What is plume delineation and how is it determined?

• A. It is the boundary of a contaminated area defined by exceedance of cleanup criteria or statistical confidence; determined using sampling data, modelling, and risk-based criteria
• B. It is the boundary of the contaminant plume within the aquifer computed solely by a single sample
• C. It is the measured depth to groundwater
• D. It is the projected future spread based on a single observation

Answer: (A)

Plume delineation is the process of defining the geographic extent of a contaminant plume by identifying where concentrations exceed cleanup criteria or meet statistical confidence limits. It is determined by integrating multiple data sources and methods: sampling data collected across the site to map concentrations, transport and fate modeling to account for groundwater flow and dispersion, and risk-based criteria that link contaminant levels to potential exposures and receptors. Together these elements establish a boundary that reflects both measured concentrations and the level of confidence in those exceedances. This is different from simply measuring the depth to groundwater, which is unrelated to contaminant extent, or projecting future spread from a single observation. Relying on a single sample can’t reliably define a plume boundary, and while modeling informs delineation, the boundary is based on exceedance of criteria or defined confidence, not a one-off projection.