NMT Rio Hondo Geochemistry, Stable Isotopes (May 31st - June 2nd 2012)

With growing concerns about declining snowpack, warmer temperatures, and land use changes, it is becoming increasingly
important to determine the sources that contribute to surface water. In western states, such as New Mexico, most of the surface
water is derived from mountainous watersheds. However, the interaction between the groundwater and the surface water within these
mountain systems is poorly understood. Geochemical data collected from a mesoscale (~200 km2) watershed in northern New Mexico
indicate there may be significant groundwater contributions to the surface water that have largely been ignored in previous studies.
Stable isotopic analysis of δ18O and δ2H and Piper diagrams for surface water, groundwater, and spring water are not geochemically
distinct. Surface water solute concentrations for most constituents increase as a function of the drainage area while the stable
isotopic signature remains constant, suggesting that the water is sourced from similar areas but has undergone differing degrees
of geochemical evolution along different flow paths. Plots of SiO2 vs Ca2+, Na+, Mg2+, and K+ show evidence of spatial evolution
of groundwater with solute concentrations from the headwaters to the watershed outlet. We hypothesize that the increasing solute
concentrations in the surface water are controlled by inputs from deep, more geochemically evolved groundwater. This is similar
to what Frisbee et al. (2011) saw in the Saguache Watershed, though our watershed is significantly smaller and has a different
geological setting. Due to the chemical kinetics involved, this more geochemically evolved groundwater would require longer residence
time along a given flow path to achieve the observed chemical compositions. Significant contributions of old groundwater to surface
water could result in the surface water system having increased buffering capacity against climate change. This deep groundwater
component in watersheds has largely been unexplored. Our research provides support for our hypothesis and indicates that deep
groundwater contributions to surface water may occur at even smaller scales than previously published.