Groton — About 400 gallons of oil spilled at Groton Oil and Marina on Thursday morning, when an employee accidentally delivered oil to the wrong underground storage tank, state Department of Energy and Environmental Protection spokesman Chris Collibee said.

Collibee said the 53 Thames St. location has three underground storage tanks. The employee was supposed to deliver 1,000 gallons to the tank that holds nontaxable diesel, Collibee said, but accidentally connected to the taxable diesel tank.

The oil that flowed out of the tank traveled down the hill and pooled at a retaining wall, soaking into the soil and impacting groundwater, Collibee said. About 10 gallons of oil also broke onto the Thames River, creating a sheen.

The DEEP Emergency Response Unit and the U.S. Coast Guard are working with the marina owner and his contractor to clean the property. Crews have deployed floating barriers to contain and absorb the spill.