A PHREEQC Model of the Geochemical Variation across a Freshwater and Saline Water Interface, Edwards Balcones Fault-Zone Aquifer, South-Central Texas




Webster, Philip

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In south central Texas, an area of brackish-water has been proposed to have developed by mixing in a zone of convergent flow of freshwater from the Edwards Balcones Fault-Zone aquifer and saline water from further downdip in the Edwards Group. To test the hypothesis, results of a geochemical model, developed using PHREEQC, are statistically analyzed and compared to analyses of chemical composition from monitoring wells along a transect from freshwater to saline water. Modeling geochemical variation across the interface can be used to explore mixing effects between freshwater and saline water and help highlight areas in the conceptual model that need further research. Simulation results are compared to monitoring wells to assess error, but the model is not calibrated in the traditional sense, rather all parameter combinations are simulated to establish a best-fit.

The model included mixing of multiple water end-members and reactions between groundwater, calcite, dolomite, and gypsum. Water-mineral reactions were constrained by target saturation index of the minerals. Each parameter was assigned a range and interval. Simulations with different end-member combinations yielded different trends in total dissolved concentration (TDS) and ionic ratios across the simulated brackish-water zone, whereas changing percent contribution of end-members affected variability around the trends. Best-fit lines of the geochemical trends are calculated to compare different trends. Overall, geochemical trends across the simulation target saturation index values had the most significance.

Mixing of freshwater and saline water causes an oversaturation with respect to calcite and, in most simulations, undersaturation to dolomite. The latter environment favors dedolomitization. Simulated supersaturation of groundwater with respect to calcite was controlled by changes in activity coefficients of calcite (Ca2+) and carbonate (CO32--) ions between the freshwater and saline end members. The extent of the simulated mixing zone that is undersaturated with respect to dolomite is dependent on the PCO2 of the saline endmember. Dolomite is undersaturated throughout the whole simulated mixing zone when using the most representative PCO2; as PCO2 decreases, the dolomite saturation curve increases causing parts of the simulated mixing zone to be oversaturated.


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