New Mexico Geological Society Annual Spring Meeting — Abstracts

Geochemical tracers can be very useful in investigating solute sources, mixing of ground and surface waters, and tracking geothermal inputs in any given system. There are many different geochemical tracers that can be applied to a system depending on the inputs and water-rock interactions that affect the system. The geochemical tracers used in this study include major ion chemistry, trace element chemistry, and stable and radiogenic isotope signatures. We analyzed spring and well waters from several different regions in New Mexico and apply geochemical models to estimate subsurface temperatures (geothermometry), as well as mixing models to quantitatively test flowpath hypotheses. When combined at a single system or study area, these tracers can prove to be immensely powerful tools in analyzing surface and groundwater.

Chemical geothermometers take advantage of temperature dependent water-rock reactions. The geothermometers applied in this study used concentrations of dissolved silica, Na, K, Ca, Mg, and Li. Geothermometers can be used to assess possible reservoir temperatures from chemical data of surface waters. Geothermometers have some limitations when solute concentrations are affected by processes such as: mixing with other waters, dissolution or precipitation, ion exchange with surrounding materials, and residence time. We use models compiled by Powell and Cumming (2010) to estimate subsurface fluid reservoir temperatures and equilibration state.

Preliminary 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 test specific subsurface flowpath models.

We applied these tracers in several locations in New Mexico, but we will be looking in detail at the waters sourced from the Valles Caldera system and the upper Jemez river system. We present a combined tracer model (eg. Cl concentration and Sr isotopic composition) that shows excellent agreement for flowpath and mixing. This encourages the use of multiple tracers in order to better define complex subsurface flowpaths.

References:

1. Powell, T. and Cumming, W., 2010, Spreadsheets for thermal water and gas geochemistry,: PROCEEDINGS, Thirty-Fifth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, SGP-TR-188.