25 Years of Monitoring, What Have We Learned? Nutrient Levels in Massachusetts Bay and Boston Harbor

摘要

The Massachusetts Water Resources Authority (MWRA) was created in 1984 by the state legislature to address pollution caused by the discharge of inadequately treated sewage into Boston Harbor. The Boston Harbor Project included reconstruction of the Deer Island Treatment Plant (DITP) to include secondary treatment, and construction of a deep water outfall. Since 2000, wastewater effluent has been diverted from the harbor to the new outfall 15 km (9.5 miles) offshore and 30 m (100 feet) deep in Massachusetts Bay (Mass Bay). The outfall is regulated under a National Pollutant Discharge Elimination System permit from the U.S. Environmental Protection Agency (USEPA) and Massachusetts Department of Environmental Protection. The narrative nutrient criteria in the DITP permit stipulate that nutrients in the effluent "shall not cause accelerated or cultural eutrophication", that is, an increase in organic enrichment of an ecosystem that causes change to that system. Improvements in harbor water quality due to decreased nutrient loads since outfall relocation have been dramatic. In more recent years there have been modest increases in annual loadings of total nitrogen (TN) in DITP effluent the bay receives. Effluent TN levels are proportional to plant influent levels, indicating that the changes are associated with population growth and not due to operational treatment processes. Monitoring observations, a multi-parameter eutrophication index, and realistic simulations using a water quality model coupled to a hydrodynamic model all demonstrate that effects of outfall TN on Mass Bay water quality are minor. In water quality model scenario simulations with substantially increased outfall TN load there is no eutrophication or change to bay ecological function, mainly because exchange with offshore waters provides approximately 15 times more TN to the bay than the outfall.