Determining groundwater recharge for quantifying PFAS mass discharge from unsaturated source zones

Soil‐to‐groundwater contaminant mass discharge (Md) is the authoritative metric defining source strength at sites impacted by per‐ and polyfluoroalkyl substances (PFAS) and is increasingly being reported. Accurate estimates of groundwater recharge at representative spatial scales, however, is critical to quantitatively estimating Md, which to date has received comparatively little attention relative to PFAS‐specific partitioning and retention processes within unsaturated zone source areas despite a plethora of available literature. The objective of this review is to summarize the concept of Md as it applies to PFAS‐impacted sites, present standardized terminology, and collate published literature on groundwater recharge for a practitioner‐level summary of practical methods. Critically, the primary aim is to reduce uncertainty in recharge (and, thus, Md) estimates commensurate to a given site management application (i.e., the data quality objective process). Therefore, we propose a tiered system of increasing cost, complexity, and certainty depending on specific project requirements and site conditions and recommend applicable recharge methods for each tier based on these factors. Ultimately, a framework is presented for the assessment of PFAS‐impacted source areas based on the concept of Md.