A government contractor tasked with cleaning out the Mall’s storm water sewer system admitted Thursday that he often had employees dump waste into the Potomac River — a federal crime that could land him in prison for about four years, authorities said.

Patrick Brightwell, 48, of Bogart, Ga., pleaded guilty in U.S. District Court in D.C. to one count of violating the Clean Water Act by knowingly discharging a pollutant without a permit and one count of presenting false claims to the United States, authorities said.

Prosecutors said Brightwell, who managed a company tasked with cleaning the Mall’s sewer system, told employees to illegally dump waste that their vacuum trucks collected into a storm drain near a parking lot in East Potomac Park, just across from the Potomac River. The waste was supposed to be discarded at a disposal facility in Fort Washington, prosecutors said.

Brightwell’s misdeeds spanned from 2009 to 2011, and his employees and subcontractors disposed of waste properly only about a third of the time, prosecutors said. His company received about $406,000 in payments from the National Park Service related to the contract during that period, prosecutors said.

Brightwell is scheduled to be sentenced Sept. 3. Federal sentencing guidelines call for him to receive a prison term of 46 to 57 months, prosecutors said.

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