New Mexico Geological Society Annual Spring Meeting — Abstracts

Alpine watersheds are increasingly relied upon in the western U.S., requiring a thorough understanding of annual hydrologic patterns and geologic controls on water quality. The hydrogeology of the Jemez River of central New Mexico is characterized by geothermal inputs from the Baca hydrothermal system associated with the 1.2 Ma Valles caldera, as well as groundwater and surface water mixing, fresh surface water input, and dissolution of local carbonates. Fifteen sites along a 35 km reach of the Jemez River were sampled between 2006 and 2010. Discharge ranged from 10-876 cfs over this period. Geochemical data collected include temperature, conductivity, pH, dissolved oxygen, major ions, trace elements, and stable isotopes. Modeling and time series analyses performed using PHREEQC, Geochemist’s Workbench, and MATLAB produced a model describing the hydrology and geochemistry of the Jemez system. Results show that geothermal springs are the primary contributors to dissolved load. Solute concentrations spike where the river crosses the Jemez fault system that localizes discharge of hot spring brines from the hydrothermal system. Solute loading from these geothermal inputs is intensified by low flows in the river observed on the annual hydrograph during late winter, and diluted during periods of high flow such as spring runoff and late summer monsoonal rains. Importantly, solute concentrations were found to increase downstream regardless of season. Stable isotope analyses support water chemistry control by geothermal systems, with enriched values of δ¹⁸O for thermal waters and near-meteoric values for river waters. Analyses completed during the spring of 2010 indicate that TDS and arsenic exceed EPA drinking water standards at low flows (<30 cfs). This suggests that the Jemez River may be susceptible to unacceptable degradation of water quality under scenarios of decreasing snowpack connected to climate change. Continuous records of temperature, conductivity, pH, and D.O. data collected from a water quality sonde installed in March 2010 also illustrate the effects of discharge fluctuations. Water chemistry of this system directly affects recreational use of the Jemez River and shallow aquifer recharge, and must be considered by communities along the river in planning for domestic and municipal use in the future.