Topic Area: 7i. Improved Conceptional Site Models that Include Molecular Biological Data

Formatted Title:

Using qPCR Data on the bssA Biomarker to Select Appropriate Rate Constants for Biodegradation of Toluene in Groundwater

Background/Objectives:

At many fuel spill sites, particularly in urban environments, plumes of BTEX compounds in groundwater extend past the boundaries of the site and encroach on land owned by other parties. Often, it is difficult to obtain access to install the monitoring wells that are necessary to adequately characterize the extent of the plume. In this situation, one approach to evaluate risk from the spill is to apply rule-of-thumb criteria for a minimum distance from the spill to the vulnerable receptors. Another approach is to apply simple analytical models or computer models that assume a typical rate constant for biodegradation of the BTEX compounds. The models forecast a concentration of the compound of concern when the impacted groundwater reaches a receptor. The forecast is compared to the regulatory standards for the site. For many years, qPCR data on the abundance of biomarkers such as the benzyl succinate synthetase (bssA) gene for anaerobic biodegradation of toluene have been used to show that microorganisms are present in the aquifer that can degrade the compound of interest. However, the data on the abundance of the biomarkers are not evaluated to select an appropriate rate constant for biodegradation of the compound of concern. If the qPCR data on the biomarkers could be used to select an appropriate rate constant, the qPCR data would strengthen the credibility of forecasts based on assumed rate constants.

Approach/Activities:

In most impacted aquifers, the active microbes that are sampled in groundwater represent from 10% to 0.01% of the active microbes in the aquifer. For this reason, many groundwater scientists and engineers have been reluctant to use the abundance of biomarkers in groundwater to predict the rate of biodegradation. To address this concern, we use the abundance of the biomarker to determine whether the microbial community at the site is acclimated for degradation of the compound of interest. If the microbial community is shown to be acclimated, then it is appropriate to select a field-scale rate constant extracted from monitoring data at benchmark sites in the literature.

To determine if a site is acclimated, we use the abundance of the bssA biomarker and the concentration of toluene and kinetic parameters in the Monod Equation to calculate a first order rate constant for degradation that would be expected in the groundwater. A rate constant of 0.3 per year is often used to a lower boundary on the “generally useful” rate constant for biodegradation. If 10% of the active bacteria in the aquifer are in the groundwater, they should provide a rate constant of 0.03 per year. If the calculated rate constant is greater than or equal to 0.03 per year, the microbial community is considered to be acclimated. To be conservative, we selected an appropriate field-scale rate constant of 2.6 per year. This is the lower 25 percentile of field-scale rate constants from available benchmark sites.

Giovanni Pilloni

Affiliation

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Using qPCR Data on the bssA Biomarker to Select Appropriate Rate Constants for Biodegradation of Toluene in Groundwater