Biomass Protein Decay Is a Source of Precursors for Microbially Mediated p-Cresol and Toluene Production in Groundwater Undergoing Enhanced Remediation for Treatment of Chlorinated Solvents

Bioremediation approaches that involve subsurface injection of fermentable substrates have become common for achieving anaerobic transformation of halogenated alkanes (e.g., 1,2-dichloroethane) and alkenes (e.g., vinyl chloride) to nontoxic final products (e.g., ethene) in groundwater. It was previously reported that subsurface microbial communities could produce toluene as an unintended consequence of subsurface molasses injection. Here, we report a combination of field monitoring data and laboratory studies that demonstrate that subsurface microbial communities can also produce p-cresol (4-methylphenol) as an unintended consequence of enhanced anaerobic bioremediation strategies targeting chlorinated solvents. Following the subsurface injection of agricultural feed-grade cane molasses, p-cresol was observed to transiently accumulate in groundwater sampled from multiple wells, reaching concentrations as high as 63.2 mg/L. Enrichment cultures established with site groundwater consistently produced p-cresol when provided with 4-hydroxyphenylacetic acid. The results from additional experiments with molasses-grown biomass and two casein-derived products with varying aromatic amino acid composition provide compelling evidence that anaerobic transformation of proteins from biomass produced following the addition of a readily fermentable substrate into groundwater can supply a sufficiently large reservoir of aromatic amino acids to account for p-cresol and toluene concentrations observed at the field scale.

Automated summary

This study demonstrates that subsurface microbial communities can produce p-cresol as an unintended consequence of enhanced anaerobic bioremediation strategies targeting chlorinated solvents. The injection of agricultural feed-grade cane molasses resulted in transient accumulation of p-cresol in groundwater, reaching concentrations as high as 63.2 mg/L. Additional experiments suggest that the anaerobic transformation of proteins from biomass produced following the addition of a fermentable substrate into groundwater can supply a reservoir of aromatic amino acids, contributing to the observed p-cresol and toluene concentrations at the field scale.