APPLICATION OF GEOGRAPHIC INFORMATION SYSTEMS TO GROUNDWATER MONITORING NETWORK DESIGN1

ABSTRACT: Effective monitoring configurations for contaminant detection in groundwater can be designed by analyzing the spatial relationships between candidate sampling sites and aquifer zones susceptible to contamination. Examples of such zones are the domain underlying the contaminant source, zones of probable contaminant migration, and areas occupied by water supply wells. Geographic information systems (GIS) are well-suited to performing key groundwater monitoring network design tasks, such as calculating values for distance variables which quantify the proximity of candidate sites to zones of high pollution susceptibility, and utilizing these variables to quantify relative monitoring value throughout a model domain. Through a case study application, this paper outlines the utility of GIS for detection-based groundwater quality monitoring network design. The results suggest that GIS capabilities for analyzing spatially referenced data can enhance the field-applicability of established methodologies for groundwater monitoring network design.     

Spatial and temporal trends of nitrate,chloride, and bromide concentration in an alluvial aquifer,North‐Central Texas,USA

The Seymour aquifer consists of unconfined outcrops of sand and gravel in a semiarid, agricultural region of north‐central Texas in the United States of America. Most water samples collected from the aquifer in 2015 had nitrate concentrations above the drinking water standard of 44.3 milligrams per liter (mg/L). Generally, areas with high nitrate concentration in 2010 remained high in 2015, although the median dropped by 3.9 mg/L. The largest decreases in nitrate concentration—up to 97 mg/L (60%)—were observed in wells with depths less than the median of 13.1 meters (m). However, other wells, including depths above and below the median, showed increases in nitrate concentration of up to 40 mg/L (42%). In 2015, chloride concentrations in six wells exceeded the secondary contaminant level of 250 mg/L, and one well had a chloride concentration of 1,810 mg/L. Past and ongoing agricultural practices, including cultivation of native grassland, application of fertilizer, and irrigation with nitrate‐contaminated groundwater, help sustain overall high nitrate concentrations within the aquifer. Local conditions governing nitrogen inputs and dilution result in significant improvement or worsening of the nitrate problem over relatively short timeframes. The pumping of groundwater from the aquifer may facilitate mixing with groundwater of increased salinity that has been affected by the dissolution of evaporites in underlying Permian bedrock.     

CONTRASTING WATER QUALITY FROM PAIRED DOMESTIC/PUBLIC SUPPLY WELLS,CENTRAL HIGH PLAINS1

ABSTRACT: Closely located domestic and public supply wells were sampled using identical sampling procedures to allow comparison of water quality associated with well type. Water samples from 15 pairs of wells with similar screened intervals completed in the central High Plains regional aquifer in parts of Kansas, Oklahoma, and Texas were analyzed for more than 200 water quality constituents. No statistically significant differences were observed between the concentrations of naturally‐derived constituents (major ions, trace elements, and radon) in paired wells. However, differences in water quality between paired wells were observed for selected anthropogenic compounds (pesticides and tritium), in that some public supply wells produced water that was more recently recharged and contained constituents derived from surface activities. The presence of recently recharged water and compounds indicative of anthropogenic activities in some public supply wells was likely due to operational variations (pumping rate and pumping cycles), as demonstrated in a particle tracking simulation. Water containing surface‐derived anthropogenic compounds from near the water table was more quickly drawn to high volume public supply wells (less than five years) than domestic wells (greater than 120 years) with small pumping rates. These findings indicate that water quality samples collected from different well types in the same area are not necessarily directly comparable. Sampling domestic wells provides the best broad‐scale assessment of water quality in this aquifer setting because they are less susceptible to localized contamination from near the water table. However, sampling public supply wells better represents the quality of the used resource because of the population served.     

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.     

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.     

Sustainable three‐well pumping strategy for efficient detection monitoring near waste impoundments

Efficient monitoring systems addressing the difficulty of detecting narrow contaminant plumes originating from unknown point sources are needed for modern landfills. A low‐discharge extraction and accompanying injection wells could potentially address this problem. This hypothetical computer‐modeling study involved a three‐well detection system consisting of one extraction and two injection wells at a rectangular landfill in a shallow, unconfined aquifer. The extraction and injection wells were located near the landfill's downgradient and cross‐gradient corners, respectively. Each injection well pumped at half the rate of the extraction well. A minimum pumping rate of 1.1 cubic meters per day was determined for the three‐well system; at this rate, all contaminant plumes originating within the landfill's footprint entered the extraction well prior to reaching a downgradient property boundary. In comparison, five passive (not pumped) wells detected all contaminant releases from the landfill. Results of this study suggest that a low‐discharge extraction well with accompanying injection wells may be an effective contaminant detection strategy at some waste impoundments.     

Patterns in groundwater quality: Selected observations in Iowa

Groundwater quality in Iowa varies with depth, location, ownership of well, time of sampling, and geologic features. Samples from deep wells (>30.48 m or 100 ft) are highly mineralized with sulfates and carbonates (calcium and magnesium), whereas the mineral content in samples from shallow wells (<30.48 m or 100 ft) is relatively much lower. Nitrate as a percent of dissolved solids is negligible in samples from deep wells and reaches as high as 3 to 5% in samples from shallow wells. Shallow wells, in particular non-public wells, are highly susceptible to nitrate contamination (with an average concentration of 27.8 mg/l) compared with any other category of wells studied. In shallow non-public wells, the concentration of nitrate from recent geologic deposits (31.61 mg/l) is more than twice the level found in corresponding public wells from similar deposits (13.35 mg/l). Shallow non-public wells are also subject to sharp seasonal fluctuations, with a peak nitrate concentration of 55.81 mg/l in the month of May, possibly because of spring runoff from agricultural and other surface sources. These observations are further supported by the results of nitrate analyses from a large number (over 44,000) of private wells in Iowa. Eighteen percent of these private wells were found to exceed the maximum contaminant level of 45 mg/l nitrate (NO₃ ^?). A higher proportion of the shallow wells (<30.48 m or 100 ft) exceed the maximum contaminant level of 45 mg/l (3867 out of 13,625 or 28.4%). It is proposed that the observed variability in groundwater quality be used as a first step in developing a strategy for strengthening state-wide groundwater quality monitoring programs. Strong state-wide programs would be of considerable assistance to policy makers in the resolution of major groundwater quality issues.     

Analyzing the Effects of Groundwater Pumping on an Urban Stream‐Aquifer System

When the cone of influence of a pumping well reaches a nearby river, the resulting hydraulic gradient can induce enhanced seepage of streamflow into the aquifer. The rate of seepage is often modeled using analytical solutions that are simple to apply but may not reproduce field data due to mathematical assumptions not being met in the field. Furthermore, the appropriateness of such models has not been investigated in detail due to difficulty in measuring streamflow loss in the field. In this study, a field experiment was conducted on a reach of the South Platte River near Denver, Colorado to estimate pumping‐induced streamflow loss. A network of stream gauges, monitoring wells, and in situ measurements was used to observe streamflow rates, groundwater levels, and temperature to assess if pumping wells have a significant impact on streamflow, and to compare observed streamflow depletion against analytical solutions. Data collected suggest that pumping wells have a noticeable impact on streamflow. The analytical solutions proved accurate if streamflow was low and constant but performed poorly if streamflow was high and variable. Therefore, for this reach, the use of analytical solutions to predict streamflow may only be appropriate under low‐flow, constant‐flow conditions. Methods and results can be used to guide other streamflow depletion studies and to inform cases of pumping‐induced streamflow depletion, particularly in regard to water rights.     

BRACKISH GROUNDWATER DESALINATION: A COMMUNITY'S SOLUTION TO WATER SUPPLY AND AQUIFER PROTECTION1

The City of Cape May, New Jersey, draws its primary water supply from the Cohansey Aquifer, a unit serving residential, community, and industrial users throughout the Coastal Plain. By the year 2000, projected population growth will impose a peak water demand beyond available supplies. In addition, regional over-pumping threatens the Cohansey with saltwater intrusion, placing the city wells at risk by 1998. In the early-to mid 1990s, three broad categories of water-supply alternatives were evaluated by regional, state, and federal agencies — additional pumping from the Cohansey, conjunctive use of the Cohansey with other aquifers, and desalination of brackish groundwater. An approach was adopted in 1996 which derives up to 2 MGD from desalination of brackish groundwater, with the remaining peak demand satisfied by short-term pumpage from existing wells in the Cohansey. The first of two wells has been completed, yielding 1.4 MGD of brackish groundwater. Similar performance from the second well will exceed the design goal. When the initial system comes on line during the summer of 1998, New Jersey will have its first public water supply derived from desalinated groundwater. The use of desalinated groundwater balances competing demands for water resources in the southern Cape Region of New Jersey, allowing continued economic growth while reducing human impacts on a threatened aquifer.     

IDENTIFICATION OF AN OPTIMAL GROUNDWATER MANAGEMENT STRATEGY IN A CONTAMINATED AQUIFER1

A groundwater hydraulic management model is used to identify the optimal strategy for allocating limited fresh-water supplies and containing wastes in a hypothetical aquifer affected by brine contamination from surface disposal ponds. The present cost of pumping from a network of potential supply and interception wells is minimized over a five-year planning period, subject to a set of hydraulic, institutional, and legal constraints. Hydraulic constraints are formulated using linear systems theory to describe drawdown and velocity variables as linear functions of supply and interception well discharge decision variables. Successful validation of the optimal management strategy suggests that the model formulation can feasibly be applied to define management options for locally contaminated aquifer systems which are used to fulfill fresh-water demands.     

INTERSTATE AND INTERNATIONAL AQUIFERS1

Many important groundwater aquifers cross state and national boundaries. The flow of water in these aquifers is not influenced by the boundaries but may be materially influenced by mans activities on one or both sides of a boundary. Interstate and international problems may develop because of excessive groundwater lowering on one side of a boundary affecting water users on the opposite side of the line. Similarly, intensive groundwater development along a surface stream may influence the amount of surface water that flows across a boundary. A third type of problem may develop when pumping on one side of the boundary induces poor quality water into an aquifer on the other side of the boundary. Several specific interstate and international aquifer problems are briefly described.     

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.     

What is a static head measurement? Evaluating structural error and implications for regional groundwater modeling

Most groundwater modelers avoid using static heads measured from active production wells because they can introduce a bias into model calibration. However, in the deep confined Cambrian-Ordovician Sandstone Aquifer System in the Central Midcontinent of North America, dedicated observation wells are sparse and remote from areas of most concentrated pumping. As a result, in areas where drawdown is the greatest and modeling is most needed, only static heads from production wells are available for calibration. This paper evaluates two leading sources of discrepancies in using production well data, spatial and temporal structural error (S.E.). A simple Theis solution is used to evaluate the potential magnitude of spatial S.E. when calibrating a regional MODFLOW model with coarse cell resolution. Despite theoretical analyses indicating that spatial S.E. could be significant, statistical analysis of the model results suggests that temporal S.E. is dominant. Long (ranging over decades) or frequent (monthly) head datasets are key in understanding temporal S.E., to better capture water-level variability. In this study, the range in static head observations impacted estimates of the remaining time a well could extract water from the aquifer by 0.1 to 16.0 years. This uncertainty in future water supply is highly relevant to stakeholders and must be assessed in hydrographs depicting risk.     

The history and evaluation of saltwater intrusion into a coastal aquifer in Mersin, Turkey

The Mersin-Kazanli region is a densely industrialized region. The factories and towns cover their water demand from groundwater. With the increased water demand, saltwater intrusion has occurred. The chloride concentration of the water samples from some wells has been analysed periodically since these wells were drilled. The results of these analyses and electrical conductivity measurements were used to show the history and development of saltwater intrusion up to the year 2000. The Cl(-) concentration of the water within the alluvial aquifer increased to over 3000 mg/l in 1999 and the wells were closed completely. In 2001 new wells were drilled more than 1 km away from the sea and old well field. With the results of the analyses conducted in 2001, the current groundwater quality was determined. The ground water is of the magnesium-calcium-bicarbonate type and this composition is controlled by the interaction of the water with the sediments of alluvial deposits.     

MULTICRITERION ANALYSIS OF GROUNDWATER CONTAMINATION MANAGEMENT1

ABSTRACT: Multicriterion decision making (MCDM) techniques were used to analyze a groundwater contamination management problem from the viewpoint of conflicting multiple objectives. The groundwater management model was used to find a compromise strategy for trading off fresh water supply, containment of the waste, and total pumping cost in a hypothetical confined aquifer affected by previous waste disposal action. A groundwater flow model was used to formulate the hydraulic constraints. A linear system model was used to describe drawdown and velocity as functions of the decision variables which were pumping rates. The model determines the pumping location and rates. A modified c-constraint method was used to generate the set of nondominated solutions which were the alternative compromise strategies. Three different MCDM techniques, Compromise programming (CP), ELECTRA II and MCQA II, were used to select a “satisficing” alternative. Analysis of the results showed that, although these techniques follow different principles, the same preferred strategies were reached. Also, it was noticed that maintaining high groundwater velocities is expensive and difficult. In order to meet a two year target date, large amounts of water had to be pumped. Therefore, rapid restoration results in large pumping volumes and high costs.     

Comparison of Aquifer Sustainability Under Groundwater Administrations in Oklahoma and Texas1

Mittelstet, Aaron R., Michael D. Smolen, Garey A. Fox, and Damian C. Adams, 2011. Comparison of Aquifer Sustainability Under Groundwater Administrations in Oklahoma and Texas. Journal of the American Water Resources Association (JAWRA) 1‐8. DOI: 10.1111/j.1752‐1688.2011.00524.x Abstract: We compared two approaches to administration of groundwater law on a hydrologic model of the North Canadian River, an alluvial aquifer in northwestern Oklahoma. Oklahoma limits pumping rates to retain 50% aquifer saturated thickness after 20 years of groundwater use. The Texas Panhandle Groundwater Conservation District’s (GCD) rules limit pumping to a rate that consumes no more than 50% of saturated thickness in 50 years, with reevaluation and readjustment of permits every 5 years. Using a hydrologic model (MODFLOW), we simulated river‐groundwater interaction and aquifer dynamics under increasing levels of “development” (i.e., increasing groundwater withdrawals). Oklahoma’s approach initially would limit groundwater extraction more than the GCD approach, but the GCD approach would be more protective in the long run. Under Oklahoma rules more than half of aquifer storage would be depleted when development reaches 65%. Reevaluation of permits under the Texas Panhandle GCD approach would severely limit pumping as the 50% level is approached. Both Oklahoma and Texas Panhandle GCD approaches would deplete alluvial base flow at approximately 10% development. Results suggest periodic review of permits could protect aquifer storage and river base flow. Modeling total aquifer storage is more sensitive to recharge rate and aquifer hydraulic conductivity than to specific yield, while river leakage is most sensitive to aquifer hydraulic conductivity followed by specific yield.     

Using a Mass Balance Model to Evaluate Groundwater Budget of Seawater‐Intruded Island Aquifers1

Jang, Cheng‐Shin, Chen‐Wuing Liu, Shih‐Kai Chen, and Wen‐Sheng Lin, 2011. Using a Mass Balance Model to Evaluate Groundwater Budget of Seawater‐Intruded Island Aquifers. Journal of the American Water Resources Association (JAWRA) 48(1): 61‐73. DOI: 10.1111/j.1752‐1688.2011.00593.x Abstract: The study developed a mass balance model to evaluate the groundwater budget of seawater‐intruded island aquifers using limited available data. The Penghu islands were selected as a study area. As sparse observed data were available in the islands, methods of combining water and chloride balances were used to determine the amounts of groundwater pumping, seawater intrusion, aquifer storages, and safe yields in the shallow and deep aquifers. The groundwater budget shows that seawater intrusion to freshwater aquifers was 1.38 × 10⁶ and 0.29 × 10⁶ m³/year in the shallow and deep aquifers, respectively, indicating that the seawater intrusion is severe in the both aquifers. The safe yield of the shallow aquifer was 14.56 × 10⁶ m³/year in 2005 which was four times higher than that of the deep aquifer (3.70 × 10⁶ m³/year). However, the annual pumping amounts in the shallow and deep aquifers were 4.77 × 10⁶ and 3.63 × 10⁶ m³/year, respectively. Although the safe yield of the shallow aquifer is enough for all water resources demands, only 55% of exploitation amount was extracted from the shallow aquifer due to its poor water quality. Groundwater exploitation in the deep aquifer should be significantly reduced and regulated by a dynamic management of pumping scheme because the annual pumping amounts are close to the safe yield and seawater intrusion occurs continually. Additionally, to alleviate further aquifer salination, at least half of the current annual groundwater abstraction should be reduced.     

Assessment of intrinsic vulnerability to contamination for Gaza coastal aquifer, Palestine

Gaza coastal aquifer (GCA) is the major source of fresh water for the 1.5 million residents of Gaza Strip, Palestine. The aquifer is under deteriorating quality conditions mainly due to the excessive application of fertilizers. The intrinsic vulnerability of GCA to contamination was assessed using the well-known DRASTIC method. Detailed analysis of the intrinsic vulnerability map of GCA was carried out and did consider different relationships between the vulnerability indices and the on-ground nitrogen loadings and land use classes. In addition, correlation between vulnerability values and the nitrate concentrations in GCA was studied. Based on the vulnerability analysis, it was found that 10% and 13% of Gaza Strip area is under low and high vulnerability of groundwater contamination, respectively, while more than 77% of the area of Gaza Strip can be designated as an area of moderate vulnerability of groundwater contamination. It was found that the density of groundwater sampling wells for nitrate concentration is high for the moderate and high vulnerability zones. The highest first quartile, median, mean, and third quartile of nitrate concentrations are reported in the high vulnerability zones. Results of sensitivity analysis show a high sensitivity of the high vulnerability index to the depth to water table.     

UTILIZATION OF A GEOGRAPHIC INFORMATION SYSTEM TO IDENTIFY THE PRIMARY AQUIFER PROVIDING GROUND WATER TO INDWIDUAL WELLS IN EASTERN ARKANSAS1

ABSTRACT: A Geographic Information System (GIS) was used to develop an automated procedure for identifying the primary aquifers supplying ground water to individual wells in eastern Arkansas. As mandated by state law, water-use data are reported by ground-water withdrawers annually to the Arkansas Soil and Water Conservation Commission, and stored in the Arkansas Site-Specific Water-Use Data System provided and supported by the U.S. Geological Survey. Although most withdrawers are able to provide the amount of water withdrawn and the depth of their wells, very few are able to provide the name of the aquifer from which they withdraw water. GIS software was used to develop an automated procedure for identifying the primary aquifers supplying ground water to individual wells in eastern Arkansas. The software was used to generate a spatial representation of the bottom boundary for the Mississippi River Valley alluvial aquifer (the shallowest aquifer) in eastern Arkansas from well log-data collected by the U.S. Geological Survey. The software was then used to determine the depth of the aquifer bottom at reported well locations to ascertain whether the Mississippi River Valley alluvial aquifer or a deeper aquifer was the primary aquifer providing water to each well. The alluvial aquifer was identified as the primary aquifer for about 23,500 wells.     

GROUNDWATER MANAGEMENT ON THE TEXAS HIGH PLAINS1

ABSTRACT The effects of major water management practices on the pumping requirement from the Ogallala aquifer are discussed. Demand on the aquifer may be reduced as much as 15 percent by recycling irrigation runoff, 25 percent by recycling irrigation runoff and irrigating with water from playas, and 29 percent by recycling irrigation water in combination with irrigation from playas and artificial recharge of playa water to the aquifer. Other practices that can result in more efficient use of precipitation and groundwater are limited irrigation, land forming, soil profile modification, and improved irrigation systems, thereby reducing the pumping demand on the Ogallala. Additional water supplies can possibly be obtained by water harvesting, weather modification, and water importation. Conclusions reached were that the overdraft on the aquifer can be reduced by the application of sound water management practices on an area-wide basis.