Estimation of Sequential Biodegradation Product Rate Constants

摘要

The use of single-species analytical solute transport modeling approaches (e.g., Buscheck and Alcantar method) can lead to underestimated values of biodegradation product rate constants because these approaches do not include biodegradation product accumulation along the groundwater flow path. In order to improve estimates of daughter product biodegradation rate constants, the steady-state solution of the one-dimensional advection-dispersion equation for a point source with first-order chain reaction of three species was derived. This solution can be calibrated to concentration versus distance data of approximate steady-state plumes of both parent and daughter products in order to estimate their field-scale rate constants. An analysis of the analytical solution also indicates that the downgradient locations of both cross-over points (where parent and daughter concentration versus distance data intersect) and maximum points (distance where highest concentration levels occur) can be used to directly interpret field data and estimate daughter product rate constants. As an application, this one-dimensional, multispecies analytical solution was calibrated to quasi-steady state trichloroethene (TCE), cis-l,2-dichloroethene (DCE), and vinyl chloride (VC) plume conditions at a site in Palm Bay, FL, in order to estimate their field-scale, first-order biodegradation rate constants. The estimated first-order rate constant values for the steady-state plumes by the analytical solution were 0.88 yr~(-1), 0.82yr~(-1), and 0.87 yr~(-1) for TCE, DCE, and VC, respectively. These rate constant values are upper-bound estimates because the assumption of a point source release with negligible transverse dispersion leads to higher calculated plume centerline concentrations with concomitant higher calibrated rate constants in order to match the observed data. Although the source width and transverse dispersivity values are not typically well defined in the field, a two- or three-dimensional model could be applied when justifiable in order to reduce the uncertainty of these estimated rate constants.