BACTERIAL FE(III) OXIDE REDUCTION IN INTACT COAST PLAIN AQUIFER SEDIMENTS

Bacterial Fe(III) oxide reduction was examined in intact coastal plain aquifer sediments from the Delmarva Peninsula at a field site near Oyster, VA. Intact core segments (15 cm) were loaded into glass reactors with stainless steel endpieces. The cores were pumped with anaerobic artificial groundwater containing organic electron donors and inorganic nutrients. Some cores (referred to as static) were pumped for 3 d and then incubated for several months; others (referred to as dynamic) were pumped continuously (ca. 2-d residence time) over a 2-3 month period, during which time the dissolved Fe(II) content of the column effluent was measured at 1-2 d intervals. At the end of the core incubations, sediments were removed from the reactors and analyzed for dissolved and solid-phase Fe(II) and Fe(III) content (0.5M HCl and citrate-dithionite extractions). Changes in sediment Fe(II) content and the total amount of Fe(II) exported from the dynamic cores were compared to the Fe(III) oxide content of sediments immediately adjacent to the core segments introduced into the reactors. Total Fe(II) production was significantly correlated with initial total (r2=0.62) and amorphous (r2=0.51) Fe(III) oxide content of the sediments, and accounted for an average of 62 ± 14 % (n=9) of the amorphous Fe(III) oxide content in the dynamic core reactors versus 51 ± 16 (n=12) in the static cores. Aqueous Fe(II) accumulation, however represented only 6.2 ± 5.0 % of total Fe(II) generation in the static core incubations. Similarly, cumulative Fe(II) export accounted for only 20 ± 15 % of total Fe(II) production in the dynamic core reactors. Rates of soluble Fe(II) export varied by more than an order of magnitude and were not correlated with initial amorphous or total Fe(III) oxide concentrations. However, soluble Fe(II) export rates as well as the fraction of total Fe(II) production accounted for by Fe(II) export (or soluble Fe(II) accumulation in the static cores) was positively correlated with hydraulic conductivity. Our results indicate that the potential for total Fe(II) production in aquifer sediments is directly related to Fe(III) oxide abundance, and emphasize the need for consideration of solid-phase Fe(II) formation in quantitative studies of subsurface bacterial Fe(III) oxide reduction.