GROUNDWATER CONTROLS ON WATER CHEMISTRY IN THE JEMEZ RIVER

The Jemez river in north-central New Mexico is a snowmelt-dominated system serving as the outlet for water discharging from the Valles Caldera, with a smaller but significant contribution from springs which discharge close to or directly into the river. The groundwater inputs contain a geothermal component which is high in salinity, trace metals and CO₂, and leads to degradation of water quality. This degradation is seen to increase downstream and multiple natural tracers highlight the extent of the fluid mixing, as far as San Ysidro 40 miles to the south. Climate models predict a reduction in snow pack, leading to a decrease in river discharge and relative increase in contribution from springs with associated alterations to water chemistry/water degradation. The variations in water quality will affect local stake holders, as well as habitat.

Salinity/Conductivity and [As] show an increase downstream from the headwaters of the Jemez River in the Valles Caldera to the boarder of the study area at San Ysidro. Peak concentrations are seen at Soda Dam, a location where multiple springs discharge into the river, and at San Ysidro below several other spring inputs. Major ion chemistry indicates that spring water a Soda Dam is a mixture of geothermal water from the Valles Caldera and local meteoric water.

Geochemical mixing models (including ⁸⁷Sr/⁸⁶Sr and stable isotopes of H, O, and S) indicate water-rock interaction with basement granites either through deep circulation or mixing with fluids from depth and several tracers indicate dissolution of carbonates and evaporates in the distal regions is adding Sr with a non-radiogenic ⁸⁷Sr/⁸⁶Sr value. Combining ⁸⁷Sr/⁸⁶Sr with conservative tracers can help to establish mixing volumes of spring and river water. These vary with river discharge: during low flow conditions, at 17 cubic feet per second (cfs), the relative component of spring contribution at Soda Dam is up to 5% of total river discharge. This work highlights the evolution of groundwater along the flow path from the Valles Caldera to as well as potential alterations to river water chemistry/degradation.