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-conta minated 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 monito ring wells. A mass-transport model was calibrated to contaminant plume s inferred from different sets of measured chloride concentrations. Se ts of calibration parameters thereby attained were used to make separa te, 50-year contaminant distribution predictions. The resulting distri butions contrasted significantly, particularly at the downgradient mar gin of the landfill where predicted chloride concentrations ranged fro m approximately 150-230 ppm. The results of this study suggest that: ( 1) Contaminant concentrations in shallow unconfined aquifers can fluct uate significantly in response to short-term changes in hydrologic con ditions; and (2) the selection of a set of ''observed'' concentrations , which is representative of an overall stage in the evolution of a pl ume, rather than indicative of short-term extremes in contaminant conc entrations, is an important step in calibrating a mass-transport model .