What is monitored natural attenuation (MNA) and how is it documented?

Monitored Natural Attenuation (MNA) relies on natural processes to reduce contaminant concentrations without active intervention. Natural attenuation processes: Biodegradation - microbial destruction (primary mechanism for many organics). Dispersion/dilution - physical spreading reduces concentrations. Sorption - binding to soil/organic matter retards migration. Volatilization - transfer to air phase. Chemical transformation - abiotic reactions (hydrolysis, oxidation). Documentation requires lines of evidence: 1) Primary evidence - declining contaminant concentrations over time; statistical trend analysis. 2) Secondary evidence - geochemical indicators (electron acceptor depletion, metabolic byproducts); for petroleum: depleted oxygen, elevated CO2, presence of Fe(II). 3) Tertiary evidence - microbial or molecular evidence of degradation capability (gene probes, microcosm studies). MNA appropriate for sites with: demonstrated attenuation rates, acceptable timeframes, no imminent receptor risks, and lower risk contaminants. Long-term monitoring required (decades). USEPA OSWER Directive 9200.4-17P provides guidance.