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Before we continue with our investigation, it will be useful at this point to begin forming our conceptual model. We can refine it as we obtain more information. Let's begin by summarizing what we know to this point: a small continuous leak at a dispenser valve has resulted in a thin NAPL plume approximately 30' x 160' in area. Due to a recent rise in the water table, the residual, sorbed phase NAPL was smeared along a minimum three foot vertical zone. As the NAPL lens formed, it became elongated in the direction of groundwater flow and eventually encountered the basement of the first residence.
The lighter components of the gasoline began to transfer from the NAPL lens and the impacted soil to the vapor phase. The vapors then migrate along the unsaturated zone to areas of low pressure such as basements, crawl spaces and sewer lines. Soluble components of the gasoline such as BTEX and MTBE begin to leach out of the sorbed and free phase NAPL to the groundwater, and move along with groundwater flow in the downgradient direction.
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