Tritium and ¹⁴C Concentrations in Unsaturated-Zone Gases at Test Hole UZB-2, Amargosa Desert Research Site, 1994-98

By David E. Prudic, Robert G. Striegl, Richard W. Healy, Robert L. Michel, and Herbert Haas

ABSTRACT

Tritium concentrations have been determined yearly since April 1994 from water-vapor samples collected at test hole UZB-2. The hole was drilled about 100 m (meters) south of the southwest corner of a commercial burial site for low-level radioactive wastes in September 1993. UZB-2 is equipped with ten 2.5-cm (centimeters) diameter air ports permanently installed in the unsaturated zone between the depths of 5.5 and 108.8 m below land surface. Depth to ground water is about 110 m. Additional sampling ports were driven by hand to depths of 0.5, 1.0 and 1.5 m in May 1997. Initial samples of water vapor collected in April 1994 showed elevated tritium concentrations of more than 100 TU (tritium units) from all 10 air ports, with a maximum concentration of 762±10 TU from an air port at a depth of 24.1 m. Subsequent tritium concentrations increased in all air ports, although tritium concentrations at depths of less than 34.1 m have remained relatively constant since July 1995. The largest observed increase in tritium has been at a depth of 47.9 m. There, tritium concentration has increased from 198±5 TU in April 1994 to 2,570±30 TU in June 1998. Large increases also have been measured in samples collected from air ports at depths of 106.4 and 108.8 m, just above the water table.

During September and October 1998, carbon dioxide samples were collected from all ten air ports in UZB-2 and at a depth of 1.5 m, and analyzed for radioactive carbon-14 (¹⁴C). ¹⁴C concentrations are highest in air ports at depths less than 6 m where they exceed 2,000 pmc (percent modern carbon). Concentrations decrease rapidly in air ports at depth and are about 20 pmc below 94.2 m. However, at 47.9 meters, the ¹⁴C concentration is 205±1 pmc, which is 2 to 4 times higher than concentrations in air ports immediately above and below. This depth corresponds to the largest tritium increase in UZB-2. Concentrations of both tritium and ¹⁴C are greater than what could be expected from atmospheric fallout. The distribution of tritium and ¹⁴C likely represent a complex pattern of lateral and vertical transport through the unsaturated zone from buried wastes to UZB-2.