Basically the cleanup plan entails rounding up tailings material that has escaped and corralling it in a second waste repository. An earlier repository was completed in 2007. Remaining tailings wastes, and contaminated soils scattered on adjacent residential lands, will be removed and deposited at the new repository.
Repository “gabion”(or basket) rock walls will be installed like so:
Gabion walls are designed to be high enough to sustain 100-year flooding conditions and substantial enough to withstand expected water flow velocities during those worst case flood conditions.
Additionally, the soil cap on the repository must resist erosion but since only a small amount of water shed is expected, i.e., only the area of the cap itself heavy erosion due to storm water runoff is not expected. Rather, wind erosion may be the primary erosive loss concern on the cap itself. To resist both water and wind erosion techniques must be used to ensure the proper grade, material mix and other armoring. The armoring, or rip-rap rock in most cases, should be substantial enough to resist the wind and water velocities associated with storm events. Both wind or rain storms would generate high velocity erosion, testing the mettle of the cap. Also, the material itself should be compacted and of appropriate particle size to resist erosion.
A final word on this idea. Our engineers have modeled a particle size mix that should in fact be largely resistant to erosive forces and also be amenable to plant growth which will further mitigate erosion threats. The idea here is to create a “water balance cover” where potential plant transpiration (PET) is in a nearly 1:1 ratio with precipitation (P). Significant plant growth will, with time, remove most of the water from the cap, preventing percolation of course, but also potential liquefaction and concomitant mass wasting.