Biogeochemical processes controlling midday ferrous iron maxima in stream waters affected by acid rock drainage

The biogeochemical cycling of Fe in acid rock drainage (ARD) streams has presented ongoing challenges to reactive solute transport modeling. Previous studies have relied on the pH-dependent solubility of Fe oxides as the main control of the mid-day Fe(II) maxima concentration in ARD streams. In this study, the authors assess the potential for Fe(II)-oxidizing reactions, including the Fenton and microbial oxidation reactions, to constrain the mid-day Fe(II) maxima concentration. At mid-day, pseudo-equilibrium between Fe(II) oxidizing reactions and photoreduction was assumed in order to evaluate the observed Fe(II) maxima and develop an equation to represent this steady state scenario. This steady state condition is assumed only while light intensity, reactivity of oxides and dissolved organic matter (DOM), and microbial populations remain approximately constant. Three Rocky Mountain ARD streams with known values for Fe(II) were evaluated and average photoreduction rates ranging from 5.56×10−4 to 1.39×10−3 μM/s were found during mid-day steady state Fe(II) maxima. Application of Fe redox biogeochemistry to reactive solute transport modeling may improve predictive capabilities of various trace metal and solute interactions incorporated with the cycling of Fe within ARD streams. Further, model improvement of Fe cycling may enable more accurate remediation predictions for ARD streams.