## 3 Results and Discussion
### Water Quality and Purification Performance  
At first, chemical analysis of the pore water samples for phases with only TWWE infiltrations indicate that the COD, the oxygen consumed for the oxidation of pollutants in the TWWE, is constantly reduced by at least 50% on the way through the unsaturated soil zone in all soils (Figure 4A,D,G and Supplementary Materials S4).  
The reduction of infiltrated NH\({}_{4}\)-N is observed by a minimum of 80%. (Figure 4B,E,H). Due to the reduction of ammonium in the course of nitrification processes, an increase in nitrate concentrations in the effluent is also observed. Denitrification processes hardly occur since the water contents were predominantly not in the range of full saturation, and thus, sufficient oxygen is available in the soil pore space due to the subsequent delivery over the soil surface (Figure 4C,F,I). At real sites, however, it is to be expected that the existing nitrate will be consumed for the oxidation of the remaining COD concentration in deeper soil layers due to the insufficient availability of oxygen. At least 50% of infiltrated nutrient PO\({}_{4}\)-P is constantly reduced on the way through the unsaturated soil zone. However, the transport of remaining PO\({}_{4}\)-P to streams and lakes by groundwater, which can cause eutrophication of these waters, cannot be excluded.  
The purification performance, independent from the soil type, is almost unaffected by the additional infiltration of water originating from precipitation events (Figure 4 and Supplementary Materials S4). Analysis results of pore water indicate that the reduction of COD and PO\({}_{4}\)-P is at least 50% and the degradation of NH\({}_{4}\)-N reaches minimum 80%, similar to the phase with only TWWE infiltration. In the case of nitrate, a stronger dilution effect can be observed, resulting in a reduction of nitrate concentrations in comparison with only TWWE infiltration. However, it should be noted that the load of nitrate reaching the groundwater is not reduced at the same time. Nevertheless, with regard to the objective of the investigations, it can be stated that the joint infiltration of wastewater and rainwater does not lead to a reduction of the purification efficiency in the unsaturated soil zone.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**0.25 m Below Surface**} & \multicolumn{4}{c}{**1.15 m Below Surface**} \\ \cline{2-7}  & \multicolumn{2}{c}{**168 h Simulation Time**} & \multicolumn{2}{c}{**Precipitation Event**} & \multicolumn{2}{c}{**168 h Simulation Time**} & \multicolumn{2}{c}{**Precipitation Event**} \\ \hline
**Event** & **RMSE** & **PBIAS** & **RMSE** & **PBIAS** & **RMSE** & **PBIAS** & **RMSE** & **PBIAS** \\ \hline  & & & & **Soil 1** & & & \\
1 & 0.017 & \(-\)3.5 & 0.025 & \(-\)5.4 & 0.019 & \(-\)10.9 & 0.021 & \(-\)13.8 \\
2 & 0.019 & \(-\)3.4 & 0.033 & \(-\)5.6 & 0.024 & \(-\)11.5 & 0.042 & \(-\)16.6 \\
3 & 0.023 & \(-\)7.8 & 0.033 & \(-\)13.7 & 0.028 & \(-\)14.5 & 0.039 & \(-\)19.8 \\ \hline  & & & & **Soil 4** & & & \\
1 & 0.016 & \(-\)6.4 & 0.018 & \(-\)4.3 & 0.018 & 12.3 & 0.018 & 14.4 \\
2 & 0.019 & 1.1 & 0.037 & \(-\)3.6 & 0.022 & 11.4 & 0.037 & 5.7 \\
3 & 0.025 & 7.2 & 0.027 & 5.0 & 0.029 & 15.7 & 0.028 & 12.0 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Model fitness of water content for the total simulation time of 168 h in the course of a precipitation event and for the period most influenced by the precipitation event for the upper (0.25 m below surface) and lower (1.15 m below surface) observation points for soil 1 and 4, RMSE (V WC) and PBIAS (%).