New wastewater management measures would make great strides toward reducing nitrogen in Hog Creek in Springs, Molly Graffam and Ron Paulsen of Cornell Cooperative Extension told the East Hampton Town Board on Tuesday.
The observation was included in the findings of an investigation to locate and quantify groundwater-derived nitrogen getting into the creek, a priority water body for remediation. The study was paid for by the town’s community preservation fund water quality improvement program.
While upgrading aging septic systems to innovative alternative models “can get us most of the way there,” said Mellissa Winslow, a senior environmental analyst in the town’s Natural Resources Department, more is needed. “This investigation completed by C.C.E. is the first step to reaching that goal,” she said.
The presentation focused on below-the-surface groundwater discharge — the flow of water through permeable sediments. That water mostly moves horizontally and percolates upward, Ms. Graffam said, and has been identified as a significant source of nitrogen to surface waters.
The investigation was meant to identify areas within the water body that exhibit both elevated nutrients and discharge rates, as they are transferring the most nitrogen to surface waters. Direct groundwater, pore-water (water contained in pores in soil or rock), and surface water measurements provide the most accurate nitrogen load information. Once identified, they can be prioritized for remediation.
Groundwater temperature is relatively stable year-round, while surface water temperature varies seasonally. A probe is inserted into the first two feet of sediment on a waterbody’s bottom to measure the temperature and conductivity between pore-water and surface water, Mr. Paulsen said. “Once we identify where groundwater is coming through, we need to know the rate of it,” he said. When temperature and conductivity indicate submarine groundwater discharge, samples are collected for nutrient analysis.
In Hog Creek, pore-water had an average of 2.3 milligrams of nitrogen per liter, Mr. Paulsen said, but could range to as high as 10 milligrams per liter. Nitrogen “hotspots” were mapped, with the entrance to the creek having the highest average value. Six of nine samples collected from the entrance during a summer 2020 survey had nitrate concentrations greater than 5 milligrams per liter, he said. The headwaters region had porewater with the lowest conductivity, indicating elevated groundwater inputs, he said.
Quantifying pore-water nitrogen concentration and seepage rates is useful, Ms. Graffam said. “We use that and prioritize and rank certain areas.”
There is no one-size-fits-all solution to cure water bodies, Ms. Graffam said, and changes such as septic system upgrades will happen over the course of decades. But “we would suggest other treatments more immediately,” such as permeable reactive barriers — usually a long, narrow trench filled with a reactive material such as iron, limestone, carbon, or mulch to remove contaminants as groundwater passes through it — and nutrient bioextraction, the use of seaweed or filter-feeding shellfish. Native planting and wetland restoration are also recommended.
Cornell Cooperative Extension researchers identified five areas in Hog Creek for potential permeable reactive barriers or nutrient bioextraction. “We will be moving with a multi-phase approach,” Ms. Graffam said, “and will apply for C.P.F. funds this summer to move forward with remediation.”
