Evaluation of Groundwater Residence Time in a Karstic Aquifer Using Environmental Tracers: Roswell Artesian Basin, New Mexico

Measurements of tritium concentration in spring water samples from the lower Pecos valley are used to estimate groundwater residence time in the Roswell Artesian Basin, southeast New Mexico. Natural groundwater discharge occurs from a series of springs, seeps and sinkhole lakes at Bitter Lakes National Wildlife Refuge, located on the Pecos River floodplain east of Roswell, NM. 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 to travel through the artesian aquifer are poorly constrained, 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 needed to make informed decisions about management of water resources at Bitter Lakes. Preliminary results of tritium measurements suggest that groundwater flow rates through the aquifer may be slower than previous workers had estimated; and that water discharging into some of the sinkhole lakes is a mixture of recent and pre-modern groundwater.