Initial Urban Stormwater Runoff Pollution and Its Fate in Rain Garden Soil

Initial rainwater carries a large number of pollutants, easily causing urban nonpoint source pollution. Determining the concentration of pollutants in initial rainwater is crucial for controlling urban nonpoint source pollution. In this study, we monitored 38 and 46 rainfall events from March 2016 to September 2021 to study the concentration of pollutants in initial rainwater from roof and road runoffs. Moreover, we determined the volume of initial abandoned rainwater and analyzed the influences of stormwater concentration infiltration on soil. Results show that the concentrations of pollutants from road runoff are considerably higher than those from roof runoff. The concentrations of chemical oxygen demand (COD), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), total nitrogen (TN), total phosphorus (TP), and Zn from road runoff are 3.67, 2.37, 2.67, 1.99, 2.24, and 6.61 times higher than those from roof runoff. The concentrations of pollutants from road and roof runoffs decrease gradually with monitoring time. The initial abandoned rainwater from road runoff should be collected within 10–15 min from the start of a rain event, and that from roof runoff should be collected within 8–10 min. The concentrated infiltration of rainfall runoff has a great effect on the surface soil of rain gardens. The NH4+-N, TN, Zn, Cd, and Pb concentrations within the 0–30 cm soil layer are considerably higher than those in the 40–50 cm soil layer. However, TP and NO3−-N migrate downward with water infiltration. The rain garden showed a moderate mass load reduction for TP (67.23%) and TN (43.40%), respectively, mainly resulting from runoff volume reduction. In addition, the rain garden significantly reduced the NH4+-N and heavy metals mass loads of Cu, Zn, Cd, and Pb from rainfall runoff by 92.77%, 91.18%, 93.45%, 94.29%, and 92.06%, respectively.