URI researchers track groundwater discharges into salt ponds

Plumes of cool water are potential sources of pollution

KINGSTON, R.I. – August 31, 2020 – The movement of groundwater in aquifers deep beneath the surface often carries with it a variety of contaminants that can be traced to leaking septic systems, damaged underground infrastructure, excessive fertilizer use and other land uses. But where that groundwater and those contaminants end up is often unknown.

Using a drone with an infrared thermal imaging camera, a team of University of Rhode Island researchers led by doctoral student Kyle Young has tracked some of it to the Ocean State’s coastal ponds.

“We’re looking to quantify the amount of nutrients being brought into our estuaries and what’s happening to those nutrients,” said Young, a Coast Guard helicopter pilot and physics teacher at the Coast Guard Academy on leave to earn his doctorate. “The key nutrient is nitrate. In small amounts, nitrate is a good thing, but in larger amounts it can be degrading to the ecosystem.”

Young and his advisor, URI Associate Professor Soni Pradhanang, seek to quantify the discharge of groundwater into the salt ponds as part of an analysis of what they call a “water budget” or an accounting of all of the water that flows into and out of the area.

“We know the amount of precipitation that comes down, we can quantify how much runoff goes into stream water, but one thing that’s not easy to directly quantify is groundwater flow,” said Pradhanang. “We don’t know how much water is going from the aquifers into other water bodies.”

Since the temperature of groundwater is cooler than the salt ponds in late summer, a drone equipped with an infrared thermal imaging camera can detect a plume of cool water in the ponds that is likely a discharge of groundwater. And that’s exactly what Young and Pradhanang Lab graduate student Jeeban Panthi and undergraduate Janelle Kmetz have found at Green Hill and Ninigret ponds.

They flew their $10,000 drone at 400 feet over miles of salt pond coastline and captured several infrared images showing significant cool zones suggesting that groundwater is entering the pond from the bottom. Because groundwater is freshwater and less dense than the saltwater in the ponds, it rises to the surface, delivering a clear signal to the infrared camera.

“Just because we don’t see plumes in some areas doesn’t mean there isn’t groundwater discharge there, too,” noted Young. “There could be too small of a freshwater component for it to show up in the thermal signature, or it might not be cool enough compared to the surrounding water. But one thing we can say about the plumes we found is that they have ample freshwater, signifying waters that came from the terrestrial zone.”

What that means for the health of the coastal ponds is uncertain. Discharges such as those the researchers found have likely been going on for many years, and groundwater doesn’t always contain contaminants. But identifying their locations may be useful in tracking the movement of terrestrial pollutants into the ponds in the future.

The discovery also has implications in the context of climate change. According to Pradhanang, the groundwater affects the salinity and pH of the pond water, which is critical to many water activities like aquaculture, as well as to the plants and animals that live in the ponds.

If storm surges happen more frequently, as is predicted with climate change, they might affect the amount of groundwater entering the water bodies, changing the environmental conditions and negatively affecting the wildlife that lives there. “It could have implications at an ecosystem level,” Pradhanang said.

Now that the plume locations have been identified, Young is continuing his drone flights to see how the weather and tides affect the plumes.

“Flying highly sensitive equipment on an aircraft is high stakes research,” he said. “Quantifying how the discharge changes over time is the next step. But so far it’s nice that we’ve been able to identify the sites of possible pollution contribution to the ponds.”

Once Young returns to the Coast Guard Academy next year, Pradhanang hopes future students will take up the project to identify groundwater discharge locations and quantities into other salt ponds, coastal and freshwater bodies, Narragansett Bay, and elsewhere around the region.