Water Quality Conditions for Narragansett Bay

For this Status and Trends report, water quality is split into separate issues, such as toxics and excess nutrient impacts (eutrophication), although many areas experience impacts from an entire suite of pollutants. The separation is followed because the sources are often not the same, and management responses to remove or diminish each pollutant stressor often differ.

The Pollution Gradient in Narragansett Bay

Fig. EW 1. Pollution Gradient for Narragansett Bay. Arrows point towards increasing concentration of most pollutants, including metals and excess nutrients.

A useful way to explain water quality conditions in Narragansett Bay is to describe the pollutant gradient that exists there (Fig. EW1). This gradient is linked to the location of major cities and populations. Themajority of pollutant stressors in Narragansett Bay are concentrated in the upper reaches, entering the Bay through major industrial and Wastewater Treatment Facility (WWTF) outfalls or major stormwater drains and Combined Sewer Overflow (CSO) points. These sources discharge either directly into the Providence and Seekonk rivers, or just upstream in the Blackstone, Pawtuxet, Ten Mile, and Taunton rivers (Nixon, 1995).

Pollutants decrease down-bay due to dilution by cleaner seawater moving up the deep East Passage. In addition, many toxic pollutants drop to the sediments, and so contaminate the Bay floor in the uppermost reaches creating “hot spots”, with much less reaching the middle and lower Bay. More widespread ecological impacts tend to come from excess nutrients, which can be distributed over wide areas of the upper and even mid Bay, affecting water quality in summer months and bottom community type over the long term. However, even nutrients show a strong down-Bay gradient (Oviatt 2008, Oviatt et al 2002, Smayda & Borkman 2008) linked to the major sources (WWTFs and major rivers).

Toxics in the Bay

Narragansett Bay has a general north to south gradient of sediment quality, with the most impaired areas located in the urban regions of the upper third of the Bay, although localized “hotspots” occur elsewhere. Metals historically found at elevated levels in Narragansett Bay include copper, lead, zinc, mercury, as well as trace metals such as cadmium. Much of this material has entered the Bay through the major WWTFs as well as historical point source discharges from the textile, jewelry and plating industries (Nixon, 1995).

Fig. EW 2. Total Metals Influent – Fields’ Point WWTF, Providence RI 1981-2006. Source: Narragansett Bay Commission 2007 Pretreatment Annual Report.Management of the toxic load from metals and organics has enjoyed great success in recent decades. Between the late 1980s and late 1990s, large decreases in these sediment pollutant concentrations were observed in the upper Bay. Loadings from WWTFs have decreased by over 96% over the last 3 decades due to decreases in industrial loads and better regulatory control over point source discharges (Fig. EW2, Narragansett Bay Commission, 2007). In 2004-05, some of the heavily polluted sediments in the Providence River ship channel were dredged and deposited in Confined Aquatic Disposal cells at the top of the Providence River. However, the shallow upper reaches of the Providence and Seekonk Rivers, as well as “hot spots” like the Navy near-shore historical dump sites around the Bay still contain heavily contaminated sediments which can be redistributed down-bay by storm erosion (King and Ford 2000). In addition, poorly flushed coves, marinas, and harbors tend to capture fine-grained sediments that carry toxics from stormwater drains and other sources, leading to higher levels of contaminated sediments at these deposition areas.


Fig. EW 3. Surface sediment concentrations of mercury (Hg), lead (Pb), % organic Carbon, and % mud. Source: Murray et al. 2007.The greatest source of new toxic loads is shifting to stormwater and nonpoint sources. Highest levels of toxics are still found in the fine-grained, high organic carbon sediments in the Providence/ Seekonk Rivers and western Greenwich Bay (Figs EW3). A comparison study of the Bay (surface) sediment changes between 1989 and 1998 by Drs. John King and James Quinn ( King and Ford 2000, Quinn, 1998) shows significant decreases in the Upper Bay for both metals and organic pollutants, but small to moderate increases of some metals (Pb) mid-Bay thought due to erosion transport down-bay from severe storms, an echo of the formerly high contamination in the Upper Bay (Corbin 1989, King et al 1995, King and Ford 2000, Quinn, 1998).For some toxics such as mercury (Hg), air pollution sources are significant sources due to coal-burning, etc. Fig EW3 provides detail on the concentrations of lead (Pb) and mercury (Hg) as well as % organic carbon and % mud in Bay sediments (Murray et al. 2007)

Back      Section-index       Next