Contaminant mass flux measurement in groundwater with passive samplers - A field application
Résumé
In France and more generally speaking in Europe, passive sampling is an emerging method to measure groundwater quality and as a consequence to monitor contaminated sites. Contaminant mass flux measurement in groundwater can be of great interest for the management of contaminated sites because it can better define pollution sources and plumes and therefore improve practically all aspects of site characterization and remediation. For example, it can allow a better definition of contaminant transfer pathways, a better calculation of natural attenuation rates or a more efficient design of remediation techniques. In some countries like in Switzerland, the remediation urgency is already evaluated thanks to contaminant mass flux. So far, contaminant mass flux evaluation in groundwater is not as precise as it could be, because they are calculated from concentrations measured in snapshot groundwater samples and generic Darcy velocities and therefore do not account for possible concentration and groundwater flux variations over time and space. Passive samplers for contaminant mass flux such as passive flux meters (PFMs) can therefore be an interesting alternative to improve contaminant mass flux determination in groundwater. To date, this kind of passive sampler is not used in Europe and feedback is needed in order to promote their use and encourage consultants to use them. In this context, this work aimed at evaluating PFMs to measure contaminant mass flux in groundwater at a site contaminated with chlorinated solvents. On site, 6 monitoring wells were equipped with PFMs at different depths. 2 phases of exposure were carried out and PFMs were exposed during 3 months in each phase. The tests on site consisted in comparing the concentrations in groundwater given by the PFMs (average concentrations over the exposure time) to the average concentrations calculated from snapshot samples obtained with the conventional sampling method before the installation and after the retrieval of the PFMs, that is to say well purging prior to groundwater sampling with a pump. Concentrations coming from the PFMs were as well compared with an integrative sampler (ceramic dosimeter) having a constant sampling rate and giving average concentrations over the exposure time. Contaminant mass flux benefit was as well evaluated. Chlorinated solvent concentrations in groundwater snapshot samples taken before and after passive sampler exposure showed little variation. Therefore, concentrations given by the ceramic dosimeters were consistent with the ones obtained from the conventional sampling method. Nevertheless, PFMs showed a vertical distribution of the contamination and significantly higher concentrations were measured at some depths. These results showed that concentrations could be underestimated with the conventional sampling technique, leading to an underestimation of the risk for Human and Environment. PFMs offered complementary information because they allowed multi-level sampling in the wells and took into account variations in time and space, which could be of great value and seems very promising for contaminated sites characterization and remediation improvement.