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Evaluation of Groundwater Residence Time in a Karstic Aquifer System: Insights Gained from use of Multiple Environmental Tracers
by Lewis Land, New Mexico Bureau of Geology and Mineral Resources, and the National Cave and Karst Research Institute, New Mexico Institute of Mining and Technology, Carlsbad, NM
Abstract: We use several environmental tracers to evaluate groundwater residence time within karstic and semi-karstic aquifers in southeastern New Mexico, including tritium, tritium-helium, carbon 14, and chlorofluorocarbons (CFCs). Natural groundwater discharge occurs in the lower Pecos Valley east of Roswell from a region of karst springs, wetlands and sinkhole lakes on the northeast margin of the Roswell Artesian Basin. 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. Estimates of the time required for groundwater to travel through the artesian aquifer from recharge areas on the Pecos Slope vary widely because of uncertainties regarding karst conduit flow. Results of our tracer studies suggest that the artesian aquifer contains a significant component of water recharged within the last 10 to 50 years, combined with pre-modern groundwater originating from the underlying Yeso and Glorieta Formations, some of which may be sourced from the Sacramento Mountains to the west.
Additional tracer surveys in the southern Sacramento Mountains indicate that groundwater in the semi-karstic Yeso Formation ranges in age from less than one year to greater than 50 years, although the calculated age varies significantly depending on which tracer is used. A distinctive feature of our results is discordance among the methods used to date groundwater in the study area. We attribute this apparent ambiguity to the heterogeneous character and semi-karstic nature of the aquifer system, which may yield water from either matrix porosity, solution-enlarged fractures, conduits, or a combination of the three. The data also indicate mixing of groundwater from two or more sources, including pre-modern groundwater upwelling along fault zones. This study highlights the importance of using multiple tracers when conducting large-scale investigations of a heterogeneous aquifer system.
Bio: Lewis Land is a hydrogeologist with the New Mexico Bureau of Geology and Mineral Resources, and has been working in Carlsbad with the National Cave and Karst Research Institute since 2002. Most of his work involves regional investigations of karstic aquifers in New Mexico, but he also occasionally finds his way into caves and sinkholes. In spite of his official title as NCKRI’s karst hydrologist, all of his degrees are in geology, from the University of Oklahoma and the University of North Carolina at Chapel Hill, where his doctoral research included going for rides in a nuclear submarine while studying submarine sinkholes in the Florida Straits. For the past couple of years Lewis has been dabbling with the Institute’s new electrical resistivity equipment. He has been known to occasionally describe himself as a karst geophysicist, as long as there are no real geophysicists in the room to ask him hard questions about his work.