Saltwater contamination in freshwater systems is on the rise

Overall, the rate of annual sea level rise has more than doubled since 1993. Global sea level has increased by 4 inches in that timeframe.

“It used to be that we could predict the future based on the past trends, but now we know that there are all these changes happening,” Sujay Kaushal, a geologist at the University of Maryland, told me. “Nothing is shocking anymore.”

Kaushal mostly studies the ecology of watersheds outside the ocean, such as wetlands, streams and rivers, which provide roughly 70 percent of humanity’s drinking water. However, in a newly published study, he teamed up with oceanographers to see how salt from the ocean and land are affecting tidal fresh waters. Spoiler alert: It’s not looking good.

Essentially, salt is coming from both directions—land and the ocean—and meeting in the middle, representing a “double trouble” issue for freshwater tidal basins, Kaushal said. The salt on land is mostly coming from wastewater, fertilizers, resource extraction and road salt.

I interviewed Kaushal for a newsletter I wrote in January about how road salt is threatening water supplies and wildlife, and the data is staggering: The U.S. alone uses about 25 million tons of salt on roads each year, according to one estimate. This salt can leach into watersheds when snow melts. Typically, “dilution is the solution of pollution,” Kaushal says, referring to the fact that heavy rainfall or releasing fresh water from dams into ecosystems can decrease salinity.

But climate-fueled droughts and rising temperatures are making this strategy more difficult. During parched periods, there is less water available to dilute the system and flush out the salt, while rising seas push salt into the watershed, a process known as saltwater intrusion.

“The interaction between human activities and climate change and climate variability is very, very important,” Kaushal said. “It amplifies the salt pulses that we see from human activity.”

This issue is particularly prevalent in the Delaware River, a 330-mile waterway that winds from New York to the Atlantic Ocean at the Delaware Bay. The river supplies about 60 percent of drinking water to Philadelphia’s 1.5 million people, but saltwater intrusion and pollution is threatening this crucial resource.