Mobilization and retardation of reduced manganese in sandy aquifers: Column experiments, modeling and implications

Manganese enrichment is common in various sedimentary environments, e.g. deep ocean floor, shallow sedimentary environments where the overlying water column is oxic and phreatic aquifers, in which the Mn aquatic geochemistry is controlled by redox conditions. In many cases, aquifers recharged by organic-rich waters contain high amounts of dissolved Mn ²⁺ that was mobilized from sedimentary Mn-oxides during microbial oxidation of the organic matter under low-oxygen conditions. Thus, the identification and quantification of the processes controlling Mn mobilization and speciation is of great practical importance as a geochemical tool for sustainable management of aquifers. This study uses column experiments to quantify the role of adsorption and precipitation in retarding Mn ²⁺ mobilization in sandy aquifers under low-oxygen conditions. Flow-through columns were packed with pristine sandy sediment and were recharged by low-oxygen artificial solutions containing different Mn ²⁺ concentrations. Outflow solutions from the column experiments were analyzed and simulated using a Mn ²⁺ transport-reaction model. The results were used to demonstrate the importance of adsorption on Mn mobilization in three different aquifers. The experimental data showed that adsorption of Mn ²⁺ on the column solids was the main control on the Mn ²⁺ breakthrough behavior. Nevertheless, up to 20% of the Mn that entered the experimental columns was precipitated as MnCO ₃. The rate of MnCO ₃ precipitation (k _p=0.04h ^-1) was found to be ∼3 orders of magnitude slower than the rate of Mn ²⁺ adsorption (k _a=10-200h ^-1). Given the slow mineral-precipitation kinetics, the water flow rate is critical in determining the potential of MnCO ₃ precipitation in immobilizing Mn within an aquifer. A scale-up of the Mn retardation time found in the column experiments to natural aquifer conditions, suggests that adsorption is responsible for the prolonged retardation of Mn ²⁺ observed in three different sandy aquifers. An important practical conclusion of this study is that the environmental response to perturbation in the hydro-geochemical regime of an aquifer might be delayed for several decades.