Numerical simulation of a natural attenuation experiment with a petroleum hydrocarbon NAPL source

A three-dimensional solute transport model with biological reactions is pre sented for simulating the natural attenuation study (NATS) at the Columbus Air Force Base in eastern Mississippi. NATS consisted of the release of a p etroleum-based nonaqueous phase liquid (NAPL) and subsequent monitoring of BTEX (benzene, toluene, ethylbenzene, p-xylene), naphthalene, decane, and b romide in a shallow, unconfined aquifer. Conceptual and mathematical models were developed for NAPL source release, sequential aerobic/anaerobic biode gradation, and sorption during NATS. A multiple species, solute transport c ode (SEAM3D) was used to simulate fully three-dimensional transport and aer obic, nitrate-reducing, ferrogenic, and methanogenic hydrocarbon biodegrada tion. Simulation results matched individual BTEX concentration distribution s collected five- and nine-months following NAPL release. SEAM3D mass-balan ce calculations at t = nine months indicated that 49% of the hydrocarbon ma ss that dissolved into the aqueous phase was consumed by biodegradation, 13 % of this mass was sorbed, and the remaining 38% was present in the aqueous phase. Mass calculations at t = nine months further indicated that aerobic biodegradation accounted for the majority of hydrocarbon biodegradation (4 6% of the biodegraded mass), followed by ferrogenesis (28%), nitrate-reduct ion (21%), and methanogenesis (5%). Model results were particularly sensiti ve to the NAPL release rate, the initial ferric iron (Fe[III]) concentratio n, hydrocarbon utilization rates, initial condition for the anaerobic micro bial populations, and dispersivity.