Adjusting Covers for Climate Change

Okane designed a cover system to support an Environmental Impact Statement (EIS) update for a phosphate mine site expansion in Idaho. The objective of the cover system was to minimize the transport of selenium by limiting net percolation. After three years of monitoring performance with a large-scale lysimeter, it was observed that actual site-specific conditions during winter were significantly warmer than the 100-year climate record which was the design basis of the cover system. The winter snowpack gradually melted, releasing infiltration, and reducing cover system storage capacity from January through March.

Okane worked with the site to refine and modify the cover system to manage the greater-than-anticipated infiltration, moving from a store-and-release design to one that could manage inevitable infiltration. The revised system includes two low-permeability layers of screened and compacted overburden at the base, with one of the layers amended with bentonite. A drainage layer of coarse rock removes the vast majority of infiltration as lateral interflow. The growth medium and topsoil above the drainage layer provide for a vegetation medium. A second large-scale lysimeter with an extensive water balance monitoring system was also constructed to demonstrate the performance of the revised cover system.

The revised cover system continues to perform as designed. Following regulatory review by the US Bureau of Land Management, US Forest Service, and Idaho Department of Environmental Quality, it is an approved cover system for the mine expansion area.