T62 Global Health and Environmental Matrices in a Sustainable Earth

Tuesday, 27 October 2020
10:00 AM - 12:00 PM
Meeting Rooms

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

Paper number: T62-3
Program: Technical Sessions
Time: 10:15 AM – 10:25 AM

Authors

Abstract

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), SO42-(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 SO42- . Data shows no correlation between increasing Ba2+ or SO42- and increase SI in barite. Although none of the samples under investigation were saturated with respect to gypsum, there is correlation between SO42- 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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358000

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