Screening of Groundwater Contaminants by Travel‐Time Distributions

Abstract
Analytical procedures are proposed for estimating probability distributions of travel times for chemical waste loads to groundwater. Travel distance of the chemical is treated as a renewal process, and travel time is given by the number of annual chemical displacements or renewals required for passage through the soil unsaturated zone. The methods were evaluated for two organic chemicals for a range of site conditions near Lexington, Kentucky, and Des Moines, Iowa. It was found that groundwater contamination risk is very dependent on site‐specific soil and hydrologic conditions. Probability distributions based on both hydrologic and soil spatial variability differed markedly from those that considered only hydrologic variability. The effect of soil spatial variability is to increase travel‐time variability and risk of groundwater contamination. Travel‐time probability distributions were combined with biochemical degradation rates to estimate the expected fraction of chemical mass reaching the water table. This fraction serves as a screening criterion to identify and rank combinations of chemicals and sites that are likely to produce groundwater pollution.