The contractor hired to improve the drainage system along Jomtien beach faces legal action after an outlet released wastewater that stained the sea black near Pattaya on Monday.
Officials immediately suspended all work on the project and ensured the discharge returned to normal, according to the Department of Marine and Coastal Resources.
An inspection was ordered after photos and videos were posted on social media showing black water being discharged into the sea at Jomtien beach, south of Pattaya.
The polluting discharge came from an "improvement project" along Jomtien beach, Pattaya's mayor said on Tuesday.
The company, Dynamic Group Ltd, had been contracted to install a drainage pipeline and stormwater retention pits to prevent flooding during heavy rain.
During the work, sediment and contaminated water had accumulated inside the system. The city had instructed the contractor to pump all wastewater directly into the treatment system, the mayor said.
However, the contractor instead dug an additional sand pit and stored the wastewater for sedimentation before pumping it into the sea, causing the discolouration along the beach.
The city had filed a formal complaint with Pattaya city police, seeking legal action against the contractor.
A team would also collect seawater samples for testing for quality and any potential environmental impact.
