Application of Mineral Iron and Sulfide Analysis to Evaluate Natural Attenuation at Fuel Contaminated Site

An investigation was conducted of the natural attenuation (bioremediation) potential in a gasoline-contaminated aquifer. In natural attenuation studies, certain oxidized electron acceptors used by bacteria and reduced respiratory products are measured to evaluate the bioremediation of an organic contaminant. Typically, only aqueous ions O₂, ${\mathrm{NO}}_{\mathit{3}}^{\mathit{-}}$, Fe²⁺, and ${\mathrm{SO}}_{\mathit{4}}^{\mathit{2}\mathit{-}}$ are examined in investigations of this type; however, this study also considered mineral Fe and S species as sampled in five core holes along a longitudinal transect of the fuel plume. These samples were subjected to sequential extractions using 6 N HCl and 1 N Cr²⁺ to determine the mineral content of Fe²⁺, Fe³⁺, FeS, and FeS₂. A 0.5 N HCl solution was used to estimate available Fe³⁺ and biogenically produced authigenic Fe²⁺ minerals. Bacteria largely deplete dissolved O₂, ${\mathrm{NO}}_{\mathit{3}}^{\mathit{-}}$, and ${\mathrm{SO}}_{\mathit{4}}^{\mathit{2}\mathit{-}}$ in the most contaminated area of the plume. ${\mathrm{SO}}_{\mathit{4}}^{\mathit{2}\mathit{-}}$ and Fe³⁺ reduction produce secondary authigenic minerals retained in the aquifer. Reduced sulfide minerals (∼FeS and FeS₂) were found near the core of the plume. Direct enzymatic reduction of Fe³⁺ minerals was inferred because most of the Fe²⁺ is not associated with iron sulfides. Expressed capacity is a measurement of past contaminant oxidation. A more accurate estimation of expressed capacity was possible when Fe and S minerals were evaluated along with aqueous data. By including mineral data it was shown that most of the hydrocarbon spill was naturally attenuated at this site. The spatial distribution of Fe mineral species suggests that the hydrocarbon plume is contracting in size. This technique may be used in support of intrinsic bioremediation as part of risk-based corrective action.