The Pollution Control Department says aluminium detected in the Prachin Buri River is unlikely to pose a significant risk to health or the environment, as the water's pH level may cause the metal to settle in river sediment.
Surin Worakijthamrong, director-general of the department, said water and sediment samples had been collected from various locations along the river in Muang, Ban Sang, Kabin Buri and Si Maha Phot districts.
The highest aluminium concentration, 3.547 milligrammes per litre, was detected in tambon Bothong in Kabin Buri district.
This compares with the US Environmental Protection Agency's recommended threshold of 0.75 milligrammes per litre for surface water.
Mr Surin said aluminium toxicity tends to decrease under more alkaline conditions. The water sample had a pH of 7.58.
The Department of Fisheries has also confirmed that freshwater animals remain safe to eat.
"In my opinion, the water is safe to use, as the alkaline conditions would cause aluminium to settle," Mr Surin said.
"For those who may have developed health problems after using the water, the department is working closely with the Department of Disease Control to identify the cause. Tap water and water from other natural sources are also being tested."
Severe flooding in Prachin Buri has triggered a pollution crisis, with hazardous chemicals suspected of leaking from industrial plants. The province is a major hub for recycling facilities.
Residents and authorities first noticed an oily film on floodwater covering paddy fields several weeks ago, before reports emerged of aquatic animals dying.
"It is rather complicated to identify the source of the pollution, as aluminium can occur naturally or result from human activities. However, the source of an oil film is more obvious, and we will take legal action against any wrongdoers," Mr Surin said.
He added that the department would carefully map the boundaries of the contaminated area before carrying out environmental remediation and identifying those responsible.
Under the Enhancement and Conservation of National Environmental Quality Act, polluters are liable for cleaning up environmental damage.