A thorough investigation is needed to determine what caused aquatic animals to die in the Prachin Buri River, deputy government spokeswoman Lalida Persvivatana said.
The Pollution Control Department plans to collect sediment samples from the river and its tributaries once the flooding subsides to assess the accumulation of pollutants and identify possible sources of contamination, she said. The findings will be used to help determine the cause of the deaths.
Ms Lalida said aluminium occurs naturally in soil and rocks and can enter waterways through weathering and erosion. However, it can also come from industrial activities and wastewater, making further investigation necessary to establish its source in the river.
She also clarified an error in the Pollution Control Department’s water-quality test results released on Saturday, in which the figures for ammonia and aluminium had been transposed.
The correct figures were 0.22-0.48 milligrammes per litre for ammonia and 0.559-3.547 mg per litre for aluminium.
The department has corrected the figures and apologised to the public and the media. Ms Lalida stressed that the correction was intended to ensure the accuracy of the published data and did not indicate that aluminium levels in the river had recently risen.
She said concerns over the aluminium levels required a comprehensive assessment, as the substance’s effects on aquatic life depend on several factors beyond its concentration in the water.
These include the water’s pH, hardness and dissolved organic carbon content, as well as the form of aluminium present and the duration of exposure. These factors influence how toxic aluminium may be to aquatic organisms under particular conditions.
Ms Lalida said guidance from the United States Environmental Protection Agency (EPA) provided a scientific reference for assessing the risks. However, local water conditions must also be taken into account when interpreting the results, she added.
She stressed that the fact aluminium occurs naturally does not mean elevated concentrations cannot harm aquatic ecosystems. Equally, the presence of aluminium in the river does not, by itself, establish that industrial activity is responsible for the contamination.
She said investigators could not yet conclude whether the aluminium originated from natural sources or industrial activity, or whether it had contributed to the deaths of animals along the river.