13 March 2015
Posted in Center Information
PI: Dr. Timothy Townsend, University of Florida
Iron concentrations in groundwater monitoring wells at several landfill sites in Florida have been found to exceed the secondary drinking water limit of 0.3 mg/L and the health- based criterion of 4.2 mg/L. It has been determined that the reductive dissolution of naturally-ocurring iron oxides in the soil is a principal mechanism. This study aims to provide a tool to evaluate the potential of a landfill site to exhibit elevated iron concentrations in the groundwater. The first section of this project describes an experimental approach to determine the initial ferrous iron concentration that can be expected under the landfill, C0. The second section describes the utilization of the expected C0 to simulate the ferrous iron plume in the aquifer downgradient of the landfill.
Initially, anaerobic batch tests were performed to assess the potential of the soils from six different landfills in Florida (with reported iron issues) to release iron into the groundwater. The samples were incubated under an anaerobic environment and different biological conditions to assess the influence of microbes on the reduction reaction. Additionally, landfill leachate was added to the soil to examine the effect of different sources of organic matter. Incubation with young municipal solid waste (MSW) leachate resulted in higher ferrous iron concentrations when compared to a control, which might indicate that the organic matter composition of this type of leachate facilitates the reduction of Fe(III) to a greater extent. When comparing the different biological conditions (abiotic and biotic incubations), no significant differences were evident. Experimental data were compared to values observed in the field; the anaerobic batch tests were unable to simulate certian conditions in the field, such as the incubation period and the liquid to solid ratio in the aquifer.
Column leaching tests were conducted on two soil samples to determine the influence of liquid to solid ratio on the reductive dissolution of iron; the tests were carried out under both aerobic and anaerobic conditions. Fe(II) was not detected under aerobic conditions. However, Fe(II) concentrations ranged from 0.5 to 3.5mg/L under anaerobic conditions. A mass flux approach was developed to correlate the measured concentrations in the column experiment to Fe(II) released in the field. The maximum theoretical Fe(II) concentrations produced using the mass flux approach were found to be the best method of corrlelating labratory data (of the experiments and mechanisms used) to the observed range of Fe (II) values in monitoring wells at the two landfill sites evaluated
According to numerical simulation using TOUGHREACT, the Oxygen (O2) (aq) concentration in the groundwater and the partitioning coefficient (Kd) of Fe(II) were found to be the major factors affecting the fate and transport of Fe(II) in the aquifer, these parameters correspond to oxidation and sorption, respectively. Using 100 mg/L as C0 (based on labratory tests), the Fe(II) front extended relatively fast and traveled a longer distance when oxidation (0-10 mg/L O2 (aq)) was solely considered, reaching 235-345 m from the landfill within two years. However, when sorption was also included in addition to the oxidation, the plume migrations were significantly retarded. A partitioning coefficient of 0.25 L/kg doubled the retardation factor, reducing the plume length up to 155 m within the same two-year period. Although other minor factors were not considered in this study, two major factors affecting Fe(II) concentrations were evaluated; this approach could be used as guidance for estimating the Fe(II) plume migration in future instances.