Isotopic investigation of infiltration and unsaturated zone flow processes at Carlsbad Cavern, New Mexico

Abstract

Stable isotopes of hydrogen and oxygen together with tritium were used to investigate infiltration and unsaturated zone flow at Carlsbad Cavern. Water samples were collected from drips and pools every 4 months for a year. Seepage flow dominates infiltration at the cavern, homogenizing the isotopic composition of individual and seasonal precipitation events early along the vertical flowpath. Within the relatively narrow range in the stable isotopic ratios of cave drip water (σD of −53 tc −44%.σ 18O of −8.3 to −85%.), isotopic changes on two different time-scales were identified. A longer-term variation, probably driven by changes in the relative amount of heavy-isotope-enriched summer precipitation, is evident in the slight enrichment of drips collected at shallower depths in the cavern (above 1100 m elevation). This suggests vertical travel times to the main cave rooms of the order of decades, consistent with observed tritium activities (from less than 4.6 to 16.9 tritium units (T.U.)). Given these residence times and the narrow range in isotopic composition, a seasonal trend in the isotopic composition of cave water (coincident at all levels in the cavern) is probably related to seasonal changes in the cave climate. Active air circulation driven by differences in air temperature between the cave and outside during the winter increases the evaporation rate from cave pools and causes enrichment in heavy isotopes. This signal may be transmitted to infiltrating water via isotope exchange before the water drips into the cave, consistent with the temporary storage of drip water in limited zones of saturation around the cave rooms. These zones may develop in response to capillary-driven surface tension before infiltrating water drips into the unsaturated-flow barriers represented by the large, air-filled cave passages.