Innovative Way to Predict Saltwater Intrusion into Groundwater

To start, the team built a salinity database that gathered all the available data from groundwater wells and surface water, such as ponds and streams, in the Plymouth area and measured them for salinity. This gave them a baseline understanding of the current locations and likely sources of elevated water salinity.

Next, Boutt and Kirshen adopted an existing U.S. Geological Survey hydrogeological model, which only focused on the onshore half of the hydrogeology equation, by extending its reach five kilometers offshore. The model includes ponds, streams, terrestrial recharge—or the rate and amount of precipitation that seeps down into the aquifer—as well the various wells that draw from the aquifer and the wastewater that is returned to the aquifer via re-infiltration or septic systems.

Finally, they conducted a series of model runs that took into consideration various scenarios in terms of future precipitation, sea-level rise, groundwater usage and changes in water returned to the aquifer.

“We found that, under the high sea-level rise scenario, areas of the aquifer will increase in salinity by up to 17,000 milligrams per liter by 2100,” says Kirshen, “and the mixing zone between the ocean and freshwater will migrate inland by up to 200 meters.” While a few ponds might see significant rise in water elevation, by up to 1.8 meters, most ponds would not see their salinity increase from this source of salinization.

The team also learned that water returned to the aquifer by septic systems plays a major role in helping to limit saltwater intrusion. “About 66% of the water that gets pumped out of the aquifer ends up returning to it,” says Kirshen.

Perhaps the biggest surprise is that the highest levels of salinity today aren’t near the coast, but inland, and especially around the roads. “This surprised me,” says Boutt, “and it looks like road salt is one of the main sources of elevated salinity today.”

“In partnering with UMass Amherst, we were always thinking beyond the municipal boundaries of Plymouth,” says SEMPBA Vice President Frank Mand. “We share an aquifer and a geological foundation with over 30 communities in our ecoregion. So, though the news for Plymouth is good, more importantly we now have a scientific foundation—and new methods for evaluating susceptibility to saltwater intrusion—that are transferrable to those other communities and will help inform Plymouth’s and other communities’ planning for years to come.”

“We were not looking to science to help us recover from our mistakes,” Mand adds. “We were seeking to avoid problems in the future. That, in and of itself, was a worthy goal.”

To prepare for the future, Boutt and Kirshen recommend further, finer-grained analyses of the region’s hydrogeography, the creation of an early warning system to monitor the sites most vulnerable to saltwater intrusion, developing new wells in areas that are at the least risk of salt contamination and reconsidering practices, such as salting the roads in the winter, which are currently responsible for the majority of saltwater contamination in the area.