Use of chloride and chlorine isotopes in the unsaturated zone to characterize recharge at the Nevada Test Site

Abstract

Chloride and ³⁶C1 from 1950's weapons testing are good tracers of recent precipitation recharge in arid environments like the Nevada Test Site. Examining the distribution of chloride in soil, rock fractures and rock matrices is a simple method to clarify the dynamics of long time averaged hydrologic processes, such as occur in unsaturated zones. In general, where infiltration is rapid, chloride gets flushed into the soil or deeper into fractures. An estimate for infiltration rates can then be obtained by finding the peak of the ³⁶C1 bomb pulse in the soil chloride. Some care should be exercised while using chlorides as tracers in layered or calcified soils. Results from a numerical model that is calibrated with soil ³⁶C1 soil data also indicate that soil moisture and chloride do not necessarily follow the same path at large soil suctions. Nonetheless, the distribution of chlorine and its isotopes in unsaturated rock appears to be a useful tool to tracing historic water movement in a fracture system and perhaps qualitatively defining the matrix diffusion potential of the rock.