SODIUM AND CHLORIDE MIGRATION IMPACTS TDS IN TWO CARBONATE BEDROCK WELLS

Wells A and B are public water supply wells constructed in Triassic limestone fanglomerate bedrock in Berks County, PA. The wells displayed an increasing trend in Total Dissolved Solids (TDS) concentrations from 400 mg/L in 2007 to 650 mg/L in 2018, with peaks of 911 mg/L in June 2015 and 880 mg/L in July 2018. This study examined the hydrogeologic setting and change in TDS constituents over time, seeking to identify a source and mechanism of transport. Each well was sampled with a 50-gpm pump set at bedrock fracture zones identified on down-borehole videos. TDS concentration approximates the sum of concentrations of all dissolved ions including inorganic salts and organic substances. Samples were analyzed for TDS, alkalinity as CaCO₃, chloride, hardness, sulfate, calcium, manganese, sodium, and magnesium. Bedrock electrical resistivity data were collected to identify potential migration pathways in the vicinity of the wells.

Analyte concentrations in wells A and B did not vary by depth, indicating the constituents are not restricted to discrete zones but are diffused throughout the aquifer. Hardness, sulfate, calcium, magnesium, and alkalinity concentrations were similar to historical samples collected from other fanglomerate wells in the area. Sodium [Na⁺] and chloride [Cl^-] ranged from 10 to 20 times higher than historical samples. Geophysical data displayed a potential dissolution feature beneath a nearby stormwater recharge basin, and down-borehole videos showed the production zones are comprised mainly of dissolution cavities. These results indicate that [Na⁺] and [Cl^-] are the primary sources of increased TDS in wells A and B, and that karst features are locally present. We propose that NaCl, a common road deicer, migrates into carbonate dissolution features via runoff and infiltration and contributes to elevated TDS in groundwater.

Although this study focused on TDS constituents in public water supply wells, the implications can be expanded to private wells and other contaminants in similar hydrogeologic settings. Well owners and communities should exercise caution when locating new wells in carbonate bedrock settings and consider the potential for contaminants to mobilize and impact groundwater quality.