Experimental Study on the Retention and Interception Effect of an Extensive Green Roof (GR) with a Substrate Layer Modified with Kaolin

Extensive green roofs (GRs) often appear as pollution sources during actual rainfall events; therefore, it is necessary to study the control of nutrient leaching in the substrate layer. In this study, four extensive GR experimental devices are built: two with artificial granular structure substrate layers improved with kaolin as a binder, one with a commercial substrate layer, and one with a standard roof (SR). Based on the simulated rainfall conditions in different local recurrence periods, the delayed outflow time, rainfall retention rate, event mean concentration (EMC), and cumulative pollutant quality of NH4+, NO3−, NO2−, and PO43− in the effluents were measured and evaluated. The results of the study indicate that under simulated rainfall in all the experimental design recurrence periods, the kaolin-modified substrate layer does not exhibit a more significant retention capacity than the commercial substrate. However, it does show some suppression of the leaching effect of NO3− and PO43− in the runoff. The reduction rate of cumulative NO3− quality is 6.56%, and PO43− is 10.54%. In future practical engineering and related research, attention should be paid to the influence of the type and addition amount of the substrate layer modifier on the stability of the granular structure to prevent nutrient loss caused by soil erosion.