Fred M. Phillips, Julie L. Mattrick, Thomas A. Duval New Mexico Tech

David Elmore, Peter W. Kubik, University of Rochester

Water Resources Research 24 (1988) 1877-1891.

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 dispersivites ranging form 5 to 8 cm, much larger than most corresponding laboratory values. These larger apparent dispersivites 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.