PORTLAND – Environmental officials have mailed letters to thousands of Maine businesses warning them about being in compliance with rules on storm water runoff that can pollute rivers, bays and other waterways.

Maine’s Department of Environmental Protection sent letters to more than 4,000 businesses this fall saying they need to comply with the law or certify that their operations don’t pose a threat.

The DEP took over enforcement responsibilities this fall from federal regulators that only loosely enforced the law. The state’s crackdown is forcing many companies to create storm water control plans and change their business practices.

“We do plan to inspect businesses,” said Don Witherell, the director of the DEP’s division of watershed management.

Storm water that washes over industrial sites, parking lots and suburban lawns can turn into a hazardous mix of chemicals and metals that can wash into waterways. Although less obvious and more dispersed than discharges from factories or sewer plants, storm water runoff is considered the most common source of surface water pollution in Maine and the country.

The U.S. Environmental Protection Agency had been in charge of enforcing the runoff regulations before the duties were transferred to the DEP.

State and environmental officials said the EPA didn’t conduct regular inspections to enforce compliance with the law, and that only 330 businesses had registered with the agency, a fraction of those covered by the law.

The DEP sent out the notices to businesses several days before it officially took over enforcement responsibility Oct. 30.

About 200 businesses responded by filing notices that they intend to comply, while many others have said they have no outdoor equipment or activities and aren’t subject to the regulations.

There could be another 1,000 businesses that are subject to storm water regulation but have yet to respond, officials estimated.

It will take time to reach every business, and the DEP is not yet issuing penalties. Instead, it is talking to industry groups and businesses about storm water pollution and how to prevent it.

Still, the effort is already paying off, said Steve Hinchman, an attorney with the Conservation Law Foundation in Brunswick.

“It’s really clear that it’s already making a difference in terms of water quality, because they’re finding some simple things that have already been cleaned up,” he said.