Short‐Term Solute Fluctuations in Ground Water: Modeling Implications

Abstract

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.