Rate Constant Calculator for Toluene Biodegradation

Introduction

Based on the equation developed in Pilloni et al. (2025),¹ the calculator below provides an estimated first-order rate constant for anaerobic toluene biodegradation using toluene concentration and abundance of the benzylsuccinate synthase gene (bssA), an important gene involved in the anaerobic degradation of toluene, ethylbenzene, and xylenes.  Pilloni et al. validated the equation by comparing the estimated rate constants calculated from field-scale qPCR and contaminant data to a published compilation of rate constants from benchmark field sites.²

Since groundwater samples are commonly submitted for analysis and more convenient to collect compared to soil core samples, the calculator is based on bssA and toluene concentrations in water. Please enter the toluene concentration in units of mg/L and the bssA concentration in units of gene copies/mL. The result will be the estimated first-order rate constant for anaerobic toluene biodegradation in units of “per year.”

A rate constant of 0.3 per year is associated with a “generally useful” rate of biodegradation. Assuming that 10% of the active microorganisms are free floating in the groundwater, Pilloni et al. suggest that an estimated rate constant ≥0.03 per year indicates that the microbial community has acclimated and is stable and active. If this threshold is met, then it is appropriate to select a typical biodegradation rate constant from benchmark sites² when using models to evaluate risk at petroleum hydrocarbon sites.

Toluene First Order Rate Constant Calculator – bssA

A note about LNAPL

It is not appropriate to apply the selected benchmark rate constant to a location where residual LNAPL is present since the aqueous toluene concentration will be largely controlled by the rate of dissolution from LNAPL rather than the rate of biodegradation. Instead, it is recommended to extend the benchmark rate constant from the first well along a flowpath that is not impacted by LNAPL.

More about bssA

Quantification of the bssA gene using qPCR is commonly used to assess the genetic potential for anaerobic toluene biodegradation. The benzylsuccinate synthase enzyme complex mediates the first step in anaerobic toluene biodegradation, where fumarate is added onto the toluene methyl group to form benzylsuccinate.

The Equation

The rate of anaerobic biodegradation of toluene can be calculated using the Monod equation as described in Edwards and Grbić-Galić (1994):³

-(dM/dt)/V_L = (μ_max/Y) × X_a x [S/(K_S + S)]

where M is the mass of toluene (mg), t is the time elapsed (days), V_L is the volume of water (L), μ_max is the maximum instantaneous growth rate of the culture (mg new cells produced per mg of existing cells per day), Y is the growth yield (mg biomass produced per mg of toluene consumed), X_a is the active biomass (mg/L), S is the concentration of toluene (mg/L), and K_S is the concentration of toluene producing a growth rate equal to one-half of μ_max. The calculated value is then multiplied by 365 to be reported in units of “per year”.  Details on this equation and its individual components can be found in Pilloni et al. (2025).¹