Light Non-Aqueous Phase Liquids (LNAPLs)

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A Light Non-Aqueous Phase Liquid (LNAPL) is a groundwater contaminant such as petroleum oil, gasoline or diesel fuel that is less dense than water and is not very soluble in water. LNAPLs affect groundwater quality at many sites and are of regulatory importance because, once in the subsurface, they can be difficult to adequately assess and recover and thus can be a long-term source of vapor, groundwater and soil contamination.

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In general, LNAPLs released from tanks, pits, pipelines, or other sources will migrate vertically downward through the soil in the unsaturated zone under the influence of gravity through permeable pathways (e.g., soil pore space, fractures, and macropores), and if the release is large enough, that migration may continue until the water table is encountered. The LNAPL will then spread out laterally with some of the LNAPL above and some below the water table. As it ages a continuous LNAPL body will form “residual LNAPL” which can be conceptualized as individual LNAPL blobs trapped in individual pores. As groundwater moves through the mobile and/or residual LNAPL source areas, soluble components partition into the moving groundwater to generate a plume of dissolved contamination. Once further releases stop, these LNAPL source areas tend to slowly weather away as the soluble components are depleted.

There are a variety of LNAPL remediation approaches. In cases where mobile LNAPL (free product) removal is feasible, most regulatory agencies require site owners or responsible parties to remove the product (e.g. by pumping or skimming the LNAPL). However, at many sites complete removal of all the potentially mobile LNAPL is not feasible with available technology because the residual LNAPL is trapped in soil by strong capillary forces making it difficult to pump out.

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