The Andover Health Division is warning residents to avoid the Merrimack River for at least 48 hours following a discharge of untreated or partially-treated wastewater Lowell Wastewater Treatment Facility on Sunday evening.

The dump happened between 6:47 p.m. and 9:35 p.m. Sunday.. The sewerage discharge increases health risks from bacteria and other pollutants for people who come into contact with the water.

Overflows occur when stormwater runoff is too much for the sewage system to process.