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Scientists study flow of groundwater into bays - results may help track pollution

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July 26, 2010

On a small, homemade barge, built from the skeleton of an old ship, a gray slurry of bay bottom sand flows out of a pipe into a bucket. Two scientists, a well driller and two student interns drill a hole in the floor of the Indian River Bay. They’ll install a very long pipe into the hole and use it to monitor groundwater – how much flows into the bay, how salty it is and how many nutrients it carries with it.

University of Delaware scientist Holly Michaels stands on deck, donning an orange hard hat. She’ll take data accumulated from this and other wells, onshore and offshore, and put them into a computer model that will explain the way fresh groundwater reaches the estuary.

“Basically, we’re trying to understand how groundwater flows into the bay and what happens to the nutrients it carries,” she said.

“The big reason we’re doing this is to study the nutrients in groundwater. The big one is nitrate,” she said. Nitrate is the compound that encourages excessive algae growth in the bays and leads to low levels of dissolved oxygen, which kill fish. Michaels said she and her team hope the model they create from several years of taking data will help scientists estimate and predict the nutrients that enter the bay through groundwater.

Michaels said scientists will also test the groundwater to see how old it is. That’s something that can help scientists and policymakers understand when they should start seeing results from nutrient-reducing measures on land. Scientists rely on chemicals in water to help estimate its age, said Scott Andres, hydrologist with the Delaware Geological Survey. He said there is no way to determine water’s absolute age.

The well is tens of feet long, a skinny white pipe punctuated in seven places by fine mesh screen. At the end is a series of tubes from which scientists can take samples from each different layer.

Andres described the work as brute force. “We’ve got basically hand tools,” he said. “The water is pretty shallow – about waist deep – so we couldn’t bring a large barge in here. We’d get grounded.” The small barge did ground once and the team retreated to land to install wells there until the next high tide, he said.

Chris Bason, science and technical coordinator for the Center for the Inland Bays, said this study will help scientists understand more about pollution that enters the bay through groundwater. “We know it can be an important pathway for pollution. It’s estimated that about 75 percent of nitrogen entering Rehoboth Bay during the summertime comes in with groundwater seeping from the bay bottom,” he said.

Learning where groundwater enters the bay, how long it takes to get there and how much pollution it carries will help scientists restore the Inland Bays and give them an idea of how long it will take, he said.

Bason said Delaware’s Inland Bays are representative of most of the Mid-Atlantic, so the study can be widely used. Work on studying groundwater entering the Inland Bays began 10 years ago, said University of Toledo geologist David Krantz.

He said that project sought to examine how water and nutrients entered the bay – from the atmosphere, the land and groundwater systems. Scientists learned where groundwater sources for the bay were located during that study, he said, including two plumes of freshwater, one of which reaches more than half a mile out into the bay. They also tested the age of the water.

“So, if you apply nitrogen to a field, and it percs into the groundwater, how long does it take to get to the estuary and what happens along the way? That’s what we wanted to find out,” said Krantz.

At the time, scientists had a lot of field data, but they lacked a hydrogeological modeler. That’s where Michaels came in for this project. In addition to setting wells and taking samples, she’ll add all the data the project collects into a computer model.

Krantz said the project 10 years ago was a snapshot – it was short term. This time, scientists plan to leave the wells in place for several years to obtain long-term data and see how the seasons affect groundwater and nutrient flow into the bays.

Reference: 
For questions and information, contact DGS at
delgeosurvey@udel.edu, 302-831-2833