The average January rainfall for Bedford is 1.8 inches, according to one meteorological service.
Tuesday night into Wednesday, rainfall totaled about three inches, resulting in a flow through the sanitary sewer system that exceeded the capacity of the main pumping station at 299 The Great Road.
And more heavy rain is forecast, beginning on Friday night and going through Saturday.
“As this could become a new normal, we’re brainstorming different options to prevent, or at least minimize, this in the future,” said Department of Public Works Director David Manugian on Thursday shortly after more than a full day of diverting untreated sewage – albeit mostly groundwater – directly into the Shawsheen River, adjacent to the pump station.
“This event was approximately 25.75 hours and discharged approximately 808,000 gallons,” Manugian announced. “All the pumps were working, but we weren’t able to keep up with the flows.”
Water from the heavy rain, overflowing brooks, and melting snow penetrated the sewers through fissures in the lines and illegally connected sump pumps.
The average 24-hour flow through the pumping station is two million gallons, Manugian said. On Tuesday and Wednesday, the volume approached six million, so the system was still able to accommodate more than 85 percent of the flow.
The town is charged by the Massachusetts Water Resources Authority for the entire volume, even though most of it is groundwater. The ultimate destination of the effluent is the MWRA’s Deer Island wastewater treatment plant in Boston Harbor.
“An interesting note in this storm is the number of communities along the MWRA pipeline that are also impacted, all the way to Boston,” Manugian reported.
The director added, “The Shawsheen was flowing at such a rate that the Wilmington stream gauge measured the river peaking at 841 cubic feet per second.” That translates to more than 375,000 gallons. “Since 1965, the previous high for Jan. 11 was in 1978 at 500 cubic feet per second.”
Manugian said the town is “following approved steps for reporting” the discharge, and does not expect any “significant actions” from the state Department of Environmental Protection. “To date their concern has been making sure residents have ample opportunity to be aware of what is happening,” he said.
In a message sent out by the Bedford Health Department on Wednesday, the town “recommends the public avoid contact with [affected] water bodies [including the Shawsheen River] for 48 hours after the discharge or overflow, during rainstorms, and for 48 hours after rainstorms end, due to increased health risks from bacteria and other pollutants associated with urban stormwater runoff and discharges of untreated or partially untreated wastewater.”