Semi-Volatile Organic Compounds as Chemical Tracers for Estimating Soil Particle Biodiffusion Coefficients: Application of Polychlorinated Biphenyls in Earthworm Bioturbation at a Grassland Site

Bioturbation is a surface soil mixing process driven by the presence of macrofauna and macroflora. Currently, estimation of soil particle biodiffusion coefficients is based on measured macrofauna-driven soil particle turnover rates and the corresponding bioturbation depths. Anthropogenic chemicals such as polychlorinated biphenyl (PCB) are persistent soil contaminants because due to extremely slow degradation they are stable for decades within the environment. Therefore, they can serve as convenient tracers for studying macrofauna soil bioturbation transport in the surface layer. The objective of this work was to obtain chemical-based coefficients for comparison with those derived from bioturbation. A chemical modeling approach is demonstrated for PCB in surface soils using an advection, diffusion, and reaction equation using field data from a site representing a half-century experiment. The average values for extracted biodiffusion coefficients were 4.03 × 10, 5.81 × 10, and 5.98 × 10 m d for congeners PCB-52, 101, and 153, respectively. These chemical-based values and similar literature data are in good agreement with published physically measured coefficients based on macrofauna bioturbation estimates derived from burrowing, cast production, and solid particle turnover rates. In addition, the advection, diffusion, and reaction model provided insight into the magnitude of PCB diffusion processes in soils. The biodiffusion particle transport contributed more than 92% of the overall process, whereas PCB diffusion in the soil-air was approximately 8% and less than 1% in soil-water. These results support the important role of the particle biodiffusion process reported by others and suggest that this parameter should be included in transport models when and where bioturbation is present. Agreement between the chemical and the soil-particle–derived biodiffusion coefficients is very interesting and suggestive. However, the size of the chemical data set is small, and more studies will be needed to verify the validity of the approach presented in this work.