The International Information Center for Geotechnical Engineers

# Soil Washing

Case Histories of Soil Washing Use

Between 1984 and 2008, soil washing was chosen as the remediation technique in 1% (or 6 projects) of US EPA Superfund projects (US EPA 2010). Therefore, it is not as commonly chosen as other remedy techniques such as bioremediation or soil vapor extraction, but given certain site conditions that allow for soil washing it can be an effective technique (US EPA 2010). The use of soil washing will be reviewed for three case histories: the Twin Cities Army Ammunition Plant (Fristad 1995), the RMI Titanium Company (US DOE 1998), and the King of Prussia Technical Corporation (US EPA 1983, 1995).   These case histories are important to examine since they demonstrate the full-scale applicability of soil washing for remediation.

Twin Cities Army Ammunition Plant

The Twin Cities Army Ammunition Plant is located in New Brighton, Minnesota and is approximately 2,370 acres. The plant opened in 1941 and produced different types of ammunition for the United States during many wars. In 1982, the site was added to the National Priorities list for remediation.  An ammunition test burn area of the site required remediation due to high levels of VOCs and heavy metals. A combination of soil leaching (where the metals could be removed and recycled) and soil washing was used to successfully remediate the site.  The full scale system implemented at the site to treat more than 20,000 tons of soil is shown below in Figure 13. After examining the site it was determined that most of the contaminated soil was within the top 1 to 2 feet of soil; however, 16 trenches were excavated to a depth of 15 feet. X-ray fluorescence was used to guide the excavation. The soil was tested for treatment effectiveness after every 10 tons. The treatment of the soil occurred from Fall 1993 to Summer of 1995. The soil washing and leaching system was located approximately 100 yards from the excavation area, and was located on an existing concrete pad. The heavy metals were successfully leached out using different agents, and the soil was successfully washed in order to reuse it as backfill at the site. Overall, eight heavy metals were removed from the site and clean up criteria of less than 175 ppm were met. Initial levels of the heavy metals were as high as 86,000 ppm for lead and 100,000 ppm for copper (the average of all the heavy metals was 1,600 ppm).   The innovative system allowed for all the soil to be reused on the site, no wastewater to be generated, and the heavy metals to be recycled (Fristad 1995).

Figure 13: Schematic of Soil Leaching and Washing System (from Fristad 1995)

RMI Titanium Company

The RMI Titanium Company (RMI) of Ashtabula, Ohio operated from 1962 to 1988. During this time they performed uranium extrusion for the Department of Energy (DOE) which included working with both depleted and slightly enriched material. During the extrusion processes uranium particles ended up escaping the facility and settling on the surrounding soils in concentrations of up to 300 picocuries per gram (pCi/g) which is much greater than the total allowable limit of 30 pCi/g (USDOE 1998). As the DOE had contracted the work out to RMI, the DOE was held responsible for the site remediation and funded the RMI Decommissioning Project. In order to determine the feasibility of soil washing as a remediation strategy RMI Environmental Services (RMIES) and Alternative Remedial Technologies, Inc. (ART) were hired to conduct pilot tests on about 64 tons of soil. They set up a small scale soil washing system and proved that through a chemical leaching process using sodium carbonate extraction they were able to remove enough uranium to achieve the allowable limit of 30 pCi/g for almost all of the soil that was processed (USDOE 1998). Their treatment system consisted of a rotary batch reactor, screens, thickeners, and a filter press. The pilot plant operated from January 7, 1997 until February 14, 1997 while they processed the 64 tons of soil. The end result showed that the process was very successful at removing the uranium contaminant with removal efficiencies of 82% and a volume reduction of 95% (USDOE 1998). Not only did soil washing prove to be an incredibly efficient method but it also proved to be incredibly cost effective. The projected cost for soil washing all 20,000 tons of contaminated soil was approximately $325/ton while remediating the soil through excavation, transportation, burial and soil replacement was projected to cost$857/ton clearly making soil washing an incredibly cost effective remediation technique for the RMI site (USDOE 1998).

King of Prussia Technical Corporation

The King of Prussia Technical Corporation site is located in Winslow Township, New Jersey, and was used for waste processing from January 1971 to April 1974. It was estimated that 15 million gallons of liquid industrial waste were processed in six waste lagoons.   On September 28, 1990 a Record of Decision (ROD) was issued for the site to clean up contaminated soils and sludge. Soil washing was chosen as the remediation technique to clean 19,200 tons of contaminated soil and sludge at the site. The full-scale soil washing unit was operated from June to October 1993. The primary contaminants on-site were nickel, chromium, and copper. Prior to treatment, the concentrations of the contaminants in the soil were 8,010 mg/kg for chromium, 9,070 mg/kg for copper, and 387 mg/kg for nickel. After treatment these concentration values were reduced to 660 mg/kg, 860 mg/kg, and 330 mg/kg, respectively (US EPA 1995). These values met the cleanup goals. The soil washing unit that was used was owned and operated by Alternative Remedial Technologies, Inc. (ART), and was designed to maintain a throughput of 25 ton/hour of soil. The system included a series of hydrocyclones, conditioners, and froth flotation cells, as depicted in Figure 14. After washing, the clean sand was reused as backfill at the site and the contaminated sludge was disposed of off-site. The in-situ soil on site was ideal for soil washing since it had an acceptable level of sand content, as shown in Figure 15, which depicts the particle size distribution of the soil. X-ray fluorescence was used to identify contaminated soils for selective excavation. On-site monitoring was also used to confirm contamination. Overall, the King of Prussia site was the first full-scale application of soil washing to remediate a Superfund site in the United States. Figures 16 and 17 show some photographs of the soil washing project site. Figure 16 depicts the site as well as the overall size of the soil washing system and the loading of the initial screening apparatus. Figure 17 depicts the soil screening, hydrocyclone usage, and sand treatment by flotation. The total cost of treatment, including soil washing and disposal off-site, was approximately 7.7 million dollars (US EPA 1983, 1995).

Figure 14: Soil Washing Unit used at King of Prussia Site (from US EPA 1995)

Figure 15: Particle Size Distribution of in-situ soils at King of Prussia Site (from US EPA1995)

Figure 16: King of Prussia Site Pictures I (from ART Engineering)

Figure 17: King of Prussia Site Pictures II (from ART Engineering)