How is sampling frequency determined in a site characterization program?

• A. Based only on number of contaminants
• B. Arbitrarily chosen by budget
• C. Only by regulatory requirement
• D. Based on data gaps, spatial/temporal variability, regulatory requirements, and decision points (plume extent)

Answer: (D)

In site characterization, sampling frequency is driven by how much information you still need to reduce uncertainty and support real decisions, not by just counting contaminants or sticking to a fixed budget. You set it by identifying data gaps—areas or times where you’re uncertain and need more data—and by planning higher frequency or denser sampling where those gaps are largest. You also consider spatial and temporal variability: a highly heterogeneous system or one with concentrations that change over time requires more frequent sampling to capture the true range of conditions. Regulatory requirements establish minimums and specific intervals, so you must align with those rules. Finally, the timing and nature of decision points, such as delineating the plume extent or choosing a remediation approach, determine how much data is needed and when to collect it. Put together, data gaps, variability, regulatory constraints, and decision milestones together set the appropriate sampling frequency.

Because the frequency isn’t determined solely by how many contaminants are present, it isn’t correct to base it only on contaminant count. Budget constraints matter for planning but don’t by themselves fix how often you sample. Regulatory requirements set minimums but don’t fully account for data gaps or plume behavior.