Fossil seawater causing desalination problem

Increasing salinity in a groundwater source for desalination in North Carolina has been traced to fossil seawater, not recent seawater intrusion, as had been feared.

Avner Vengosh, professor of geochemistry and water quality at Duke’s Nicholas School of the Environment, recently directed a study to measure and analyze salinity levels in the Yorktown aquifer and identify their source. The study appears in the online version of Hydrogeology Journal, a peer-reviewed publication – Journal 10040 – Evaluating salinity sources of groundwater and implications for sustainable reverse osmosis desalination in coastal North Carolina, USA by David S Vinson, Haylee G Schwartz, Gary S Dwyer and Avner Vengosh.

Salinity levels in the aquifer are roughly 250% higher today than when the Dare County North Reverse Osmosis (RO) Water Plant in Kill Devil Hills began pumping and desalinating groundwater in the late 1980s. Using geochemical and boron isotope tracers, Vengosh’s research team found that the increase is from an upflow of old and diluted seawater, trapped long ago in the Atlantic coastal aquifers.

“As more and more water is pumped out of the Yorktown aquifer to meet growing year-round demand, the groundwater level is dropping and the relative proportion of fossil seawater, flowing up from deeper aquifers, is increasing,” Vengosh says. “As fossil seawater mixes with the remaining fresh groundwater, it is raising salinity, but at a relatively slow and steady rate that is more manageable and sustainable than the rapid increase we’d see if there was modern-day seawater intrusion.”

Tests showed that the Dare County water plant’s RO membranes still remove about 96-99% of the dissolved salts from the aquifer’s groundwater. However, the membranes have not remained as effective at removing boron and arsenic, which occur naturally in deep saline groundwater.

Tests of the water plant’s four wells found the membranes remove only 16-42% of the boron in the water, and 54-75% of the arsenic. The arsenic levels – which are below safe drinking levels after additional treatment that follows the reverse-osmosis desalination – are not expected to rise in coming decades, but boron levels likely will, in relation to the aquifer’s rising salinity.

“Boron isn’t currently regulated as a drinking water contaminant in the US, but there are international recommendations about safe levels for human consumption,” Vengosh says. “Additional treatment might be needed to remove boron from the desalinated Dare County groundwater.”