Biogeochemical characterization of the experimental site (Lake) suggests that the above-ground facilities rather than reservoir should be the target for the future nitrate treatment. The produced water from the Lake oil-water separator was used to study interactions of nitrate reduction with other microbial processes and to determine the appropriate concentration of nitrate for controlling these processes. Laboratory incubations were set up in an anaerobic glovebox, where the produced water was distributed into serum bottles and supplemented with different amounts of nitrate and other amendments. Sterile and unamended controls were also constructed. In the incubations without nitrate, sulfate reduction was the dominant terminal electron accepting process. It occurred actively especially if sulfate was unlimited and there was a supply of electron donors (Fig. 1). In the incubations supplemented with 5 mM NaNO3, sulfide was oxidized and nitrate was partially removed (Fig.1). Smaller concentrations (1 and 2 mM) did not produce a reliable effect; sulfide was not removed completely and sulfate reduction could be resumed (Fig.2). In the incubations with the water from Bebee-Konawa Field, nitrate was never used up. In the incubations where nitrate and sulfate were present in equimolar amounts, sulfide oxidation took over sulfide formation. The addition of easily degradable substrates (lactate and acetate) did not change the pattern. If nitrate was present, sulfide was oxidized, and sulfate reduction was inhibited (Fig.3). Crude oil or acetate did not stimulate sulfide oxidation. Molybdate inhibited sulfide oxidation as well as sulfide formation.
The potential rates of nitrate reduction were about 0.06 mmol/L day (Fig.4). Molybdate also completely inhibited this process. Oil did not stimulate nitrate reduction, however the effect of acetate was inconsistent. Sometimes there was stimulation; sometimes there was not. To get a clearer understanding of this phenomenon, some incubations were transferred into mineral media that is selective for DNB to enrich for this group of microorganisms. In the enrichments containing 10 mM acetate as a growth substrate, 15 mM nitrate was reduced completely in one week with a corresponding formation of nitrite (Fig. 5). Oil did not stimulate nitrate reduction, and the process in the enrichments supplemented with oil resembled the unamended control.
Fig.1
