IMPACT OF EFFLUENT FROM REVERSE OSMOSIS (RO) WATER TREATMENT PLANTS (WTP) ON WATER-QUALITY IN ALBEMARLE SOUND, NORTH CAROLINA, USA

Several North Carolina counties plan to build RO-WTP to treat salty groundwater from dominantly limestone aquifers. The process will result in 1.6-mgd discharge of brackish water into Albemarle Sound, classified as fish-spawning habitat. Government agencies requested a one-year, pre-construction study of potential effects. Concerns about Whole Effluent Toxicity encouraged us to analyze all major elements instead of relying on TDS or conductivity to indicate water quality. One county has an existing RO-WTP discharging 0.2-mgd of brine into the Pasquotank River. This site was used as a model to predict potential impacts at proposed sites. Bi-monthly samples taken from surface and bottom waters (depth ~2 meters) at 13 sites around the discharge pipe and at proposed sites were analyzed for major elements and nutrients. Ambient conditions were also assessed at each location. Data analyses included contour maps and profiles of ion concentrations, Piper diagrams, PHREEQCI analysis of mineral saturation, sediment grain-size and organic-content analysis. The effluent plume is detected in bottom waters (only within 50 meters of the pipe) as increased concentrations of major elements, and shifts its position frequently, presumably with prevailing wind and current conditions. Surface waters are not noticeably affected and show less variable chemistry than bottom waters. Sodium concentrations are ~10 times greater than other major cations and chloride concentrations are frequently as much as 10 times greater than other major anions. Aragonite is the only mineral likely to achieve saturation in any receiving water influenced by discharged brines but precipitated phases should quickly dissolve in shifting water masses. No natural waters sampled showed Ca2+/Na+ > 0.08, which is significant because toxicity studies have only exhibited high mortality rates among organisms at ratios near 15:1. Of the major-element ratios, only HCO3/Cl was significantly higher than ambient ratios and this was only for a few bottom samples nearest the discharge pipe. Because the groundwater feed to the plant is high in NH4, within a few meters of the pipe, the NH4 levels will be significantly higher than ambient, especially for estuarine locations.