VADOSE-ZONE INFILTRATION VELOCITIES: EVIDENCE FROM LECHUGUILLA CAVE POOL RADIOISOTOPES

Isolated pools in deep cave systems enable collection of large-volume vadose-zone water samples, suitable for trace-level radioisotope analyses. Detection of elevated levels of radioisotopes associated with global nuclear fallout indicates presence in the pool of significant nuclear-era (post-1950) water, and enables estimates of minimum infiltration velocities.

Lechuguilla Cave (Carlsbad Caverns National Park, New Mexico) is the deepest limestone cave in the United States. Sixteen samples from the Entrance Series, the Western Borehole and the Near East regions of the cave have been analyzed for chlorine-36, an isotope with a low cosmogenic background and an elevated "bomb-pulse" signal due to atmospheric nuclear tests in the 1950s and 1960s. Seven pools show clear evidence of bomb-pulse chloride, and nine show pre-bomb cosmogenic background levels. All four pools that are within 170 m of the ground surface contain bomb-pulse chlorine-36, while deeper pools show no clear relationship between depth and chlorine-36 levels. The three deep pools with elevated chlorine-36 apparently capture local fast infiltration pathways through the vadose zone.

Using GIS software, we have examined the locations of the bomb-pulse pools relative to surface topography, surface catchment areas, and mapped photolineaments. A simple model based on these variables can explain the observed chlorine-36 distribution. It predicts that a pool will contain bomb-pulse chlorine-36 if it is within a certain distance of the surface, is below a very large surface catchment, or is within a certain distance of a lineament intersection lying within a surface catchment area. The model, applied to as-yet-unsampled pools in Lechuguilla’s North Rift and Southwest Region, predicts that bomb-pulse chlorine-36 will be detected at Moonbeam, Dilithium, Tower Place, Atlantis and Aqua Velvet Pools, but not at Yo Acres, Lake Margaret, Pearlsian Gulf, Voids or Briny Pool. In February 2001, we will collect samples to test these predictions.