In Situ Anaerobic Bioremediation
- Three, 8-ft deep gravel-filled, surface infiltration trenches and two, 240-ft long horizontal wells with 30-ft screened intervals
- Groundwater extracted from upper horizontal well and recirculated via surface trenches and lower horizontal well at a rate of about 1.5 gpm
- Benzoate, lactate, and methanol added to recirculated water to serve as nutrients for dechlorinating bacteria
- 250,000 gallons of water circulated during pilot study over five month period
Chlorinated solvents, including trichloroethene, methylene chloride, dichloroethene, and vinyl chloride
Concentrations ranged from 10-400 mg/kg
DNAPL suspected to occur in localized areas
Total cost of pilot remediation project was $397,074, including:
- Mobilization and preparatory work - $35,000
- Monitoring, sampling, testing, and analysis - $238,310
- Groundwater collection and control - $87,536
- Biological treatment - $23,748
- General requirements - $12,480
The Pinellas STAR Center operated from 1956 to 1994, manufacturing neutron generators and other electronic and mechanical components for nuclear weapons under contract to the U.S. Department of Energy and its predecessor agencies. The Northeast site is associated with the location of a former waste solvent staging and storage area. In the 1950s and 1960s, an existing swampy area at the site was used for staging and burial of construction debris and drums, some of which contained solvents. The site consists of a shallow groundwater aquifer contaminated with a variety of VOCs, including chlorinated solvents such as trichloroethene, methylene chloride, dichloroethene, and vinyl chloride.
From February 7, 1997 to June 30, 1997 a demonstration using in situ anaerobic bioremediation was conducted at the site. The demonstration was part of a program at the Pinellas STAR Center to evaluate several innovative remediation technologies that could enhance the cost or performance of an existing pump-and-treat system. The pilot system was located in an area of the site that had total chlorinated contaminant concentrations in groundwater generally ranging from 10-400 mg/kg, with one monitoring well having concentrations in excess of 2,900 mg/kg. The bioremediation pilot system consisted of three 8-ft deep gravel-filled, surface infiltration trenches and two 240-ft long horizontal wells with 30-ft screened intervals. The horizontal wells, directly underlying and parallel to the middle surface trench, were at 16- and 26-ft depths. The study area was about 45 feet by 45 feet and extended from the surface down to a thick, clay confining layer 30 feet below the surface. Groundwater was extracted from the upper horizontal well and recirculated via the surface trenches and the lower horizontal well while benzoate, lactate, and methanol were added to the recirculated water to serve as nutrients for the dechlorinating bacteria.
During this period, groundwater was extracted and recirculated at a rate of about 1.5 gpm. Approximately 250,000 gallons of water, based on soil porosity of about two pore volumes, were circulated during the pilot study. Tracer and nutrient monitoring data indicated that nutrients were delivered to 90% of the central treatment area during operations. Where nutrient breakthrough was observed, significant declines in total chlorinated VOC concentrations were generally observed.
The cost of the pilot system totaled approximately $400,000, with over half the costs associated with sampling and analyses. Most of the sampling and analyses were discretionary and were used to verify the system concept and design. This level of sampling would not be needed during a full-scale bioremediation project. System construction costs were about $90,000, while operating costs were about $30,000 or $0.12 per gallon of water treated. The extensive modeling, hydrogeologic, nutrient transport, and operating cost data developing during this pilot system operation suggest that the Northeast Site could be remediated using nutrient injection in approximately 2-3 years at a cost of about $4-6M.