Superfund Information Systems: Site Profile

Superfund Site:

MONSANTO CHEMICAL CO. (SODA SPRINGS PLANT)
SODA SPRINGS, ID

Cleanup Activities

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Background

The 800-acre Monsanto Chemical Co. (Soda Springs Plant) site is located outside the city limits of Soda Springs, Idaho. The site includes the 540-acre Monsanto plant operating area and a 260-acre buffer area.

Monsanto purchased the property in 1952 to use local phosphate-rich ore to manufacture elemental phosphorus. Site activities and waste disposal practices contaminated soil and groundwater with hazardous chemicals and radioactive constituents. Following cleanup, operation and maintenance activities and monitoring are ongoing.

The operating area of the Monsanto Chemical Company encompasses 540 acres and processes locally mined phosphate ore to produce elemental phosphorus. Surrounding the operating area are approximately 260 acres of buffer properties. All 800 acres (540 acres in the operating area and 260 acres in the buffer properties) have institutional controls in place to prevent future land-use that could result in exposure to hazardous constituents.

The elemental phosphorous plant consists of over a dozen administrative and processing buildings plus ore piles, slag piles, by-product materials, surface impoundments and a waste landfill. Ore is stockpiled on-site prior to being processed for introduction into electric arc furnaces along with coke and silica. All process waters, with the exception of non-contact cooling water, are held and treated on site and then reused. The non-contact cooling water is discharged from the site to Soda Creek, which is used in agricultural irrigation.

Land use in the vicinity of the Monsanto facility is primarily industrial and agricultural. The plant is staffed with about 400 employees, and 3,300 residents live within 3 miles of the site. Most residents' water is supplied by the City of Soda Springs from springs unaffected by the plant. The closest surface water is Soda Creek, located approximately 2,000 feet west of the facility. Some nearby residents depend on domestic wells, but none are known to be in use for drinking water downgradient of the site.

In 1991, Monsanto agreed to an administrative order with EPA to perform the necessary Remedial Investigation Feasibility Study (RI/FS) and to pay EPA’s oversight costs. The RI/FS was completed in the summer of 1996. Process wastes, previously stored in unlined ponds or impoundments, have been pinpointed as sources of contamination to the local groundwater. Groundwater underlying the site and the surrounding vicinity is contaminated with cadmium, selenium, fluoride and nitrate. Studies to date have confirmed that Soda Springs drinking water supplies are uncontaminated and have not been affected by the site. Contaminated groundwater is not currently used for drinking. Without treatment, the groundwater is expected to recover through natural attenuation within 25-30 years. Below is the protectiveness statement included in the five-year review report: "The remedy at the Monsanto Chemical Co. (Soda Springs Plant) currently protects human health and the environment because all known groundwater and soil exposure pathways have been restricted through institutional controls or other means. However, in order for the remedy to be protective in the long-term, the following actions need to be taken: 1) the Lewis well and property needs to be evaluated for institutional controls (note: the Lewis well has not exceeded MCLs in the last 5 years and therefore IC are currently not required); 2) Monsanto needs to submit a plan for upgrading their wind dispersal program and for evaluating in the short term those localized areas where increased 226Ra soil concentrations have been observed (note: Monsanto has submitted and EPA has approved the wind despresal SOP and Monsanto is trying to improve the encapsulation of the UFS piles) ; and, 3) the U.S. Environmental Protection Agency (EPA) needs to reevaluate during the next five-year review whether the monitored natural attenuation remedy underneath the operating area is proceeding as intended in the decision documents." EPA has determined that MNA is not functioning as intened and a new remedy may need to be selected.

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What Has Been Done to Clean Up the Site?

The site is being addressed through federal and potentially responsible party (PRP) actions.

EPA has conducted several five-year reviews of the site’s remedy. These reviews ensure that the remedies put in place protect public health and the environment, and function as intended by site decision documents. In the most recent review, EPA deferred making a protectiveness determination until additional information can be obtained and additional actions are taken. These actions include conducting a remedial investigation to fully define the nature and extent of groundwater contamination; developing an institutional control plan to address areas where groundwater contaminants have migrated beyond the current institutional control boundary; conducting the next phase of the source characterization to evaluate current sources; updating the conceptual site model to evaluate if the current remedy is appropriate; and continued sediment monitoring to determine if additional remedial action may be needed.

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What Is the Current Site Status?

The site’s long-term remedy included monitored natural attenuation of contaminated groundwater and institutional controls. Remedy construction took place between 1998 and 2000. Site monitoring is ongoing.

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