Greenpeace claims two canals in Samut Prakan province - near an industrial zone and connected to the lower Chao Phya River - were tainted with hazardous chemicals at a level "high and alarming" enough to affect human hormones and cause cancer.
A Greenpeace water patrol had samples of wastewater released by the two factories into the Samrong, Bang-nang-kreng and Bangplakod canals and found two of them to be contaminated with a wide range of dangerous pollutants.
They included nonylphenols - environmental contaminants and hormone disrupting chemicals - as well as 2-naphthalenamine, a human carcinogen formed from the chemical breakdown of certain dyes.
The study also found in the wastewater tri-iso-butyl phosphate (TiBP), a toxic chemical used in the textile industry.
"The results of this study are alarming and reveal the hidden dangers from chemical industrial pollution," Ply Pirom, toxics campaigner for Greenpeace Southeast Asia, said.
" These chemicals are very difficult to detect and are even harder to clean up from the water," he added.
The use and discharge of nonylphenols and 2-naphthalenamine are regulated in other countries. Once such chemicals are released, it can be difficult to eliminate them from the environment, Ply said.
According to the study, water samples collected from Samrong canal contained nonylphenols and TiBP as well as high concentrations of copper, lead, manganese, nickel and zinc - all exceeding Thai surface water quality standards by three to eight times. Canal sediment samples contained high concentrations of chromium, copper, nickel, and lead, with zinc levels up to 30 times higher than normal in uncontaminated surface water.
Meanwhile, Bang-nang-kreng canal water samples were also contaminated with nonylphenols and TiBP, and contained copper and nickel at levels over two times Thai surface water quality standards.
Bang-nang-kreng canal sediment held the highest concentration of chromium, copper, nickel and zinc, with nickel concentrations 30 to 80 times higher than typical background levels.
The Bangplakod canal was largely uncontaminated with metals and organic pollutants.
Ply said action is needed urgently. Both the Department of Industry and the Pollution Control Department are failing to prevent chemical contamination of water resources. Additionally, the chemical management approach does not adequately address the issue of toxic chemicals used and released by Thai industries.
He said Greenpeace would send an official letter to the Department of Industry, the Pollution Control Department, and to the Samut Prakan province governor, recommending strict control over the discharging of contaminated wastewater into the canals.