Spatial variability in the naphthalene mineralization response to oxygen, nitrate, and orthophosphate amendments in MGP aquifer sediments

The feasibility of aerobic in situ bioremediation is being investigated for use in a strategy to control subsurface coal tar contamination at the site of a former manufactured gas plant. As part of this investigation, anoxic aquifer sands collected between 11 and 25 m below ground surface were assayed in batch microcosms to measure the singular and combined effects of O-2, NO3-, and PO43- on C-14-naphthalene mineralization. The influence of these additives varied considerably between sediments. A high initial concentration of O-2 (21 mg/L) promoted the greatest extent of mineralization in the majority of active sediments. NO3-(85 mg/L) was observed to enhance, inhibit, or have no effect on the rate of naphthalene mineralization, although significant denitrification was observed in nearly all the active sediments. Data suggest that PO43- complexation and/or precipitation with sediment cations limited P bioavailability. The sediments that were incapable of mineralizing naphthalene were characterized by low pH(< 4.1), high SO42-(> 500 mg/L), and moderate to high dissolved Fe(II) (30-265 mg/L) when equilibrated aerobically with water. Fe(II) likely exerted a significant O-2 demand that reduced the O-2 available as an electron acceptor for biodegradation. These experiments demonstrate that while aeration/oxygenation can be an effective strategy for enhancing subsurface bioremediation of aromatic hydrocarbons, the biodegradation response to aeration/oxygenation and nutrient addition may vary considerably within an aquifer.