## 3 Results and Discussion  
### Effect of Rainfall Intensity on the Retention of Pollutants by Bioretention  
All three groups with bioretention systems had high rates of removal of TP for the whole rainfall duration (Figure 3), indicating that the bioretention technology is able to effectively reduce the content of phosphorus in the rainfall-runoff. The conventional bioretention system (control group) had the lowest removal efficiency of TP, while the DS and SZ columns remained less than 0.4 mg/L of TP in the initial effluent and showed a higher purification effect. This could be attributed to the plant uptake, media adsorption, and chemical precipitation that occurred within the system [30]. With the increase in rainfall time, the concentration of phosphorus in the rainwater outflow increased gradually. One possible explanation is that the initial fast-reversible adsorption sites on the media surface were gradually occupied, and then the phosphate in the rainwater moved slowly towards the irreversible adsorption sites during infiltration [43]. At the same time, the released adsorption reversible sites could regain the adsorptive capacity of the media for the bioretention system before the beginning of the next rainfall. The removal rate of TP decreased with increasing rainfall time. The removal rate of TP in the SZ column was still highest under these three different rainfall intensities. When the rainfall recurrence interval was 1a, the bioretention columns with aquifers (SZ) was able to achieve a removal rate of more than 92.4% in the initial effluent. However, with an increase in rainfall intensity, the removal rate of TP in the effluent had decreased gradually. The removal of TP was mainly due to the physical and chemical adsorption of the media. At the same time, higher rainfall intensity would have led to more considerable runoff and higher total pollution load. The hydraulic retention time of rainwater in the bioretention system was shortened, and the phosphorus adsorption by the fillers with limited adsorption capacity was easily saturated. The traditional bioretention system (no saturated area) was affected by the rainfall intensity. When the recurrence interval was increased to 10a, the removal rate of TP decreased to 72.6%. Therefore, even under extreme weather conditions, the bioretention system with a saturated zone proved to be more reliable and stable. Under different rainfall intensities, the removal efficiency of NH4+-N was relatively different (Figure 4). With continued rainfall, the concentration of NH4+-N in the effluent had gradually increased, with the SZ column exhibiting the highest removal efficiency. When the rainfall recurrence interval was 1a, the removal rate of NH4+-N reached 90.1%, and NH4+-N concentration in the effluent was lower than 0.85 mg/L by the end of the rainfall. This is because higher rainfall intensities lead to greater runoffs and higher hydraulic loads. Rainwater runoff could quickly pass through the planting layer, which decreases the hydraulic retention time of rainwater runoff in the planting layer and reduces the fast adsorption and filtration of NH4+-N by the plant roots and soil layers [30,35]. At the same time, the reaction time between nitrifying bacteria and NH4+-N in the rainwater runoff is reduced, which subsequently diminishes the treatment efficiency of the bioretention system for the rainwater runoff. Figure 4: Concentration of NH4+-N using bioretention systems with the Control column, DS column, and SZ column under 1a (**a**), 5a (**b**), and 10a (**c**) rainfall intensities. Figure 3: Concentration of total phosphorous (TP) using bioretention systems with the Control column, DS column, and SZ column under 1a (**a**), 5a (**b**), and 10a (**c**) rainfall intensities. Figure 5 shows the operational effect of the bioretention system on NO3\({}^{-}\)-N under different rainfall intensities. The concentration of NO3\({}^{-}\)-N in the outflow declined with increasing sampling time.