Characteristics and influencing factors of runoff pollution from typical underlying surfaces in mountainous cities

To analyze the pollution characteristics of rainwater runoff in mountainous cities， the concentrations of SS， COD， TN， TP， and Fe in rainwater on typical underlying surfaces such as roads， squares， and parking lots of Chongqing Yuelai National Expo Center were monitored. Based on the average concentration （EMC） of rainfall runoff and the cumulative runoff-cumulative pollutant load curve ［M（V）］， the water quality characteristics of rainwater runoff and the initial flush effect were analyzed， and the correlation between underlying surface， dry period before rain， rainfall intensity， and pollutant concentration was explored. The results showed that： 1） There were significant differences in the output of pollutants on the three types of underlying surfaces. The EMC mean values of SS， COD， TN， and TP in the parking lot （55， 67， 4.97， 0.23 mg/L） were significantly higher than those in the roads （47， 60， 2.41， 0.10 mg/L） and squares （42， 36， 3.16， 0.14 mg/L）， while the North Ring Road had a higher Fe concentration （0.47 mg/L） due to its dense traffic load. 2） The initial 30% runoff of the three types of underlying surfaces carried more than 50% of pollutants， and the initial flush effect was significantly stronger than that of plain cities. In particular， the flush ratios of SS and TN in parking lots reached 83% and 82%， respectively， indicating that the steep slope areas of mountain cities need to give priority to the control of initial rainwater； the flush effect increased with the increase of rainfall intensity and dry period before rain， but it was not obvious when the rainfall duration was short， discontinuous or the catchment area was large. 3） The concentration of runoff pollutants was positively correlated with the dry period before rain （r>0.52）， and negatively correlated with rainfall intensity （ρ<-0.06）. Under the condition of rainstorm， the concentration of TN in the north exhibition field increased rapidly to 63 mg/L （significantly higher than 34 mg/L under heavy rain）， due to the short-term strong erosion， indicating that terrain had a great influence on runoff water quality.