N, P, and COD conveyed by urban runoff: a comparative research between a city and a town in the Taihu Basin, China.

Stormwater runoff containing various pollutants exerts adverse effects on receiving water bodies and deteriorates the urban aquatic environment. Although numerous studies have been conducted on runoff pollution, research comparing its characteristics in cities with those in towns is rare in the literature. To close this gap, the present study was conducted. The instantaneous concentrations of ammonia-N, TN, TP, and COD during the rainfall events in the town were higher than those in the city in most conditions. The outfall concentrations increased with the increase of rainfall intensity. EMCs (the average value of EMC) and CV (coefficient of variation) of TN and DTN in the town were higher than those in the city, which may lie in the differences of urban environment planning and management, road cleaning methods, garbage disposal methods, industrial enterprise, etc. On the one hand, EMCs and CV of TP in the city's industrial areas were lowest among three functional areas, while on the other hand, in the town it was in the commercial areas rather than the industrial areas that EMCs and CV were the lowest, which may be caused by the low level of economic development of small towns in China. The concentrations of COD in the town were generally higher than that in the city. Compared with the city, the correlation among COD and various forms of N was stronger in the town, which may illustrate a stronger similarity of pollutant sources in the town. According to the results, road runoff in the town contributed more to urban aquatic pollution; thus, further research should concentrate on this particular type of runoff.