Chlorine 36 and tritium from nuclear weapons fallout as tracers for long‐term liquid and vapor movement in desert soils

Abstract

Chlorine 36 and 3H have been widely used as laboratory tracers for the advection and dispersion of anionic solutes and water, respectively. Advances in accelerator mass spectrometry have recently made possible the analysis of 36Cl in natural soil water samples. Field investigations analogous to the laboratory experiments can now be conducted. Pulses of 36Cl and 3H released into the atmosphere by nuclear weapons testing in the 1950s and 1960s serve as the tracers. We report 36Cl and 3H measurements on desert soil profiles from New Mexico. In both cases where the two tracers were measured in the same profile, the 3H bomb pulse had penetrated between 1 and 3 m, but much of the 36Cl was retained near the soil surface. In contrast, the laboratory experiments usually show more rapid movement of 36Cl than 3H due to the anion exclusion effect. We attribute the difference to the lower water content, fluctuating temperature gradients, and longer time scale of the field situation, which may enhance vapor movement of the tritiated water relative to solute advection of the 36Cl. The spreading of the 36Cl and 3H pulses may be modeled by dispersivities ranging from 5 to 8 cm, much larger than most corresponding laboratory values. These larger apparent dispersivities are probably more a result of vertical velocity fields varying with depth and time than of soil heterogeneity. These results illustrate how the bomb pulse 36Cl and 3H can be useful links between laboratory solute transport studies and the corresponding field applications.