Factors limiting in situ bioremediation of polycyclic aromatic hydrocarbons and alkylbenzenes

A former manufactured gas plant (MGP) was used to investigate four factors affecting the bioremediation of polycyclic aromatic hydrocarbons (PAHs) and alkylbenzenes in the subsurface: (1) distribution and biodegradation capacity of bacteria; (2) type and availability of electron acceptors; (3) substrates interactions; and (4) biodegradation kinetics.; Experiments demonstrated that intrinsic bioremediation is occurring at the MGP site as evidenced by the in situ accumulation of reduced species and the widespread occurrence of bacteria capable of biodegrading PAHs and alkylbenzenes. The bacterial occurrence is heterogeneous, and sediment grain size and inhibitory chemistry control bacterial activity and distribution.; Oxygen is another significant factor affecting PAH biodegradation in the aquifer. However, in situ aeration will be confounded by the excessively high FeII in the site ground water (typically ∼300 mg/L). Aeration will cause significant precipitation of Fe(OH)₃(s) and a pH decline, both of which could serve to hinder the success of bioremediation. Citrate effectively inhibited precipitation; however, the resulting Fe III-citrate complex was easily biodegraded by the FeIII-reducing bacteria common at the site. The biodegradability of the FeIII-citrate complex introduces the possibility of excessive biomass growth plugging the aeration wells or infiltration galleries used in aerobic bioremediation.; Data indicate that nitrate-based remediation could be a viable complement to aerobic bioremediation at the site. PAHs resisted biodegradation under anaerobic conditions, but toluene, m-, o- and p-xylene all biodegraded under nitrate-reducing conditions. m-Xylene biodegradation closely followed Monod kinetics, with biokinetic parameters estimated at Y = 0. 92 mM cells/mM m-xylene, μmax = 0.091 hr−1, and $Ks=0.006$ mM m-xylene. A novel approach was developed for estimating biokinetic parameters in which the Monod model is simultaneously fit to multiple data sets.; Substrate interactions were observed among alkylbenzene in under nitrate-reducing conditions. O- and p-xylene biodegraded cometabolically with toluene as the primary substrate, while m-xylene competitively inhibited toluene biodegradation. These data indicate that substrate interactions play an important role in governing the fate of alkylbenzenes in the subsurface.; While oxygen and nitrate can stimulate in situ bioremediation, the success of bioremediation will vary depending on the type of compound, the site-specific geochemistry, and substrate interactions.