Natural Attenuation and Anaerobic Benzene Detoxification Processes at a Chlorobenzene-Contaminated Industrial Site Inferred from Field Investigations and Microcosm Studies

A five-year
site investigation was conducted at a former chemical
plant in Nanjing, China. The main contaminants were 1,2,4-trichlorobenzene
(TCB) reaching concentrations up to 7300 μg/L, dichlorobenzene
(DCB) isomers, monochlorobenzene (MCB), and benzene. Over time, these
contaminants naturally attenuated to below regulatory levels under
anaerobic conditions. To confirm the transformation processes and
to explore the mechanisms, a corresponding laboratory microcosm study
was completed demonstrating that 1,2,4-TCB was dechlorinated to 1,2-DCB,
1,3-DCB, and 1,4-DCB in approximately 2%/10%/88% molar proportions.
The DCB isomers were dechlorinated via MCB to benzene, and, finally,
benzene was degraded under prevailing sulfate-reducing conditions.
Dechlorination could not be attributed to known dechlorinators Dehalobacter or Dehalococcoides, while
anaerobic benzene degradation was mediated by microbes affiliated
to a Deltaproteobacterium ORM2, previously associated
with this activity. Unidentified organic compounds, possibly aromatic
compounds related to past on-site production processes, were fueling
the dechlorination reactions in situ. The microcosm study confirmed
transformation processes inferred from field data and provided needed
assurance for natural attenuation. Activity-based microcosm studies
are often omitted from site characterization in favor of rapid and
less expensive molecular surveys. However, the value of microcosm
studies for confirming transformation processes, establishing electron
balances, assessing cocontaminant inhibition, and validating appropriate
monitoring tools is clear. At complex sites impacted by multiple compounds
with poorly characterized transformation mechanisms, activity assays
provide valuable data to incorporate into the conceptual site model
to most effectively inform remediation alternatives.