USING MULTIPLE GEOCHEMICAL TRACERS TO EXAMINE GEOTHERMAL INPUTS TO THE JEMEZ RIVER, NORTHERN NEW MEXICO

The Valles Caldera is a volcanic-hosted geothermal system in North-Central New Mexico. Geothermal systems such as the Valles Caldera are associated with highly mineralized and fluids which can significantly impact ground- and surface water quality. This study focuses on water chemistry of the Jemez River which serves as the major drainage for the Valles Caldera area. The Jemez river serves many stakeholders, and is used for recreation, irrigation, and drinking water. Several spring complexes associated with thermal features occur along the extent of the Jemez. These springs are sourced by the Valles geothermal system, which have a strong influence on water chemistry and quality in the Jemez. Both acid-sulfate and neutral chloride brines are observed. We use multiple geochemical tracers to identify groundwater flowpaths and to quantify mixing that is occurring among multiple water members.

Samples were taken over a 40-km reach of the Jemez river, including tributaries and spring sources. The geochemical tracers used in this study include major and trace element chemistry, and stable and radiogenic isotopes. We apply geochemical models to estimate subsurface temperatures (geothermometry), as well as mixing models to quantitatively test flowpath hypotheses. Model results are also applied to continuous sensor data from the Jemez river, where specific conductance (a proxy for solute load) is inversely correlated to discharge.

Tools used for identifying site-specific mixing trends and potential solute sources were ternary diagrams (Cl-SO4-HCO3 and Na-K-Mg), Piper diagrams and variation diagrams. We analyzed a subset of waters in the Jemez river system (including springs and wells) for radiogenic strontium as well as uranium isotopic composition to develop a multiple tracer model. Strontium 87/86 values range from 0.7084 to 0.7232 and concentrations range from 0.07 to 3.0 ppm.

We present a combined tracer model (eg. Cl concentration and Sr isotopic composition) that quantifies the fraction of geothermal input. At low-flow conditions, the geothermal component from one spring contributes up to 20% of the streamflow, and the majority of the dissolved solute load. This model can be used to estimate salinity as a function of discharge, enabling the prediction of water quality impairment based on stream hydrographs.