Tritium in groundwater in the Black Hills of South Dakota

Abstract

Tritium from 1950s and 1960s atmospheric hydrogen-bomb tests was dispersed into the stratosphere and around the globe. The fallout of tritium in precipitation was especially high in the northern hemisphere, peaking during a flurry of atmospheric nuclear testing in 1963. Rainout from hydrogen-bomb tests recharged aquifers, and this maximum of tritiated water inadvertently has provided hydrogeologists with a way to establish the time of recharge because prior to hydrogen-bomb explosions, practically no tritium existed in groundwater. In 1967, the US Geological Survey took numerous Inyan Kara Group groundwater samples in the Dewey/Burdock area of the southwestern Black Hills. They found tritium concentrations exceeding 200 tritium units and determined a groundwater velocity of 4.6 m per day in the Inyan Kara aquifer, assuming the tritium originated from rainout on nearby outcrops in 1963. In 2011, the US Geological Survey took groundwater samples in the same general area as the 1967 study. These samples from the Inyan Kara Group had tritium concentrations ranging from ≤ 0 to 15.3 tritium units, much lower than the 1967 study and almost back to values expected from natural production of tritium. Because the half-life of tritium is ~ 12.3 years, only 7% of the tritium generated from tests in 1963 would still be present 48 years later in 2011. The maximum tritium values found in the 2011 study are approximately the values expected from radioactive decay only; this could be interpreted that in some locations very slow groundwater movement has occurred between 1967 and 2011, while in other locations lower tritium values show there has been much faster movement. However, the great range of tritium concentrations indicates considerable variability in groundwater dispersion has occurred. This variability is caused by the complex stratigraphic units of irregular geometry and permeability in the Inyan Kara Group.