New Mexico Geological Society Annual Spring Meeting — Abstracts

Groundwater flow in the southern Sacramento Mountains is related to regional fracture systems within the highly heterogeneous Yeso Formation. As a part of the Sacramento Mountains hydrogeology study, water from precipitation, streams, springs, and wells was sampled and analyzed for the stable isotopes of oxygen and hydrogen.

Stable isotope data for precipitation were used to develop a local meteoric water line (LMWL). Seasonal fluctuations along the LMWL define expected isotopic compositions for summer and winter precipitation. An elevation effect was observed for summer precipitation but not for winter precipitation. Over the three years that precipitation was collected, the Sacramento Mountains experienced two discrete recharge events due to above average precipitation during the summers of 2006 and 2008. These recharge events resulted in an increase in groundwater levels over most of the study area.

Stable isotope data for spring and well water in the high mountains indicate a dual porosity system. The isotopic compositions of water in the mobile hydrologic system, which is associated with fractured limestone and dolomite, plot along an evaporation line (slope ~5.5) that intersects the LMWL in the range of expected winter values. This trend suggests that the primary source of groundwater recharge is snow melt that undergoes mixing and evaporation in fractures and mountain streams.

The isotopic compositions of water in immobile regions, which are probably located in non-fractured portions or smaller fractures within the Yeso Formation, plot along the LMWL and appear to be controlled by the elevation of local precipitation. The isotopic composition of most springs sampled approximately one month after the 2006 monsoon season plotted near the LMWL. Tritium and water chemistry data indicates that most of this water was not derived from 2006 monsoons. The isotopic compositions of spring waters changed with time over the next 18 months, shifting away from the LMWL, until they plotted along the evaporation line. Then the isotopic composition of spring waters began to shift again towards the LMWL shortly after the precipitation events in 2008. We interpret the isotopic shift towards the LMWL as a result of immobile waters being flushed into the mobile hydrologic system due to these extreme summer precipitation events.