WELLFLEET — Water resources consultant Scott Horsley presented the latest Targeted Watershed Management Plan to the board of health on Nov. 9 with minimal sewerage and  myriad advanced ecofriendly technologies.

The “hybrid” plan, as Horsley called it, is estimated to cost $106.6 million, less than half of the projected cost of a traditional sewerage plan — at least $237.5 million.

The goal of the plan, Horsley said, is to mitigate water quality impairment associated with high nitrogen levels, to restore marine habitats, and to bring Wellfleet Harbor into compliance with the Clean Water Act. Nitrogen contamination from wastewater, fertilizers, and stormwater runoff has resulted in the eutrophication of coastal waters, the loss of native eelgrass habitat, and an increase in “black custard” sediments that threaten the shellfish industry.

The watershed plan is the result of a 2010 Mass. Estuary Project (MEP) study whose findings led to a determination that the nitrogen load in the Wellfleet Harbor watershed must be reduced by 31.2 percent to meet Clean Water Act requirements.

In the hybrid plan, the only sewerage would be downtown in the Duck Creek and Cove subwatersheds as well as at the 95 Lawrence Road affordable housing development. Both sewer systems would have centralized collection and treatment facilities nearby. The MEP report found that the Duck Creek and Cove subembayments contained the highest levels of nitrogen, with Duck Creek requiring a 76-percent nitrogen reduction and the Cove requiring a 79-percent reduction.

The study found that wastewater was the primary source of contamination, producing 82 percent of total nitrogen concentration. Eight percent was from stormwater, and another 8 percent from fertilizer.

The bulk of the plan listed alternatives to sewers: a stormwater management plan to be integrated into the Route 6-Main Street reconstruction project, the use of permeable reactive barriers, shellfish propagation, and a reliance on the Mayo Creek and Herring River projects, which would help reduce nitrogen levels by restoring salt marshes. Horsley also detailed the use of enhanced Innovative and Alternative (I&A) septic systems for individual households outside of the planned sewerage zones.

This new generation of septic systems includes two novel components located between the septic tank and leach field: an aeration chamber and a wood chip bioreactor. Horsley explained that wood chips are natural tools for nitrogen mitigation: “Wood chips are really effective sources of bacteria that use nitrogen as their food source and de-nitrify it,” he told the health board. The result is the removal of 80 to 90 percent of the nitrogen from wastewater, he said.

As the technology advances, this new generation of septic systems could be enough to help reduce nitrogen loads to the threshold amount. “They’re equivalent or very close to being equivalent now to a traditional sewer system,” Wellfleet Clean Water Advisory Committee Chair Curt Felix told the Independent.

Enhanced I&A septic systems also degrade and decompose other contaminants of emerging concern, such as PFAS, said Felix.

But will it be on individual homeowners to get these new septic systems installed?

“That’s the $64,000 question,” Felix said. The answer, he hopes, is some sort of betterment agreement in which the town and homeowners share the cost. “There should not be a disadvantage there,” Felix said. “They’re helping the town meet its overall watershed requirements.”

Horsley’s analysis, however, showed that if Title 5 systems are phased out in favor of enhanced I&A systems over a 25-year period, the town will still meet watershed requirements through natural attrition. “A failed system, a cesspool, or upon real estate transfers or the building of new homes all trigger the requirement for an enhanced I&A system within 25 years,” Felix said.

The use of enhanced I&A septic systems would also make the town eligible for potential grants and low-interest loans from Barnstable County, said Horsley. “There may be some very attractive funding,” he told the board.

“There is going to be a lot of other assistance available,” Felix added. An infrastructure bill proposed by the Biden administration includes a substantial amount of funding for wastewater infrastructure, said Felix. “We’re going to try to access that funding as well,” he said.

The focus of the plan, Felix said, is combining affordability and efficacy. “One of our main focuses was to make sure that we pursued the technologies that were the least expensive and had the biggest bang for the buck,” he said.

The plan also promotes the development of affordable housing, Horsley and Felix both argued. By building a wastewater treatment plant for existing Title 5 systems on the Lawrence Road property and connecting the nearby fire and police stations and elementary school to the new sewer system, nitrogen levels will fall from 26 to 35 mg/liter to 5 to 10 mg/liter. “The net result is that we can actually reduce loads in the watershed while adding affordable housing,” Felix said.

“This is a unique and significant part of this project,” Horsley told the board.

The plan was submitted to the Mass. Dept. of Environmental Protection (MassDEP) in August as part of the town’s watershed permit application. The permit, if approved, would be the second of its kind in the state.

MassDEP requires a contingency plan in which a traditional sewer system would be built if the new technologies fail. An adaptive management approach will be used to closely monitor the effectiveness of the technologies over the course of the plan’s implementation, which Horsley predicted will take up to 20 years to complete.

Despite the plan’s long time frame, “its benefits are enormous,” Felix said. Aside from capital cost savings, the project will provide economic development opportunities for local engineers and septic installers. “You’re keeping a lot of that money local,” he said.

Enhanced I&A septic systems are also significantly less energy intensive, Felix said, and “therefore have a much lower carbon footprint. But at the end of the day, we’ve got a very cost-effective approach that won’t be too burdensome on the homeowner.”