Staff collected water samples and will send the oil for testing to determine its source.
Nong Maikaen residents complained that oil was dumped into a flood- drainage canal polluting the water and creating a bad smell.
Nongprue Deputy Mayor Suwit Rachatawattanakul and sub-district environmental head Surapol Krumprasert inspected the canal May 22, ordering its immediate cleanup.
Staffers collected water samples and will send the oil for testing to determine its source.
Surapol said the oil was not engine oil but instead a lighter or hydraulic oil used in a local factory.
Locals say the water in the canal near Ban Nong Maikan 21 is used as raw water to create tap water, so oil pollution is very serious.
People have placed mattresses and cloth in the water to filter it before it flows into reservoirs in Pong that are sources for tap water production. One particularly aggrieved resident also put a sign in the middle of the canal warning against dumping.
Resident Chamoy Panprasong, 60, said the oil first appeared about two weeks ago along with dead marine animals. And, to this day, the oil continues to pour into the canal from an unknown source, she said.
The sub-district has asked police to begin investigating the pollution and prosecute the dumper.