EVALUATION OF GROUNDWATER RESIDENCE TIME IN A KARSTIC ARTESIAN AQUIFER USING NATURAL AND ANTHROPOGENIC TRACERS: ROSWELL ARTESIAN BASIN, NEW MEXICO

Several natural and anthropogenic tracers, including tritium, chlorofluorocarbons, sulfur hexafluoride, and 14C are used to evaluate groundwater residence time within the karstic San Andres limestone aquifer in the Roswell Artesian Basin, southeastern New Mexico. Natural groundwater discharge occurs from a region of wetlands, karst springs, and sinkhole lakes at Bitter Lakes National Wildlife Refuge, located on the Pecos River floodplain east of Roswell. The springs and sinkholes are formed in gypsum bedrock that serves as a leaky confining unit for an artesian aquifer in the underlying San Andres limestone, and are fed by upward seepage of groundwater from the aquifer. Because wetlands on the Refuge provide habitat for a number of threatened and endangered species, Refuge managers have expressed concern about the potential for contamination by oil and gas drilling and other anthropogenic activity in the aquifer recharge area. Estimates of the time required for groundwater, and hence contaminants, to travel through the artesian aquifer vary widely, ranging from as little as four to greater than 100 years, mainly because of uncertainties regarding the role that karst conduit flow plays in subsurface transport. A better understanding of groundwater residence time is thus required to make informed decisions about management of water resources at Bitter Lakes. Preliminary results suggest that groundwater in the artesian aquifer is 20 to 50 years old, and that water discharging into the sinkhole lakes is a mixture of recent and pre-modern groundwater, some of which may be sourced from deeper Paleozoic aquifers underlying the San Andres Formation.