Multidisciplinary subsurface monitoring for a better understanding of Soil Aquifer Treatment capacity applied on coastal operational wastewater treatment plant (Agon-Coutainville, France)

International audienceUnconfined coastal aquifers are potentially subject to both saline intrusion near the seashore and over discharge of treated wastewater in the surficial environment during the tourist season. In Agon-Coutainville (Normandy, France), managed aquifer recharge (MAR) system, combined with Soil Aquifer Treatment (SAT), was integrated as part of the full-scale operational wastewater treatment plant. Such integrated natural/engineered water treatment system ensure the sustainability of the seaside activities (seafood production, beach) and locally supply freshwater for the irrigational needs of the golf course. Concerning the MAR system, the secondary treated wastewater is infiltrated alternatively into three natural reed bed areas before reaching the sand dune aquifer and thus to enhance the quantity of freshwater in the aquifer. Treated wastewater potentially contains various compounds (chemical, virus, pathogen) which can, however, affect the groundwater quality. Nevertheless, some of these compounds are partly removed, during the SAT. To assess performance and efficiency of the integrated system in the natural environment, we have designed and performed an innovating and multidisciplinary monitoring dedicated to 1) spatial evolution of the freshwater generating by the MAR system, 2) mean residence time of water during SAT and 3) potential reactivity occurring during SAT. Spatial field campaigns and tracer tests were conducted by associating classical and innovative approaches including physico-chemical measurements and quantitative analyses, non target analysis for screening organic compounds, ecotoxicological bioassays, online biomonitoring BACTcontrol® system to detect fecal contamination and online system monitoring device dedicated to saline intrusion. Results show that the MAR system provides a freshwater barrier in the aquifer which is seasonally affected by saline intrusion. A part of the aquifer is assessed for freshwater potential production regardless of the natural and anthropogenic recharge. SAT mean residence time is around two weeks that allows SAT reactivity and thus increases quality of the pumped groundwater. This novel subsurface monitoring provides a better understanding of the SAT capacity to enhance the quantity of freshwater and improve its quality

High Resolution Monitoring of Seawater Intrusion in a Multi-Aquifer System-Implementation of a New Downhole Geophysical Tool

Monitoring of seawater intrusion is extremely important for the management of coastal aquifers, and therefore requires reliable and high-frequency monitoring tools. This paper describes the use of a new near field and downhole geophysical tool that monitors seawater intrusion in boreholes with high vertical resolution. This sensor is further used to study the impact of pumping on water electrical conductivity profiles (ECP) at the fresh-saline water interface. The new device was installed in a confined calcareous sandstone aquifer along the northern Israeli coast. The site includes two monitoring wells and one pumping well located at distances of 50, 75 and 125 m from shoreline, respectively. The new geophysical tool, called the subsurface monitoring device (SMD), was examined and compared to water an electric conductivity profiler (ECP) and a conductivity temperature depth (CTD) driver’s data. All methods show similar salinity trends, and changes in pumping regime were clearly identified with both the SMD and CTD. The advantage of using the SMD tool is the high temporal and spatial resolution measurement, which is transferred via internet and can be analyzed and interpreted in real time. Another advantage of the SMD is that it measures the electrical resistivity of the aquifer mostly outside the well, while both water ECP and the CTD measure in-well electrical conductivity; therefore, are subjected to the artefact of vertical flow in the well. Accordingly, while the CTD shows an immediate and sharp response when pumping is stopped, the SMD provides a gradual electric conductivity (EC) change, demonstrating that stability is reached just after a few days, which illustrates, more precisely, the hydrological response of the aquifer

Surveillance hybride du système naturel de filtration aquifère des eaux usées traitées d'Agon-Coutainville (France)

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