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Steve Karnowski, Associated Press
In this Dec. 15, 2011 photo, Heather Andersen of Bloomer, Wisc. looks at frac sand piled up at the EOG Resources Inc. processing plant in Chippewa Falls, Wis. Largely overlooked in the national debate over fracking is the emerging fight in the U.S. heartland over mining frac sand, which has grains of ideal size, shape, strength and purity. Mining companies say the work provides good jobs in rural areas, but some residents fear the increase in mining could harm human health and the environment.

CHIPPEWA FALLS, Wis. — Rolling hills and scenic bluffs along the Wisconsin-Minnesota border hide a valuable resource that's sparked what's been called a modern-day gold rush.

But it's not gold companies are after. It's a soft sandstone that drilling companies need to get at underground natural gas and oil supplies in a process called hydraulic fracturing, or fracking.

The sand has grains of ideal size, shape and purity for use in fracking. And mining companies say the work provides good jobs in rural areas. But some people worry fine silica dust from mines and plants will make people sick, spoil landscape and contaminate ground water.

Nearly three-fourths of "frac sand" comes from the Midwest. One expert says he expects the tonnage of sand mined will have doubled when the most recent data is released.