Recharge estimates for the Hanford Site 200 Areas Plateau

The magnitude of recharge from natural precipitation was assessed on the Hanford Site 200 Areas Plateau. Two sources of data were used in the evaluation: (1) moisture accumulation in an 18-m-deep 200 East Area, closed-bottom lysimeter for 13 years, and (2) distribution of Cesium-137 in a solid-waste burial ground in which contaminated soil had been buried for 10 years. A borehole was core drilled in 1985 to the bottom of the 200 East area closed-bottom lysimeter and the moisture content was determined gravimetrically. The 1985 moisture content of the lysimeter was compared with its initial moisture content to estimate the recharge. There was no apparent net loss of moisture during the 13-year experiment. There has been no detectable moisture accumulation in the bottom of the lysimeter since 1972. Thus, the recharge rate was approximately zero over the 13-year period. Cesium-137 contaminated soil, resulting from a multimolar, NH{sub 4}(+) waste spill, was placed in a 200 West Area solid-waste burial ground in 1970. After ten years the distribution of Cesium-137 was determined from core-drilled samples. No Cesium-137 above background was detected beneath the trench bottom. Thus, there was no indication of measurable downward movement. However, Cesium-137 was detected above the trench,more » most likely resulting from evapotranspiration. Based on the above observations, there appears to be very little potential for downward movement of mixed waste contaminants in the upper vadose zone beneath much of the 200 Areas Plateau under natural or minimally disturbed conditions. 24 refs., 6 figs.« less