A Computer Model for Simulating Water Quality and Quantity in a Wellfield in an Alluvial Aquifer

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Abstract
A surface‐water/ground‐water interaction computer model was developed for the Grand Island, Nebraska municipal wellfield. This finite‐difference model, which is two‐dimensional in plan view, includes both quantity and quality components. The quantity component is a modified version of the Prickett‐Lonnquist model. Modifications account for the effects of the Plattc River channels, which flow through the modeled area, and adjust water levels at pumping wells to account for the difference between the well radius and the grid size. The quality component of the model calculates nitrate concentrations in pumping wells penetrating aquifers which arc vertically stratified with respect to nitrate. The model also estimates amounts of recharge from the river to the aquifer and the direction and velocity of movement of ground water in several nitrate‐contaminated areas adjacent to the wellfield. Nitrate is assumed to be a conservative pollutant, and nitrate transport is assumed to be solely due to advection. The model was calibrated using field data collected in 1984 and then verified with field data from two other independent events in 1978 and 1984. After calibration, the difference between measured and simulated heads at individual wells was less than one foot. Predicted nitrate concentrations in the pumping wells ranged from 0.07 to 0.40 mg/1 NO3‐N which is within the measured range of 0.00 to 0.58 mg/1 NO3‐N. River recharge predictions ranged from 176 to 200 cfs, whereas measured values were 168 to 232 cfs. The model was used to investigate the impacts of river diversions upstream on the quantity and quality of water in the wellfield. Simulation results indicate that zero‐flow conditions in the river cause drawdown to increase greatly, cause the nitrate concentration in the pumping wells to increase slightly, and cause the ground water in the nitrate‐contaminated area north of the wellfield to begin moving toward the wellfield.

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