Ahtna Engineering Services, LLC (Ahtna) was tasked to evaluate mitigation alternatives for a vapor intrusion pathway resulting from the removal a leaking heating oil underground storage tank from a property adjacent to a residence located at 1534 ½ Stacia Street in Fairbanks, Alaska. Through our analysis, we provided alternatives that included one or more of the following approaches: reduce the contaminant source, prevent contaminant entry into the building, or treat or remove contaminants from the building.
To execute the work, Ahtna performed a site inspection in June 2013 with ADEC to evaluate and understand the feasibility of implementing vapor intrusion mitigation alternatives. Our objectives for the site inspection included the following:
- Understand the general site layout and collect measurements and photographs
- Request a utility locate to determine if utilities would conflict with any mitigation alternatives
- Identify where power could be obtained for remedial options
- Inspect the crawl space to understand how the layout would affect various mitigation alternatives
- Understand the function and layout of the heating and ventilation systems to determine their effect on mitigation alternatives
- Inspect the building for secondary vapor sources (i.e., heating oil leaks, fuel cans, small engines, etc.)
After completion of the site inspection, Ahtna identified six mitigation alternatives to achieve the project’s mitigation action objectives (MAO) and performed a comparative analysis to evaluate the alternatives based on effectiveness, feasibility of implementation, and cost effectiveness. Our analysis provided the following:
- A description of each alternative
- An assessment of each alternative against the evaluation criteria
- A comparative analysis of the alternatives to assess the relative performance of each using specific evaluation criteria
We applied the threshold and balancing criteria described in Section 121(b) of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the National Contingency Plan to evaluate our six proposed mitigation options using a qualitative and numerical scoring system for each criteria. Our cost evaluation was based on a 5-year operating period for each alternative, which represents the timeframe within which we assume that the source of vapor intrusion would be characterized and remediated, and eliminating the need for vapor mitigation.
Our six mitigation alternatives were as follows:
- No Action
- Crawl Space Ventilation
- Air Purification and Filtration
- Soil Vapor Extraction
- Vapor Barrier Trench
- Source Removal (via Excavation)
Overall, our comparative analysis suggested that the Contaminated Source Removal alternative (#6) is the most effective and has a reasonable cost when compared to the other alternatives. However, due to the site specific limitations with implementing each option, a combination of two of the alternatives may be the best option for this site. For instance, complete source removal is limited at the site by tight access and proximity to the building foundation. The readily accessible portion of the source area could be removed at the site, which reduces the volume of source and also minimizes cost, and then the building could be protected though additional crawl space ventilation or filtration.
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Period of Performance