USING PARAMETER ESTIMATION METHODS TO CALIBRATE A GROUND-WATER FLOW MODEL WHERE MUNICIPAL PUMPING OCCURS FROM A LAKE

A basin-wide ground-water flow model was constructed to evaluate the impact of municipal pumping from a geologically complex carbonate-rock aquifer system in the Valley and Ridge Province of Virginia. The setting is unique in that pumping occurs from quarries (now lakes) that were excavated on a plunging limb of an anticline. A recent five-year drought in conjunction with large pumping rates caused the pumped quarry to go dry. Furthermore, widespread drawdowns associated with municipal pumping have created great concern about long-term sustainability of both municipal and homeowner water resources. Parameter estimation methods were used to calibrate the model. Measured water levels and streamflow hydrographs were implemented as weighted observations. Various conceptual models were evaluated using sensitivity analysis and were tested using nonlinear regression to evaluate hydraulic conductivity, anisotropy, recharge, general-head boundaries, and riverbed conductance. By far the most important parameter was determined to be recharge. Further analysis indicates that recharge for this geologic setting and at the model scale is based on depth to the water table, land use, and seasonal variations in evapotranspiration. Using a combination of parameter estimation and estimated seasonal changes in transpiration, a sinusoidal-type function for recharge was shown to yield the best calibrated model. Results suggest that during average-precipitation years, municipal pumping should not produce significant drawdown of the quarry. However, during drought years the best management practice will be to avoid concentrated pumping from one location (quarry), and instead pumping should be spread over an area of at least 10 square miles.