Radium, found in soil, rock and water, accumulates in the filtered silt.
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At a deserted gas station in a remote North Dakota town, local officials recently found the latest example of the shale-oil boom’s unintended consequences: hundreds of garbage bags filled with mildly radioactive waste.

These bags (pictured), which were discovered late February in Noonan, N.D., contained what are known as “oil socks”: three-foot-long, snake-like filters made of absorbent fiber that the shale-oil industry uses to capture silt from waste water resulting from hydraulic fracturing.