ABSTRACTChlorine-36, including the natural cosmogenic component and the component
produced during atmospheric nuclear testing in the 1950's and 1960's (bomb
pulse), is being used as an isotopie tracer for groundwater infiltration
studies at Yucca Mountain, a potential nuclear waste repository. Rock
samples have been collected systematically in the Exploratory Studies
Facility (ESF), and samples were also collected from fractures, faults, and
breccia zones. Isotopie ratios indicative of bomb-pulse components in the
water (36Cl/Cl values > 1250 × 10-15), signifying
less than 40-yr travel times from the surface, have been detected at a few
locations within the Topopah Spring Tuff, the candidate host rock for the
repository. The specific features associated with the high
36Cl/Cl values are predominantly cooling joints and syngenetic
breccias, but most of the sites are in the general vicinity of faults. The
non-bomb pulse samples have 36Cl/Cl values interpreted to
indicate groundwater travel times of at least a few thousand to possibly
several hundred thousand years. Preliminary numerical solute-travel
experiments using the FEHM (Finite Element Heat and Mass transfer) code
demonstrate consistency between these interpreted ages and the observed
36Cl/Cl values but do not validate the interpretations.