Frank Carini: Nitrogen from houses threatens coastal salt ponds
Mashchaug Pond, in Westerly, one of southern Rhode Island's salt ponds, also called lagoons.
From ecoRI News (ecori.org)
CHARLESTOWN, R.I. — People flock to live on and visit the coast here, but in the town’s most densely developed area, excess nitrogen from all those coastal dwellings is threatening the health of three salt ponds that connect to Block Island Sound and are the foundation of the local tourism industry.
This salt-ponds watershed makes up 33 percent of the town but contains 63 percent of all Charlestown dwellings. Dense development around the Ocean State’s coastal salt ponds isn’t limited to Charlestown.
The salt-pond region of southern Rhode Island extends from Maschaug Pond in Westerly to Point Judith Pond in Narragansett and forms the natural boundary between the Atlantic Ocean and a shallow freshwater aquifer. This watershed is so built up that vital ecosystems are under enormous pressure.
“A burgeoning population and increasing competition among activities threatens to overwhelm the capacity of the salt ponds to absorb wastes, provide shelter for boats and vessels, attract residents and tourists and underpin premium real estate values,” according to the Coastal Resources Management Council’s Salt Ponds Region Special Area Management Plan. “Large areas of the salt ponds are poorly flushed, which makes them valuable as fish and shellfish nurseries, but, also particularly susceptible to eutrophication and bacterial contamination.”
Studies and surveys by the Environmental Protection Agency (EPA) and by state agencies across the country have found that nonpoint source pollution, such as excess fertilizers, herbicides, and insecticides, oil, grease, and toxic chemicals from runoff, and bacteria and nutrients from faulty septic systems, causes the most harm to rivers, streams, lakes, estuaries, coastal waters, and wetlands.
In Rhode Island’s salt-pond region, cesspools and failing and substandard septic systems are the largest source of bacterial and nutrient contamination. In Charlestown, work done by the University of Rhode Island's Cooperative Extension and Nonpoint Education for Municipal Officials has found that 80 percent of groundwater nitrate is from onsite wastewater treatment systems.
In the town’s coastal-pond watershed sampling of private wells has found nitrogen concentrations that approach or exceed the EPA established maximum contaminant level of 10 milligrams per liter. The EPA action level is 5 mg/L. Charlestown relies exclusively on groundwater for drinking.
Besides contaminating drinking-water wells, this nitrogen-enriched groundwater also eventually flows into Charlestown’s three coastal salt ponds, where it causes eutrophication and increases the risk of hypoxia.
Since 1994, Green Hill Pond and eastern Ninigret Pond have been closed to shellfishing because of “significantly deteriorated water quality.”
The total average annual influx of nitrogen to Charlestown’s three coastal ponds is nearly 1.2 million pounds, according to the Rhode Island Department of Environmental Management (DEM). Here is the breakdown: Green Hill Pond, 46,903 pounds; Ninigret Pond, 44,794; Quonochontaug Pond, 24,579.
Matt Dowling, hired in 2008 as Charlestown’s first full-time onsite wastewater manager — only a handful of Rhode Island municipalities have a such a position — recently told ecoRI News that reducing nitrogen loading is a major priority from both a public-health and environmental-protection perspective.
Charlestown began addressing the problem in earnest four years before Dowling, an environmental scientist, was hired.
In the town’s coastal-pond watershed sampling of private wells has found nitrogen concentrations that approach or exceed EPA established maximum contaminant levels.
Filling in holes
In 2004, three years before a Rhode Island enacted a law to address a statewide problem, Charlestown officials took a bold step by requiring the elimination of all cesspools, which are merely holes in the ground that do nothing to treat human waste. These 55-gallon or so concrete- or stone-lined pits with holes in which sewage is flushed offer no treatment for pathogens, harmful bacteria, and nutrients, most notably nitrogen.
Cesspools and failing septic systems contaminate groundwater, the only source of drinking water in Charlestown. Conventional septic systems, meanwhile, do little to address the problem of nitrogen loading.
The town’s forward-thinking ordinance that required all cesspools be removed and replaced by 2009 was later modified. A zoned phase-out was implemented that mandated cesspools to be replaced by June 2014. Mission nearly accomplished.
Of the nearly 1,000 cesspools that once marked the local landscape, only 12 remain, according to Dowling.
The removal of the remaining cesspools is being managed through the town’s municipal court or by the Wastewater Management Commission’s waiver program that grants relief to property owners with financial hardships.
Statewide, DEM identified 1,084 cesspools subject to Rhode Island Cesspool Act of 2007 provisions requiring the replacement of those within 200 feet of a coastal shoreline feature, within 200 feet of a public drinking-water well, or within 200 feet of a public drinking-water reservoir, according to an agency spokeswoman.
She recently told ecoRI News that at last check 752 of those cesspools were replaced with a septic system and 147 were removed from service because the property connected to a sewer line, leaving 185 cesspools that haven’t yet come into compliance with the law.
“It is worth noting that these numbers are a snapshot,” she wrote in an e-mail. “There are sites working through the permitting process all the time so it is likely that more cesspools have been removed from service than are reported here.”
There are more than 3,000 onsite wastewater treatment systems in Charlestown’s salt ponds watershed and nearly 84 percent are within a ‘Lands Developed Beyond Carrying Capacity’ area.
Nutrient overload
Conventional septic systems typically have final effluent nitrogen concentrations of 44 milligrams per liter. For a three-bedroom home, this amounts to a nitrogen discharge of some 21 pounds annually.
In Charlestown, there are 3,008 onsite wastewater-treatment systems in the salt ponds watershed. Of those, nearly 84 percent, including 138 systems classified as unpermitted and/or substandard and installed before 1968, are within a Coastal Resources Management Council “Lands Developed Beyond Carrying Capacity” area, which frequently means one residential or commercial unit per one-eighth to half an acre.
In Charlestown, Dowling noted that the housing density is 8-10 dwellings per acre, and each lot has a septic system and well.
“Such intense development was the major source of contamination to groundwater and the salt ponds,” according to the Salt Ponds Region Special Area Management Plan. “High nutrient loadings and contaminated runoff waters were resulting in a high incidence of polluted wells and increasing evidence of eutrophic conditions and bacterial contamination in adjoining salt pond waters.”
Septic systems with nitrogen-reducing technology, however, are designed to lower nitrogen concentrations in wastewater by 50 percent. Dowling and the town are working with property owners to install this technology in the coastal ponds watershed. In the past eight years about 35 septic systems with this technology have been installed in Charlestown.
In 2016, Charlestown received an EPA grant to, among other things, help upgrade 15 substandard septic systems with nitrogen-reducing technology. The chosen homeowners will be compensated 75 percent of the total cost. The $674,201 grant also is funding a quarterly effluent sampling program for up to 50 property owners with denitrification systems.
To make sure that these expensive systems — $25,000 on average — are working properly and not underperforming, Dowling said data are needed to measure nitrogen output and to adjust the systems to meet their optimal nitrogen-reducing capacity. Barnstable County on Cape Cod is running a similar sampling program to help ensure a substantial decrease in nitrogen loading.
If the town were to be sewered — an expensive proposition that would require an agreement with South Kingstown — more property would be opened up to development, Dowling explained. Less nitrogen would be discharged to groundwater and the coastal ponds, but other development pressures would increase.
The four-year grant is also being used to help the town’s voluntary Recommended Landscaper Process partner with Save The Bay to install six demonstration rain gardens on public property, and partner with the Salt Ponds Coalition to establish two surface water sampling stations in Green Hill Pond to track nutrient impacts.
Modeling has demonstrated that fertilizer use contributes as much as 20 percent of the groundwater nitrogen in densely areas where high-maintenance lawns are clustered, according to Dowling.
Lawn chemicals aimed at killing pests, insect or plant, also take a toll on the ponds' health. During a recent visit to the area, two adjacent homes on Powaget Avenue had "Lawn Chemicals Applied" signs in their front yards.
Frank Carini is editor of ecoRI News.