Distinguishing Iron‐Reducing from Sulfate‐Reducing Conditions

Abstract

Ground water systems dominated by iron- or sulfate-reducing conditions may be distinguished by observing concentrations of dissolved iron (Fe(2+)) and sulfide (sum of H(2)S, HS(-), and S(=) species and denoted here as "H(2)S"). This approach is based on the observation that concentrations of Fe(2+) and H(2)S in ground water systems tend to be inversely related according to a hyperbolic function. That is, when Fe(2+) concentrations are high, H(2)S concentrations tend to be low and vice versa. This relation partly reflects the rapid reaction kinetics of Fe(2+) with H(2)S to produce relatively insoluble ferrous sulfides (FeS). This relation also reflects competition for organic substrates between the iron- and the sulfate-reducing microorganisms that catalyze the production of Fe(2+) and H(2)S. These solubility and microbial constraints operate in tandem, resulting in the observed hyperbolic relation between Fe(2+) and H(2)S concentrations. Concentrations of redox indicators, including dissolved hydrogen (H(2)) measured in a shallow aquifer in Hanahan, South Carolina, suggest that if the Fe(2+)/H(2)S mass ratio (units of mg/L) exceeded 10, the screened interval being tapped was consistently iron reducing (H(2) approximately 0.2 to 0.8 nM). Conversely, if the Fe(2+)/H(2)S ratio was less than 0.30, consistent sulfate-reducing (H(2) approximately 1 to 5 nM) conditions were observed over time. Concomitantly high Fe(2+) and H(2)S concentrations were associated with H(2) concentrations that varied between 0.2 and 5.0 nM over time, suggesting mixing of water from adjacent iron- and sulfate-reducing zones or concomitant iron and sulfate reduction under nonelectron donor-limited conditions. These observations suggest that Fe(2+)/H(2)S mass ratios may provide useful information concerning the occurrence and distribution of iron and sulfate reduction in ground water systems.

Minor differences in sand physicochemistry lead to major differences in bacterial community structure and function after exposure to synthetic acid mine drainage

“…This substantiates the hypothesis that microbial iron reduction dominated in the group BCD mesocosms. The results are consistent with the fact that the organic carbon requirement for microbial sulphate reduction is substantially higher than that for microbial iron reduction [39].…”

Functional microbial diversity explains groundwater chemistry in a pristine aquifer

“…So despite sulfate reducers and iron reducers competing for the same electron donors in the Mahomet aquifer, by working together they prevent product inhibition. Therefore, rather than being excluded due to thermodynamic constraints by iron reducers as is often suggested [19,20], sulfate reducers seem to be thriving alongside them in the Mahomet aquifer. The relative richness of iron-reducing bacteria as a proportion of total OTUs only exceeded that of sulfate reducers when sulfate concentrations were below 0.2 mM.…”

“…This aquifer contains a diverse community of iron-reducing and sulfate-reducing bacteria in which sulfate has been proposed as a key discriminant of bacterial community structure [18]. Specifically, in high sulfate wells, sulfate reducers have been shown to co-exist with iron reducers throughout the aquifer [18], contrary to previous notions that sulfate reduction is excluded under iron-reducing conditions [19-21]. Previous studies focused exclusively on bacterial populations, leaving the distribution of archaeal populations such as methanogens unexplored.…”

Ecological dynamics in the riverine aquifers of a gaining and losing river

“…This groundwater may be decades older and more depleted in bioavailable DOC than the runoff-derived groundwater in the losing section. The refractory DOC compounds in old groundwater are metabolized very slowly and do not strongly stimulate bacterial growth (Ladd et al 1982, Chapelle et al 2009). …”