Use of chlorine-36 data to evaluate fracture flow and transport models at Yucca Mountain, Nevada

Abstract

An extensive set of 36 Cl data has been collected in the Exploratory Studies Facility (ESF), an 8 km long tunnel at Yucca Mountain, Nevada, for the purpose of developing and testing conceptual models of flow and transport at this site. These data have been used in conjunction with a numerical model to establish upper and lower bounds on infiltration rates, estimate groundwater ages, evaluate hydrologic parameters for fractured volcanic tuff, and develop a conceptual model for the distribution of fast flow paths. At several locations, the measured signals are high enough to be unambiguous indicators of at least a small component of bomb-pulse 36 Cl fallout from atmospheric testing of nuclear devices in the 1950s and 1960s, implying that some fraction of the water traveled from the ground surface to the level of the ESF during the last 50 years. Characterization of the structural settings of these samples as well as predictive modeling generally support the conceptual model that a through-cutting fault in conjunction with sufficient infiltration would be required to transmit bomb-pulse 36 Cl to the sample depth in less than 50 years. Away from such fault zones, the ages of water samples at the ESF appear to be significantly controlled by the thickness of the nonwelded tuff between the ground surface and the ESF.