## 1 Introduction  
Only about 86% of households are connected to the central public wastewater treatment system in the Free State of Saxony, Germany [1]. In sparsely populated regions, especially with small municipalities or stand-alone residential and leisure facilities, a centralized wastewater treatment plant is not always available or the connection through, e.g., pipes is uneconomic and/or technically not realizable. In general, the connection to a central system often comes with high expenses for the installation of pipeline networks. Furthermore, the constant energy and maintenance costs of necessary pumping stations are a critical aspect [2; 3; 4; 5; 6; 7]. Small wastewater treatment plants (SWWTP) represent an alternative, economic method for the on-site treatment of wastewater in these regions. According to German DIN standard [8], SWWTTPs are defined as systems for the treatment of wastewater from domestic use up to a volume of 8 m\({}^{3}\) per day, which is roughly the amount of wastewater produced by about 50 residents. After treated wastewater leaves the SWWTP, it is normally discharged into a river or stream. The acceptable maximum contaminant levels (MCLs) for specific dissolved constituents, such as chemical oxygen demand (COD) and biological oxygen demand (BOD) are defined in [9]. However, especially in rural regions, a sewer system is not available, nor is the discharge into receiving waters possible. In these cases, the disposal of the treated wastewater effluent (TWWE) takes place by direct infiltration into the soil by means of infiltration elements such as soakaways, trenches, drainage pipes, or swales [8]. As a result, direct infiltration into the ground contributes to controlling the sustainable recharge of an aquifer and attempts to maintain a constant groundwater level. Regarding climate change, this has particular importance to overcome frequent and significantly longer dry periods, which can be expected in the future [10; 11; 12]. However, for the disposal of TWWE by direct infiltration into the soil system, the aspects of subsurface purification and groundwater pollution risk have to be taken into account, especially with regard to European Comission Directive [13]. It is specified that all member states must achieve a good chemical and ecological status of all waterbodies. In particular, this includes reduction of dangerous substances released into groundwater, protection and improvement of the current state of aquatic ecosystems, and promotion of sustainable use of the groundwater resources. In order to meet the EU Water Framework Directive [13], the infiltration of TWWE must be feasible for both present and future climatic and hydrogeological conditions. Numerous field and laboratory studies indicate that several organic compounds, such as nitrogen, phosphorus, bacteria, and viruses, within the TWWE can be removed substantially by percolating them through the unsaturated soil zone [14; 15; 16; 17]. However, this removal process can be influenced by irregular and additional infiltrations originating from intensive precipitation events at the locations of installed TWWE infiltration systems. At a pilot site in Schoneck, Germany, this influence was observed at an SWWTP followed by on-site infiltration of TWWE through subsurface ditches. Due to large quantities of water originating from intensive precipitation events, flooding occurred in the installed ditches. A temporary impact on the hydraulic conditions in the unsaturated soil zone was apparent due to observed water contents close to saturation [18]. As a result, risks and uncertainties arise about the degradation of residues contained in the TWWE, the hydraulics of the unsaturated soil zone, and the overall pollution of groundwater resources. To assess potential groundwater contamination, it is necessary to evaluate the advective and reactive transport processes as well as their interaction in the unsaturated soil zone below the infiltration system.