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.