Insights into the mechanisms of iron reductive dissolution in vadose zone soils and implications for landfill activities: Predicting the potential for groundwater pollution

Anomalously high iron (Fe) concentrations have been measured in groundwater samples collected from monitoring wells downstream of several landfill units in Florida. Based on monitoring data, vadose zone soils and aquifer sediments impacted by landfills could be sources of Fe that pollutes the groundwater, and not landfilled wastes. Therefore, the objectives of this study were to investigate the reductive dissolution of soil-Fe as a function of biotic and abiotic processes; and to lay the groundwork for the development of a geochemical solubility model for Fe.;Soil samples were collected from different locations in North Florida, using a sampling strategy that included samples with a gradient in the degree of Fe-mineral crystallization. After characterization, soil samples were used in biotic (using bacteria from an anaerobic digester) and abiotic (using sulfide as electron donor) Fe-reductive dissolution batch studies. Fe(II) was released from all soils in concentrations that exceeded the secondary drinking water limit of 0.3 mg/L, when treated with bacteria and organic carbon (OC) or with sulfide but under abiotic conditions. Microbial respiration of OC in soils appears to be a major pathway and could lead to Fe-reductive dissolution in Fe-rich soils that interact with OC-rich waters or leachates. The rates of Fe-reductive dissolution were positively correlated with soil Fe content when glucose was used as the OC source, while no relationship was obvious when landfill leachate was used as OC. Sulfide driven Fe-reductive dissolution was positively correlated with soil-Fe content. However, despite the initial high release of Fe(II) into the aqueous phase in soil spiked with sulfide, temporal trends of Fe-reductive dissolution rates suggest that this pathway may not be as significant as the biotic process due likely to the precipitation of solid FeS species.;Finally, batch studies were conducted using hematite, an Fe-oxide mineral, to investigate the effects of pH, OC and ionic strength on Fe-reductive dissolution rates. Obtained data were used to develop a geochemical solubility model for soil-Fe; which was then validated using soil samples. However, an efficient fine tuning and validation of the model was not fully accomplished as more experimental and field data are still needed.