Attenuation of a discharging chlorinated ethene (CE) plume: use of streambed point velocity probes (SBPVP), streambed passive flux meters (SBPFM) and contaminant mass discharge (CMD)

Description

Chlorinated ethenes (CE) are abundant groundwater contaminants and pose risk to both groundwater and surface water bodies, as plumes can migrate through aquifers to streams. After release to the environment, CE may undergo attenuation. The hyporheic zone is believed to enhance CE attenuation, however studies contradicting this have also been reported. Since dilution commonly reduces contaminant concentrations in streams to below quantification limits, use of mass balances along the pathway from groundwater to stream is unusual.

Our study is conducted at the low-land Grindsted stream, Denmark, which is impacted by a contaminant plume. CE have been observed in the stream water; hence our study site provides an unusual opportunity to study attenuation processes in a CE plume as it migrates through the groundwater at the stream bank, through the stream bed and further to the point of fully mixed conditions in the stream.

The study undertook the determination of redox conditions and CE distribution from bank to stream; streambed contaminant flux estimation using streambed Passive Flux Meters (SBPFM); and quantification of streambed water fluxes using streambed Point Velocity Probes (SBPVP). The advantage of the sPFM is that it directly measures the contaminant flux without the need for water samples, while the advantage of the SBPVP is its ability to measure the vertical seepage velocity without the need for additional geological parameters. Finally, a mass balance assessment along the plume pathway was conducted to account for any losses or accumulations.

The results show consistencies between contaminant mass discharge estimates at the bank, through the streambed and in the stream channel. Furthermore, parent-metabolite compound ratios indicate limited degradation between the bank and the point of fully mixed stream water, leaving the dominant attenuation process to be mixing in the stream channel. Since the plume at the bank mainly consists of cis-DCE and vinyl chloride, this implies high and persistent stream water concentrations of these compounds. Finally, this study demonstrates the usefulness and complementary nature of SBPFM and SBPVP measurements for assessing the attenuation processes through mass balance calculations.

Period	11 Dec 2017
Event title	2017 AGU Fall Meeting
Event type	Conference
Location	New Orleans, United States, LouisianaShow on map
Degree of Recognition	International