Evaluating salinity sources of groundwater and implications for sustainable reverse osmosis desalination in coastal North Carolina, USA

The natural and pumping-induced controls on groundwater salinization in the coastal aquifers of North Carolina, USA, and the implications for the performance of a reverse osmosis (RO) desalination plant have been investigated. Since installation of the well field in the Yorktown aquifer in Kill Devil Hills of Dare County during the late 1980s, the groundwater level has declined and salinity of groundwater has increased from ∼1,000 to ∼2,500 mg/L. Geochemical and boron isotope analyses suggest that the salinity increase is derived from an upflow of underlying saline groundwater and not from modern seawater intrusion. In the groundwater of four wells supplying the plant, elevated boron and arsenic concentrations were observed (1.3–1.4 mg/L and 8–53 μg/L, respectively). Major ions are effectively rejected by the RO membrane (96–99% removal), while boron and arsenic are not removed as effectively (16–42% and 54– 75%, respectively). In coming decades, the expected rise of salinity will be associated with higher boron content in the groundwater and consequently also in the ROproduced water. In contrast, there is no expectation of an increase in the arsenic content of the salinized groundwater due to the lack of increase of arsenic with depth and salinity in Yorktown aquifer groundwater.