GROUNDWATER CONTAMINATION AT U.S. DEPARTMENT OF ENERGY’S HANFORD SITE

Past disposal of liquid waste to the ground at the U.S. Department of Energy’s Hanford Site has created contaminant plumes in the unconfined aquifer. These plumes have spread over the years, some discharging to the Columbia River. Each year, the Department samples ~650 wells, some of them quarterly or monthly, to track these plumes, meet multiple regulatory requirements, and assess the effectiveness of groundwater and vadose zone remediation. Sampling and analysis are coordinated to eliminate redundancy when wells are sampled for more than one objective, and data are stored in a central database. The most widespread contaminant is tritium from sources in the central part of the site. The portion of the tritium plume originating in east-central Hanford flows through highly permeable sediments and has traveled more than 20 km. Plumes originating in west-central Hanford, where the aquifer is less permeable, have moved only ~3 km. Because there is no practical treatment method for tritium, the plumes will probably be left to attenuate naturally. The tritium plumes have shrunk since 1990. Carbon tetrachloride beneath west-central Hanford forms the largest organic contaminant plume. Remediation in the vadose zone and unconfined aquifer limit the spreading of the plume’s center. There is limited evidence of greater lateral spread at depth than near the water table in some locations. Hexavalent chromium is a contaminant of concern near the Columbia River in the former reactor areas. It moves readily through coarse-grained sediment and discharges to the river at concentrations above the drinking water standard. Though the sources are being remediated, chromium levels remain high in groundwater and four sites are being treated to reduce the amount of chromium reaching the river. Strontium-90 plumes tend to be smaller because strontium sorbs to sediment in the vadose zone and aquifer. However, concentrations exceed the drinking water standard in riverbank springs near one reactor area. Strontium-90 does not lend itself to cleanup via pump-and-treat, so alternative remedial methods are being explored. Uranium, present in the southeastern part of the site near the river also interacts with sediment grains, slowing its movement in groundwater.