Isotopic Tools to Characterize Potential Sources of Brackish Water: The Capitan Reef Complex Aquifer, Texas

In the light of dwindling freshwater supplies in an arid region—Far West Texas—the Capitan Reef Complex Aquifer is a potential source of brackish water for desalination. This aquifer underlies fresh groundwater resources that are increasingly utilized by municipalities up to 100 miles away and consequently face the possibility for depletion. The underlying Capitan Reef Complex Aquifer is not currently widely utilized.

This work is an initial phase of a project to construct a groundwater flow model for part of the Capitan Reef Complex Aquifer. This model will be a tool to aid with regional-scale groundwater resource management. Isotopes provide information on groundwater hydrology important to developing conceptual models of aquifers. Groundwater carbon-13 compositions reflect soil and rock signatures that indicate relatively short and long residence in the aquifer, respectively. Groundwater carbon-14 and tritium compositions indicate where recharge is recent or ancient. Stable oxygen and hydrogen isotopes indicate the spatial and seasonal distribution of recharge due to the effects that altitude, temperature, and amount of precipitation have on the composition of recharging water.

The carbon-13 and carbon-14 isotopic compositions of Capitan Reef Complex Aquifer groundwater indicate recharge zones in the Guadalupe and Glass Mountains but little recharge in the Apache Mountains—all areas where the aquifer crops out. The carbon-13 and carbon-14 isotopic compositions also indicate recharge associated with faults at the southern margin of the Delaware Mountains. Groundwater tritium compositions suggest that the most recent recharge occurred at the southern margin of the Delaware Mountains. The stable oxygen and hydrogen isotopes indicate a relatively simple flow system in the eastern arm of the Capitan Reef Complex Aquifer characterized by a single recharge zone, while in the west, there is a more complex flow system with recharge taking place under a range of conditions.