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MEASUREMENT OF BIOAVAILABLE IRON AT TWO HAZARDOUS WASTE SITES
Citation:
Wilson*, J T., C. Lebron, AND P. Evans. MEASUREMENT OF BIOAVAILABLE IRON AT TWO HAZARDOUS WASTE SITES. Battelle Conf. 7th Int'l. Symposium on In-Situ & On-Site Bioremediation, Orlando, FL, 06/2-5/2003.
Description:
In the past, the concentrations of iron II in monitoring wells has been used to evaluate natural attenuation processes at hazardous waste sites. Changes in the aqueous concentrations of electron acceptors/products are important to the evaluation of natural biological attenuation, but this information can be misleading in the case of iron. The question of concern to regulators is not whether biological processes have destroyed contaminants in the past, but whether the processes can be sustained to destroy contaminants in the future. Aqueous concentrations of ferrous iron give no indication of the remaining reservoir of bioavailable iron present in the aquifer matrix. Various methods have been proposed to estimate the amount of bioavailable iron in soil, including extraction methods using weak and strong acid, sophisticated instrumental analysis for particular iron minerals, and bioassays.
A commercially available assay for biologically available iron in sediment was evaluated at two hazardous waste sites on the U.S. Coast Guard Support Center at Elizabeth City, North Carolina. The first site is a plume of petroleum hydrocarbons and MTBE emanating from residual JP-4 jet fuel. The second site is a plume of PCE emanating from a fire training area.
To conduct the assay, a sample of sediment was incubated with microbiological culture medium and a culture of iron reducing bacteria at room temperature for 30 days. Samples of sediment that have been incubated with the culture medium and samples that have not been incubated were extracted into 0.3 N HCl for two days at room temperature. The amount of iron II extracted from the sample that was incubated with culture media was compared to the amount of iron II extracted from the sample that was not incubated with culture media to determine the additional amount of iron II released by biological activity on the sediment. To estimate the amount of iron III in the sediment that might be biologically available, the amount of iron II that was extracted from the sediment that was not incubated with culture medium was subtracted from the amount of total iron that was extracted. To evaluate the bioassay, the amount of extracted iron III was compared to the biologically available iron.
The ground water down gradient of the JP-4 jet fuel spill contains BTEX compounds and the ground water is strongly iron reducing. The ground water typically contains from 50 to 100 mg/liter of iron II. The concentration of biologically available iron up gradient of the spill varied from 580 to 610 mg/kg. Compared to concentrations up gradient, the concentration of biologically available iron in sediment from the plume of contamination was reduced by 120 to 340 mg/kg. In each sediment sample, the amount of biologically available iron recovered by the assay was greater than the amount of iron III extracted by 0.3 N HCl; on average the assay recovered 2.5 times as much iron. Currently, the ground water down gradient of the fire training is not strongly iron reducing, the ground water typically contains from 0.3 to 3 mg/liter of iron II. The concentration of biologically available iron in uncontaminated material side gradient of the spill varied from 2000 to 2800 mg/kg. Compared to concentrations side gradient of the plume, the concentration of biologically available iron in sediment from the plume of contamination was reduced by 620 to 1600 mg/kg. In each sediment sample, the amount of biologically available iron recovered by the assay was greater than the amount of iron III extracted by 0.3 N HCl; on average the assay recovered 1.6 times as much iron.