The International Information Center for Geotechnical Engineers

Bioremediation - Case History: Brown Wood Preserving Site

Case History: Brown Wood Preserving Site

Site location and geology

The Brown Wood Preserving Site (BWPS) is located 2 miles west of Live Oak, Florida. The site covers a 51-acre area in which sinkholes are a common geologic hazard due to the karst terrain (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995). The karst consists of limestone and dolomite estimated to be 2,500 ft thick. This limestone forms the upper region of the Floridian Aquifer which has an extensive network of connected cavities allowing quick movement of ground water. The site is not located in an area prone to flooding and the surrounding land is considered rural. Locally there are four water wells in private use, the closest located 1000ft south of BWPS. Public water supplies are drawn from wells that are 2 miles away and uphill.

Site history

BWPS operated for 30 years from 1948-1978 (USEPA Region IV, 1995) and (Sharma & Reddy, 2004). Pressure treatment of timber with creosote and occasionally pentachlorophenol was carried out at BWPS. Waste water produced in the process was treated and discharged to a lagoon. In 1981 a former site opertator informed EPA that the site had handled hazardous material and samples collected in 1982 found soil and sludge around the treatment tanks and lagoon to be contaminated with PAHs (USEPA Region IV, 1995). In 1982, the site was placed on the National Priorities List and in 1988 a Record of Decision (ROD) was signed (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995). The ROD identified a cocktail of 6 PAHs to be used as Total Carcinogenic Indicator Chemicals (TCICs) with a goal of 100mg/kg of TCICs within two years (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995). The PAHs selected were the 6 out of the 200 compounds that make up creosote that the EPA determined to pose the highest risk, these included benzo(a)anthracene, benzo(a)pyrene, benzo(b)fluorothene, chrysene, dibenzo(a,h)anthracene and indeno(1,2,3-cd)pyrene (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995). The EPA considered 7 different treatment options to be implemented at BWPS that included:

1. No treatment action.

2. Incineration on site.

3. Incineration off site.

4. Land treatment.

5. On site treatment of the sludge and off site disposal of any waste.

6. Treatment and off site disposal of sludge with land treatment of soils.

7. Biological treatment of sludge in reactors and then land treatment of bio-sludge and soils.

Land treatment was selected as the best remediation option based on cost and technical feasibility (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995). Although some of the sludge found initially at the site was pumped and transported to a hazardous waste landfill at Emelle, Alabama (USEPA Region IV, 1995).

Contamination and soil description

The total concentration of the 6 TCICs in the soil varied from 100mg/kg to 208mg/kg. The soil conditions in the lagoon varied from clay to silty clay and fine sands (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995).

Cleanup requirements

The EPA ROD required a number of goals be achieved. The total TCIC concentration had to be reduced to 100mg/kg within two years. The final concentration was selected so that the risk of soil ingestion by a child would be 1x10-6. Once the site was decontaminated the land treatment area had to be re-vegetated.

Land treatment system and results

The land treatment system consisted of several components. A clay liner of thickness 1-3ft. A berm of compacted clay of height ranging from 2.5ft-7ft surrounding the lagoon and an additional berm surrounding the stockpiled soil of height 3ft. Water drains to prevent surface water entering the site. A separate subsurface drainage system under the treatment area to collect runoff water. A 750,000 gallon retention pond to contain any runoff water. A sprinkler system able to deliver water at 0.5inches an hour. Figure 17 shows the site layout.

Figure 17 – Site layout of the land treatment system (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995)

The land treatment area (LTA) had an area of four acres and was split into 8 half-acre treatment plots. The treatment was completed in 3 lifts with a sample from each plot being collected periodically; when the TCIC concentration for that lift fell bellow 100mg/kg a new lift was treated (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995).

The first lift was placed in January 1989 and consisted of 3300 yd3 of soil placed in a layer 5-7 inches thick. The LTA was treated twice a week with microorganisms capable of biodegrading the PAH TCICs in the contaminated soil. The LTA was tilled once every two weeks and the water content of the soil was maintained at 10% using the sprinkler system (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995). Initial TCIC concentrations varied from 103-258mg/kg. After 8 months of treatment the concentration of TCICs in each plot was reduced to 100mg/kg or less. Removal rates varied from 58mg/kg a month to 13mg/kg a month.

The second lift was placed in September 1989. Around 3000 yd3 was placed in a layer 9-12inches thick. Again the same treatment and tilling process was followed. Initial TCIC concentrations were similar to those in lift 1 and within 3 months the TCIC concentrations were below the 100mg/kg target.

The final lift was 1800 yd3 placed in a layer 4-7inches thick. The same treatment and tilling process was followed. Final TCIC concentrations after treatment was completed in July 1990 varied from 23mg/kg to 92mg/kg. Once remediation was completed 90% of the LTA was successfully covered with native grass species. The final cleanup goal was achieved in 18 months, 6 months ahead of the ROD 24 month deadline (USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995).

Cost

Costs for similar projects conducted in the past varied from $50-$100 per cubic yard of soil treated, provided a minimum volume of 3000 yd3 was treated. This cost of soil treatment was a total of $565,406 to treat 8100 yd3 of soil, meaning the cost per volume was$70 per cubic yard. Cost breakdown showed that of the total cost $58,039 was for ‘before treatment’ operations and$9,827 was for ‘post treatment’ operations.

Observations and lessons

• Tilling of the soil proved difficult after heavy rainfall when it could take up to 2 weeks for the ground conditions to be suitable for tractor access.
• Treatment in 3 lifts reduced TCIC concentrations below the 100mg/kg threshold and 90% of the LTA was vegetated with native grasses.
• Final TCIC concentrations varied from 23mg/kg to 92mg/kg.
• Biodegradation rates varied from 13mg/kg per month to 58mg/kg per month.
• Half of the total cost was for ‘short term’ (up to three years) operations. The cost of treatment was \$70 per cubic yard.
• Target clean up levels where achieved 6 months ahead of schedule.
• The BWPS project saw some of the earliest applications of land treatment bioremediation at a creosote contaminated superfund site.

(USEPA Office of Solid Waste and Emergency Response & Technology Innovation Office, 1995).