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Mystic River Watershed

Environmental Challenges for the Mystic River Watershed


Bacterial Challenges

Reducing high levels of bacteria in the Mystic River has been a top priority for The Mystic River Watershed Initiative.

Elevated bacteria levels are primarily caused by the following sources:

  • Illicit sewage discharges to storm drain systems

    Illicit discharges are generally any discharge from a storm drain system that is not composed entirely of stormwater. Illicit discharges are a problem because, unlike wastewater which flows to a wastewater treatment plant, stormwater generally flows to waterways without any treatment. Illicit discharges often include sewage, bacteria, viruses, phosphorus and nitrogen (nutrients), surfactants, and various toxic pollutants.

    EPA enforcement efforts have stopped over 31,000 gallons per day of sewage from being discharged to the watershed through illicit connections.

  • Combined Sewer Overflows (CSO)
    • CSOs occur when wastewater containing untreated human waste, industrial waste and other debris is carried through the stormwater pipes and discharged into the River. Boston's sewer system was originally designed to carry sewage and stormwater in the same pipe to a sewage treatment plant. After heavy rainfall or snowmelt events, however, the wastewater volume can be more than the sewer system or treatment plant can handle. For this reason, combined sewer systems were designed to have safety valves that allow overflow after rain events, which results in wastewater being discharged directly into the River. Visit EPA's National Pollutant Discharge Elimination System web site for additional information on Combined Sewer Overflows (CSOs)
    • Massachusetts Water Resources Authority (MWRA) information on CSOs in Greater Boston area Exit
  • Uncontrolled urban stormwater runoff that contains pet and animal waste

    Stormwater runoff occurs when precipitation from rain or snowmelt flows over the ground. Impervious surfaces like driveways, sidewalks, and streets prevent stormwater runoff from naturally soaking into the ground. Stormwater can pick up sediments, oil, nutrients like phosphorus and nitrogen that can cause algae, debris, chemicals, dirt, and other pollutants, which flow into a storm sewer system or directly to a lake, stream, river, wetland, or coastal water. Anything that enters a storm sewer system is discharged untreated into the waterbodies we use for swimming, fishing and sometimes drinking water. Pet waste can be a major source of bacteria and excess nutrients in local waters.

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Nutrient Challenges

Nutrients, primarily phosphorus, are a chief culprit for dramatic algae blooms that plague the River with blue-green algae during the summer months.

These "blue green" algae blooms, are a form of bacteria known as Cyanobacteria, whose cells may release a toxin when they die. Exposure to the toxin can cause skin rashes and irritate the nose, eyes or throat, and if ingested can lead to serious liver and nervous system damage. Other harmful effects of the algae include reduced water clarity, nuisance scum, and reduced oxygen in the water which is necessary for a healthy fish habitat.

For more information on Nutrient Challenges.

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