Experimental Solutions to Reduce Nitrogen Pollution on Cape Cod

The Problem: Nitrogen Pollution

For many towns on Cape Cod, decades of nutrient pollution—especially nitrogen—in waterways from septic system effluent and fertilizer runoff threaten drinking water supplies and lead to toxic algae blooms, fish kills and beach closures.

Nitrogen is a key element in our natural systems and is essential for plant growth. But too much can be a bad thing. In our developed landscape, excess nitrogen from agriculture, home fertilizer use, and traditional septic systems that weren’t designed to remove nitrogen fuel the growth of plants like algae that out-compete beneficial plants and can be toxic to marine life and humans. Worse, elevated nitrate levels can be dangerous to human health, especially for children. When these excess nutrients flow into groundwater, they contaminate wells used for drinking water.

As the oceans absorb the increasing carbon dioxide and heat in our atmosphere, the waters warm and acidify, which stacks up the challenges for coastal ecosystems. Nutrient pollution-fueled algae blooms use up much of the oxygen in the water and add even more carbon dioxide—and the carbonic acid it produces—exacerbating stress on our waterways.

This damage isn’t just bad for the environment, it’s costly as well. Nitrogen-fueled algal blooms can close beaches, threatening a key income source for communities that rely on beachgoers and tourism. For families whose home wells are contaminated with nitrates, the costs of filtering or purchasing bottled water for drinking and cooking quickly add up. The consequences for marine health impact fisheries and aquaculture, a challenge for the seafood and tourism economies of these coastal towns.

What’s the innovative solution? Working with nature itself, with a little help from technology, too.

Nature is a Solution to Nitrogen Pollution

Nature-based solutions use natural features or processes, sometimes alongside manmade structures, to adapt to the impacts of climate change, enhance community resilience or improve water quality. In addition to addressing complex challenges, they often provide many other benefits, like wildlife habitat, flood protection, recreation opportunities, cost reductions and other community improvements. For example, restoring wetlands can help reduce stormwater flooding, filter pollutants to improve water quality and provide key habitat for fish, birds, amphibians—and nature lovers!

Nature vs. Nitrogen Pollution

To mitigate these nitrogen pollution challenges, we are advancing nature-based nutrient management solutions and exploring opportunities for innovative, long-term funding and financing to implement these fixes. TNC is working with multiple collaborators, including the Town of Barnstable, Barnstable Clean Water Coalition (BCWC), the Massachusetts Alternative Septic System Test Center (MASSTC) and the U.S. Environmental Protection Agency (EPA).

Traditional septic systems remove viruses and bacteria from wastewater but allow large quantities of nutrients and other contaminants to filter into groundwater and surface water supplies. Though many towns on Cape Cod are planning to expand their sewer networks, these plans will require billions of dollars and several decades to implement. TNC and BCWC, with support from EPA and the U.S. Geological Survey, conducted research and engaged with communities to identify nitrogen-reduction alternatives which help address the problem more quickly and cost-effectively. We are now piloting two of the most promising solutions in the Three Bays Watershed in Barnstable:

• Cranberry Bog Restoration: Restoring the Marstons Mills cranberry bog complex into an ecologically functional wetland designed to soak up nitrogen from water as it flows through the wetlands on its way to the Marstons Mills River and the ocean. In addition to traditional wetland restoration, the project includes a bioreactor, which uses woodchips to help remove additional nitrogen from the system.
• Innovative/Alternative (I/A) Septic Systems: Installing and monitoring the performance of woodchip-based alternatives which remove nitrogen far more effectively than traditional septic systems.

The results of these pilots will inform future steps and broader solutions in conjunction with the community.

Pilot Program: Restoring Cranberry Bogs and Enhancing Nitrogen Filtration

Much of the nitrogen polluting the Three Bays estuary flows through the Marston Mills cranberry bog complex. Restoring the natural wetland ecosystem processes that existed prior to cranberry cultivation will help filter out the nitrogen before it empties into Barnstable’s coastal waters. The wetlands will also improve wildlife habitat, carbon sequestration and recreational space for people.

In July 2020, a small-scale experiment designed to aid with nitrogen removal from the bog system was built in partnership with the bog owner, TNC, BCWC and EPA’s Office of Research and Development (ORD). This experiment filled part of a cranberry bog ditch at the head of the Marston Mills River with woodchips. Known as a bioreactor, this design has been used to remediate nutrient pollution in agricultural streams. Through this pilot, the team is learning valuable lessons to determine how bioreactors might be included in the ecological restoration to optimize nitrogen removal.

In early 2022, conceptual designs for the restoration were completed. These plans, designed by Inter-Fluve with input from the Native Land Conservancy, would restore Atlantic white cedar swamps on the site and would reshape channels and wetland habitats to enhance its ability to filter out excess nitrogen. Successful restoration of the area could provide 20% of the required nitrogen reduction in the watershed. The next phase of engineering design on the bogs is now underway and expected to be complete by fall 2023, supported by a five-year Pilot Watershed Initiative grant from EPA’s Southeast New England Program.

Newer, enhanced I/A septic systems have been shown to successfully remove nitrogen in wastewater before it is released into the ground. Many of these systems rely on natural materials—like woodchips and limestone—as part of a system that hosts beneficial bacteria that turn nitrogen from the water into atmospheric nitrogen, which is much less of an issue in excess.

“Removing nitrogen using these next-generation septic systems can help us address these critical water quality concerns more quickly and affordably than sewer expansion in many places, but both are important tools,” says Emma Gildesgame, climate adaptation scientist for TNC in Massachusetts. “The hope is that systems which remove significant amounts of nitrogen will become the new standard for installation, leading to widespread adoption amongst the community. That’s where TNC and our partners come in.”

Barnstable Clean Water Coalition is piloting the replacement of traditional septic systems with these I/A systems. By Spring 2022, 14 homeowners in the Shubael Pond neighborhood were upgraded to these systems. EPA and partners are monitoring the resulting changes in groundwater nitrogen concentration for three years to determine the impacts of having them clustered in a single neighborhood.

In order to make pollution-removing septic systems the new normal, these new technologies must demonstrate that they meet the rigorous approval standards set by the Massachusetts Department of Environmental Protection. The Department is also proposing requiring better nitrogen reduction technologies in nitrogen sensitive areas, which includes much of Cape Cod, which could be promising for I/A system implementation.

“Proving out these nature-based solutions will allow for wider spread adoption and faster water quality improvements not just in Barnstable, but across the region and for other coastal communities,” says Chatelain.

Gildesgame adds, “As we move to scale these projects, testing and demonstrating the value of nature-based alternatives to remove nitrogen will ensure the goals we build into funding mechanisms are appropriate and achievable.”