Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6c. LNAPL Recovery/Remediation Technology Transitions

Formatted Title:

LNAPL Polishing Following Active Recovery

Background/Objectives:

It is common to rely heavily on LNAPL recovery rate data to support the decision to transition from a technology that focuses on LNAPL recovery volume to one that assesses mass removal. Meanwhile, decisions regarding the continued operation of active versus passive LNAPL management corrective applications require metrics other than recovery rates.

Additional technical metrics useful in determining LNAPL recovery success include: graphing groundwater elevation against LNAPL thickness to assess whether successful recovery is achievable in only limited vertical zones; LNAPL transmissivity assessments to support understanding of recovery potential; collection of intact soil cores to confirm limited recovery potential (if LNAPL is observed primarily at residual levels); fluid level measurements collected from surrounding wells to assess LNAPL presence or absence outside of the recovery well network or significant changes in groundwater elevation that may impact saturation levels within the smear or vadose zone; collection of BTEX concentration data in the general vicinity of active recovery points to assess whether the active recovery had positively affected areas between recovery points; photoionization detector screening and laboratory analytical testing of influent, mid, and effluent locations along an active treatment train to provide data regarding reduction efficacy; or failure(s) associated with prior active recovery attempts at a given location.

There are also often non-technical considerations that influence the decision to continue active recovery or to transition to passive corrective actions. Examples of these non-technical site-specific factors include conflicting surface or subsurface obstructions, lack of operational infrastructure (e.g., power); aesthetic concerns (e.g., noise, odor); and property transactions. These non-technical factors should be included in any review prior to any final decisions regarding the transition from active to passive LNAPL recovery.

Approach/Activities:

In spite of large-scale LNAPL recovery efforts across the site boundary, LNAPL recovery was limited in an area where the recovery system was parallel to the groundwater flow direction and the groundwater had a flatter gradient. Additionally, fluctuations in groundwater elevation (natural and influenced by shutdown of nearby groundwater recovery wells) contributed to periods when LNAPL recovery was more effective. These technical and non-technical considerations, along with a review of areas where additional infrastructure could or could not be added for a polishing phase, contributed to the final decision to transition to a passive LNAPL corrective action approach.

Results/Lessons Learned:

The polishing phase was expected to be applied for only a limited period of time and the effort was determined to be successful in achieving some polishing reductions. Though significant quantities of LNAPL were not recovered, this reinforced the understanding of a lack of meaningful recoverable LNAPL. A complete review of the site-specific LNAPL recovery to date as well as various technical and non-technical factors knitted together the understanding of what is occurring within the influence of the active system and within the surrounding subsurface to inform future LNAPL recovery efforts.

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Kate Fogarty