Formation of Nonextractable Soil Residues:  A Stable Isotope Approach

Stable carbon isotopic measurements were employed to characterize the transformation of a 13 C-labeled polycyclic aromatic hydrocarbon (PAH), anthracene, in a closed soil bioreactor system. The 13 C-label was used to calculate a carbon mass balance including mineralization and the formation of nonextractable soil-bound residues. Similar results were obtained from 13 C-labeled carbon and 14 C-labeled carbon mass balance calculations for separate batch experiments with labeled anthracene. In concentration ranges typical for real PAH-contaminated sites, the sensitivity of the 13 C tracer method meets the requirements of classical radiotracer experiments. Therefore, our balancing method based on stable isotope-labeled chemicals may supplement or substitute radiotracer experiments under many circumstances. One major advantage of using stable isotope-labeled tracers is the possible application in transformation studies where the use of radioactive substances is of environmental concern. The transformation of 13 C-labeled PAH into nonextractable residues clearly depends on the metabolic activity of the soil microflora and occurs during an early phase of biodegradation. Successive contamination of the soil by anthracene leads to a progressive adaptation of the microflora to a complete mineralization of anthracene in the soil. The extent of residue formation is controlled by the capability of the microflora to degrade the contaminant. Results of long-term experiments indicate that nonextractable residues are relatively stable over time.