Using geochemical modeling (PHREEQC.) to determine Edwards volume fraction in mixed groundwaters from the Edwards and Carrizo aquifers at SAWS Aquifer Storage and Recovery (ASR) site, Bexar County Texas

Geochemical mixing between Edwards and Carrizo waters at the Aquifer Storage and Recovery (ASR) site operated by the San Antonio Water System (SAWS) was modeled using the software program PHREEQC. Applying measurements of the water-quality parameters of alkalinity, pH, hardness and total iron from five end-member mixing runs yielded values for determining multiple regression coefficients, which were used to estimate Edwards volume fraction (EVF) of water samples from the ASR site. EVF correlates strongly with modeled volume fraction (R2 = 0.98 to 0.99) and standard error is between 3.0 and 4.3. The appropriate formula was applied to 287 recovered water samples collected over time from ASR wells having any combination of three or four of the water-quality parameters.

The average EVF in recovered groundwater samples at the ASR site appears to be about 83 percent; this is not a volumetric-weighted average of EVF at the site. Hydrographs suggest EVF changes through time in complex relation to ASR well phase (injection, storage or recovery) in operation at any particular time, cumulative volume of Edwards water in storage (net volume of injected minus recovered water), and location in the ASR well field.