Monitored natural attenuation (MNA) is a cost-effective remediation approach applicable to many sites and contaminants. Enhanced anaerobic bioremediation (EAB), an engineered remedial action, is another cost-effective remediation approach for chlorinated solvents. In order to evaluate the applicability of MNA and EAB, concentrations of terminal electron acceptors must be measured. Dissolved hydrogen (DH) is another parameter that allows actual terminal electron accepting processes occurring in situ during MNA and EAB to be determined. Dissolved ferrous iron (Fe II), typically measured to assess ferric iron (Fe III) reduction, underestimates the extent of this important terminal electron accepting process because most Fe II remains bound to the soil. Understanding the magnitude of this electron accepting process is important because it can contribute to the biodegradation or transformation of contaminants including vinyl chloride, benzene, toluene, nitroaromatics, MTBE, and uranium. There is a clear need for standardized and cost-effective analytical technologies to support MNA and EAB efforts. The objective of this project was to demonstrate and validate two innovative analytical tools that can identify bioavailable Fe III (BAFeIII) and DH.
Two analytical technologies were evaluated: a BAFeIII assay and a DH analyzer. The BAFeIII assay involves the addition of soil to a tube that contains iron-reducing bacteria (FeRB), lactate as an electron donor, and a mineral salts medium supplemented with reagents that accelerate the assay. The tube is incubated anaerobically for 1 month. During the incubation period, the FeRB consume lactate and reduce BAFeIII present in the soil sample. At the end of the incubation period, Fe II is quantified using a Hach kit. The initial concentration of Fe II present in the soil is subtracted from the result to calculate the BAFeIII concentration. The DH analyzer is a field instrument that strips DH from groundwater into a carrier gas, conditions the carrier gas, and then uses a solid-state sensor to detect the hydrogen concentration. A microprocessor converts the result into a dissolved hydrogen concentration.
The BAFeIII assay was validated using a series of confirmatory analyses conducted on samples collected from four different sites. The results indicated that the assay is precise and more robust than conventional chemical extraction methods. Also, the assay provides results that are representative of true bioavailability as opposed to chemical extractability. The test kit costs $30 to $74 to purchase depending on the quantity, and the total analysis cost is about $210 per sample.
The DH analyzer was tested at three sites and was determined to be negatively affected by interfering gases. These interferences may include hydrogen sulfide and hydrocarbons. Further research is needed on sensors that are not affected by these interferences.
Analysis of soil samples using the BAFeIII assay is recommended in addition to analysis of groundwater for dissolved FeII. This approach will give a more comprehensive and realistic picture of iron reducing processes and electron acceptor demand at contaminated sites. The net benefit will be more accurate estimation of cleanup costs and timeframe along with a more defensible position with regulatory agencies. (Project Completed - 2004)