Temporal variations in natural attenuation of chlorinated aliphatic hydrocarbons in eutrophic river sediments impacted by a contaminated groundwater plume.

Chlorinated aliphatic hydrocarbons (CAHs) often discharge into rivers as contaminated groundwater baseflow. Biotransformation, sorption and dilution of CAHs in the impacted river sediments have been reported to reduce discharge, but the effect of temporal variations in environmental conditions on the occurrence and extent of those processes in river sediments is largely unknown. We monitored the reduction of CAH discharge into the Zenne River during a 21-month period. Despite a relatively stable influx of CAHs from the groundwater, the total reduction in CAH discharge from 120 to 20 cm depth in the river sediments, on average 74 ± 21%, showed moderate to large temporal variations, depending on the riverbed location. High organic carbon and anaerobic conditions in the river sediments allowed microbial reductive dechlorination of both chlorinated ethenes and chlorinated ethanes. δ(13)C values of the CAHs showed that this biotransformation was remarkably stable over time, despite fluctuating pore water temperatures. Daughter products of the CAHs, however, were not detected in stoichiometric amounts and suggested the co-occurrence of a physical process reducing the concentrations of CAHs in the riverbed. This process was the main process causing temporal variations in natural attenuation of the CAHs and was most likely dilution by surface water-mixing. However, higher spatial resolution monitoring of flow transients in the riverbed is required to prove dilution contributions due to dynamic surface water-groundwater flow exchanges. δ(13)C values and a site-specific isotope enrichment factor for reductive dechlorination of the main groundwater pollutant vinyl chloride (VC) allowed assessment of changes over time in the extent of both biotransformation and dilution of VC for different scenarios in which those processes either occurred consecutively or simultaneously between 120 and 20 cm depth in the riverbed. The extent of reductive dechlorination of VC ranged from 27 to 89% and differed spatially but was remarkably stable over time, whereas the extent of VC reduction by dilution ranged from 6 to 94%, showed large temporal variations, and was often the main process contributing to the reduction of VC discharge into the river.

Assessment of the intrinsic bioremediation capacity of an eutrophic river sediment polluted by discharging chlorinated aliphatic hydrocarbons: a compound-specific isotope approach.

At a field site in the industrial area of Vilvoorde, Belgium, we investigated the capacity of the indigenous microbial community of a eutrophic river sediment to biodegrade chlorinated aliphatic hydrocarbons (CAHs) originating from discharging, polluted groundwater using a compound-specific isotope approach. We specifically targeted the site's major pollutants cis-1,2-dichloroethene (cis-DCE) and vinyl chloride (VC). Analysis of Rayleigh correlation plots enabled us to assess the extent to which microbial and abiotic natural attenuation processes contributed to the mitigation of a pollution of the surface water due to discharging CAH-contaminated groundwater. Our results provide evidence for (i) the occurrence of biodegradation of cis-DCE and VC by reductive dechlorination in parts of the aquifer and at several positions in the river sediment (ii) the presence of river sediment zones exhibiting attenuation of chloroethenes by a combination of biodegradation and dilution through unpolluted water, (iii) the existence of zones in the river sediment lacking significant biodegradation, and thus (iv) a pronounced spatial heterogeneity in the occurrence and extent of biodegradation in the aquifer and river sediment. We conclude that at many investigated positions in the river sediment the indigenous microbial community failed to facilitate complete biodegradation of the groundwater-sourced chloroethenes. The overall intrinsic bioremediation capacity of the river sediment was thus not high enough to completely prevent the release of these pollutants into the surface water. These findings and conclusions are thus in agreement with those of our companion paper (1), which investigated the river sediments at the Vilvoorde study site by a combination of stable hydrogen and oxygen isotope analysis of water and the detection of chlorinated aliphatic hydrocarbons (CAHs) and their dechlorination products.

Factors determining the attenuation of chlorinated aliphatic hydrocarbons in eutrohic river sediment impacted by discharging polluted groundwater.

This study explored the potential of eutrophic river sediment to attenuate the infiltration of chlorinated aliphatic hydrocarbon (CAH)-polluted groundwater discharging into the Zenne River near Brussels, Belgium. Active CAH biodegradation by reductive dechlorination in the sediment was suggested by a high dechlorination activity in microcosms containing sediment samples and the detection of dechlorination products in sediment pore water. A unique hydrogeochemical evaluation, including a delta2H and delta18O stable isotope approach, allowed to determine the contribution of different abiotic and biotic CAH attenuation processes and to delineate their spatial distribution inthe riverbed. Reductive dechlorination of the CAHs seemed to be the most widespread attenuation process, followed by dilution by unpolluted groundwater discharge and by surface water mixing. Although CAHs were never detected in the surface water, 26-28% of the investigated locations in the riverbed did not show CAH attenuation. We conclude that the riverbed sediments can attenuate infiltrating CAHs to a certain extent, but will probably not completely prevent CAHs to discharge from the contaminated groundwater into the Zenne River.