Measurement of Tritium in Gas Phase Soil Moisture and Helium-3 in Soil Gas at theHanford Townsite and 100 K Area

摘要

In 1999, eight soil gas sampling points ranging in depth from 4.9 ft to 32 ftbelow ground surface (bgs) in two clusters were installed adjacent to well 699-41-1, south of the Hanford Townsite. Fifteen soil gas sampling points, ranging in depth from 7.0 ft to 10.4 ft bgs, were installed to the north and east of the 100-K East Reactor facility. Gas phase soil moisture samples were collected using silica gel traps from all eight sampling locations adjacent to well 699-41-1 and eight locations at the 100-K Area. Soil gas samples for helium-3 measurements were collected at all sampling points. No detectable tritium (<240 pCi/L) was found in the soil moisture samples from either the Hanford Townsite or 100-K Area sampling points. This behavior suggests that tritiated moisture from groundwater is not migrating upward to the sampling points and there are no large vadose zone sources of tritium at either location. Helium-3 analyses of the soil gas samples showed significant enrichments relative to ambient air helium-3 concentrations with a depth dependence consistent with a groundwater source from decay of tritium. Helium-3/helium-4 ratios (normalized to the abundances in ambient air) at the Hanford Townsite ranged from 1.012 at 5 feet below ground surface (bgs) to 2.157 at 32 feet bgs. Helium-3/helium-4 ratios at the 100-K area ranged from 0.972 to 1.131. Based on results from the 100-K area, we believe that a major tritium plume does not lie within that study area. The data also suggest there may be a tritium groundwater plume or a source of helium-3 to the southeast of the study area. We recommend that the study be continued by the placement of additional soil gas sampling points along the perimeter road to the west and to the south of the initial study area.