BRACKISH GROUNDWATER IN BEXAR COUNTY TEXAS: SUITABILITY FOR REVERSE OSMOSIS (RO) DESALINATION

Brackish groundwater containing TDS in the range of 1–10 g/L is an important water resource (8.5×10⁶ km³) that can be used for potable and non-potable applications after desalination in US. Currently, the brackish groundwater desalination is mainly achieved by RO treatment processes and its efficiency is limited by scaling and fouling of membranes and issues associated with the disposal of brine. Current RO-based desalination technology can achieve an 85% (7.2×10⁶ km³) of freshwater recovery. Photobiological silica, calcium, and microbial sulfate removal may allow an additional 10% recovery, increasing the total recovery and decreasing volume of waste by 8.5×10⁵ km³. We studied the aqueous chemistry of 45 brackish groundwater wells in Bexar County, TX. Geochemical data was obtained from the USGS national brackish groundwater database (McMahon et al., 2016) and saturation index (SI) of minerals were calculated using PHREEQC.

Average well depth (~256m) and water temperature (29±6 °C) varied greatly among the wells sampled. The pH varied from 5.3 to 7.9 with an average of 6.9±0.4. TDS range between 1 - 5 g/L and are composed of mainly: Na⁺ (270±185 mg/L), K⁺ (15±9 mg/L), Ca²⁺(274±183 mg/L), Mg²⁺(82±76 mg/L), Ba²⁺(18±13 µg/L), total Fe (0.2± 0.3 mg/L), HCO₃^- (261±73 mg/L), Cl^-(597 ± 738 mg/L), SO₄^2-(1,037 ± 765 mg/L), and SiO₂ (16±7 mg/L).

Halite is undersaturated in all samples and Na⁺ increase has a strong logarithmic correlation (R²=0.93) with increase SI of halite. Barite is undersaturated among samples despite high SO₄^2- . Data shows no correlation between increasing Ba²⁺ or SO₄^2- and increase SI in barite. Although none of the samples under investigation were saturated with respect to gypsum, there is correlation between SO₄^2- and Ca²⁺ increase and SI (SI, R²=0.67, R²=0.72 respectively). Calcite and dolomite had moderate correlations with pH (R²=0.35, 0.34 respectively) indicating SI increasing as pH increases. The pH had strong correlations with the following: Fe(OH)₃ (R²=0.80), hematite (R²=0.80), and goethite (R²=0.80). As pH is increasing, Fe-minerals became supersaturated. At lower pH values (6.5) goethite, hematite, Fe(OH)₃, calcite, and dolomite remain in solution and dissolve faster. Further sampling will allow for more precise correlations to be established between ion concentrations, pH, and SI.