EVALUATING THE POTENTIAL FOR OFFSITE MIGRATION OF CONTAMINANTS THROUGH THE UPPER BASALT AQUIFER SYSTEM IN THE SOUTHERN PART OF THE HANFORD SITE, WASHINGTON

Most groundwater contamination beneath the Hanford Site is within saturated sediments that overlie the basalt bedrock. However, lower concentrations of contaminants are also present within the upper basalt-confined aquifer, which is composed of sedimentary interbeds and interflow zones within the Saddle Mountains Basalt Formation. Because groundwater in this aquifer flows to the southeast, it has been identified as a potential pathway for contaminants to migrate across the southern boundary of the Site and into areas where the aquifer is used as a source of water supply. The situation is complicated by application of imported irrigation water just south of the Site boundary and pumping from the upper basalt-confined aquifer. Hydrogeologic and hydrochemical data, including new samples from offsite wells, were examined to evaluate the potential for offsite migration of contaminants. Geological information was used to create cross sections showing the configuration of the upper basalt-confined aquifer. And hydraulic heads were measured and mapped to indicate flow direction. Based on the available data, it was determined that contaminants have not moved offsite through the upper basalt-confined aquifer. However, the potential for future offsite migration is affected by current and future pumping of the upper basalt-confined aquifer and recharge from imported irrigation water.