Natural Attenuation in Streambed Sediment Receiving Chlorinated Solvents from Underlying Fracture Networks

Contaminant discharge from fractured
bedrock formations remains
a remediation challenge. We applied an integrated approach to assess
the natural attenuation potential of sediment that forms the transition
zone between upwelling groundwater from a chlorinated solvent-contaminated
fractured bedrock aquifer and the receiving surface water. In situ
measurements demonstrated that reductive dechlorination in the sediment
attenuated chlorinated compounds before reaching the water column.
Microcosms established with creek sediment or in situ incubated Bio-Sep
beads degraded C1–C3 chlorinated solvents
to less-chlorinated or innocuous products. Quantitative PCR and 16S
rRNA gene amplicon sequencing revealed the abundance and spatial distribution
of known dechlorinator biomarker genes within the creek sediment and
demonstrated that multiple dechlorinator populations degrading chlorinated
C1–C3 alkanes and alkenes co-inhabit
the sediment. Phylogenetic classification of bacterial and archaeal
sequences indicated a relatively uniform distribution over spatial
(300 m horizontally) scale, but Dehalococcoides and Dehalobacter were more abundant in deeper sediment, where
5.7 ± 0.4 × 105 and 5.4 ± 0.9 × 106 16S rRNA gene copies per g of sediment, respectively, were
measured. The microbiological and hydrogeological characterization
demonstrated that microbial processes at the fractured bedrock–sediment
interface were crucial for preventing contaminants reaching the water
column, emphasizing the relevance of this critical zone environment
for contaminant attenuation.