Lechuguilla Cave Pool Chemistry, 1986-1999

In May 1986, cavers dug into Lechuguilla Cave, in southeastern New Mexico, USA. Subsequent exploration and research have demonstrated that Lechuguilla is a world-class cave, both in size and in speleological importance. Of particular interest to hydrologists and geochemists are the numerous isolated pools throughout much of the cave. Since 1986, close to 200 water samples have been collected and subjected to over 2000 individual analyses. Results of these analyses are collected and published here for the first time. Dominant cations in the pool waters are calcium and magnesium; dominant anions are bicarbonate and sulfate. These characteristics reflect the limestone/dolomite host bedrock of the cave, modified to varying degrees by the cave’s massive gypsum deposits, associated with the cave’s early development. The overall chemistry of the water can be explained by a small number of geochemical processes, starting with evaporation and concentration of local rainfall, and dissolution of soil CO2 and local bedrock. Within the cave, excess CO2 is degassed, leading to precipitation of CaCO3 and increased Mg2+:Ca2+ ratios. In some areas of the cave, infiltrating water encounters and dissolves gypsum, leading to increased CaCO3 precipitation and increased SO42-:HCO3- ratios. In at least one location, massive evaporation has created a magnesium sulfate brine. Geochemical characteristics seem to confirm that the pool located at Lechuguilla’s current deep point is actually the regional aquifer, suggesting that the cave’s maximum air-filled depth has been reached