Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6e. Surfactant-Enhanced Remediation

Formatted Title:

Lessons Learned on In Situ Remediation of Very Deep LNAPL

Background/Objectives:

A remote filling station was used for security vehicles at a military facility in the southwest. A leaking product line from an underground storage tank (UST) resulted in gasoline releases. Site geology consists of layered silt and sand which is underlain by weathered, fractured granite bedrock. Depth to groundwater at the site is 65 to 70 feet below ground surface (bgs), compared with depth to bedrock of approximately 70 feet bgs. Depth to groundwater has been observed to drop nearly 10 feet during the past decade. The USTs were removed in 1995. Remediation has been ongoing since 1995, including product recovery, soil vapor extraction, air sparging, passive bioventing, and multiphase extraction (MPE). Past activities reduced light non-aqueous phase liquid (LNAPL) to 0.03 to 0.6 feet in monitoring wells within a focused area approximately 600 square feet.

Approach/Activities:

To further reduce petroleum hydrocarbon mass and LNAPL thickness a remediation approach was selected that consisted of surfactant injection, fluid recovery, and in situ chemical oxidation. The primary remediation objective was to eliminate measurable LNAPL. A pilot-test surfactant injection/fluid recovery event using existing MPE wells was completed over one week. Fine-grained soils and thin overburden water column resulted in rapid drawdown that limited fluid recovery. As a result, weekly fluid recovery was performed over a period of 3 months. Approximately 6 months after the surfactant injection, in situ chemical oxidation was performed for additional destruction of petroleum hydrocarbons.

Results/Lessons Learned:

Following fluid recovery and chemical oxidation, no further LNAPL has been observed in all site wells except two wells, and BTEX concentrations were reduced by 80% in wells outside of the former LNAPL area. Additional surfactant injection with fluid recovery was performed in a targeted area to support LNAPL dissolution across the water column. Following the in situ remediation program, measurable LNAPL has not been observed in any well, and the site has been approved for long-term monitoring.

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Steve Geiger