CHARLESTON, W.Va. (AP) -- A stormwater collection trench overflowed Thursday evening at the site of a Charleston company that spilled chemicals into West Virginia's largest water supply.

The spill sent a "small but undetermined" amount of water into the Elk River, according to the state Department of Environmental Protection. West Virginia American Water detected no traces of the chemical that spilled in January in treated and untreated water at its treatment plant Thursday night.

DEP said inspectors discovered the issue at 5 p.m. The department notified the public at 10:25 p.m.

The water treatment plant is 1.6 miles downstream from Freedom Industries. The company's January spill turned water taps off to 300,000 people for days.

Freedom is in the midst of bankruptcy proceedings. The company also is under multiple state and federal investigations.

The water testing was conducted in the Elk River near the treatment plant's intake, as well as elsewhere.

The water company says it is "constantly monitoring the situation" and is taking precautionary steps, including having additional personnel monitor untreated water.

The company will start tearing down its tanks in Charleston by the end of the month, per state orders.

The state Department of Environmental Protection warned that the process could stir up the black licorice chemical smell that enveloped the valley in January and lingered in people's tap water long after.

West Virginia American Water has replaced each of its 16 filters after the January spill. Results returned early Thursday showed no chemical traces in water heading to people's homes and businesses.

Low amounts of the chemical had been leaching from the filters for months. In March, the chemical appeared in treated water at levels 2,000 times less concentrated than health officials deem safe to drink.