Examples of Cleanup Approaches
    Union Pacific Railroad Company, Pocatello, ID

Union Pacific used a pit 620 ft long by 58 ft wide for disposal of sludges from their wastewater treatment facility. The pit has two longitudinal concrete retaining walls approximately 450 ft long but no engineered bottom. Groundwater under the site, which is used for drinking water, has been contaminated with nonaqueous phase liquids, polyaromatic hydrocarbons, metals, and other chemicals such as chlorinated solvents.

The stratigraphy at the site consists of fill— loose black cinders and cinders mixed with silt; recent alluvium—stiff, brown silt; older alluvium—sandy gravel, and gravel with some sand; Michaud Gravel—gravel, cobbles, and boulders; American Falls Lake Beds clay—stiff silty clay; and Pleistocene gravel— dense sandy gravel. Groundwater in the sludge pit vicinity occurs in two distinct water bearing deposits (Upper Aquifer and Lower Aquifer) separated by a less permeable clay layer. The Lower Aquifer is very productive and is used as a water source by local, private residents, businesses, and the City of Pocatello (Supply Well No. 32). No water supply wells in the immediate area have been found to utilize the Upper Aquifer, which is contaminated with chemicals that have migrated downward from the sludge, through the silt and soil, to the groundwater surface. Contaminants have also been identified in the Lower Aquifer but are below Safe Drinking Water Act standards.

The cleanup goals were to prevent direct contact with sludges and contaminated soils, remove contaminants from the vadose zone under the pit to the extent possible, and restore the upper aquifer to MCLs or risk-based levels when MCLs are absent. (ROD dated 9/10/91)

Contaminated sludge and soils were excavated and disposed of at an offsite landfill. The excavated area was backfilled with clean soil and covered with a low permeability cap. A pump and treat system was installed to treat contaminated water in the upper aquifer with the effluent discharged to a local publicly owned treatment plant. An infiltration gallery was installed prior to capping to allow clean water to be pumped into the underlying gravels to flush soil contaminants not excavated to the pump and treat system. Institutional controls were imposed and would remain for as long as needed to achieve cleanup goals.

The removal of 13,821 tons of sludge and contaminated soils combined with capping effectively removed the source. After pumping and treating over 62 million gallons of groundwater, contaminant levels fell below MCLs and other cleanup goals over the entire plume. When there was no rebound, the site was considered restored and deleted from the NPL.

Complete restoration of a drinking water aquifer.

Record of Decision (498K/168pp/PDF)