Hanford Site Vadose Zone Studies: An Overview

899 The Hanford Site is 1517 km2 (586 mi2) in size, located in a sparsely populated area in the rain shadow of the Cascade Mountains, adjacent to the Columbia River in southeastern Washington State (Fig. 1). The Hanford Site was set aside in the early 1940s by the U.S. Government to perform a top-secret mission, the Manhattan Project, for production of plutonium for atomic weapons used to end World War II and later to support the Cold War with the Soviet Union. Starting in 1943– 1945, Hanford employed thousands of workers on the largest construction project in the world at that time. The workers built a series of nuclear reactors along with large monolithic concrete buildings used to reprocess wastes. Plutonium production continued at Hanford for over 40 yr. During the late 1980s, in the aftermath of the Chernobyl accident, plutonium production was suspended at Hanford (Gephart, 2003). Since then, efforts at Hanford have focused on cleaning up the legacy of wastes stemming from years of nuclear-weapons production. More than 91,000 Mg (100,000 tons) of nuclear fuel were reprocessed between 1944 and 1990. Reprocessing required between 2100 and 16,500 L (550–4360 gal) of water per tonne of fuel. In addition, hundreds of thousand of tonnes of chemicals, including acids, solvents, nitrates, ammonia, and carbon tetrachloride, were used in the reprocessing plants. While some waste streams, containing various suites of radionuclides and chemicals, were put into storage tanks, more than 454 million L (120 million gal) of the waste was directly discharged to the ground (Gephart, 2003). Radioactive wastes from the Hanford Site mission are stored in 177 buried singleand double-shell carbon-steel tanks grouped together in what are called “tank farms” (Zachara et al., 2007). Of the 149 single-shell tanks, 67 are suspected to have leaked more than 3.8 million L (1 million gal) of tank waste to the vadose zone (Gephart, 2003). Hanford now contains as much as 28,300 m3 (1 million ft3) of soil contaminated with radionuclides in liquid wastes released near processing facilities. Figure 2 schematically shows discharges to the vadose zone and the Columbia River from various sources. The total radioactivity discharged to ground is estimated to be nearly 2 million Ci (accounting for decay through the year 2000; Gephart, 2003). This large quantity of radioactivity, combined with 100,000 to 300,000 tonnes of toxic chemicals now residing in the vadose zone (Gephart, 2003), poses an ongoing environmental challenge. All of the contaminants that are not in some way attenuated in the subsurface (or do not decay away soon enough) will potentially be transported to the Columbia River (Gephart, Hanford Site Vadose Zone Studies: An Overview