The State Fire Marshal has determined what caused an explosion at the WellsDairy south ice cream plant in Le Mars last month.Officials say a check valve ruptured releasing ammonia that caused andexplosion. There was seven-million dollars in damage to the plant and twoemployees were injured in the blast. A spokesman for the plant says they arestill trying to determine why the valve went bad. Employees at the planthave been working at the company’s north ice cream plant to keep upproduction. Repair work will begin in the next week at the south plant andit will likely take two months.