A stall of cis-1,2-DCE and vinyl chloride (VC) is frequently observed during bioremediation of groundwater chloroethenes via reductive dechlorination. These chloroethenes may be oxidised by aerobic methanotrophs or ethenotrophs co-metabolically and/or metabolically. We assessed the potential for such oxidation at 12 sites (49 groundwater samples) using hydrochemical and molecular biological tools. Both ethenotroph (etnC and etnE) and methanotroph (mmoX and pmoA) functional genes were identified in 90% of samples, while reductive dehalogenase functional genes (vcrA and bvcA) were identified in 82%. All functional genes were simultaneously detected in 78% of samples, in actively biostimulated sites in 88% of samples. Correlation analysis revealed that cis-1,2-DCE concentration was positively correlated with vcrA, etnC and etnE, while VC concentration was correlated with etnC, etnE, vcrA and bvcA. However, feature selection based on random forest classification indicated a significant relationship for the vcrA in relation to cis-1,2-DCE, and vcrA, bvcA and etnE for VC and no prove of relationship between cis-1,2-DCE or VC and the methanotroph functional genes. Analysis of hydrochemical parameters indicated that aerobic oxidation of chloroethenes by ethenotrophs may take place under a range of redox conditions of aquifers and coincide with high ethene and VC concentrations.

A series of field measurements were used to evaluate the mutual interaction between the perched water zone and the regional aquifer under the specific site conditions of the contaminated site. The perched water zone apparently developed after the demolition of a building used for chemicals by leaving low permeability relics of the building slab after demolition and the presence of a relatively thin clayey horizon (approx. 2 m thick). The regional aquifer consists of weathered shale bedrock beneath the perching layer. The difference between the watertables of the perched and regional aquifers is between 0.5 and 1.2 m. The perched water was found to contain elevated concentrations of volatile organic compounds during initial sampling. A study was conducted to examine the perched water more closely and to describe the (1) controls on perching behavior, (2) contaminant flux through the unsaturated zone (soil gas) and to the regional aquifer, and (3) differences in the water quality of the perched groundwater and that in the regional aquifer. The data evaluation showed that the perched aquifer system is dynamic both in vertical and horizontal flow. Nonetheless, the hydrochemical data from the two aquifers were distinct in chemical composition despite their close proximity.