Arsenic naturally occurs in the Missouri River, and some arsenic is inherent in Montana soils. That vaporized arsenic was deposited atop soils throughout the area. Still, arsenic is prevalent throughout the valley for another reason the historical smelting practices by Asarco involved direct releases of arsenic-laden water into Prickly Pear Creek. For decades, farmers used that contaminated water to irrigate their fields. Ranchers used it for their stock.
People spread it on their gardens.
It s an amazing situation. You have water that s high in arsenic, high in other things, like heavy metals, and people are pulling it from the creek and feeding it to their cattle, sprinkling it on their crops, said Kevin Keenan, a former water quality enforcement officer with the state. They ve taken it from one media and are putting it onto another, from water contamination to soil contamination to contamination in the food. High arsenic concentrations were found in 1993 near the Grandview subdivision just south of Canyon Ferry Drive in the Helena Valley. Almost 1,000 samples were taken, and many revealed soils with less than 100 parts per million of arsenic.