SHORT-TERM SOLUTE FLUCTUATIONS IN GROUND-WATER - MODELING IMPLICATIONS

AS a preliminary phase in a predictive mass-transport modeling study, ground-water samples were collected from an unconfined, landfill-contaminated aquifer over a five-month time period. The dilution effect of recharge from a single precipitation event caused short-term decreases of up to 56% in chloride concentrations measured at individual monitoring wells. A mass-transport model was calibrated to contaminant plumes inferred from different sets of measured chloride concentrations. Sets of calibration parameters thereby attained were used to make separate, 50-year contaminant distribution predictions. The resulting distributions contrasted significantly, particularly at the downgradient margin of the landfill where predicted chloride concentrations ranged from approximately 150-230 ppm. The results of this study suggest that: (1) Contaminant concentrations in shallow unconfined aquifers can fluctuate significantly in response to short-term changes in hydrologic conditions; and (2) the selection of a set of ''observed'' concentrations, which is representative of an overall stage in the evolution of a plume, rather than indicative of short-term extremes in contaminant concentrations, is an important step in calibrating a mass-transport model.