The Monsanto Chemical Co. plant in Soda Springs, Idaho processed phosphate ore on 800 acres and was added to the National Priorities List (NPL) in 1990. Site operations left soil and groundwater contaminated with hazardous chemicals. The plant continues to operate as a phosphorus refining facility under current operator P4 Production, and several farmers own portions of the property.
Contaminants in soil include arsenic, beryllium, cadmium, manganese, radium-226, and thorium-230. Groundwater contains arsenic, cadmium, fluoride, manganese, molybdenum, nickel, nitrate, and selenium. Process wastes stored in unlined ponds and impoundments are the identified contamination sources. Selenium concentrations in Soda Creek currently exceed aquatic life water quality standards, meaning the cleanup has not yet fully protected the environment.
The original remedy, selected in 1997 and completed in 2000, relied on monitored natural attenuation of groundwater along with institutional controls across all 800 acres to restrict future land use. In 2018, EPA determined the remedy was not working as intended and found the contaminated groundwater area was larger than originally identified. In April 2021, EPA, the Idaho Department of Environmental Quality (IDEQ), and P4 Production agreed to conduct a supplemental remedial investigation and focused feasibility study. Environmental data collection is continuing through 2025. A Record of Decision Amendment is estimated for completion between September and November 2028.
EPA's 2023 five-year review concluded the remedy is not currently protective. Groundwater contamination continues to migrate beyond the original area. Human exposure status has not been fully determined due to insufficient data. The city of Soda Springs' public drinking water comes from unaffected springs and is safe. No domestic wells are known to be in use for drinking water in the downgradient area.
Community members can stay informed by contacting EPA's Community Involvement Coordinator or the Remedial Project Manager. For state-related questions, contact IDEQ.
