Hydrogeophysical Monitoring of Water Infiltration in the Context of Soil Aquifer Treatment at the Shafdan Site (Israel)

Soil Aquifer Treatment (SAT) is a sustainable technology in modern waste water management. The Shafdan SAT facility in Israel, one of the largest in the world, is being successfully operated for many years. However, due to increasing amounts of waste water the infiltration capacity approaches its limit. Our study aims at better understanding the infiltration process and hereupon improving the efficiency of the infiltration management using Electrical Resistivity Tomography (ERT) in combination with hydrological methods. We installed three permanent ERT lines within one infiltration pond close to the city of Yavneh, south of Tel Aviv. Each line comprises 96 electrodes, separated by 2 m in one line and 0.5 m in the other two lines. The sediments below the pond are mainly composed of sands and porous sandstone. In order to calibrate the ERT results, water content, temperature, and electrical conductivity were measured in-situ at different depths in shallow regions of the pond, and cores from Geoprobe drillings were taken for lithological analyses. Continuous ERT monitoring was conducted over two months spanning infiltration scenarios characterized by different flooding sequences. The averaged apparent resistivity data over time show a clear response which can be related to the overall water content dynamics in the vadose zone (over 30 m). The inverted ERT images indicate that the infiltration is strongly affected by subsurface heterogeneity. With a view to setting up a hydrological model, we analyzed the transient behavior of the local drying curves after breakthrough of the infiltration front for each pixel of the ERT image sequence. This approach represents a new, "dynamic" method for subsurface hydraulic zonation based on time-lapse ERT. Our study shows that ERT helps to better understand the dynamics of water infiltration processes in the context of SAT under real-world conditions, and by this may contribute to enhancing the efficiency of SAT facilities.