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.     

SIMULATING THREE-DIMENSIONAL GROUND WATER RESPONSE IN A SMALL MOUNTAINOUS WATERSHED1

ABSTRACT: Snowmelt from deep mountainous snowpacks is seldom rapid enough to exceed infiltration rates; thus, the source of streamflow in many mountainous watersheds is snowmelt recharge through shallow ground water systems. The hydrologic response and interaction between surface and sub-surface flow processes in these watersheds, which is controlled by basin structure, the spatial distribution of snowmelt, and the hydrogeology of the subsurface, are not well understood. The purpose of this study was to test a three-dimensional ground water model using simulated snowmelt input to simulate ground water response to spatially distributed snowmelt on the Upper Sheep Creek Watershed located within the Reynolds Creek Experimental Watershed in Southwestern Idaho. The model was used to characterize the mountainous aquifer and to delineate the subsurface flow mechanisms. Difficulty in finding a reasonable combination of grid spacing and time stepping within the model was encountered due to convergence problems with the Picard solution to the non-linear variably saturated ground water flow equations. Simulation results indicated that flow may be either unconfined or confined depending on inflow rate and hydrogeologic conditions in the watershed. The flow mechanism had a much faster response time when confined flow occurred. Response to snowmelt from a snow drift approximately 90 m away took only a few hours when flow was confined. Simulated results showed good agreement with piezometer measurements both in magnitude and timing; however, convergence problems with the Picard solution limited applicability of the model.     

APPLICATION OF ENHANCED ANNEALING TO GROUND WATER REMEDIATION DESIGN1

ABSTRACT: A methodology for ground water remediation design has been developed that interfaces ground water simulation models with an enhanced annealing optimizer. The ground water flow and transport simulators provide the ability to consider site‐specific contamination and geohydrologic conditions directly in the assessment of alternative remediation system designs. The optimizer facilitates analysis of tradeoffs between technical, environmental, regulatory, and financial risks for alternative design and operation scenarios. A ground water management model using an optimization method referred to as “enhanced annealing” (simulated annealing enhanced to include “directional search” and “memory” mechanisms) has been developed and successfully applied to an actual restoration problem. The demonstration site is the contaminated unconfined aquifer referred to as N‐Springs located at Han‐ford, Washington. Results of the demonstration show the potential for improving groundwater restoration system performance while reducing overall system cost.     

Changes in ground water quality in an irrigated area of southern Alberta

Few studies have documented spatial and temporal variations in ground water quality in areas with high densities of animal farming operations (AFOs), or the long-term effects on surface-water quality. Changes in ground water quality were characterized in an irrigated area with a high density of AFOs in southern Alberta, Canada to evaluate the effect on ground water quality of manure application to fields. Fifty-five piezometers in the oxidized zone were sampled once or twice annually from 1995 to 2001, and temporal changes were analyzed using mixed model analysis. Average NO3- -N increased significantly from 12.5 to 17.4 mg L(-1) and average Cl- increased significantly from 19.4 to 34.4 mg L(-1) in piezometers installed in an unconfined sand aquifer at locations receiving fertilizer and manure. Compared with these manured locations, nitrate and chloride concentrations were significantly lower in shallow aquifer water in areas of pasture or native range, and concentrations did not change significantly with time. Nitrate and chloride concentrations in shallow ground water in fine-textured manured locations did not change significantly. Ground water below about 6 m in till and fine lacustrine sediments contains 18O signatures indicative of recharge under preirrigation or glacially influenced conditions, suggesting this ground water has a low vulnerability to agricultural contamination. Evaluations suggest that shallow ground water discharge will cause NO3- -N and Cl- in the Oldman River to increase by factors of at least 4.3 and 1.3, respectively, with more significant effects in smaller streams and under low-flow conditions.     

GEOCHEMICAL CHARACTERIZATION OF SHALLOW GROUND WATER IN THE EUTAW AQUIFER,MONTGOMERY, ALABAMA1

ABSTRACT: Ground water samples were collected from 30 wells located in, or directly down gradient from, recharge areas of the Eutaw aquifer in Montgomery, Alabama. The major ion content of the water evolves from calcium‐sodium‐chloride‐dominated type in the recharge area to calcium‐bicarbonate‐dominated type in the confined portion of the aquifer. Ground water in the recharge area was under saturated with respect to aluminosilicate and carbonate minerals. Ground water in the confined portion of the aquifer was at equilibrium levels for calcite and potassium feldspar. Dissolved oxygen and nitrite‐plus‐nitrate concentrations decreased as ground water age increased; pH, iron, and sulfate concentrations increased as ground water age increased. Aluminum, copper, and zinc concentrations decreased as ground water age and pH increased. These relations indicate that nitrate, aluminum, copper, and zinc are removed from solution as water moves from recharge areas to the confined areas of the Eutaw aquifer. The natural evolution of ground water quality, which typically increases the pH and decreases the dissolved oxygen content, may be an important limiting factor to the migration of nitrogen based compounds and metals.     

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.     

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 groundwater potential in obiaruku and environs using surface geoelectric sounding

The interpretation of ten resistivity curves in Obiaruku and environs indicates that the area has a great groundwater potential. Correlation of the curves with the lithologic log from a nearby borehole in Ghana quarters, reveals the lithologic succession as an extensive sandy unit between the range of 20 m and 136 m. The medium grained sand unit, which is the aquiferous zone, has a resistivity range of between 300 Ω m and 600 Ω m. The result of the interpreted data and the lithologic log from the borehole indicates three to five geoelectric layers except at Adonishaka, which has a confined aquifer in the third layer and Umukwata that has a confined aquifer in the second layer, the study area generally has an unconfined aquifer, which is in the second layer. In the event of pollution, the groundwater may be contaminated. Sinking of water borehole is not recommended in these areas. Boreholes for potable water are therefore recommended at locations within Adonishaka and Umukwata areas. The present study acts as a guide for future groundwater exploration and exploitation.     

OPTIMAL MANAGEMENT OF A REGIONAL AQUIFER IN EASTERN SAUDI ARABIA1

ABSTRACT: This paper describes the formulation and application of a ground-water hydraulic management model to determine the optimal development and operating policies of a regional aquifer in the Eastern Province of Saudi Arabia. The hydraulic response of the aquifer system is represented by a simulation model that is linked to an optimization management model using response functions. Yearly optimal ground-water extraction rates over a planning horizon of 15 years are determined for four scenarios, each reflecting alternative ground-water development policies. The results are presented in the form of tradeoff curves, relating drawdowns to optimal pumpage, which may enhance the decisionmaker's ability to select the best development policy from a set of alternatives. The results illustrate how various optimal management schemes can be devised to increase the total withdrawal from the aquifer while preventing excessive de-watering.     

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.     

SHALLOW AQUIFERS RELATIVE TO SURFACE WATER,LOWER NORTH PLATTE RIVER VALLEY,WYOMING1

ABSTRACT. The occurrence of ground water in the lower North Platte Valley, Goshen County, Wyoming, was studied to determine safe yield within the alluvial aquifer under varying discharge and recharge conditions. The alluvium of the North Platte is extensively developed for irrigation purposes and the effects of large-scale pumpage are of major concern. Actual withdrawals are estimated to be 46,000 acre-feet. Should pumping reach potentially higher levels an overdraft is expected. Effect of ground water withdrawals are established from projections of the flow regime within the alluvial aquifer. A time dependent, numerical model was employed to predict aquifer response to increased withdrawals. The results suggest that more efficient use of surface waters and/or increased use of ground water will reduce the annual subsurface return flow to the North Platte River and its tributaries by an amount equal to the reduced ground water recharge increment. Alternatives are available for management of the lower North Platte alluvial aquifer. The preferred course is to correlate surface and subsurface water rights, in light of convenience, economics, and best means of storage for maximum utilization of the single water resource.     

ENHANCEMENTS OF NONPOINT SOURCE MONITORING OF VOLATILE ORGANIC COMPOUNDS IN GROUND WATER1

ABSTRACT: The U.S. Geological Survey (USGS) has compiled a national retrospective data set of analyses of volatile organic compounds (VOCs) in ground water of the United States. The data are from Federal, State, and local nonpoint‐source monitoring programs, collected between 1985–95. This data set is being used to augment data collected by the USGS National Water‐Quality Assessment (NAWQA) Program to ascertain the occurrence of VOCs in ground water nationwide. Eleven attributes of the retrospective data set were evaluated to determine the suitability of the data to augment NAWQA data in answering occurrence questions of varying complexity. These 11 attributes are the VOC analyte list and the associated reporting levels for each VOC, well type, well‐casing material, type of openings in the interval (screened interval or open hole), well depth, depth to the top and bottom of the open interval(s), depth to water level in the well, aquifer type (confined or unconfined), and aquifer lithology. VOCs frequently analyzed included solvents, industrial reagents, and refrigerants, but other VOCs of current interest were not frequently analyzed. About 70 percent of the sampled wells have the type of well documented in the data set, and about 74 percent have well depth documented. However, the data set generally lacks documentation of other characteristics, such as well‐casing material, information about the screened or open interval(s), depth to water level in the well, and aquifer type and lithology. For example, only about 20 percent of the wells include information on depth to water level in the well and only about 14 percent of the wells include information about aquifer type. The three most important enhancements to VOC data collected in nonpoint‐source monitoring programs for use in a national assessment of VOC occurrence in ground water would be an expanded VOC analyte list, recording the reporting level for each analyte for every analysis, and recording key ancillary information about each well. These enhancements would greatly increase the usefulness of VOC data in addressing complex occurrence questions, such as those that seek to explain the reasons for VOC occurrence and nonoccurrence in ground water of the United States.     

Nitrate distributions and source identification in the Abbotsford-Sumas Aquifer,northwestern Washington State

The Abbotsford-Sumas Aquifer is a shallow, predominantly unconfined aquifer that spans regions in southwestern British Columbia, Canada and northwestern Washington, USA. The aquifer is prone to nitrate contamination because of extensive regional agricultural practices. A 22-month ground water nitrate assessment was performed in a 10-km2 study area adjacent to the international boundary in northwestern Washington to examine nitrate concentrations and nitrogen isotope ratios to characterize local source contributions from up-gradient sources in Canada. Nitrate concentrations in excess of 10 mg nitrate as nitrogen per liter (mg N L(-1)) were observed in ground water from most of the 26 domestic wells sampled in the study area, and in a creek that dissects the study area. The nitrate distribution was characteristic of nonpoint agricultural sources and consistent with the historical documentation of agriculturally related nitrate contamination in many parts of the aquifer. Hydrogeologic information, nitrogen isotope values, and statistical analyses indicated a nitrate concentration stratification in the study area. The highest concentrations (> 20 mg N L(-1)) occurred in shallow regions of the aquifer and were linked to local agricultural practices in northwestern Washington. Nitrate concentrations in excess of 10 mg N L(-1) deeper in the aquifer (> 10 m) were related to agricultural sources in Canada. The identification of two possible sources of ground water nitrate in northwestern Washington adds to the difficulty in assessing and implementing local nutrient management plans for protecting drinking water in the region.     

DECISION SUPPORT SYSTEM FOR MANAGING GROUND WATER RESOURCES IN THE CHOUSHUI RIVER ALLUVIAL IN TAIWAN1

ABSTRACT: Ground water is a vital water resource in the Choushui River alluvial fan in Taiwan. A significantly increased demand for water, resulting from rapid economic development, has led to large scale ground water extraction. Overdraft of ground water has considerably lowered the ground water level, and caused seawater intrusion, land subsidence, and other environmental damage. Sound ground water management thus is essential. This study presents a decision support system (DSS) for managing ground water resources in the Choushui River alluvial fan. This DSS integrates geographic information, ground water simulation, and expert systems. The geographic information system effectively analyzes and displays the spatially varied data and interfaces with the ground water simulation system to compute the dynamic behavior of ground water flow and solute transport in the aquifer. Meanwhile, a ground water model, MODFLOW‐96, is used to determine the permissible yield in the Choushui River alluvial fan. Additionally, an expert system of DSS employs the determined aquifer permissible yield to assist local government agencies in issuing water rights permits and managing ground water resources in the Choushui River alluvial fan.     

SENSITIVITY OF GROUNDWATER FLOW MODELS TO VERTICAL VARIABILITY OF AQUIFER CONSTANTS1

ABSTRACT: The Ogallala aquifer in the Oklahoma Panhandle is in need of better management because of increased groundwater demand which has caused declines in static water levels at an alarming rate. A groundwater management computer model was developed for the Ogallala aquifer in the Texas Panhandle and treats the aquifer as a homogeneous system. In this study, the computer model has been modified in order to evaluate the effects of vertical layering on semi-static water level changes which occur during the dewatering of a single unconfined aquifer. The modified model was applied to a study area near Guymon, Oklahoma, using both the homogeneous and the multilayered cases. The aquifer is characterized by a saturated thickness of 400 feet. The accumulated drawdown values of the homogeneous and the multilayered cases demonstrate that an average difference of approximately 22% of the original saturated thickness occurs between the two cases before the base of the aquifer is encountered. Approximately 25% more time is required to dewater the layered aquifer. Thus, vertical variations of lithology in an aquifer such as the Ogallala should be considered when prediction is made relative to groundwater management.     

GROUND WATER DROUGHT MANAGEMENT BY A FEEDFORWARD CONTROL METHOD1

ABSTRACT: Management of a regional ground water system to mitigate drought problems at the multi‐layered aquifer system in Collier County, Florida, is the main topic. This paper developed a feedforward control system that consists of system and control equations. The system equation, which forecasts ground water levels using the current measurements, was built based on the Kalman filter algorithm associated with a stochastic time series model. The role of the control equation is to estimate the pumping reduction rate during an anticipated drought. The control equation was built based on the empirical relationship between the change in ground water levels and the corresponding pumping requirement. The control system starts with forecasting one‐month‐ahead ground water head at each control point. The forecasted head is in turn used to calculate the deviation of ground water heads from the monthly target specified by a 2‐in‐10‐year frequency. When the forecasted water level is lower than the target, the control system computes spatially‐varied pumping reduction rates as a recommendation for ground water users. The proposed control system was tested using hypothetical droughts. The simulation result revealed that the estimated pumping reduction rates are highly variable in space, strongly supporting the idea of spatial forecasting and controlling of ground water levels as opposed to a lumped water use restriction method used previously in the model area.     

RELATIONSHIP OF NITRATE CONCENTRATIONS TO DISTANCE OF WELL SCREEN OPENINGS BELOW CASING WATER LEVELS1

ABSTRACT: Data from 54 well in central and eastern Kansas developed in unconsolidated deposits of Quaternary age indicate that nitrate concentrations are inversely related to the depth of the well screen opening below the water level in the well casing because this relationship was found to exist in an area of Nebraska and in a large area of Kansas, the relationship is generally valid over a wide geo- graphical area. In addition, the data indicate that the incidence of nitrate concentrations exceeding 45 milligrams per liter and nitrate concentrations, in general, are significantly lower in water from wells with screens deeper than 25 feet below the water table in unconfined aquifers or where screens are placed in deep confined aquifers. No concentrations of nitrate greater than 45 milligrams per liter were in obseved wells where screens were deeper than 60 feet below the casing water level. These findings suggest that general placement of well screens as far below the water table as possible in unconfined unconsolidated aquifers in Kansas. and possibly other areas of the Midwest, may be an effective measure in preventing undesirable nitrate concentrations in ground water supplies. particularly in areas where nitrate is a problem.     

APPLICATION OF TABU SEARCH TO GROUND WATER PARAMETER ZONATION1

ABSTRACT: The spatial distribution of hydrogeologic parameter is an important issue in ground water simulation. One of the methods is to divide an area into several zones such that parameters are assumed to be constant within zone. The purpose of this study is to apply Tabu Search (TS) to find the best zonation of parameters that can result in the best ground water simulation. The initial zonation can be determined as the Thiessen method, and then zonation is optimized by T.S. The mean square error between simulated and observed hydraulic heads was used as the objective function. A designed confined aquifer with known zonation was used as an example to test the proposed method. Results indicated that Tabu Search can locate the optimal zonation successfully and avoid being trapped by local optimal zonations. Besides this, four other arbitrary initial zonations can be directed to the optimal zonation by TS, which proves the robustness of the proposed method. The method proposed in this study is feasible and expected to work well in the field problems with sufficient sampling of concerned parameters.     

A DIGITAL OVERLAY TECHNIQUE FOR EVALUATION OF POTENTIAL INFILTRATION AND RECHARGE1

ABSTRACT: A common problem encountered during regional planning and development of ground water dependent communities is the difficulty in deciding which areas should be preserved for aquifer recharge purposes. This paper describes the development and application of a digital overlay technique for objective evaluation and ranking of potential infiltration and potential recharge areas. Equations are developed which relate the hydrologic parameters pertaining to infiltration and recharge in a surface aquifer-confined aquifer system. These equations make use of discrete data, yet by application in a digital overlay technique results are obtained in the form of spatial distributions in order for regional trends and conditions to be examined. An application of this procedure to the 551,000 acre region of central Florida, known as the Green Swamp, is discussed. The results are presented in the form of computer generated maps which identify and rank areas of potential recharge to the aquifer system.     

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.