Practice question

Intrinsic bioremediation also termed natural attenuation relies exclusively on innate capacity of subsurface microbial populations to reduce mass, toxicity, mobility, volume or concentration of contaminants without engineered human intervention beyond periodic monitoring to demonstrate effectiveness. Observed processes include aerobic and anaerobic biodegradation by heterotrophs, abiotic transformation such as hydrolysis, sorption onto soil organic matter, dispersion and dilution of plume. Contaminated aquifers containing benzene, toluene, ethylbenzene and xylene as well as chlorinated ethenes often show stable or shrinking plumes over years as native Pseudomonas and Geobacter sulfurreducens expressing benzylsuccinate synthase bssA activate anaerobic toluene oxidation coupled to nitrate, Fe(III) or sulfate reduction. Regulatory acceptance under EPA Directive 1999 requires three lines of evidence: documented plume stability or receding via concentration vs time graphs, geochemical footprints such as depleted dissolved oxygen below 1 mg per L, increased dissolved ferrous iron, manganese, methane and chloride indicating biodegradation, and microbiological data showing daughter product formation and presence of degrading genes. Distinguished from engineered bioremediation that injects oxygen or nutrients, intrinsic approach passive, low cost but necessitates long-term monitoring and institutional controls to ensure protectiveness.