## 4 Conclusions  
A significant increase in water content is observed by the infiltration of water originating from heavy precipitation events below on-site TWWE infiltration systems at sites with installed decentralized SWWTP. This effect raises uncertainties on the purification efficiency of TWWE constituents while percolating through the unsaturated soil zone. Performed laboratory column experiments show that water contents of soils are only briefly affected by the additional infiltrated water. The elevated water contents below TWWE infiltration systems drop quickly to their initial values at the end of the precipitation events. Moreover, a fully saturated condition, which negatively influences biological activity, is never observed even when the extreme events are applied. Realistic numerical reproduction of the flow behaviors of percolated TWWE in the laboratory experiments supports the plausibility of these observations. Figure 4: Vertical chemical oxygen demand (COD)-concentration profiles, vertical NH4-N concentration profiles, and vertical NO3-N concentration profiles for soil 2 before and 24 h and 72 h after the precipitation event 1 (**A–C**), event 2 (**D–F**), and event 3 (**G–I**). Observations also indicate a constant reduction of COD and phosphor as well as ammonium concentrations by a minimum of 50% and 80%, respectively, in the case of a single TWWE infiltration as well as for joint infiltration of TWWE and additional water originating from heavy precipitation events. No negative influence in the purification efficiency of TWWE constituents within the unsaturated soil zone was observed. Ammonium degradation correlates with an increase in nitrate concentrations due to nitrification processes. Denitrification processes do not occur due to sufficient oxygen available in the soil pore space. Joint infiltration causes the dilution of the substance concentrations, especially for nitrate, but the load and thus mass transfer into lower layers or groundwater bodies in general is not affected. Ultimately, this study reveals that, at least for the considered conditions, a negative effect on the purification efficiency regarding TWWE constituents in case of joint infiltration of both TWWE and additional water originating from precipitation events was not observed. Based on these observations, the overall risk for deterioration of the chemical status of the groundwater is assessed to be very low. Especially for shallow and therefore more vulnerable aquifers, the constant degradation of wastewater constituents for both single TWWE infiltration and joint infiltration shows that there is only a small risk arising for contamination of groundwater. However, the results of the presented works only provide insights about the effects of joint infiltration of TWWE and additional water originating from precipitation events on the purification efficiency of TWWE constituents and should not be generalized. Therefore, future works should verify the presented findings by means of field investigations as real-world conditions including more complex subsurface features. With regard to future laboratory investigations, the determination of a wider number of parameters (e.g., NO\({}_{2}\)-N, Total N) should be performed. Thus, a better description of chemical processes taking place by the calculation of substance-specific mass balances is possible. In case of the availability of a sufficient database, software solutions such as PhreeqC can be recommended for that purpose.