Quantifying biodegradation rate constants of o-xylene by combining compound-specific isotope analysis and groundwater dating.

The objective of this study is to estimate hydraulic conductivities and biodegradation rate constants in a coal-tar contaminated aquifer by compound-specific isotope analysis (CSIA) and tracer-based (H-He) groundwater dating (TGD). In two observation wells downgradient from the contaminant source in situ biodegradation of o-xylene, toluene and naphthalene under sulfate-reducing redox conditions could be demonstrated using CSIA. Median biodegradation rate constants for o-xylene ranging between 0.08 and 0.22 a were estimated. By using tracer-based groundwater dating in these two wells, hydraulic conductivities could be also estimated, which are in a similar range as k-values derived from sieve analysis, a pumping test and a calibrated groundwater flow model. These results clearly demonstrate the applicability of tracer-based groundwater dating for the determination of in situ hydraulic conductivities in aquifers without pumping contaminated groundwater. Finally, a sensitivity analysis is performed using a Monte Carlo simulation. These results indicate high sensitivities of the assumed effective porosity for the estimation of the hydraulic conductivity and the selected isotope enrichment factor for the biodegradation rate constant, respectively. Conversely, the outcome also evidently demonstrates the main limitations of the novel combined isotope approach for a successful implementation of monitored natural attenuation (MNA) at such field sites.