Quantification of Groundwater Recharge Rates in New Mexico Using Bomb 36CL, Bomb-3H, and Chloride as Soil-Water Tracers

Significant amounts of chlorine-36 (36C1) and tritium (3H) were released into the environment as a result of nuclear weapons testing in the 1950s and 1960s. These anthropogenic radionuclides were used to estimate natural groundwater recharge rates and to determine soil dispersive properties in arid climates near Socorro and Las Cruces, New Mexico.

Ground water recharge estimates based on a mass balance of the chloride ion indicate that only about one percent of the average annual precipitation becomes recharge. These results are considerably lower than those estimated by a soil physics study at the same location near Socorro. Discrepancies may be attributed to lateral components of flow and differences in liquid and solute transport.

Relative positions of the 3H and 36C1 profiles indicate that moisture movement by vapor transport is significant. Tritiated water moves in both the liquid and vapor phases while chloride moves only as a dissolved constituent in the liquid phase. A combined liquid and vapor flux may drive 3H deeper into the soil than chloride.

One-dimensional finite-element models were used to simulate transport of 3H and 36C1 through unsaturated soils near Socorro. Dispersivities equal to 5 cm and 8 cm provided the best fit of the computed curve to the observed 3H and 36C1 profiles, respectively.