EPA Superfund Site

MONSANTO CHEMICAL CO. (SODA SPRINGS PLANT)

ID | EPA ID IDD081830994

About this site’s exposure pathways: As of October 2024, the Monsanto Chemical Co. (Soda Springs Plant) Superfund site is considered “Insufficient Data to make a Human Exposure evaluation” (HEID). Groundwater and surface water are impacted by cadmium, fluoride, nitrate, selenium, and manganese. Potential exposure pathways exist at the Site for current and future industrial workers, current and future residents, and current and future recreation. The site was previously categorized as Human Exposure Under Control (HEUC). Site activities and waste disposal practices contaminated groundwater, surface water, sediments, offsite soils, byproduct/source piles, and air. EPA issued a Record of Decision (ROD) in 1997. The ROD selected remedial actions for the media listed above and established remediation goals for the following contaminants of concern (COCs) at the Site: cadmium, fluoride, nitrate as NO3/Nitrate as N, selenium, manganese, and radium-226. The selected remedy included monitored natural attenuation (MNA) with institutional controls (ICs) for groundwater, ICs for offsite soil in the form of land use restrictions placed in deeds, and no further action for surface water, source piles, air, and sediments. The third and fourth Five-Year Reviews determined the nature and extent of groundwater plumes of site-related COCs are not well defined. Concentrations of COCs in groundwater and surface water remain above Remedial Goals and have exceeded Remedial Goals beyond the P4 Production LLC (one of the potentially responsible parties at the site) property boundary.
EPA uses this Human Exposure status when there is a lack of evidence to suggest that actual or reasonably anticipated human exposures are occurring and that those exposures are above acceptable risk-based levels. The planned activities to collect sufficient information to evaluate the exposure pathways and contaminants are in progress. EPA, Idaho Department of Environmental Quality (IDEQ), and P4 Production LLC entered into an Administrative Settlement Agreement and Order on Consent (ASAOC) in April 2021 for performance of a Supplemental Remedial Investigation (SRI) and Focused Feasibility Study (FFS) after the 2013 and 2018 Five-Year Reviews found the selected remedy to not be protective. Current activities include evaluating the effectiveness of the three pump-back wells located at the southern end of the Plant site to capture groundwater containing elevated COCs and the effects of pumping on groundwater and surface water quality concentration trends downgradient of the Plant. Near-continuous pumping from pump-back wells have intercepted a significant portion of groundwater at the southern fence-line and the extent of the plume has been significantly reduced. Impacted groundwater no longer migrates off-site. EPA currently anticipates that there will be sufficient data to make a Human Exposure evaluation by December 2025 because the Human Health Risk Assessment and Supplemental Remedial Investigation will be complete.