Ongoing uncertainty about why LNAPL accumulates in wells—even when transmissivity is extremely low—drives misinterpretation of LNAPL well thickness and unrealistic remediation expectations, underscoring the need for clearer understanding of why LNAPL accumulates in monitoring wells.  This document addresses that problem by explaining the principles behind transmissivity and residual saturation, and why accumulations of LNAPL do not necessarily indicate significant mass or recoverable product.  It brings together current understanding of low‑mobility behaviour, including the effects of soil heterogeneity, groundwater fluctuations and changing LNAPL properties, to help practitioners interpret field observations more accurately.  Ultimately, the document provides practical, science‑based advice to support stronger conceptual site models, better risk‑based decisions, and more robust remediation planning at LNAPL-impacted sites.

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