Quantification of Vapor Phase - Related Natural Source Zone Depletion Processes Natural source zone depletion (NSZD) has emerged as an important concept within the realm of environmental re mediation. NSZD is a term used to describe the collective, naturally occurring processes of dissolution, volatilization, and biodegradation that results in mass losses of light non - aqueous phase liquid (LNAPL) petroleum hydrocarbon constituents from the subsurface. This document provides practical gu idance on NSZD theo ry, application, measurement methods, and data interpreta tion. It is intended to be used by practitioners to help plan, desi gn, and implement NSZD monitoring programs in support of petroleum hydr ocarbon site remediation