Bomb chlorine-36 analysis in the characterization of unsaturated flow at a proposed radioactive waste disposal facility, Chihuahuan Desert, Texas

Abstract

The distribution of anthropogenic chlorine-36 in the unsaturated zone is used to estimate the moisture flux at a study site in the Chihuahuan Desert, Texas, for a potential low-level radioactive waste disposal facility. The unsaturated zone at the study site is approximately 150 m thick; the uppermost section consists of 15 m of coarse-grained sediment that overlies 140 m of clay. The 36 Cl Cl depth profile includes a well-defined peak ratio of 6.6×10 −12 at a depth of 0.5 m. A background 36 Cl Cl ratio of 0.46 × 10 −12 below a depth of 1.25 m agrees with the predicted natural fallout of 0.50×10 −12 for this latitude. The measured total inventory of bomb 36Cl is 2.5 × 10 −12 atoms m −2, which is 73% of the predicted fallout at this latitude. The specific moisture flux based on the 36 Cl Cl peak depth is 1.4 mm a −1, or 0.5% of the mean annual precipitation rate in the region. Because the bomb pulse is restricted to the root zone, it is not possible to determine if this infiltrating water will recharge the deep water table. The movement of liquid and vapor phases of water in the shallow, unsaturated zone is complex and includes both downward- and upward-directed flux components controlled by infiltration and evapotranspiration; therefore, Cl data that delineate the net movement of water over a long time period are critical in evaluation of the potential for radionuchde transport from disposal facilities.