VADOSE ZONE HYDROGEOLOGY OF THE HANFORD SITE

The U.S. Department of Energy’s Hanford Site is an inactive nuclear production complex in southeastern Washington State. The Site was built along the Columbia River in 1943 in the semiarid Pasco Basin of the Columbia Plateau. During the operational period, nuclear and chemical wastes were released into the environment. Significant waste quantities currently reside in the vadose zone and will continued to be disposed there in engineered facilities. Vadose zone hydrogeology is the primary control on the ultimate fate of those wastes.

The thickness of the vadose zone ranges from less than one meter near the river to more than 100 meters beneath the central production facilities. Most of the sediments are glacio-fluvial sands and gravels of the Hanford Formation with complex and highly variable structure. In some areas, the deeper vadose zone consists of the fluvial-lacustrine sediments of the Ringold Formation. The whole is mantled by weakly developed soils that tend to be slightly finer in texture than the underlying sediments.

Water is the primary agent for mobilizing and transporting waste in the vadose zone. Water sources include precipitation, runoff, and artificial recharge (dilute and hypersaline solutions). After Site closure, natural recharge will be the primary water source. Part of the site cleanup strategy is to deploy surface barriers above waste disposal sites to minimize water infiltration.

Numerous studies have been conducted to understand the vadose zone: measurements of physical and hydraulic properties and their variability, characterization of specific features such as clastic dikes, measurements of recharge rates under natural and engineered conditions, and model predictions of contaminant fate and transport.

Challenges, however, remain. How do we represent features that cause water to diverge from strictly vertical gravity-driven flow, complicating the movement of contaminants? How do we adequately represent properties in numerical models when the size and extent of the vadose zone dwarfs the measurement domain of studies conducted to date? How will potential runoff from surface barriers impact the vadose zone, and thus the waste, beneath the barriers? How long will surface barriers perform as designed? These and other issues are being considered and will be addressed before Site closure.