Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6g. LNAPL Sites: Understanding and Managing Risks

Formatted Title:

Risk Abatement Strategy for LNAPL in UST Pits

Background/Objectives:

Most leaking underground petroleum storage tank cases which are not able to achieve regulatory closure in a timely manner based upon risk-based scenarios typically share the common element of persistent light non-aqueous phase liquids (LNAPLs) on groundwater or in the underground storage tank (UST) pit. A significant proportion of sites have persistent LNAPL present within the UST pit, in either recoverable or residual form. In those cases, LNAPL held within the UST pit can be problematic as a secondary source impacting the less porous native soil and/or groundwater near the UST pit. Because of this, most regulatory agencies have a requirement to remove LNAPL to the maximum extent practicable or to a specific low measurable thickness. In some cases, LNAPL in UST pits can also be problematic due to surfacing periodically during high water events that may impact nearby utilities or surface water.

Typically, UST pits are installed with higher porosity backfill material (i.e., pea gravel or sand) compared to the less porous surrounding soils or bedrock. This creates the conditions for LNAPL to be limited to the UST pit. While most regulatory agencies don’t consider tank pit water or backfill to be an environmental media, remediation of the tank pit may be necessary to achieve No Further Action status. Most common traditional methods of LNAPL removal from tank pit backfill incorporate fluid withdrawal through tank pit observation wells (vacuum truck, MDPE, skimmer pumps, etc.). This evacuates only the mobile LNAPL within reach of the tank pit observation well(s). Residual LNAPL held within tank pit backfill may continue to accumulate in the tank pit wells preventing regulatory closure. In some cases, nothing short of complete product removal can prevent the potential for LNAPL surfacing.

Approach/Activities:

To address persistent residual LNAPL in UST pits, aggressive tank pit flushing and recirculation incorporating selective non-ionic surfactants were used to desorb petroleum hydrocarbons from tank pit backfill and recover mobile LNAPL quickly and cost effectively.

Three case studies are summarized: one in Texas threatening a nearby drinking water reservoir, one in Oklahoma potentially impacting surface water, and one in Alabama where surfacing impacted an adjacent sanitary sewer lift station. In each case, treatment was designed to recover and eliminate measurable LNAPL to prevent recurrence of LNAPL impact to the surface or adjacent utilities. Regulatory acceptance and broad approval of the process has proven readily obtainable. The timeframe to implement field work varies based upon UST pit size, access, and thickness of LNAPL. However, most sites can be completed in 1 to 5 days on site.

Results/Lessons Learned:

Thorough assessment of the extent of LNAPL impact is critical to successful treatment and to ensure LNAPL does not recur following treatment. In most cases, one treatment event is sufficient to adequately remove mobile and residual LNAPL. Most cases with persistent LNAPL in UST pits historically utilized more traditional LNAPL recovery methods usually based upon low initial cost. This approach normally satisfies the regulatory agency but may not achieve closure in a timely manner and may increase risk by only partially addressing LNAPL. When considering the complete closure costs of remediation, an aggressive UST pit surfactant flush can achieve closure more quickly and often for a lower overall project cost. Starting this process early after release discovery and leak repair has been a highly cost-effective method of eliminating a common secondary source and has resulted in significantly lowered risk presented by sites that had persistent LNAPL in tank pits.

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R. George