The Department of Pollution Control (PCD) has warned people not to use water from 18 sources during the Songkran holidays because of bacteria.
The unsafe water sources include river sections that pass through Bangkok and Nonthaburi.
Pornsak Phuim of the PCD said water samples from 61 locations across the country had been collected ahead of the Songkran Festival (April 13-15), when vast quantities of water are used for splashing.
Tests showed that samples from three of the locations were of very good quality; 35 were of good quality; 21 were fine and nine other locations had poor quality water, Mr Pornsak said.
Poor-quality water sources tend to be located along the mouth of the river where industrial plants and crowded communities are located, he said, adding most of the water sources are still safe for use.
Nonetheless, the tests showed that 18 river locations are unsafe, as the water contains bacteria.
The 18 rivers include the Chao Phraya passing through Bangkok and Nonthaburi's Bang Kruai and Pak Kret districts, as well as rivers in Nan, Lop Buri, Phetchaburi, Chanthaburi, Chumphon and Rayong.
Mr Pornsak said the PCD also collected samples from 77 beaches in the East and South of Thailand in February and March, adding most were clean and safe for Songkran.
