An ANS-C Lecture provided by Dr. Lewis Land (Karst hydrogeologist, National Cave and Karst Research Institute) at the Carlsbad Environmental & Monitoring Center (CEMRC), 1400 University Drive, Carlsbad, NM.
Evaluation of groundwater residence time in a high mountain aquifer system, southern Sacramento Mountains, New Mexico: Insights gained from use of multiple environmental tracers
Personnel with the National Cave and Karst Research Institute and the New Mexico Bureau of Geology and Mineral Resources have conducted a regional investigation of groundwater residence time within the southern Sacramento Mountains aquifer system using multiple environmental tracers. Results of the tracer surveys indicate that groundwater in the southern Sacramento Mountains ranges in age from less than one year to greater than 50 years, although the calculated ages contain uncertainties and vary significantly depending on which tracer is used. A distinctive feature of the results is discordance among the methods used to date groundwater in the study area. This apparent ambiguity results from the effects of a thick unsaturated zone, which produces non-conservative behavior among the dissolved gas tracers, and the heterogeneous character and semi-karstic nature of the aquifer system, which may yield water from matrix porosity, fractures, solution-enlarged conduits, or a combination of the three. The data also indicate mixing of groundwater from two or more sources, including recent recharge originating from precipitation at high elevations, old groundwater stored in the matrix, and pre-modern groundwater upwelling along fault zones. The tracer data have also been influenced by surface-water/groundwater exchange via losing streams and lower elevation springs (groundwater recycling). This study highlights the importance of using multiple tracers when conducting large-scale investigations of a heterogeneous aquifer system, and sheds light on characteristics of groundwater flow systems that can produce discrepancies in calculations of groundwater age.