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

Khris B. Olsen, Greg W. Patton, P. Evan Dresel, John C. Evans

Pacific Northwest National Laboratory, Richland, WA and

Robert Poreda

University of Rochester, Rochester, New York

This study measured helium-3 and tritium concentrations in soil gas samples to detect and delineate groundwater tritium plumes. This approach is a modification of a successful technique developed in the late 1960’s used to age-dating deep ocean water as part of the GEOSECS ocean-monitoring program and applied to shallow aquifers in the late 1980’s by Poreda (1989). All these studies were based on the presence of tritium, which decays to a stable, inert isotope, helium-3, i.e.:

³H à ³He + â^- t_½ = 12.32 yr

When tritiated water is released from a source into the subsurface environment and migrates downward to mix with groundwater, it daughter isotope, helium-3, begins to build up in the vadose zone and groundwater at the rate of tritium decay. The helium-3 then diffuses away from the source and toward the surface. Throughout this process, helium-3 acts as a non-reactive tracer moving through the vadose zone. By contrast, tritium, as tritiated water in soil gas, is a somewhat reactive tracer, exchanging with water adsorbed on the surface of sediments in the vadose zone. Tritium is thus retarded, to some degree, in its movement through the vadose zone.

In 1999, eight soil gas sampling points ranging in depth from 4.9 ft to 32 ft below 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.