Arsenic contamination of well water in the town of Kamisu, Ibaraki Prefecture, came from an arsenic compound that was mixed into blocks of concrete and abandoned there by an unknown party, not from wartime chemical weapons as previously assumed, an Environment Ministry panel concluded Wednesday.

The state has paid medical allowances to residents who suffered health problems due to the contamination after it surfaced in March 2003 based on the earlier assumption, but will continue to do so despite the panel's conclusion, ministry officials said.

Three concrete blocks, weighing a combined 52 tons, were found buried 1 to 4 meters deep about 90 meters southeast of a contaminated well.

They were found to contain a high amount of diphenylarsinic acid, an organic arsenic compound, equivalent to about 180 kg of arsenic. About 100 kg is estimated to have seeped into the ground.

Since the substance was mainly used to produce poisonous gas for the wartime Japanese military, the government believed the contamination originated from abandoned chemical weapons.

But the panel of experts said in a report Wednesday that the substance contained in the concrete is believed to have melted into groundwater that flowed into the well.

The panel said poisonous gas agents have not been detected in the contaminated area.

An empty drink can produced in 1993 has been found in one of the blocks, suggesting the concrete blocks were dumped there in or after 1993.

The arsenic in the concrete, however, may still be connected to the former military, as there is no record that it was mass-produced after the war.