Compare soil sampling vs groundwater sampling in site characterization.

• A. Soil sampling characterizes source zones and vadose zone processes; groundwater sampling characterizes dissolved plumes and aquifer transport
• B. Soil sampling characterizes dissolved plumes and aquifer transport
• C. Both methods provide identical information
• D. Groundwater sampling is obsolete when soil sampling is performed

Answer: (A)

In site characterization, you’re looking at two connected but distinct zones: the vadose (unsaturated) zone in the soil above the water table and the saturated groundwater zone. Soil sampling is focused on source zones and vadose zone processes. It reveals where contaminant mass is stored near the surface, how it interacts with soils (sorption, retardation), and how infiltration and diffusion drive downward movement toward the groundwater. This helps identify potential sources, concentrations near the surface, and early-stage transport pathways into the aquifer.

Groundwater sampling, on the other hand, targets dissolved contaminants in the aquifer itself. It characterizes dissolved plumes, their extent, concentration distribution, and how groundwater transport—advection, dispersion, dilution, and interactions with the aquifer matrix—shapes plume shape and movement. This is essential for understanding actual transport within the aquifer and assessing long-term exposure and risk.

The other statements don’t fit because dissolved plumes and aquifer transport reside in the groundwater zone, not primarily in soils; soil and groundwater provide different, non-identical information about contamination and movement; and groundwater measurement remains necessary even when soil data exist, since groundwater is a key exposure pathway and transport medium.