Fossil Seawater affects Coastal Aquifer

Rising salinity in the primary source for desalinated tap water in North Carolina’s Outer Banks has been traced to fossil seawater and not, as some feared, to seawater intrusion. Duke University researcher Avner Vengosh says the saline groundwater in the Yorktown aquifer can remain a valuable source of desalination for decades to come, without having to switch to the costly alternative of desalinating seawater.

Vengosh, professor of geochemistry and water quality at Duke’s Nicholas School of the Environment, recently directed a study to measure and analysesalinity levels in the Yorktown aquifer and identify their source. The study appears in the on-line version of Hydrogeology Journal, a peer-reviewed publication.

 Salinity levels in the aquifer are roughly two-and-a-half times higher today than when the Dare County North Reverse Osmosis Water Plant in Kill Devil Hills began pumping and desalinating groundwater in the late 1980s. Some feared the rise was caused by seawater seeping into the aquifer. However, by using geochemical and boron isotope tracers, Vengosh’s research team found that the increase is from an upflow of old and diluted seawater which was trapped long ago in the Atlantic coastal aquifers.

 That can be viewed as good news, Vengosh explains. “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,” he says. “As fossil seawater mixes with the remaining fresh groundwater, it is raisingsalinity 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 reverse osmosis membranes still remove about 96–99% of the dissolved salts from the aquifer’s groundwater. The membranes haven’t 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 between 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) aren’t expected to rise in coming decades, but boron levels likely will as the salinity of the aquifer rises.

“Boron isn’t currently regulated as a drinking water contaminant in the United States, 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.”

 Even with the additional costs, desalinated groundwater remains a bargain compared to desalinating seawater, Vengosh says. Desalinating seawater requires substantial additional capital investments and advanced filtrationtechnology, largely because of the quantity of salts that must be removed. Seawater contains about 35 ppt of dissolved salt. The groundwater currently used by Dare County has a dissolved salt content of about 5 ppt.

 “Given a choice, groundwater desalination is the way to go, as long as you take care of other contaminants such as arsenic,” Vengosh says.

 The new study is the first to directly link fossil seawater to rising salinity in a groundwater aquifer. “It is intriguing that the coastal aquifers in the southeastern US contain a large volume of brackish water that could sustainably be used for desalination or any other applications that can tolerate their relatively low salinity, particularly as other water sources are at risks due to climatic change and human stress,” he says. “The implications of this study may extend far beyond the Outer Banks.”

The lead author of this paper is doctoral student David S. Vinson who will graduate this spring. Other co-authors were senior research scientist Gary S. Dwyer from Duke’s Nicholas School and former undergraduate Haylee G. Schwartz, who earned a Bachelor of Arts in Earth and Ocean Sciences from Duke in 2009 and is now at the UCLA School of Law. Schwartz’s participation in the study was funded by a grant from the Duke University Undergraduate Research Support Office.