthly, quarterly, and even tri annual water‐level monitoring was found to effectively estimate mean water table depth and standard deviation to within 30 cm. Quarterly and tri annual sampling were found more cost‐effective than monthly observations, and electronic monitoring more effective than manual measurements for sampling made more frequently than monthly. Results have broad application for studies involving surficial aquifers in humid regions of the world where annual rainfall exceeds 50 cm and is reasonably distributed throughout the year. Governmental agencies and universities have a wealth of water table monitoring records, collected either through ambient monitoring or as part of a permit requirement or study. These sparse data sets hold the promise of significantly increasing the hydro geologic knowledge of water table behavior in many regions of the world.
Quantifying Macropore Recharge: Examples from a Semi‐Arid Area
Authors: Wood Warren W. , Rainwater Ken A. , Thompson David B. 
Published: August 2005

AbstractThe purpose of this paper is to illustrate the significantly increased resolution of determining macropore recharge by combining physical, chemical, and isotopic methods of analysis. Techniques for quantifying macropore recharge were developed for both small‐scale (1 to 10 km2) and regional‐scale areas in arid and semi‐arid areas. The Southern High Plains region of Texas and New Mexico was used as a representative field site to test these methods. Macropore recharge in small‐scale areas is considered to be the difference between total recharge through floors of topographically closed basins and interstitial recharge through the same area. On the regional scale, macropore recharge was considered to be the difference between regional average annual recharge and interstitial recharge measured in the unsaturated zone. Stable isotopic composition of ground water and precipitation was used as an independent estimate of macropore recharge on the regional scale. Results of this analysis suggest that in the Southern High Plains recharge flux through macro pores is between 60 and 80 percent of the total 11 mm/y. Between 15 and 35 percent of the recharge occurs by interstitial recharge through the basin floors. Approximately 5 percent of the total recharge occurs as either interstitial or matrix recharge between the basin floors, representing approximately 95 percent of the area. The approach is applicable to other arid and semi‐arid areas that focus rainfall into depressions or valleys.
Development of a Preliminary Ground Water Flow Model for Water Resources Management in the Pingtung Plain, Taiwan
Authors: Ting Cheh‐Shyh , Zhou Yangxiao , Vries J.J.de , Simmers I. 
Published: August 2005

AbstractPingtung Plain is formed by Quaternary alluvial fan material from the three main rivers: Kaoping, Tungkang and Linpien. Ground water is the major water supply source on the plain. This is principally extracted from two aquifers. The natural ground water source is derived mainly from direct rainfall percolation and infiltration from the three rivers, with their catchments lying partly outside the plain. Rainfall characteristics are therefore the main factors controlling water resources availability. Pingtung Plain is an important primary production area for southern Taiwan, the comparatively warm climate allowing a long growing season, diversified cropping and the rearing of aquacultural produces. Approximately 75 percent of irrigation and domestic water supplies are derived from ground water. A water balance for the entire plain indicates that ground water resources, under optimized management, are sufficient to meet the existing multi‐purpose uses.Development of a hydrogeological conceptual model is the first phase of a numerical ground water flow simulation. Preliminary results are encouraging, with the final simulations affording better insight to the hydraulic behavior of the aquifer system. Data input requirements for model operation fall into three categories: hydrological stresses, hydrogeological parameters and boundary conditions.After the model is built, the normal numerical modeling process requires significant calibration and sensitivity analyses for the hydrogeological parameters and stresses which are the most sensitive, but the least well defined.A well‐calibrated simulation model can lead to a reliable and realistic management model. With this in mind, the calibration processes detailed are presented, and these data are introduced as initial values in the calibration process.
How Good Are Estimates of Transmissivity from Slug Tests in Fractured Rock?
Authors: Shapiro Allen M. , Hsieh Paul A. 
Published: August 2005

AbstractSlug tests in fractured rock usually are interpreted with models that assume homogeneous formation properties, even though hydraulic properties of fractures can vary by many orders of magnitude over the length of boreholes. To investigate the impact of heterogeneity on the interpretation of slug tests in fractured rock, slug tests were conducted over large intervals of boreholes in crystalline rock in central New Hampshire, and interpreted using a homogeneous model. The results of the slug tests were then compared with estimates of transmissivity from fluid‐injection tests conducted over shorter intervals in the same boreholes. The fluid‐injection tests showed transmissivity to vary more than six orders of magnitude over the length of the boreholes; however, the sum of the transmissivities from the fluid‐injection tests were within an order of magnitude of the transmissivity estimated from the slug tests. Although the two estimates of transmissivity were within an order of magnitude of each other, the water level responses during the slug tests did not exactly match the responses predicted by the homogeneous model. To investigate the effect of heterogeneity on water level responses during slug tests, a Laplace‐transform solution was developed for slug tests conducted in boreholes containing multiple fractures with hydraulic properties that vary over the length of the borehole. A comparison of this solution with the homogeneous model shows no difference between the shape of water level responses in a homogeneous formation and a (layered) heterogeneous formation. Furthermore, the transmissivity estimated using a homogeneous model is within an order of magnitude of the prescribed transmissivity in the heterogeneous model. Thus, differences between responses predicted from a homogeneous model and measured water levels during slug tests can be attributed to phenomena such as nonradial flow in the vicinity of the borehole, and not heterogeneous hydraulic properties over the length of the borehole. The experimental results of this investigation show that even when conditions such as nonradial flow are present in the vicinity of the borehole, interpretations of slug tests using a homogeneous model provided order‐of‐magnitude estimates of transmissivity in the crystalline rock terrane under consideration.
How Uncertain Is Our Estimate of a Wellhead Protection Zone?
Authors: Evers Sarah , Lerner David N. 
Published: August 2005

AbstractProtection zones for wells are defined through the use of pathline tracing in ground water flow models. Traditional approaches to ground water flow modeling focus on obtaining a single best model, occasionally supplemented by the use of sensitivity analyses. In most situations this approach is inappropriate because the ground water flow model is so poorly determined that a variety of different boundary conditions and parameter values would give similar predictions of heads. However, the range of feasible models may well give radically different predictions of the variable for which the model has been built. For example, the area, orientation, or many other descriptive variables of a catchment may be much more sensitive to the parameters used in a ground water flow model than are the values of head commonly used to calibrate the model.This paper outlines a procedure to determine the range of predictions of catchments which would arise from alternative calibrations of a model. The range of catchments is used to identify zones of certainty and uncertainty, leading to alternative definitions of the protection zone for differing purposes. An example is presented, based on Bestwood Pumping Station, Nottinghamshire, which is located in the Triassic Sandstone Aquifer of the northern East Midlands of the United Kingdom. A trial and error calibration of a ground water flow model is used to determine an acceptable level for automatic calibration in the subsequent step. Only 12 measurements of ground water head were available, and these were used to determine the fit of a variety of model structures. A search routine revealed a range of feasible models and superposition of their catchments delineated the zones of certainty and uncertainty, which had areas of 7.4 and 15.2 km2, respectively.
Ground Water Age and Nitrate Distribution Within a Glacial Aquifer Beneath a Thick Unsaturated Zone
Authors: Johnston C.T. , Cook P.G. , Frape S.K. , Plummer L.N. , Busenberg E. , Blackport RJ. 
Published: August 2005

AbstractThe impact on ground water quality from increasing fertilizer application rates over the past 40 years is evaluated within a glacial aquifer system beneath a thick unsaturated zone. Ground water ages within the aquifer could not be accurately determined from the measured distribution of 3H and as a result, chlorofluorocarbon (CFC) and 3H/3He dating techniques were applied. Beneath a 25 m thick unsaturated zone, ground water ages based on CFC‐11 concentrations were greater than 3H/3He ground water ages by 6 to 10 years, due to the time lag associated with the diffusion of CFCs through the unsaturated zone. Using the corrected CFC‐11 and 3H/3He ground water ages and the estimated travel time of 3H within the unsaturated zone, the approximate position of ground water recharged since the mid‐1960s was determined. Nitrate concentrations within post mid‐1960s recharge were generally elevated and near or above the drinking water limit of 10 mg‐N/L. In comparison, pre mid‐1960s recharge had nitrate concentrations <2.5 mg‐N/L. The elevated NO3− concentrations in post mid‐1960s recharge are attributed mainly to increasing fertilizer application rates between 1970 and the mid‐ to late 1980s. Anaerobic conditions suitable for denitrifkation are present within pre mid‐1960s recharge indicating that removal of DO is a slow process taking tens of years. Over the next 10 to 20 years, nitrate concentrations at municipal well fields that are currently capturing aerobic ground water recharged near the mid‐1960s are expected to increase because of the higher fertilizer application rates beginning in the 1970s and 1980s.
Drawdowns in Coupled Aquifers with Confining Unit Storage and ET Reduction
Authors: Denis Roberto E. , Motz Louis H. 
Published: August 2005

AbstractThis paper presents a nonsteady coupled aquifer solution in which transient and steady‐state drawdowns due to pumping can be calculated in one or both of two aquifers separated by a semipermeable confining unit in a confined aquifer system. The water pumped is obtained from storage in the aquifers and the confining unit, and a reduction in evapotranspiration (ET) due to a decline in the water table. This solution is different from existing analytical solutions in that it includes both confining unit storage and a drawdown‐dependent ET reduction term. A Laplace space solution obtained for the differential equations subject to the initial and boundary conditions is inverted to the time domain using the Stehfest numerical algorithm. The presence of the ET reduction term in the solution allows the drawdowns to reach a steady‐state condition, and the presence of the confining unit storage term delays the response of the aquifer system to pumping, particularly in the unpumped aquifer. Use of the solution to calculate drawdowns for a representative field case is illustrated, along with verification of the solution using a numerical ground water flow model.
Conducting and Interpreting Slug Tests in Monitoring Wells with Partially Submerged Screens
Authors: Binkhorst Gordon K. , Robbins Gary A. 
Published: August 2005

AbstractConducting and interpreting slug tests in wells with screen sections and sand packs that span the water table are severely complicated by sand pack drainage and resaturation. Sand pack drainage greatly reduces the actual head difference between the well and the formation. Resaturation of the drained sand pack must be properly accounted for, or the formation hydraulic conductivity will be underestimated. The magnitude of error is a function of the well geometry, sand pack properties, and the model chosen to interpret the data. A method has been developed to correct for sand pack resaturation by calculating the specific yield of a drained sand pack using the early recharge data of a slug test. Slug tests were conducted in one well in which the water table varied over time, creating both partially and fully submerged screen and sand pack conditions for comparative testing. The mid‐time, log‐linear portion of slug test data was corrected using the calculated specific yield of the drained sand pack to yield essentially the same hydraulic conductivity value as the fully submerged screen results. Not accounting for sand pack resaturation would have yielded a hydrauli c conductivity value that was lower than the submerged screen results by a factor of seven.
Utilization of Intrinsic Boron Isotopes as Co‐Migrating Tracers for Identifying Potential Nitrate Contamination Sources
Authors: Leenhouts James M. , Basset R.L. , III T. Maddock 
Published: August 2005

AbstractThe stable isotopes of the conservative element boron, 11 B and 10 B, have been employed as co‐migrating isotopic tracers to trace potential sources of nitrate observed in ground water pumped from a large capacity 0.167 m 3/s irrigation well in the Avra Valley of southeastern Arizona. The isotopic ratios provided an identifying signature for two nitrogen carrying source waters: municipal waste water and agricultural return flow. Additional chemical parameters were also examined to corroborate the isotopic indications.Boron isotopes provided a superior delineation of mixing processes in the system compared to the general inorganic chemical parameters. Findings of this investigation indicate that the water pumped by the study well at the beginning of the 1993 irrigation season was composed of a mixture of approximately 25 % municipal waste water and 75 % background ground water. As the irrigation season progressed, an increasing proportion of water was contributed by irrigation return flow from neighboring agricultural fields.
Predicting the Probability Distribution of Yield from Multiple Boreholes in Crystalline Bedrock
Authors: Banks David 
Published: August 2005

AbstractIn many hard rock lithologies, the distribution of borehole yields can often be regarded as being derived from a log‐normally distributed population, characterized by the median yield (m1), and standard deviation of In‐transformed values (σ1. By means of numerical techniques using standard spreadsheet software, the distribution of total yield from two or more boreholes can be calculated, assuming that the yields of the boreholes are statistically independent variables and that there is no significant hydraulic interference between boreholes. The results are not necessarily obvious: (1) the median total yield for a group of y noninterfering boreholes (my) is greater than y.mt. The ratio my/y.m1 increases with increasing y and with increasing σ1; and (2) the standard deviation σy of In‐transformed yield distributions for water systems consisting of y multiple boreholes is less than the standard deviation σ1 of the In‐transformed yield distribution for single boreholes. The standard deviation decreases as the number of boreholes in the group increases.
Simplified Method of “Push‐Pull” Test Data Analysis for Determining In Situ Reaction Rate Coefficients
Authors: Haggerty R. , Schroth M.H. , Istok J.D. 
Published: August 2005

AbstractThe single‐well, “push‐pull” test method is useful for obtaining information on a wide variety of aquifer physical, chemical, and microbiological characteristics. A push‐pull test consists of the pulse‐type injection of a prepared test solution into a single monitoring well followed by the extraction of the test solution/ground water mixture from the same well. The test solution contains a conservative tracer and one or more reactants selected to investigate a particular process. During the extraction phase, the concentrations of tracer, reactants, and possible reaction products are measured to obtain breakthrough curves for all solutes. This paper presents a simplified method of data analysis that can be used to estimate a first‐order reaction rate coefficient from these breakthrough curves. Rate coefficients are obtained by fitting a regression line to a plot of normalized concentrations versus elapsed time, requiring no knowledge of aquifer porosity, dispersivity, or hydraulic conductivity. A semi‐analytical solution to the advective‐dispersion equation is derived and used in a sensitivity analysis to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push‐pull tests in a homogeneous, confined aquifer with a fully‐penetrating injection/extraction well and varying porosity, dispersivity, test duration, and reaction rate. A numerical flow and transport code (SUTRA) is used to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push‐pull tests in a heterogeneous, unconfined aquifer with a partially penetrating well. In all cases the simplified method provides accurate estimates of reaction rate coefficients; estimation errors ranged from 0.1 to 8.9% with most errors less than 5%. In an example application, the method is used to estimate reaction rate coefficients for a microbiological process (denitrification) using data from field push‐pull tests performed in a petroleum‐contaminated, unconfined alluvial aquifer.