Ordinarily, storm water is sent to a wastewater treatment plant before it is emptied to a river. However, during high rainfall events, treatment plants may not be able to process all wastewater. Combined sewer overflows (CSOs) were built as a way to deal with this excess wastewater, essentially bypassing the treatment plant. However, the Massachusetts Department of Environmental Protection (MassDEP) is working to reduce the amount of active CSOs across the state in an effort to reduce water pollution. MassDEP’s Western Regional Office maintains a list of active CSOs within the region.

Between 2014 and 2016, the number of CSOs draining to the Connecticut River or its tributaries fell from 61 to 59, for an average decrease of 3.3% per year. Both of the CSOs that were removed were located in the City of Chicopee. All remaining CSOs within the region are located throughout Hampden County.