A chemical spill in a tributary of the Songhua River in Jilin province poses threats to the northeast's biggest river and water supplies to a city of 2 million people. China Central Television yesterday reported the toxic spill occurred on Sunday night when a factory upstream discharged a lot of untreated waste water into the Mangniu River near Jilin city . The report said the factory processed food additives in Jiaohe city , an area under Jilin city's administration, and seven people - five factory workers and two drivers - had been detained for their involvement. 'The toxic slick has been stopped around 14km from the Songhua River, which has so far not been affected,' it said. The Mangniu River is Jilin city's main industrial and drinking water source and flows into the Songhua River, which was heavily polluted last year in a large-scale disaster. A resident of Harbin , a Heilongjiang province city downstream from the slick, said people were stockpiling drinking water, fearing their city's supplies were again under threat. The Hong Kong China News Agency reported that water quality remained poor in the Mangniu River, even though authorities had started to build a dam and were using activated carbon to filter pollutants, mainly aniline, a poisonous benzene derivative. Jilin city had a population of nearly 2 million in 2003, according to the local government website. The city government has issued a notice banning factories, farmers and other residents from using water from the polluted river. A Jiaohe government official said her 470,000-strong city had not been affected by the pollution. An investigation led by Lu Xinyuan , a top environmental supervision official from the country's green watchdog, had been launched on Tuesday, CCTV reported. The report did not give details about the discharge or its impact on Jilin's water supplies. Toxic industrial discharges have become a main factor in the country's worsening water pollution and remains a big headache for authorities despite repeated official crackdowns. A chemical spill at a Jilin petrochemical plant on the Songhua River in November resulted in massive water contamination, affecting millions of people in Harbin and other cities, and straining diplomatic ties with Russia.