New Mexico Geological Society Annual Spring Meeting — Abstracts

Carlsbad Cavern contains dozens of pools of standing water that lie at depths ranging from 500 feet to 1100 feet below the land surface. The pools are recharged by drip water that enters through the cave ceiling, and water is lost from the pools by a combination of evaporation from the pool surface, leakage from the bottom, and overflow from the edges. Flowing streams are not present in the cave, except followiIlg intense precipitation events.

The chemistry of the pools is determined by the chemical composition of the infIltrating drip water, as well as the geochemical and physical processes operating in the cave, such as mineral precipitation/dissolution, and evaporation/condensation. The mean residence time of water in the pools determines the relative importance of these processes in altering pool water chemistry.

Water samples from 10 pools throughout Carlsbad Cavern have been collected and analyzed on a quarterly basis for pH, temperature, electrical conductivity, alkkalinity ,and the concentrations of major and trace inorganic solutes. The temperature, relative humidity, and carbon . dioxide concentration of the cave air adjacent to each of the pools were also measured.

Different pools w'ere found to vary markedly in chemical composition from one another. The waters range from very fresh (TDS≈200 mg/L) to saline (TDS≈10,000 mg/L). The salinity of the pool waters does not con-elate with depth beneath the land surface, and the pool at the lowest elevation in the cave (Lake of the Clouds) is among the pools containing the lowest concentrations of dissolved solutes. The most dilute pool waters are of the calcium-bicarbonate type, whereas the more saline pools contain water of a magnesium-sulfate type.

The geochemical equilibrium speciation model PCWATEQ was used to determine the chemical species present in the pool waters, as well as the saturation status of various cave minerals. The model simulations indicate that most pools are saturated or supersaturated with respect to calcite, and a few pools are also saturated with gypsum or hydromagnesite.

The chloride concentrations of most of the pool waters is very low, generally less than 5 mg/L. Using a mass balance approach in which chloride is assumed to be a conservative tracer derived from precipitation, and assuming a chloride concentration of 0.4 mg/L in meteoric precipitation, the pool water chloride concentrations suggest that ground-water recharge to the pools is on the order of 10% of total precipitation, with the remainder lost to evaporation in the overlying vadose zone. The chloride and bromide concentrations in one pool and those in the nearby drip water that recharges the pool are nearly identical, indicating little evaporative concentration of solutes in the pool. Given that the relative humidity above the pool is approximately 92%, and thus evaporation is occurring, we can infer that the evaporation rate from the pool surface is small compared with the leakage rate of water through its, bottom. Hence the mean residence time for water in this pool must be small, probably on the order of days. This conclusion is in agreement with previous isotopic studies in the cave.

The carbon dioxide concentration of the cave atmosphere above the pools ranges from that of fresh outside air (350 ppm) to approximately 5 times greater (1750 ppm). The lowest carbon dioxide concentrations are observed during the winter months when cold dense outside air sinks into the natural entrance, thereby freshening the cave atmosphere. The partial pressures of carbon dioxide in the pool waters themselves are 2 to 10 times greater than those of the cave atmosphere, indicating that the pools represent a continuous source of CO₂ to the cave air.