Government workers and local residents began cleaning up Hua Hin beach on Wednesday after the Prachuap Khiri Khan coastline was fouled with oil, tar balls and petroleum-coated garbage.
The cleanup started at 9am with employees from the Hua Hin municipality, district office and civilian volunteers.
Hua Hin mayor Nopporn Wuthikul said the hardest-hit areas were a stretch of about 10 kilometres from Khao Takiap to the Hua Hin fishing pier, and in front of the Klai Kangwon Palace.
He said participants scooped up blackened sand and oil-coated objects, disposing of them in black bags hauled away by garbage trucks.
- Related: Inquiry begins into oil spill
On Wednesday morning, oil slicks in the bay were not as thick as yesterday.
It was initially believed the oil had spilled from a large vessel far offshore. Authorities were investigating, he said.
Mr Nopporn said people should not go swimming at Hua Hin beach until an official notice is posted advising that water quality has returned to normal. First, the entire beach must be cleared of all pollution, he said.
เทศบาลหัวหินเตรียมทำความสะอาดคราบน้ำมัน ยาว 10กม. ตลอดแนวชายหาด เร่งตรวจสอบหาที่มาคราบน้ำมัน คาดถูกปล่อยจากเรือ pic.twitter.com/gsWU7UfVz0
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On Tuesday, naval personnel and officials from the provincial natural resources and environment office used a patrol boat to search for the source of the spill.
Jirat Pranprasit, an environment expert from the office of natural resources and environment, said it was still unclear whether the oil came from a drilling platform or from a commercial vessel.
Surachai Sinprakorb, an official from the provincial energy office, said the Mineral Fuels Department would send representatives to collect samples of the oil from Hua Hin and Cha-am beaches for tests. He expected the results within two weeks and the source would be revealed then.