Deep Vadose Zone Contaminant Flux Evaluation at the Hanford BY-Cribs Site Using Forward and Imposed Concentration Modeling Approaches

Abstract

For sites with vadose zone contamination, the nature and extent of groundwater contaminant plumes are related to the flux into groundwater. In thick vadose zones at arid sites, transport is likely relative slow, complicating direct measurement of contaminant flux. A predictive numerical analysis was applied to estimate contaminant flux to groundwater at the Hanford BY Cribs site. The site received large amounts of waste water in the 1950’s, containing contaminants such as technetium-99 and nitrate. Recently, large increases in groundwater concentrations have been observed in site groundwater wells and, for potential remediation purposes, vadose zone fluxes into groundwater needed to be evaluated. The analysis approach uses forward simulations, tracking contaminants during and after disposal, and imposed concentration (IC) simulations, where concentrations, derived from electrical conductivity data obtained in 2007, are imposed on the flow field. A total of three IC methods, relating electrical conductivity to nitrate concentrations, are developed and tested. The numerical analysis predicts high fluxes during and directly after disposal, with decreasing trends beginning in 1970. The IC simulations also show declining nitrate fluxes over time. The predicted fluxes of the forward and IC simulations after 2007 are in reasonable agreement. The observed concentration increases in the groundwater below the BY Cribs site cannot be explained by an increase in contaminant flux from the vadose zone. An alternative explanation, based on the observed decrease in aquifer thickness below the site, is presented.