By ecoRI News staff
NARRAGANSETT, R.I. — A team of oceanographers from the University of Rhode Island has completed a 10-year study of water quality in Narragansett Bay and found that reductions in nitrogen discharged into the bay from sewage treatment plants has resulted in much clearer water and fewer algae blooms.
However, stormwater runoff from heavy rains still leads to poor water quality.
Regulations required that Rhode Island’s 11 coastal wastewater treatment plants reduce their discharges of nitrogen into Narragansett Bay by 50 percent. These facilities achieved a 30 percent reduction by 2005, when the URI study began, and the remainder by 2013, according to URI Graduate School of Oceanography professor Candace Oviatt.
“The objective of our study was really to better understand hypoxia — a lack of oxygen — in the bay and determine how it was related to high nitrogen concentrations,” Oviatt said.
Nitrogen from wastewater treatment plants is largely responsible for the excessive growth of algae, which then sinks to the bottom and rapidly uses up all the available oxygen in the water. It can lead to large-scale fish kills, such as happened in Greenwich Bay in 2003.
The researchers collected data from a summer network of 12 instruments in upper Narragansett Bay that measure water temperature, salinity, oxygen, chlorophyll and pH. They also analyzed monthly nutrient samples collected from 13 locations, and they created a series of computer models to predict how Narragansett Bay would respond to reduced nitrogen.
Oviatt said the data confirmed that the wastewater treatment plants achieved the 50 percent reduction in nitrogen discharges. That reduction has resulted in a 15 percent decrease in phytoplankton growth in the upper bay and 25 percent to 30 percent decrease in the mid-bay.
As a result, the water in Narragansett Bay has become noticeably clearer. Water in the upper bay is now as clear as in the lower bay, according to Oviatt. It’s so clear, in fact, that many fishermen are worried that the water has been poisoned, she said.
“The water is clear, that’s for sure,” Oviatt said. “But there is no pollutant in the water that is killing things and making it clear. It’s due to the lack of nutrients.”
One objective of the reduction in nitrogen discharges was to control the hypoxia episodes in Narragansett Bay, but Oviatt said hypoxia isn’t yet completely under control. During dry years, such as 2014, low oxygen still occurs but in shorter and less-intense events. In rainy years such as 2013, however, freshwater runoff creates a cap over the salt water and enhances the likelihood of hypoxia episodes.
“I think we’re making progress, but in rainy years we’re still going to have hypoxia,” Oviatt said. “The next step is to keep an eye on how habitats respond. Are there more fish in the upper bay? Is there enough food available for aquaculture? The level of nutrients in the water now probably isn’t causing harm, but we should continue to assess it.”
She noted that the one area of Narragansett Bay where water quality remains poor is Greenwich Bay, but it has little to do with wastewater discharges.
“The treatment plant there achieved its reductions, at least intermittently, but other nitrogen is coming into the bay through the groundwater that the plant has no control over,” she said. “The groundwater is picking up nutrients from cesspools and septic fields, and it will be a while before it wears out. The water quality there is still improved, but it’s got a way to go.”