Comparative analysis of rainfall runoff pollution characteristics between hilly and plain towns in the Yangtze River Basin

With the acceleration of urbanization, the problem of urban non-point source pollution caused by rainfall runoff in the Yangtze River basin has become increasingly prominent. Some old and new urban areas were selected as the research objects in the hilly towns of Yongchuan District in Chongqing City and the plain towns of Jintan District in Changzhou City in the Yangtze River basin. The water quality and quantity of rainfall runoff in the four research areas were monitored. The event mean concentrations (EMC) and comprehensive runoff coefficient methods were used to calculate the average concentration and pollution load of rainfall-runoff pollutants on each underlying surface. The b-parameter method was used to analyze the first flushing effect of rainfall runoff. The results showed that: (1) The EMC of SS, COD, TP, NH₃-N and TN was mainly affected by topography, rainfall characteristics and early drought days. The EMC of SS and COD in traffic roads in hilly towns was 1.10-7.05 times and 0.86-8.98 times that of other underlying surfaces, respectively. The EMC of SS and COD in traffic roads in plain towns was 1.01-3.70 and 1.07-6.70 times that of other underlying surfaces, respectively. (2) The first flushing effect of rainfall runoff in hilly towns was stronger than that in plain ones, and the first flushing intensity of SS and COD was higher, with b values ranging from 0.25 to 1.20 and 0.50 to 1.28, respectively. (3) The average output coefficient of the pollution load of each underlying surface in hilly towns was greater than that in plain ones. The average output coefficient of the pollution load of each index in the old urban area of hilly towns was 0.37-16.13 times that in plain towns, and the average output coefficient of the pollution load of each index in the new urban area of hilly towns was 0.38-7.11 times that in plain towns. The study reveals the characteristics of rainfall-runoff pollution in hilly and plain towns in the Yangtze River basin, which can provide reference for the control of rainfall-runoff pollution in these two types of towns.