The U.S. Coast Guard is unsure of what caused pools of a "thick, purple diesel-type product" to appear at three Charleston area marinas.

Coast Guard pollution responders traveled to the Bristol, Ashley River and Charleston City marinas Saturday after a boater who keeps his vessel at one of the marinas reported the pollution at about 6:20 p.m. More reports soon followed.

Joshua Nichols, command duty officer at Coast Guard Sector Charleston, said Sunday that the heaviest pockets were within the City Marina, and that a mile-long and quarter-mile wide sheen was visible in the Ashley River from the U.S. Highway 17 bridge to Tradd Street.

City Marina employees deployed additional boom and absorbent pads Sunday morning.

Nichols said most of the product had dissipated by Sunday morning and had become unrecoverable because of heavy rainfall so it was not necessary to use the Oil Spill Liability Trust Fund as Coast Guard officials had initially intended.

Petty Officer 1st Class Lauren Jorgensen said Sunday evening that the source of the pollution remained unknown but that whatever had caused the chemical spills was not continuing to flow. Jorgensen said the exact type of chemical spilled was unknown but that it had a "strong diesel odor."

Coast Guard pollution responders left the area Sunday morning after the chemical had begun to dissipate, Jorgensen said.