Case Study 6: Study of PFAS migration and remediation in a PluriMetrics experimental pilot

In PROMISCES' Case Study 6, PFAS migration and remediation in a PluriMetrics experimental pilot are investigated. The plurimetric experiment involved a pilot (5.2 m long, 3.6 m wide, and 3 m deep) located at BRGM (Orléans, France). The goal was to simulate PFAS contamination from AFFF (aqueous film-forming foam) in a soil-groundwater continuum, mimicking soil and groundwater contamination followed by the demonstration of several remediation technologies trains at a semi-real scale. The experiment lasted eleven months, nine of which were devoted to observing PFAS migration in the soil, with a focus on vertical infiltration in the soil, unsaturated zone, and horizontal transport in the saturated zone. The remediation stage lasted two months.

The tank was filled with a man-made geomedia designed to mimic soil. It consisted of sandy materials with additions of urban green waste compost, clay materials, and oxides. The plurimetric pilotwas designed in two domains: the upper part was unsaturated (1 m thick), while the lower part was fully saturated (2 m thick). Monitoring was carried out using 15 tensiometers and 15 time-domain reflectometry (TDR) devices to track soil moisture and soil water potential in the vadose zone. Pressure sensors were used to monitor water table depth. Water samples were taken using 70 suction cups, and nine piezometers were positioned along the main flow lines at different locations and depths. This grid was selected to ensure robust calculations of PFAS distribution in the pilot. Physical and chemical parameters, including pH, redox, temperature, and conductivity, were measured, along with more specific analyses for bromide (tracer), organic fluoride, and PFAS concentrations. More details concerning the protocol used for these chemical analyses can be found in the Work Package 1 deliverables.

During the PFAS migration stage, water flow and mass transport conditions were fixed throughout the experiments. Steady-state conditions were maintained in the saturated zone for water flow and transport, while transient boundary conditions were maintained at the top of the unsaturated zone. The contamination stage can be divided into two periods. During the first three weeks, AFFF containing various PFAS compounds, such as 6:2 FTSA, 6:2 FTAB, and 6:2 FTSAam, was repeatedly infiltrated into an infiltration square at the surface of the pilot. This infiltration square was located upstream of the horizontal flow. Afterward, PFAS-free water was infiltrated weekly for seven months to simulate natural recharge.

Furthermore, a model has been built using the model train developed within the framework of PROMISCES to simulate PFAS migration, showing its potential for use in similar real-world scenarios. However, the complexity of modelling PFAS transport lies in accounting for the various physical and chemical processes that affect their movement in soil and groundwater, highlighting the need for an accurate conceptual model.

Following the PFAS contamination stage, the experimental setup is now being used to test the remediation techniques developed as part of the project, in collaboration with the involved partners BRGM, COLAS, IPGP, and SINAPTEC. To date, we are testing PFAS remobilization using Non-Newtonian Foam, PFAS degradation with reductive chemicals, and effluent treatment via sono-cavitation. The entire monitoring system is now applied to track in situ PFAS remediation. This approach allows us to test the performance of our techniques at a semi-real scale.