The Shafdan Soil Aquifer Treatment (SAT) is the largest and most efficient tertiary treatment system in Israel. Water quality is improved in a various ways, TOC is reduced by an order of magnitude, extensive nitrification takes place, and microbial risks are reduced to zero. For many years, much prior to this research, it was noted that most of the TOC removal as well as nitrate formation in the Shafdan SAT take place in the intermittently wetted zone. Nitrogen transformations and DOC oxidation in the vadose zone were never properly studied at any Artificial Groundwater Recharge (AGR) site, let alone the SAT due to monitoring difficulties. The need for careful research and optimization of organics
removal in the vadose zone is accentuated by the emerging manganese problem in the SAT recovery wells. Some 20 recovery wells in the Yavne SATs are contaminated by manganese, resulting in Mn-oxide blockage of irrigation pipes, which poses the largest threat for the future of the Shafdan SAT. Since manganese (IV) reduction in the aquifer is the only possible source for dissolved manganese (i.e. Mn2+), the problem is inherently connected to the oxygen supply and DOC removal in the SAT. The removal of these variables takes place predominantly in the vadose zone, which is the subject of this study. We have demonstrated that over 90% of the micropollutants removal takes place in the unsaturated zone. A preliminary estimate showed that over 40 mg/L of oxygen is needed in order to achieve such oxidation, a remarkable figure considering that the dissolved oxygen level in the secondary effluent feed is less than 1 mg/L. How is a need to increase the reliability of this estimate in view of two factors: 1) The aquifer measurement was conducted in a well that has perforation between 10-20 m below the water table, and thus, depending on water streamlines, it collected water that has much higher retention time than the few week retention in the vadose zone. 2) The unsaturated zone influx was taken from Station six of the Shafdan main effluent header and not from the percolation lagoons. The oxygen balance has to take into account oxygen intake from the air and by algal population, which were not accounted for by the rough preliminary estimate. In view of the importance of the oxygen balance for the understanding of the removal mechanisms in the unsaturated zone and in view of the importance of oxygen supply in this zone to prevent deep anoxic zone formation and manganese reduction, we have carried out extensive research to assess and confirm oxygen balance over the unsaturated zone. In the first phase of this project we have focused attention on several subprojects relating to the processes that take place in the vadose zone below the Shafdan percolation lagoons.