CHLORINE-36 VALIDATION STUDIES AT YUCCA MOUNTAIN, NEVADA

Ratios of chlorine-36 to total chloride (³⁶Cl/Cl) in leachates of crushed tuff were used to trace unsaturated-zone flow within the proposed high-level radioactive waste repository block at Yucca Mountain. Ratios greater than 1,250 ×10^-15 were previously reported from the northern Exploratory Studies Facility (ESF) tunnel and were interpreted to have a bomb-pulse source indicating percolation to depths of 200 to 300 m below land surface in the last 50 years. In contrast, samples from the southern ESF had ³⁶Cl/Cl ratios near the Holocene meteoric value of 500 ×10^-15, despite generally similar hydrogeologic conditions. Attempts to reproduce high ratios yielded mixed results. To resolve the issue, the U.S. Department of Energy requested an independent validation study (VS), organized by the U.S. Geological Survey, to corroborate the presence of bomb-pulse ³⁶Cl/Cl ratios.

The VS used core primarily from 4-m-long boreholes drilled across a fractured zone where 11 of 18 ESF tunnel-wall samples and 8 of 10 core samples were previously reported to have bomb-pulse ³⁶Cl/Cl ratios. New VS samples were crushed and sieved between 2- and 19-mm screens and passively leached for 1 hr in deionized water. Leaching experiments showed that small variations in grain size and leach times had only minor effects on soluble Cl extractions. The short leach time resulted in low Cl concentrations and large ³⁶Cl/Cl errors, but maximized the probability of detecting bomb-pulse ³⁶Cl/Cl signatures. Leachates of 40 VS samples had a mean Cl concentration of 0.141 (±0.026 2× standard error) mg/kg rock and a mean ³⁶Cl/Cl ratio of 337 (±45 2× standard error) ×10^-15 (range of 137 to 717 ×10^-15). Low Cl concentrations that do not correlate with ³⁶Cl/Cl ratios are consistent with a single, low-³⁶Cl/Cl meteoric Cl source rather than mixtures of bomb-pulse and rock (or other) sources. Thus, the VS did not independently corroborate earlier results despite efforts to maximize extraction of bomb-pulse Cl and efforts to use core from some of the same boreholes used previously. Differences in sampling, leaching, and analysis are not likely causes for the discrepancies.