A five-year study in Chippewa County, Wis., has transformed a reclaimed frac sand mine into a successful wild prairie, according to Wisconsin Public Radio.
Researchers are hopeful that lessons learned can be used at other mining operations around the state beginning to fill in their pits.
In a rare collaboration, researchers from the University of Wisconsin-River Falls worked with Superior Silica Sands and Chippewa County’s Department of Land Conservation and Forest Management to learn how sand mining impacts soil that is stripped away, stored and replaced after mining operations wrap up.
Since 2013, students led by UW-River Falls geology and soil science professor Holly Dolliver have been taking hundreds of samples from land owned by Superior Silica Sands in the town of Auburn.
They started by measuring undisturbed soil at the top of ridges which contain the silica sand prized by oil companies drilling in places like Texas and North Dakota. Dolliver said they found that topsoil on these ridges was very thin – only a foot deep in some places – but still rich in diversity of microbial life.
Dolliver’s team then went to work sampling soils on land that had been reclaimed by Superior Silica Sands. Under Wisconsin’s mine reclamation statutes, companies must fill areas in once active mining has been completed.
There are rules for how steep the contours must be, along with performance standards for vegetation. But Dolliver said the law doesn’t really set standards for the health of the soil itself being placed on top of retired mines.
“Often soils are overlooked in the reclamation process,” Dolliver said. “Sadly, they get pushed off to the side and there’s an assumption made that we’ll just lay them back down, the plants will grow and we’ll be good. And that mindset is even more apparent today because we have all sorts of things like fertilizers and amendments to help us out when natural soil quality can’t do it.”
Dolliver said her students found the soil at the reclamation site had been compacted in a similar way to what happens on farm fields when heavy machinery is used, which makes it harder for water to filter through.
Dolliver also worried that the naturally sandy soil would be less than ideal for retaining water and fostering lush plant growth. A decision was made to try adding muds and fine silt separated from silica sand during processing as a way to increase the water retention of the soils in the test area.
That didn’t seem to make any difference in plant growth, Dolliver said, but it did have the effect of adsorbing naturally occurring metals leaching from topsoil and subsurface bulk material used in reclamation.
“So we’re actually finding that the metal concentrations in water are actually lower where we use the belt press fines than where we didn’t,” she said.
In June 2015, a mix of wild prairie flowers and grasses were planted at the reclamation test plot. The seed mix came from Neil Diboll, president of the private company Prairie Nursery in Westfield.
Diboll said there were winners and losers during the three years that followed. Some species of prairie flowers did well in the first year but didn’t return while a number of prairie grass species thrived.
“Although sand mining is an inherently destructive process, we were able to reclaim within three years a reasonably diverse and fully vegetated native prairie on these pretty low nutrient, difficult soils,” said Diboll.