Published online by Cambridge University Press: 03 September 2012
Chlorine-36, including the natural cosmogenic component and the componentproduced during atmospheric nuclear testing in the 1950's and 1960's (bombpulse), is being used as an isotopie tracer for groundwater infiltrationstudies at Yucca Mountain, a potential nuclear waste repository. Rocksamples have been collected systematically in the Exploratory StudiesFacility (ESF), and samples were also collected from fractures, faults, andbreccia zones. Isotopie ratios indicative of bomb-pulse components in thewater (36Cl/Cl values > 1250 × 10-15), signifyingless than 40-yr travel times from the surface, have been detected at a fewlocations within the Topopah Spring Tuff, the candidate host rock for therepository. The specific features associated with the high36Cl/Cl values are predominantly cooling joints and syngeneticbreccias, but most of the sites are in the general vicinity of faults. Thenon-bomb pulse samples have 36Cl/Cl values interpreted toindicate groundwater travel times of at least a few thousand to possiblyseveral hundred thousand years. Preliminary numerical solute-travelexperiments using the FEHM (Finite Element Heat and Mass transfer) codedemonstrate consistency between these interpreted ages and the observed 36Cl/Cl values but do not validate the interpretations.