WAYNESBURG — A southwestern Pennsylvania borough is trying to figure out who dumped 3,000 to 4,000 gallons of hydraulic fracturing water into its sewer system.

Waynesburg officials said the water was dumped between 8 and 9 a.m. on Sept. 30.

The borough’s sewage plant manager noticed a milky substance right after flow meters indicated a spike in the normal amount of sewage flowing through the system, assistant borough manager Bryan Cumberledge said.

Hydraulic fracturing is a process in which chemically-laced water is injected into underground shale formations to break them up and free natural gas for drillers. The water can include briny substances and traces of chemicals used in the process.

The borough hasn’t accepted gas well water since 2009, when the state Department of Environmental Protection ordered sewage plants to reduce how much wastewater they processed after the Monongahela River was found to be contaminated with substances the plants couldn’t properly treat.