Modeling As release and attenuation by considering a threshold energy for microbially mediated redox reactions

One of the mechanisms accepted for As mobilization in several alluvial aquifers is the reductive dissolution of Feoxide driven by organic matter (OM) oxidation. It is assumed that the OM is degraded into simpler molecules: CO2 and electron donors (e.g. H2, acetate, formate,) among which H2 is dominant. These products are then used as electron donors by different terminal electron accepting processes (TEAPs). The TEAPs are often mediated by bacteria that require a minimum energy available to conduct the reaction, in order to sustain their life functions. Implementing this threshold energy in transport modeling is currently rarely used. This work presents a 1D reactive transport modeling of As release by reductive dissolution of Fe-oxide and subsequent attenuation by coprecipitation in iron sulfides considering a threshold energy for Fe-oxide reduction, sulfate reduction and methanogenesis. This model is further extended by implementing an energy gap for the reaction pair Fe(III) reduction/Fe(II) oxidation. The model simulates a vertical 1D transect representing the multilayer aquifer in Cremona, Po Plain (N. Italy) and is calibrated by measured groundwater concentrations at different depths. The model, implemented using PHREEQC, is based on a partial equilibrium approach and redox reactions are written considering the H2 as reductant instead of electrons. In order to implement the threshold energy, the logK value is shifted to lower or higher values, depending on the reaction direction. The reversibility of the reaction pair Fe(III) reduction/Fe(II) oxidation implementing a threshold energy for both directions is given in the model using a BASIC algorithm within the kinetic feature of PHREEQC. The model fitted to measured data showed that the As concentration is mainly governed by the concomitant equilibrium between Fe-oxide and sulfate reduction and FeS precipitation. The optimal threshold energy fitted resulted in 3.76, 4.50 and 1.60 kJ/mol e- for Fe-oxide reduction, sulfate reduction and methanogenesis, respectively. The threshold energy became 2.10 kJ/mol e- for Fe(III) reduction and Fe(II) oxidation when the model is extended by implementing an energy gap for this reaction pair. The fact that these threshold energy values are close to each other is consistent with the occurrence of different TEAPs close to a concomitant equilibrium, i.e., different microorganisms would be able to survive with a similar energy yield