China is sending investigators to Hunan province after local officials were accused of faking data at a water monitoring station, state media report.
The officials are alleged to have placed sensors intended to measure the water quality of Lujiang River inside bottles of mineral water instead.
The river, in Zhuzhou, is badly polluted by sewage water, reports say.
There is widespread suspicion that some local officials and companies in China ignore environmental policies.
The environment ministry says it is investigating in Zhuzhou and “will seriously punish” any “violations”.
One monitoring sensor was even placed in a cup of tea instead of the Lujiang River, Xinhua news agency says.
Water monitoring currently takes place at 2,050 sites in the country, China Daily reports.
The Chinese government has vowed to improve its efforts to monitor and combat pollution – but there continues to be concern about air and water quality in China.
In 2016, one government report said more than 80% of rural wells in the north-east contained water unsafe for drinking.
Meanwhile, a separate 2017 government survey found more than 13,000 companies in China failed to meet environmental standards.