The Pollution Control Department (PCD) has finished drafting a plan to clean up several tracts of land in Tak's Mae Sot district contaminated with cadmium.
The plan will be submitted to the National Environment Board (NEB) for approval next month, PCD chief Wichien Jungrungruang said.
Cadmium, a toxic heavy metal, was detected in three tambons in early 2004 by foreign scientists from the International Water Management Institute.
They found that several hundred villagers in the tambons had high levels of cadmium in their blood. The scientists believe the contamination came from eating cadmium-tainted rice.
State agencies inspected the area and concluded the cadmium contamination in the soil occurred naturally as the area was located near cadmium and zinc deposits.
Villagers, however, accused Padaeng Industry, a giant zinc mining firm nearby, of polluting the area.
Mr Wichien said the proposed clean-up operation covers about 35,660 rai across the three tambons.
Of the contaminated land, 248 rai had high levels of cadmium contamination, with more than 30 miligrammes per kilogramme of soil; 3,566 rai had medium-level contamination; and the rest of the area had low-levels, or a level of contamination less than 3mg/kg. The contaminated soil is 30cm deep.
The clean-up plan includes four options _ absorb the cadmium with sugarcane plantations; remove the contaminated soil; dilute the cadmium with other chemicals; or cover the soil with fresh soil.
The NEB will decide which option is best, Mr Wichien said. The authorities may use a combination of tactics, he added.
Mr Wichien said the PCD also detected low cadmium contamination in sediment samples collected from Ma Tao and Mae Ku creeks. Other lab tests, however, found no contamination in fish taken from the creeks.
In 2009, residents filed a complaint with the Administrative Court, demanding the PCD clean up the cadmium from the area.
The affected villagers also filed a civil case against two mining firms: Padaeng Industry and Tak Mining Co, demanding more than 3 billion baht in compensation.
The cases have yet to go to court.
Since the cadmium contamination was exposed, authorities have encouraged villagers in the three tambons to switch from growing edible crops.
They say the farmers should switch from rice to sugarcane, which can be sold for ethanol production.
The authorities say producing tainted rice could damage the country's rice export stocks and prospects.
However, the villagers said they suffered losses from farming sugarcane and want another solution from the government.