CHARLESTOWN — Environmental scientist and on-site wastewater manager Matthew Dowling presented an update to the Town Council this month on an ongoing project to measure the nitrogen-removing performance of advanced septic systems.
Funded by a $674,201 grant from the Environmental Protection Agency, the study, which began in 2016, is part of the Charlestown Coastal Watershed Protection and Restoration Program and also involves the University of Rhode Island, the Salt Ponds Coalition and Save The Bay.
“One of the tasks under the grant consisted of monitoring the effluent quality coming out of newly installed or recently installed nitrogen reducing septic systems within our watershed,” Dowling told the council at its April 8 meeting.
Advanced wastewater treatment systems, also referred to as denitrification systems because they reduce nutrients such as nitrogen, are required by the state for new buildings on the coast. Despite the more stringent regulations, however, groundwater nitrate levels remain elevated in several Charlestown neighborhoods and in the three salt ponds.
High nitrogen levels cause algal blooms, which in turn can cause low oxygen levels that can kill fish and shellfish. Dowling said it was important, therefore, to document the effectiveness of the advanced wastewater treatment systems.
“We chose to examine effluent quality coming out of these nitrogen reducing systems basically for two reasons,” he said. “One is that they’re required in our coastal watershed map for every new septic system that gets installed, and over the past 10 years, we have had about 40 annual installs of this technology and we currently have about 700 of these technologies in the ground right now and we want to have an understanding as to how they’re functioning.”
Rhode Island has set an upper limit for nitrogen of 19 milligrams per liter of effluent, but does not require the monitoring of effluent from nitrogen reducing septic systems.
The only way to determine whether a system is functioning is to collect and analyze the effluent. Dowling explained that in a previous study, URI researchers had collected samples from advanced septic systems throughout Rhode Island and compared the data to samples from Barnstable County, Massacusetts — the Cape and Islands — where regular effluent monitoring is required. Not surprisingly, the study found that regular, mandatory monitoring had a positive impact on reducing nitrogen levels.
“Same geology, same water table, same type of development, same use as Rhode Island where the systems aren’t required to be sampled, and lo and behold, the systems in Barnstable County are functioning much better than those technologies in Rhode Island where the sampling is not required,” Dowling said.
University of Rhode Island Ph.D. candidate Bianca Ross, of the laboratory of soil ecology and microbiology, is the lead investigator in the current study.
Ross explained that the 46 Charlestown study sites were about evenly split between year-round and seasonal residences.
“I wanted to know, do the seasonal sites and their total overall nitrogen values differ significantly from the year-round sites,” she said. “There really is no difference so far …. The lack of difference here is really encouraging for us. Of course, homeowners who live at these homes year round are paying the same amount of money for their systems as those who only live there in the summer months.”
Ross and her team did find that the type of technology used in the advanced systems had an impact on nitrogen removal efficiency. While all four of the advanced septic systems studied are capable of meeting the state’s nitrogen standard, three performed to expectations and a fourth did not.
“The next question in my research is, why is this happening?" she said.
Ross said she planned to relay her findings to the manufacturers and work with them to improve performance.
“Ideally, this monitoring that I’m doing is going to trigger some sort of corrective action,” she said. “I have about a year’s worth of sampling left. In subsequent months, we’re going to target what did they change and did the change improve performance overall.”
A study conducted by URI in 2014 determined that septic systems contribute 80 percent of the nitrogen in the salt ponds. Advanced treatment systems are required only for new construction, major rebuilds, or if an existing conventional system fails, so there are still many conventional septic systems on coastal properties. Many septic systems in Charlestown are older systems that do not remove nitrogen, and at the time of the study, 138 systems were un-permitted or substandard.
In densely populated Quonochontaug, where groundwater is the principal source of drinking water, nitrate pollution of wells has become a problem that becomes more acute during the summer, when the demand for water increases along with septic use and homeowners apply lawn fertilizers containing nitrogen.
Ross said she was confident that nitrogen removing treatment systems, when properly calibrated and maintained, would perform to expectations.
“As we get more and more of these systems and as we improve the performance of these systems with my study, with other studies, with monitoring efforts, we’re definitely hoping to reduce the nitrogen overall,” she said.