HYDROCHEMISTRY OF SULPHUR CREEK AND ALAMO CANYON, VALLES CALDERA: EFFECT OF GEOTHERMAL SYSTEMS ON SURFACE WATER QUALITY

The Valles caldera in northern New Mexico is a large, Quaternary silicic volcanic complex (1.25 Ma to 40 ka) containing a liquid-dominated geothermal system. Sulphur Creek and Alamo Canyon in the southwestern part of the caldera contain acidic geothermal features. There are also other snow fed tributaries such as Redondo creek that feed into the Jemez river. In this study, we examine the hydrochemistry and diffuse gas input into the acid-sulfate waters and examine their influence on water quality of the surface waters draining the Valles caldera.

Waters from several tributaries as well as the Jemez river were sampled in multiple campaigns in 2015. These results were compared with prior studies at the same locations. We report hydrochemical results for 23 waters. Sampled waters exhibit pH ranging from 2.5 to 8.5, temperature from 16 C° to 21 C°, and total dissolved solids range from less than 100 ppm at Redondo up to 1370 ppm at Soda Dam Spring. Major, minor and selective trace elements were also quantified in each sample. Using solute concentrations (including sulfate, chloride and bicarbonate) we are able to quantify the mass loading of geothermal constituents to the stream system. Elevated sulfate levels correspond with low pH in the waters and serve as a proxy for geothermal input in the upper reaches.

In addition to the water parameters, diffusive CO₂ flux measurements have been taken in the study area to quantify the CO₂ released in association with thermal features. The instrument used is a PP Systems EGM-4 Portable CO₂ Gas Analyzer, coupled to a respiration chamber. Acid waters are spatially associated with regions of high CO₂flux.

The acidic geothermal contributions have a major effect on the water quality in streams and shallow groundwater systems; especially pH, temperature, sulfate and metal content in the upper stream reaches of Alamo and Sulphur creeks. The water quality improves as waters mix with low TDS waters from Redondo and the East Fork Jemez. This study has implications for quantifying influence of geothermal inputs to the Jemez river with changes in discharge.