ANALYSIS OF PUMPING TESTS: SIGNIFICANCE OF WELL DIAMETER,PARTIAL PENETRATION,AND NOISE1

ABSTRACT: The nonlinear least squares (NLS) method was applied to pumping and recovery aquifer test data in confined and unconfined aquifers with finite diameter and partially penetrating pumping wells, and with partially penetrating piezometers or observation wells. It was demonstrated that noiseless and moderately noisy drawdown data from observation points located less than two saturated thicknesses of the aquifer from the pumping well produced an exact or acceptable set of parameters when the diameter of the pumping well was included in the analysis. The accuracy of the estimated parameters, particularly that of specific storage, decreased with increases in the noise level in the observed drawdown data. With consideration of the well radii, the noiseless drawdown data from the pumping well in an unconfined aquifer produced good estimates of horizontal and vertical hydraulic conductivities and specific yield, but the estimated specific storage was unacceptable. When noisy data from the pumping well were used, an acceptable set of parameters was not obtained. Further experiments with noisy drawdown data in an unconfined aquifer revealed that when the well diameter was included in the analysis, hydraulic conductivity, specific yield and vertical hydraulic conductivity may be estimated rather effectively from piezometers located over a range of distances from the pumping well. Estimation of specific storage became less reliable for piezometers located at distances greater than the initial saturated thickness of the aquifer. Application of the NLS to field pumping and recovery data from a confined aquifer showed that the estimated parameters from the two tests were in good agreement only when the well diameter was included in the analysis. Without consideration of well radii, the estimated values of hydraulic conductivity from the pumping and recovery tests were off by a factor of four.     

DETERMINATION OF UNCONFINED AQUIFER HYDRAULIC PROPERTIES FROM RECOVERY TEST DATA1

ABSTRACT: This paper analyzes the sensitivity of drawdown to four hydraulic parameters in unconfined aquifers: horizontal and vertical hydraulic conductivity K_r and K_z, storage coefficient S, and specific yield S_y. Sensitivity coefficients indicate that the sensitivity vanes with time for each aquifer parameter, and K_r, K_z, S, and S_y are identifiable from recovery test data. An inverse method was used to calculate the four parameters from residual drawdowns. Results of application examples demonstrate that residual data provide valid information in the determination of unconfined aquifer hydraulic parameters.     

OBSERVATION WELL NETWORK DESIGN FOR PUMPING TESTS IN UNCONFINED AQUIFERS1

ABSTRACT: This paper presents a method for determining locations of observation wells to be used in conducting pumping tests in unconfined aquifers. Sensitivity coefficients, the distribution of relative errors, and the correlation coefficients between four aquifer parameters (horizontal and vertical hydraulic conductivities K_r and K_z, storage coefficient S, and specific yield S_y) are used as the criteria for the design of observation well networks and the interpretation of pumping tests. The contours of the relative errors over a vertical profile are very useful in selecting the “best” location of an observation well. Results from theoretical analyses suggest that a wide range of locations is suitable for the determination of K_r and that good locations for the determination of K_z and S may be poorly suited for the determination of S_y. Consideration must be given to the position and lengths of the pumping well screen in the selection of observation well locations. For a given location, the quality of test data can be improved by using high pumping rates and frequent sampling of drawdowns. We found that a minimum of two and preferably three observation locations are needed along a given transect. Results of the four parameters from a single well analysis may contain higher uncertainties. However, composite analyses of multiple observation wells can reduce the correlation between the four aquifer parameters, particularly between K_r and S_y, thus, improving the quality of parameter estimation. Results from two pumping tests conducted at sites located in Nebraska were examined with regard to the proposed methodology.     

EVALUATION OF A STREAM AQUIFER ANALYSIS TEST USING ANALYTICAL SOLUTIONS AND FIELD DATA1

ABSTRACT: Considerable advancements have been made in the development of analytical solutions for predicting the effects of pumping wells on adjacent streams and rivers. However, these solutions have not been sufficiently evaluated against field data. The objective of this research is to evaluate the predictive performance of recently proposed analytical solutions for unsteady stream depletion using field data collected during a stream/aquifer analysis test at the Tamarack State Wildlife Area in eastern Colorado. Two primary stream/aquifer interactions exist at the Tamarack site: (1) between the South Platte River and the alluvial aquifer and (2) between a backwater stream and the alluvial aquifer. A pumping test is performed next to the backwater stream channel. Drawdown measured in observation wells is matched to predictions by recently proposed analytical solutions to derive estimates of aquifer and streambed parameters. These estimates are compared to documented aquifer properties and field measured streambed conductivity. The analytical solutions are capable of estimating reasonable values of both aquifer and streambed parameters with one solution capable of simultaneously estimating delayed aquifer yield and stream flow recharge. However, for long term water management, it is reasonable to use simplified analytical solutions not concerned with early‐time delayed yield effects. For this site, changes in the water level in the stream during the test and a varying water level profile at the beginning of the pumping test influence the application of the analytical solutions.     

Modeling the Potential Impact of Seasonal and Inactive Multi‐Aquifer Wells on Contaminant Movement to Public Water‐Supply Wells1

Johnson, R.L., B.R. Clark, M.K. Landon, L.J. Kauffman, and S.M. Eberts, 2011. Modeling the Potential Impact of Seasonal and Inactive Multi‐Aquifer Wells on Contaminant Movement to Public Water‐Supply Wells. Journal of the American Water Resources Association (JAWRA) 47(3):588‐596. DOI: 10.1111/j.1752‐1688.2011.00526.x Abstract: Wells screened across multiple aquifers can provide pathways for the movement of surprisingly large volumes of groundwater to confined aquifers used for public water supply (PWS). Using a simple numerical model, we examine the impact of several pumping scenarios on leakage from an unconfined aquifer to a confined aquifer and conclude that a single inactive multi‐aquifer well can contribute nearly 10% of total PWS well flow over a wide range of pumping rates. This leakage can occur even when the multi‐aquifer well is more than a kilometer from the PWS well. The contribution from multi‐aquifer wells may be greater under conditions where seasonal pumping (e.g., irrigation) creates large, widespread downward hydraulic gradients between aquifers. Under those conditions, water can continue to leak down a multi‐aquifer well from an unconfined aquifer to a confined aquifer even when those multi‐aquifer wells are actively pumped. An important implication is that, if an unconfined aquifer is contaminated, multi‐aquifer wells can increase the vulnerability of a confined‐aquifer PWS well.     

DETERMINATION OF HYDRAULIC C.ONDUCTWITY TENSOR USING A NONLINEAR LEAST SQUARES ESTIMATOR1

ABSTRACT: Reliable and consistent estimation of the components of the hydraulic conductivity tensor provides information needed to make proper decisions regarding clean up and restoration of contaminated aquifers. In this study, the nonlinear least-squares estimation technique was applied to drawdown versus time data from three or more observation wells to determine a theoretical ellipse of equal drawdown. The angle of rotation of this ellipse with respect to the working coordinate axes was determined by a procedure based on contouring the drawdowns at a given time. This ellipse, in turn, was used to estimate the directions and magnitudes of the horizontal components of the hydraulic conductivity tensor. The technique is applicable to confined, as well as leaky, aquifers. Sources of error in this technique include nonhomogeneity of the aquifer and partial penetration of the pumping and observation wells into the aquifer. The procedure presented may be used as an additional tool to verify computations of hydraulic conductivity anisotropy based on other techniques.     

GEOSTATISTICAL ESTIMATION OF HORIZONTAL HYDRAULIC CONDUCTIVITY FOR THE KIRKWOOD‐COHANSEY AQUIFER1

ABSTRACT: The Kirkwood‐Cohansey aquifer has been identified as a critical source for meeting existing and expected water supply needs for southern New Jersey. Several contaminated sites exist in the region; their impact on the aquifer has to be evaluated using ground water flow and transport models. Ground water modeling depends on availability of measured hydrogeologic data (e.g., hydraulic conductivity, for parameterization of the modeling runs). However, field measurements of such critical data have inadequate spatial density, and their locations are often clustered. The goal of this study was to research, compile, and geocode existing data, then use geostatistics and advanced mapping methods to develop a map of horizontal hydraulic conductivity for the Kirkwood‐Cohansey aquifer. Spatial interpolation of horizontal hydraulic conductivity measurements was performed using the Bayesian Maximum Entropy (BME) Method implemented in the BMELib code library. This involved the integration of actual measurements with soft information on likely ranges of hydraulic conductivity at a given location to obtain estimate maps. The estimation error variance maps provide an insight into the uncertainty associated with the estimates, and indicate areas where more information on hydraulic conductivity is required.     

ESTIMATING AQUIFER PARAMETERS FROM A HORIZONTAL WELL PUMPING TEST IN AN UNCONFINED AQUIFER1

ABSTRACT: A pumping test on a phreatic glacial till aquifer was performed near Ames, Iowa, in November 1990. The head in a horizontal well was pumped down rapidly and then held constant for the duration of the 30-hour test. Throughout the test, the flow rate at the pumped well and the head at an adjacent vertical observation well were recorded. The pumping test data were used to determine the hydraulic conductivity and specific yield of the aquifer. The results indicate a hydraulic conductivity of 2.23 × 10^?5 cm/s and a representative specific yield of 0.03. Hydraulic conductivity was calculated by a simple integration of Darcy's Law after extrapolating the data to steady state. Specific yield was determined by use of several methods from the literature and a new method proposed by the author. The results show that specific yield increases with time, and that each method is within an order of magnitude of the others.     

Estimating Hydraulic Properties of Volcanic Aquifers Using Constant‐Rate and Variable‐Rate Aquifer Tests1

Abstract: In recent years the ground‐water demand of the population of the island of Maui, Hawaii, has significantly increased. To ensure prudent management of the ground‐water resources, an improved understanding of ground‐water flow systems is needed. At present, large‐scale estimations of aquifer properties are lacking for Maui. Seven analytical methods using constant‐rate and variable‐rate withdrawals for single wells provide an estimate of hydraulic conductivity and transmissivity for 103 wells in central Maui. Methods based on constant‐rate tests, although not widely used on Maui, offer reasonable estimates. Step‐drawdown tests, which are more abundantly used than other tests, provide similar estimates as constant‐rate tests. A numerical model validates the suitability of analytical solutions for step‐drawdown tests and additionally provides an estimate of storage parameters. The results show that hydraulic conductivity is log‐normally distributed and that for dike‐free volcanic rocks it ranges over several orders of magnitude from 1 to 2,500 m/d. The arithmetic mean, geometric mean, and median values of hydraulic conductivity are respectively 520, 280, and 370 m/d for basalt and 80, 50, and 30 m/d for sediment. A geostatistical approach using ordinary kriging yields a prediction of hydraulic conductivity on a larger scale. Overall, the results are in agreement with values published for other Hawaiian islands.     

Hydraulic characterisation of the Stuttgart formation at the pilot test site for CO2 storage, Ketzin, Germany

The paper presents an approach for the interpretation of hydraulic tests of a CO₂ storage reservoir. The sandstone reservoir is characterised by a fluviatile channel structure embedded in a low-permeability matrix. Pumping tests were carried out in three wells, with simultaneous pressure monitoring in each well.The hydraulic parameters (permeability and storativity) and the boundary configurations were calibrated using three different approaches: (i) parameter calibration and type curve interpretation for single-hole tests, (ii) calibration of the entire build-up phase for cross-hole tests, and (iii) calibration of the initial pressure response for cross-hole pumping tests. In addition, the arrival time of the pressure response was determined and provides additional information about the pathways of hydraulic connection.The measured pumping test permeabilities of the formation were much lower than those measured on the cores, which is very unusual. The pumping test permeabilities are mainly between 50 mD and 100 mD (millidarcy), while core samples show a mean aquifer permeability of 500–1100 mD. Based on this it was concluded that some kind of continuous low-permeability structure exists, which was supported by core material. Three possible aquifer configurations were considered. The first and second were derived from traditional pumping test analysis and were conceptualised using flow boundaries. Each of the analyses provides a different result. A method was developed in which these differences were resolved by interpreting the pressure response with respect to its spatial and temporal sensitivity. This solution lead to a third configuration which was mainly based on spatially-variable permeabilities. Taking into account the pumping test results, the geological background and the behaviour of injected CO₂, we consider only the third configuration to be realistic. The results are in good agreement with modelled CO₂ arrival times and pressure history.     

NUMERICAL ANALYSIS OF PUMPING FROM CONFINED-UNCONFINED AQUIFERS1

ABSTRACT: A numerical method is presented for the analysis of a pumped well in a homogeneous aquifer with allowance made for the decrease in saturated depth, vertical components of flow, the possibility of regions of the aquifer changing between the confined and unconfined states and the effect of different outer boundaries. The method is based on a discrete space, backward difference time, approximation. A particular example considered in detail concerns heavy pumping from one of a regular array of wells in an unconfined aquifer until the drawdown in the well reaches a critical value. Non-dimensional curves are presented relating the time and volume dewatered to the quantity discharged from the well. A further example investigates the effect of an initial confining pressure on the aquifer behaviour.     

MODELING CONCENTRATION VARIATIONS IN HIGH-CAPACITY WELLS: IMPLICATIONS FOR GROUNDWATER SAMPLING1

ABSTRACT: High-capacity wells are used as a convenient and economical means of sampling groundwater quality. Although the inherent limitations of using these wells are generally recognized, little has been done to investigate how these wells actually sample groundwater. A semi-analytical particle tracking model is used to illustrate the influence of variable vertical contaminant distributions and aquifer heterogeneity on the composition of water samples from these wells during short pumping periods. The hypothetical pumping well used in the simulations is located in an unconfined, alluvial aquifer with a shallow water table and concentration gradients of nitrate-nitrogen contamination. This is a typical setting for many irrigated areas in the United States. The main conclusions are: (1) high-capacity wells underestimate the average amount of contamination within an aquifer; (2) shapes of concentration-time curves for high-capacity wells appear to be governed by the distribution of the contaminant and travel times to the well; (3) variables such as well construction, pumping rate, and hydrogeologic properties contribute to the magnitude of the concentration-time curves at individual high-capacity wells; and (4) a sampling strategy using concentration-time curves based on the behavioral characteristics of the well rather than individual samples will provide a much better framework for interpreting spatial contaminant distributions.     

Model Development and Calibration of a Saltwater Intrusion Model in Southern California1

Abstract: A nine‐layered confined‐unconfined flow and transport model is developed for the Alamitos saltwater intrusion barrier in Southern California. The conceptual model is based on the geological structure of the coastal aquifer system. The key parameters in the flow and transport models are calibrated using a two‐phase procedure which matches the types of data available for calibration. Because of the abundance of point measurements of hydraulic conductivity, the heterogeneous and random hydraulic conductivity field for each of the five aquifers is estimated by the geostatiscal method of natural‐neighbor‐kriging in Phase 1. In Phase 2, the longitudinal and transverse dispersivities in the transport model are estimated by a traditional inverse procedure that minimizes the least‐squares error for concentration (LSE‐CON). The minimum LSE‐CON is achieved near 15.2 and 1.52 m for the longitudinal and transverse dispersivities, respectively. Additional simulations with increasing transport parameter complexity did not yield significant improvements in LSE‐CON. Also, tracking least‐squares error for head while parametrically varying the transport parameters revealed there is a negligible interaction between predicted head and transport parameters.     

NUMERICAL MODELING OF SALTWATER INTRUSION IN THE NORTHERN GUAM LENS1

ABSTRACT This paper deals with the application of a two-dimensional, saltwater intrusion model to the aquifer in Northern Guam. The model used finite element theory and the Galerkin, weighted-residual technique as its basis. The Northern Guam lens was discretized into 299 linear, triangular elements and 189 nodes. The model was calibrated using 1978 hydrologic data. The output of the model was compared with measured water levels in six observation wells. The calibrated values of permeability and porosity were then used to verify the model using 1979 data. A calibrated and verified model can be used to make an infinite variety of management and planning studies. In this study, three applications are provided that would be considered typical management runs. Steady state runs were made to compare the four conditions of no pumping, 1978 pumping levels, twice 1978 pumping levels, and five times 1978 pumping levels. The water levels due to these conditions are shown in plan and in cross sections of the aquifer. The effect of zero recharge to the aquifer is next demonstrated for the pumping levels existing during 1978. The final run shows how long the aquifer takes to reach steady state when the pumping rate is increased from the 1978 pumping level to twice that value. The program can be used for numerous other studies for management and planning purposes.     

RESULTS OF ACIDIZING WATER WELLS IN ARIZONA1

Two wellfields have been developed to provide water for a coal fired electric generating station in Arizona. Wellfield No. 1 penetrates the unconfined Coconino Sandstone aquifer, and wellfield No. 2 penetrates the composite Kaibab Limestone-Coconino Sandstone aquifer where ground water occurs under confined conditions. A well in each wellfield was pumped and water level drawdown data were collected before and after acidizing. The drawdown data at the various pumping rates were analyzed to determine the potential benefits of acidizing production water wells in both wellfields. After acidizing, the specific capacity of the well in wellfield No. 1 was improved about 50 percent at water production rates ranging from about 200 to 500 gallons per minute (gpm) (13 to 32 liters per second (lps)). After acidizing, the specific capacity of the well completed in wellfield No. 2 was improved about 100 percent at pumping rates ranging from about 1,250 to 2,200 gpm (79 to 139 lps). An annual saving of approximately 11 percent in pumping costs can be realized in wellfield No. 2, and savings are approximately four percent in wellfield No. 1. Acidization is beneficial for wells that can produce more than 500 gpm (32 lps), and is of marginal value for those that produce less than that amount.     

OPTIMIZATION OF INTERMITTENT PUMPING SCHEDULES FOR AQUIFER REMEDIATION USING A GENETIC ALGORITHM1

ABSTRACT: Using a genetic algorithm (GA), optimal intermittent pumping schedules were established to simulate pump‐and‐treat remediation of a contaminated aquifer with known hydraulic limitations and a water miscible contaminant, located within the Duke Forest in Durham, North Carolina. The objectives of the optimization model were to minimize total costs, minimize health risks, and maximize the amount of contaminant removed from the aquifer. Stochastic ground water and contaminant transport models were required to provide estimates of contaminant concentrations at pumping wells. Optimization model simulations defined a tradeoff curve between the pumping cost and the amount of contaminant extracted from the aquifer. For this specific aquifer/miscible contaminant combination, the model simulations indicated that pump‐and‐treat remediation using intermittent pumping schedules for each pumping well produced significant reductions in predicted contaminant concentrations and associated health risks at a reasonable cost, after a remediation time of two years.     

ESTIMATING TRANSMISSIVITY AND HYDRAULIC CONDUCTIVITY OF CHICOT AQUIFER FROM SPECIFIC CAPACITY DATA1

ABSTRACT: Specific capacity data obtained from Well Construction reports which are available from USGS offices, can provide useful estimates of tranamissivity (T), and hydraulic conductivity (K), of an aquifer. The Chicot Aquifer in Louisiana is one of the largest sources of fresh ground water in North America. Hydrologic data collected for the Chicot Aquifer indicate that specific capacity tests can be used in estimating local and regional values for T and K, if the Cooper-Jacob equation for transient flow is used with proper corrections for well loss and partial penetration. Where full scale pumping test data are scarce, specific capacity test data that are adequately distributed spatially can be used to map changes in T and K values and can be summarized statistically to indicate applicable regional values. A computer program called “TGUESS” which is available from International Ground Water Modeling Center, Holcomb Research Institute, was used in this study. The contour maps for T and K values are prepared for different well depth intervals to avoid wide variation of values.     

STREAMBED HYDRAULIC CONDUCTIVITY FOR RIVERS IN SOUTH‐CENTRAL NEBRASKA1

ABSTRACT: This paper presents hydraulic conductivities of streambeds measured in three rivers in south‐central Nebraska: the Platte, Republican, and Little Blue Rivers. Unlike traditional permeameter tests in streams that determine only the vertical hydraulic conductivity (K_v), the extended permeameter methods used in this study can measure K in both vertical and horizontal as well as oblique directions. As a result, the anisotropy of channel sediments can be determined from streambed tests of similar sediment volumes. Sandy streambeds with occasional silt/clay layers exist in the Republican and Platte Rivers. The average K_v values range from about 15 to 47 m/day for the sandy streambed and about 1.6 m/day for the silt/clay layers. Statistical analyses indicated that the K_v values of sand and gravel in the Platte and Republican Rivers essentially have the same mean; but the K_v values from the Little Blue River have a statistically different mean. K_v is about four times smaller than the horizontal hydraulic conductivity (K_h) for the top 40 cm of sandy streambed. Measured K_h values of the sandy streambed are in the same magnitude as the K_h of the alluvial aquifer determined using pumping tests. The smaller K_v value in the whole aquifer is the result of interbedded layers of silt and clay within the sand and gravel sediments.     

An approach for delineating drinking water wellhead protection areas at the Nile Delta, Egypt

In Egypt, production has a high priority. To this end protecting the quality of the groundwater, specifically when used for drinking water, and delineating protection areas around the drinking water wellheads for strict landuse restrictions is essential. The delineation methods are numerous; nonetheless, the uniqueness of the hydrogeological, institutional as well as social conditions in the Nile Delta region dictate a customized approach. The analysis of the hydrological conditions and land ownership at the Nile Delta indicates the need for an accurate methodology. On the other hand, attempting to calculate the wellhead protected areas around each of the drinking wells (more than 1500) requires data, human resources, and time that exceed the capabilities of the groundwater management agency. Accordingly, a combination of two methods (simplified variable shapes and numerical modeling) was adopted. Sensitivity analyses carried out using hypothetical modeling conditions have identified the pumping rate, clay thickness, hydraulic gradient, vertical conductivity of the clay, and the hydraulic conductivity as the most significant parameters in determining the dimensions of the wellhead protection areas (WHPAs). Tables of sets of WHPAs dimensions were calculated using synthetic modeling conditions representing the most common ranges of the significant parameters. Specific WHPA dimensions can be calculated by interpolation, utilizing the produced tables along with the operational and hydrogeological conditions for the well under consideration. In order to simplify the interpolation of the appropriate dimensions of the WHPAs from the calculated tables, an interactive computer program was written. The program accepts the real time data of the significant parameters as its input, and gives the appropriate WHPAs dimensions as its output.     

A GIS-based DRASTIC vulnerability and net recharge reassessment in an aquifer of a semi-arid region (Metline-Ras Jebel-Raf Raf aquifer, Northern Tunisia)

This paper aims to elaborate new generic DRASTIC aquifer vulnerability maps of the coastal aquifer of Metline-Ras Jebel-Raf Raf (Northeast of Tunisia) using the GIS technique, making the data analyses easier to handle and providing better capabilities of dealing with large spatial data. A similar study was carried out in 1999 in the same aquifer using a method based on the soil water balance equation to determine the net recharge parameter. Unfortunately, the lack of data in the study area made the results unsatisfactory. By applying the Williams and Kissel equation and the Rao relationship, we intend to demonstrate that we could correctly evaluate the net recharge parameter. Moreover, new data related to the aquifer hydraulic conductivity, the soil cover and the vadose zone lithology have become available, and allowed us to elaborate suitable DRASTIC maps.