Predictive Analysis of Geochemical Controls in an Alpine Stream

Alpine watersheds are increasingly relied upon for use in the American West, necessitating a more complete understanding of annual hydrologic patterns and geologic influences on water chemistry. The Jemez River is a fifth order stream in central New Mexico that flows from its source in the Jemez Mountains to its confluence with the Rio Grande north of the town of Bernalillo. Designated uses of the Jemez River include domestic water supply, recreation, and agriculture. Geothermal uses are currently being considered as well. The river recharges shallow aquifer waters used by several communities, including tribal lands of the Jemez Pueblo. The hydrogeology of the Jemez system is characterized by geothermal inputs from the Baca hydrothermal system associated with the 1.2Ma Valles caldera, as well as groundwater and surface water interactions. Freshwater input from the Rio Guadalupe and several ephemeral tributaries also influences the water chemistry of the Jemez system. Fifteen sites along a 35 km reach of the river were sampled between 2006 and 2010. Discharge of the Jemez River ranged from 10-876 cfs over the study period. The annual hydrograph is affected by annual snowmelt in the Jemez Mountains as well as surges due to monsoonal rains in July and August. Geochemical data collected over this period include temperature, conductivity, pH, dissolved oxygen (D.O.), major ions, trace elements, and stable isotopes. Continuous records of temperature, conductivity, pH, D.O. and turbidity data were collected from a water quality sonde installed in March 2010. Geochemical modeling and time series analysis were performed using PHREEQC, Geochemist’s Workbench, and MATLAB. Empirical data collected during this study gave rise to several models describing the hydrology and geochemistry of the Jemez system. Our data suggest that springs are the primary contributors to dissolved load, and that solute loading from geothermal inputs is intensified by low flows observed on hydrographs during late winter, as well as on the falling limb of flow during summer. Cation and anion concentrations experience significant declines during periods of high flow, though loadings remain high. Solute concentrations were found to increase downstream regardless of season. Downstream increases take place abruptly where the river crosses fault systems that localize discharge of hot spring brines from the hydrothermal system. Analyses completed during the spring of 2010 indicate that arsenic greatly exceeds EPA drinking water standards at low flows (<30 cfs). TDS and sulfate concentrations in the Jemez also exceed these standards at similar discharge. Stable isotope analyses demonstrate contributions from geothermal systems, with isotopically enriched values of δ18O for thermal waters, and near-meteoric values for most river waters. A model predicting solute concentrations as a function of snowmelt demonstrates that the Jemez River is susceptible to significant degradation of water quality under scenarios of decreasing snowpack. Fluctuations in water chemistries of this system directly affect recreational use and water quality of the Jemez River and shallow aquifer recharge, and must be considered for any proposed domestic or municipal use in the future.