Year
2004
Abstract
Pacific Northwest National Laboratory sampled and analyzed soil gas for helium-3 and helium-4 concentrations from the vicinity of the 618-11 burial ground. Helium-3, the first daughter product from tritium decay, was used to investigate the source and extent of tritium contamination in the area. This study began in January 2000 after elevated tritium was detected in groundwater near the burial. Seventy soil-gas sampling points were installed around the perimeter of the 618-11 burial ground, approximately 122 meters downgradient of well 699-13-3A, and in four transects downgradient of the burial ground to a maximum distance of 946 meters. Helium-3/helium-4 ratios determined from the sampling points showed significant enrichments compared to the ambient air helium-3/helium-4 ratio. These results suggest there is a vadose zone source of tritium within the burial ground. This vadose zone source is likely the source of tritium in the groundwater. The helium-3/helium-4 ratios also suggested the tritium groundwater plume is traveling eastnortheast from the burial ground, the highest groundwater tritium value may be to the north of well 699-13- 3A, and there are no significant upgradient sources of tritium affecting the burial ground. The use of helium isotopes in soil gas provided a rapid and cost-effective technique to define the shape and extent of tritium contamination from the 618-11 burial ground.