GROUNDWATER EXTRACTION AND SOURCE CONTROL FOR ELEVATED PHOSPHORUS CONCENTRATIONS IN GROUNDWATER AND SURFACE WATER AT THE J.R. SIMPLOT COMPANY DON PLANT, SOUTHEASTERN IDAHO

The J.R. Simplot Company Don Plant, located near Pocatello, Idaho, produces phosphoric acid and fertilizers from phosphate ore. The primary byproduct is gypsum, which is disposed on a 400-acre stack. Primary sources of phosphorus to groundwater are the gypsum stack and the Phosphoric Acid Plant. Groundwater discharges to the Portneuf River, resulting in phosphorus concentrations above target levels. As such the facility is subject to both CERCLA and TMDL agreements to control sources and extract and reuse groundwater.

The facility is located at the base of the Bannock Range along the western flank of the Portneuf Valley. A shallow aquifer in coarse-grained fluvial sediments of the Michaud Gravel is locally unconfined. A deeper aquifer is comprised of interfingering alluvial fan and fluvial sediments and is confined by the American Falls Lake Beds aquitard. To the north, this aquitard is absent and no vertical flow barrier is present. This area is a regional discharge zone to the Portneuf River.

The remedy being implemented entails: (1) lining the gypsum stack to allow continued placement of gypsum on top of the liner; (2) control of sources in the Phosphoric Acid Plant; and (3) extraction of groundwater downgradient of source areas. A three dimensional hydrostratigraphic model was constructed to support remedy design including groundwater mass flux, constituent attenuation and hydraulic influence of extraction wells. Groundwater travel times from source areas to the river range from 0.5 to 13 years. Interactions between groundwater and surface water have also been studied extensively. Currently the groundwater extraction system pumps approximately 800 gpm, and will be increased to 1,000 gpm in 2011. One of the three gypsum stack cells was lined in 2010, with overall liner completion scheduled for 2014. Source controls in the Phosphoric Acid Plant reduced loading of phosphorus from this source by 75% in 2009 compared to 2008. Phosphorus concentrations at the point of compliance in the Portneuf River have decreased from a baseline average of 1.2 mg/L to below the interim TMDL goal for December 31^st 2013 of 0.625 mg/L. A predictive model of phosphorus concentrations in the Portneuf River was developed and indicates that continued improvements are expected as on-going remedial actions are implemented and become effective.