Site Assessment and Remediation
Core concepts
Site assessment follows a conceptual site model: source–pathway–receptor. Phase I ESA is a records/reconnaissance review (ASTM E1527) and generally no sampling; Phase II uses borings, monitoring wells, soil vapor, and chemical analysis to confirm impacts. Know hydrogeology: porosity, hydraulic conductivity, Darcy's law (q = Ki), seepage velocity (v = q/n), gradients from three wells, LNAPL/DNAPL behavior, retardation (R = 1 + Kdρb/n), and Henry's law. Sampling QA/QC includes chain of custody, holding times, preservation, blanks, duplicates, and data validation. Field screening with PID/OVA and proper decontamination prevent false positives. Know when to use low-flow sampling and how to purge wells. Remediation options include excavation, pump-and-treat, SVE, air sparging, ISCO, bioremediation, thermal treatment, PRBs, and monitored natural attenuation. Risk-based closure and institutional controls often matter.
Worked example
K = 1×10^-4 cm/s, gradient i = 0.01, porosity n = 0.30. Specific discharge q = Ki = 1×10^-6 cm/s. Seepage velocity v = q/n = 3.3×10^-6 cm/s. Trap: using q as the velocity at which a contaminant travels; use v for pore-water travel time. For a conservative solute, time = distance/v.
Rule of thumb
If the plume is LNAPL, it floats near the water table; DNAPL sinks. Do not assume a monitoring well detects dense NAPL by sampling only the water table.