The Environmental Protection Agency is testing out a new technique for keeping heavily-used river banks from eroding into the water.
The EPA wants to see if these living retainer walls can contain lead pollution at a Superfund site in north Idaho.
To make these “soil burritos,” a construction crew pours a layer of dirt into what looks like a car-sized piece of burlap. Then they put down willow branches. If all goes according to plan, those willows will start taking root in the dirt next spring.
“You end up, in very short order, with a matrix of roots that grab the soil and hold it like crazy,” said Jo Christensen, a federal restoration biologist who's helped perfect the soil burrito. The EPA brought her to this campground along the Coeur d'Alene River because erosion was exposing old pollution from silver mining.
In places like this, federal agencies often use rip-rap — the piles of big gray rocks you've probably seen before. But Christensen said the problem is all those smooth rock faces can lead to more erosion downstream.
“They actually speed up the water,” she said. “What this design does, and what a natural stream bank does is all of these stems slow the water down.”
Christensen has helped agencies install the more-natural walls at about 50 sites across the West, including in the Klamath and Willamette river basins in Oregon.
Usually Christensen's focus is on improving fish habitat. But the EPA's biggest concern along the Coeur d'Alene River is containing decades-old lead pollution.
“Your greatest exposure risk in this basin is either ingestion, which is eating or inhaling lead dust,” said Ed Moreen, the EPA's project manager on cleanup in the Coeur d'Alene Basin. “And if you have soils that have high lead level and it's exposed then you have no barrier to that. Vegetation is one of the greatest barriers that prevents that.”
Moreen said at this site by the Kahnderosa Campground, the soil exposed by erosion has more than 10 times the maximum level of lead the EPA considers safe.