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LNAPL – It’s Not Just Unconfined

written by Andrea

LNAPL-Not Just UnconfinedAbstract of presentation to be given by Mike Hawthorne of H2A (coauthors Andrew Kirkman of AECOM and Mark Adamski of BP America) at the LNAPL Site Management Strategies Session of the 2011 Battelle Bioremediation and Sustainable Environmental Technologies Conference, Reno, Nevada, June 27-30, 2011.

Background. Existing LNAPL distribution literature is heavily biased towards ideal unconfined conditions representing static water-table elevations.  Mobile LNAPL has been documented at multiple sites to be submerged below the water-table as well as existing perched above the water-table.  Given the substantially exaggerated apparent NAPL thicknesses (ANT) that can occur in wells under these conditions, the identification of confined or perched LNAPL at a site is critical to correctly characterize the mobile LNAPL interval thickness and stratigraphic occurrence in the LNAPL Conceptual Site Model (LCSM).  A more accurate LCSM will lead to better metrics for both LNAPL characterization and remediation.  For example, ANT in wells is a poor metric for remediation because the ANT represents LNAPL confining pressure rather than recoverability or thickness of the mobile LNAPL interval.  The presence of confined or perched LNAPL may also have a profound effect on LNAPL remediation technology selection.

Approach. LNAPL flow vertically is inhibited due to non-wetting phase capillary pressure limitations rather than soil permeability limits.  However, LNAPL confining capillary pressures typically coincide with reduced grain size in unconsolidated sediments.  Therefore, an improved understanding of microstratigraphy results in more precise identification of potential confining or perching layers.  Improved characterization tools (e.g., laser induced fluorescence or LIF, baildown testing, soil core analyses) simplify identification of confined or perched mobile LNAPL intervals.  By combining microstratigraphic characterization tools (e.g., soil core analyses) with mobile LNAPL interval identification tools (e.g., baildown testing, diagnostic gauge plots), stratigraphic intervals containing mobile LNAPL may be identified even under confined, perched or interbedded conditions.

Results. This presentation will discuss methods to identify confined and perched LNAPL, and will include a review of site-specific examples of both simple and complex confined and perched LNAPL sites.  The tools discussed include hydrostratigraphs, diagnostic gauge plots, visual soil boring data, CPT and laser induced fluorescence borings, soil core photography and core analyses, and baildown test data.

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