REGIONAL INVESTIGATION OF GROUNDWATER RESIDENCE TIME USING MULTIPLE TRACERS: SOUTHERN SACRAMENTO MOUNTAINS, NEW MEXICO, USA

We have conducted a regional investigation of groundwater residence time within the southern Sacramento Mountains aquifer system using multiple environmental tracers, including tritium, chlorofluorocarbons (CFCs), noble gases, and carbon 14. Results of the tracer surveys indicate that groundwater in the southern Sacramentos 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 the disconcordance among the methods used to date groundwater in the study area. We attribute this apparent ambiguity to four factors, all of which reflect the heterogeneous character of the southern Sacramentos aquifer system: (1) Atmospheric gases present within epikarst in a thick unsaturated zone, which can produce conflicting results when dating groundwater using CFC and noble gas methodologies; (2) the karstic nature of the aquifer system, wherein wells and springs may yield water from either matrix porosity, solution-enlarged fractures, conduits, or a combination of the three; (3) recycling of groundwater that discharges from high mountain springs, then re-enters the groundwater system along losing reaches of streams at lower elevations; and (4) 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.