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When Groundwater Meets the Surface: Measuring the Invisible

It was a wet afternoon, but equipped with rain gear, I followed Professor Audrey Sawyer and her team from the School of Earth Sciences at The Ohio State University out to Darby Battelle Metro Park to learn more about their research and get a first-hand view of science in action.  Today’s objective was for the team to test out some equipment they planned to use in an upcoming research trip. We arrived at the Big Darby Creek to find it was almost completely frozen over, making liquid water inaccessible.

They were able to use their temperature probe in the riverbed next to the ice. Two students on Audrey’s team, Amelia and Alexandra, placed the probe into the ground several inches, and there it was – a temperature change indicative of groundwater. Groundwater is usually different in temperature than the surface water – sometimes in the year it is colder, sometimes warmer. In the winter, it is warmer. By measuring the change in temperature, the team is able to detect where groundwater is flowing.

After hearing a very audible ice crack, the team decided to look for a more thawed stream so readings could be taken in the stream bed itself (without worrying about falling through the melting ice). A little further down the road, and after a short hike through the muddy terrain, we found a small stream of flowing water.  They set to work measuring the stream width and using markers to grid out the area in which they were going to insert the probe. From the determined temperature differences, they are able to map where the groundwater was discharging.

As the groundwater flows up, it could carry with it nutrients and contaminants that feed into the rivers and streams, which end up in a larger body of water. Quantifying this information can help us understand how much groundwater contributes to what we see in our rivers and lakes. In the project funded by the Ohio Water Resources Center, the Sawyer team measured how much groundwater was seeping into Lake Erie to determine if the groundwater was contributing significant nitrogen and phosphorus – the key nutrients that lead to harmful algal blooms. Mapping out an area of groundwater discharge can help put eyes on the “hidden” contributions groundwater makes to nutrient pollution.

The team worked diligently to attain accurate readings at each point in the grid. The water and sediments were almost freezing – they were getting readings as low as 0.1 degrees Celsius, so they each took turns inserting the probe into the river sediment while the other two would record the temperature and let their hands warm up. This data, combined with their expertise of river formation and characteristics, will contribute to the production of usable maps – a way to “see” groundwater and keep its contributions in mind.