Formatted Title
Lessons Learned: Long-Term Effectiveness of Bioremediation in a Low Permeability Aquifer
Background/Objectives
The goal is remediation at a former waste handling facility to allow for site development and re-use in a prime mixed use (commercial/residential/industrial) area of Seattle, Washington. The site was primarily contaminated with chlorinated VOCs (including 1,4-dioxane) with suspected DNAPL source. The target aquifer consists of discontinuous interbedded fine sand, silty sand, and sandy silt lenses. An interim measure was installed around the source area in 2004, comprised of a hydraulic control system with low flow pump and treat system and a subsurface barrier wall tied into a silt aquitard approximately 60 to 90 feet below ground surface. Bioremediation was selected as the remedial option to polish groundwater concentrations of chlorinated solvents following remedial excavation outside/upgradient of the barrier wall area and was utilized for mass reduction internal to the barrier wall.
Approach/Activities
Although bioremediation was selected for implementation, different application methods were selected due to site constraints (limited routine access, active rail yard, capping differences, excavation limitations). In the area upgradient of the containment area, bioremediation through placement of a one-time dose of emulsified vegetable oil (EVO) was completed through a large infiltration gallery installed following excavation of the vadose zone and placement of quarry spalls. This single application treatment targeting residual chlorinated solvents remaining in the subsurface following excavation. It was expected to reduce groundwater concentration to below cleanup levels for up to 5 years.
In the area internal to the containment area, a network of extraction and injection wells was installed to perform recirculation and dosing of substrate to promote reductive dechlorination of solvents. Dosing was performed semi-annually over four years (eight events) to distribute substrate. Both high fructose corn syrup (HFCS) and EVO were utilized as substrates over the four years.
Long-term monitoring was performed following the large single dose application outside/upgradient of the of the barrier wall and monitoring was performed throughout the four year injection/recirculation period inside the containment area to understand the effectiveness of both application methods. Bioaugmentation for 1,4-dioxane was also piloted.
Results/Lessons Learned
We will compare and contrast the site constraints that led to the different implementation methods and the long-term effectiveness of each. Review of up to 10 years of operation and performance monitoring data, including mass removal and cost/benefit analysis for each implementation method will be presented. Lessons learned will be highlighted by discussion of:
- Comparison of implementation methods and their long-term effectiveness;
- Substrate effectiveness and impacts to biofilm generation, including fouling of remediation wells;
- Vinyl chloride generation impacts on shallow groundwater;
- Bioaugmentation effectiveness in a low permeability aquifer; and
- Complications due to the threat of vandalism.