STABLE ISOTOPIC COMPOSITION OF WATERS IN THE SACRAMENTO MOUNTAINS, NEW MEXICO: IMPLICATIONS FOR GROUNDWATER RECHARGE MECHANISMS

The Sacramento Mountains, located in southeastern New Mexico, are a north-south trending mountain range with several peaks over 9,000 feet above sea level. In the southern Sacramento Mountains, San Andres limestone and dolomite overlie the Yeso Formation, which is composed of limestone, siltstones and shale and is the primary aquifer in the mountains. The extreme vertical and lateral heterogeneity of the Yeso Formation results in a karst hydrologic system characterized by a series of perched carbonate aquifers that are interconnected by regional fracture systems and incised surface water drainages. A significant portion of precipitation that falls in these mountains makes its way through this groundwater system to the east and southeast to eventually recharge aquifers in the Roswell Artesian and Salt Basins. As part of the Sacramento Mountain hydrogeology study, the stable isotopes of oxygen and hydrogen were used to trace precipitation as it travels from the surface through the karst aquifer system in the high mountains.

The stable isotopic compositions of local precipitation samples define a local meteoric water line (LMWL) and show an inverse correlation with elevation. Multiple samples collected from springs and wells between 2006 and 2009 provide evidence for two recharge mechanisms. Groundwater samples with isotopic values that plot along an evaporation line represents high elevation precipitation, primarily snow melt, that mixes in fractures and mountain streams where it undergoes evaporation, then percolates through stream beds to recharge the groundwater system. Isotopic shifts from the evaporation line towards the LMWL associated with extreme monsoons in 2006 and 2008 provide evidence of vadose zone storage in epikarst below the thin soils. This epikarst water, which plots along the LMWL and is controlled by elevation was apparently flushed into the saturated zone as a result of large monsoon rains. The monsoon events resulted in significant increases in groundwater levels in the study area. Infiltration of water through losing mountain streams appears to be the primary recharge mechanism. However, the periodic flushing of epikarst waters into the saturated system can significantly contribute to groundwater recharge.