SHANGHAI, Jan. 13 (Xinhua) -- Improper loading resulted in a petrochemical leak that has polluted a port in Shanghai, local authorities said Sunday.

Workers at the Jueshi Port failed to fully shut down a valve while loading C9, a dangerous petrochemical, onto a cargo ship around 10 a.m. Thursday, resulting in the chemical spilling into the port's waters, the municipal work safety administration said.

Local residents reported a pungent odor near the port around 6:30 p.m., leading authorities to discover the leak.

Police have taken four people into custody for their alleged involvement in the incident.

Three companies involved in the spill were found to have broken regulations pertaining to the sale, transport and loading of the chemical.

Local authorities have sent 20 vessels to clean up the leaked chemicals. Two local drinking water processing plants have stopped siphoning water from the contaminated area.

A further investigation and environmental evaluation are both under way.