and Massachusetts Bay
Total Suspended Solids (TSS)
A mix of silts and organic waste particles, tinier than grains of sand, give raw sewage its cloudy appearance. These suspended solids are largely removed (85% or more) from wastewater by secondary treatment. Suspended solids in MWRA sewage discharged to Boston Harbor averaged about 160 tons per day in 1988, about 20 tons per day in 2000, and virtually nothing in 2001. Sewage solids discharges have affected the harbor both directly, by clouding the water, and indirectly by the BOD and toxic contaminants associated with the solids.
SOLIDS POLLUTION AND WATER CLARITY
The aesthetic impact of solids discharged by two under-functioning treatment plants in the 1980s on Boston Harbor was severe: five nearshore wastewater plumes and a large sludge plume were highly visible. Water clarity in the black sludge plume was often zero. Aesthetics are important, but water clarity also has a major impact on marine ecosystems. Many important species of plants, including seagrasses, require adequate light. If poor water clarity means insufficient light penetrates to the seafloor, where seagrasses are rooted, the plants will die off. When this happens, the bottom sediments are no longer bound and stabilized by the roots, and can be more easily stirred up, further exacerbating poor water clarity.
New England waters are normally rather turbid (compared, for example to tropical waters) because this highly productive coastal environment supports rich populations of phytoplankton and zooplankton. Natural turbidity is highest in spring through fall. Other natural sources of suspended solids include particles brought in by rivers, and sediments stirred up into the water by storms or normal wave action. In Boston Harbor, water clarity varies from region to region, with the clearest water found at the mouth of the harbor. More cloudy waters occur in shallow regions like Dorchester and Quincy Bays, where bottom sediments are easily resuspended. Many observers, from marina owners to lobstermen, report improved water clarity throughout the harbor over the past decade; these anecdotal observations have been confirmed scientifically at the former site of Nut Island effluent discharges.
Eventually, water clarity should improve so that the areas of seagrasses can expand from their limited habitat in Hingham Bay to other areas of the harbor.
SOLIDS POLLUTION AND SEAFLOOR COMMUNITIES
Because much of sewage-related solids are organic materials (see Organic Material, BOD) which use up oxygen, the effects of solids pollution on the seafloor mirrors the effect of BOD; too much sewage solids will deplete the oxygen available to marine life on the bottom. A second effect is that of toxic contamination; again, toxic contaminants tend to stick to particles and accumulate on the bottom. These toxic materials adversely affect sensitive marine organisms by inhibiting growth and reproduction. Toxic contaminants can also be taken up by the tiny worms and shellfish living on the bottom. When larger fish feed on these tiny animals, the toxic contaminants can bioaccumulate in the larger organisms. Eventually, concentrations of toxic contaminants can be high enough to be a health threat to human consumers of seafood. (See Toxic Contaminants.)
The dramatic drop in sewage solids discharged to the harbor over the past decade has been associated with an encouraging recovery of the small shrimp-like amphipod Ampelisca, which is moderately pollution-intolerant. In 1989, Ampelisca covered about 25% of the harbor floor and a decade later covers more than 60% of the harbor floor, indicating a recovery from severe pollution. The trend to a more normal seafloor community should continue as discharge of sewage solids to the harbor ends.