On February 6th, EPA was notified of a subsidence in Interstate 90 (I-90) both directions at milepost 48.9 near Kellogg, Idaho. The subsidence is currently estimated at approximately 6 inches and has been increasing since it was first discovered on February 4th. The area of the subsidence is adjacent to visible sediment seeps in the South Fork of the Coeur d’Alene River discovered in late December. As part of the Superfund Remedial Action for the Bunker Hill Mining and Metallurgical NPL site, EPA and the Corps of Engineers recently completed construction of a subsurface groundwater cutoff wall in this same area. I-90 through this area was constructed on top of historic mine waste, groundwater levels are naturally high and there is a direct hydraulic connection with the river. As a result, roadway subsidence and groundwater seeps are not uncommon. I-90 is a major east-west transportation corridor through northern Idaho.
The Idaho Department of Transportation is aware of the condition of the road surface and is coordinating with the EPA and Corps of Engineers. Signs warning drivers of the subsidence have been posted on the highway and they are prepared to do emergency patching if necessary.
Region 10 Remedial Project Managers Kim Prestbo and Ed Moreen have been working closely with the Corps and Engineers and contractors involved with the groundwater cut off wall construction since the seeps were discovered in late December to determine whether EPA construction activities have contributed. They are also working closely with Idaho Department of Environmental Quality and the U.S. Geological Survey to investigate the nature and source of the seeps, and now the subsidence, and sharing information other partners including the Coeur d’Alene Tribe and the Panhandle Health District, and the public.
Due to the recent developments related to the highway subsidence, EPA OSC Vanderboom from the Idaho Operations Office and Emergency Management Program contractors have mobilized to the scene to support the Remedial Cleanup Program’s investigation of the situation and initiation of mitigation measures, if they are determined to be necessary.