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Landfilling is the most widely used method of waste disposal in the world including the USA. However, contamination of groundwater by landfill activities is becoming increasingly problematic. In recent years, groundwater contamination with iron (Fe) in landfill impacted aquifers has been on the increase in Florida, where Fe concentrations above the secondary drinking water limit of 0.3 mg/L have been measured in a number of groundwater monitoring wells. Interestingly, groundwater monitoring data collected from the same wells at or downstream of landfill sites indicated that detected Fe may not originate from the actual landfill, based on the behavior of key tracers common to landfill leachates. In fact, it now believed that Fe-reductive dissolution of Fe-(hydr)oxide minerals naturally occurring in vadose soil appears is the main source of dissolved Fe(II) detected in polluted groundwater. To investigate this hypothesis, laboratory batch experiments were conducted and the ability of native vadose soils to behave as source of Fe polluting the aquifer was tested. Vadose soil samples were collected from different sites falling along a transect with an apparent gradient in both organic matter content and degree of crystallization of Fe-(hydr)oxide minerals. The results show that organic-rich leachate can fuel the reductive dissolution of soil’s Fe-(hydr)oxide minerals as organic matter undergoes oxidation during microbial anaerobic respiration by iron reducing bacteria. In addition to this microbial driven process, a non-biological process does exist as well. In the latter, reduced sulfur compounds (e.g. H2S) appear to be responsible for the Fe-reductive dissolution. However, the contribution of these biotic and abiotic processes under natural conditions remains to be determined. The ultimate goal of this project is to develop a geochemical model which incorporates key soil’s physicochemical characteristics to help predict the potential of a given soil to respond to landfill activities with regard to Fe-reductive dissolution. |