CONTINUOUS MONITORING OF GROUNDWATER MOVEMENT ALONG THE NACIMIENTO FAULT, NM - IMPLICATIONS FOR UNDERSTANDING ENDOGENIC INPUTS INTO GROUNDWATER SYSTEMS

High temporal resolution monitoring of water (depth, temperature and specific conductance) can reveal processes controlling fluid movement along faults. Faults are known to act as conduits bringing deep fluids to the surface. In many springs of the western US, deeply sourced fluids carry mantle volatiles, are CO₂ rich and deposit travertine, are highly saline and have poor quality due to elevated trace metals (e.g. arsenic).

Regional climate change predictions suggest a climate scenario with less snowmelt as a contributor to NM streams. In the arid southwest, spring discharge can contribute significant volume to surface water and with less of a spring snowmelt pulse they can become the main contributions.

Our monitoring sites are a series of travertine depositing CO₂ rich springs which occur in a north south trend along the Nacimiento fault. Helium and carbon isotope data indicate deep fluid connections, while electrical resistivity imaging has indicated symmetrical 100 meter scale electrically conductive features, interpreted as groundwater convection, inside of and on either side of the fault. Stable isotope data suggest a component related to the distal effects of the Valles Caldera geothermal system, while water chemistry data indicate sodium and chloride dominated waters.

Continuous monitoring of temperature, depth and specific conductance in these spring was carried out with resolutions set as small as 5 minute intervals. This continuous monitoring provides baseline data showing near constant temperatures. Results indicate connectivity between three of the four monitoring sites, that are co-aligned along the fault suggesting this fault acts as a conduit for vertical and lateral migration of fluids. Strongest connection is seen in water depth, where similar temporal variations can be seen. The most distal of the sites shows the least connectivity, however depth measurements are punctuated periodically, possibly due to local groundwater pumping in the nearby town of San Ysidro.