New Mexico Geological Society Annual Spring Meeting — Abstracts

Quantifying natural rates of transport of water and solutes through fractured unsaturated rock is difficult to accomplish without the expense of establishing a network of unsaturated zone sampling devices at many locations and depths in the region of interest. We have been studying a system that provides these sampling locations naturally - the unsaturated fractured rock of the Permian reef in southeastern New Mexico. We access the system through Lechuguilla cave, where numerous isolated drip-fed pools allow direct sampling of large volumes of vadose-zone water.

To study unsaturated-zone moisture transport, we have analyzed samples from the cave pools for chlorine-36, an isotope with a low cosmogenic background and an elevated "bombpulse" signal due to atmospheric nuclear tests in the 1950s and 1960s. Detection of bomb-pulse levels of chlorine-36 indicates the presence of significant nuclear-era water, and enables estimates of minimum infiltration velocities. Thus far, seven pools show clear evidence of bombpulse 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.

We hypothesize that a simple model based on the locations of the bomb-pulse pools relative to surface topography, surface catchment areas, and mapped photolineaments 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, predicts that bomb-pulse chlorine-36 will be detected at Dilithium, Tower Place, Atlantis and Aqua Velvet Pools, but not at Yo Acres, Lake Margaret, Pearlsian Gulf, Voids or Briny Pool. We are now collecting samples to test these predictions.