Study On Water Quality Characteristics Of Stormwater Runoff And Initial Stormwater Definition And Control Strategy Of A City In Northwest China

With the acceleration of the process of urbanization in China,city hardening of the ground increased unceasingly,waterlogging frequent,non-point source pollution is serious.In order to solve the increasingly serious problems of urban water safety,water environment,water resource and water ecology,our country put forth effort to promote the construction of sponge city.Since 2015,it has carried out pilot construction and demonstration construction of sponge city,and proposed to systematize the whole-scale promotion of sponge city construction.Therefore,exploring the water quality characteristics of urban stormwater runoff,explaining the initial erosion effect of stormwater runoff,defining the initial stormwater,and proposing theoretical basis and technical support for sponge city construction have important practical significance for controlling urban non-point source pollution.In this paper,six different underlying surfaces of flat roof,inclined roof,municipal trunk road,municipal secondary trunk road,green space and parking lot are selected as research objects,SS,COD_Cr,NH₃-N,TN and TP are selected as typical pollutants.According to the field monitoring data,The characteristics of stormwater runoff were studied from the aspects of water quality characteristics of stormwater runoff,initial erosion effect of stormwater runoff and the definition of initial stormwater,and the results were as follows.Firstly,according to the analysis and research results of rainwater runoff quality characteristics,it is shown that:（1）The variation range of pollutant concentration in stormwater runoff on each underlying surface in the study area is large.The mean concentration of each pollutant decreases gradually with the increase of runoff duration and eventually tends to be flat.（2）The pollution intensity characteristics of rainwater runoff indicate that nitrogen pollutants are the main pollutants in the stormwater runoff of flat roof and sloping roof,suspended matter and organic matter are the main pollutants in the stormwater runoff of municipal trunk road and municipal secondary dry road,and suspended matter is also the main pollutants in the stormwater runoff of green space and parking lot.（3）The pollutant quality assessment shows that the improved Nemero pollution index for rainwater runoff from flat roofs,sloping roofs,municipal trunk roads,municipal secondary trunk roads,green spaces,and parking lots is 2.63,1.72,28.35,13.73,24.45,and 7.32,respectively,with corresponding assessment results being cleaner,cleaner,heavily polluted,heavily polluted,and moderately polluted;The pollution situation of rainwater runoff on each underlying surface is as follows:main road>lawn>secondary main road>parking lot>flat roof>inclined roof.（4）Correlation analysis of stormwater runoff pollutants showed that SS and COD_Cr were significantly positively correlated in all underlying surfaces except green Spaces,SS and NH₃-N were significantly positively correlated in all underlying surfaces except flat roofing,and SS and TP were significantly positively correlated in municipal main roads,municipal secondary trunk roads and parking lots.SS and TN showed significant positive correlation in flat roof,sloping roof and parking lot.Secondly,according to the research on the initial scouring effect of rainwater runoff,it is shown that:（1）The qualitative analysis of the M（V）curve shows that there are differences in the initial scouring effects of each underlying surface.Quantitative analysis based on the MFF₃₀method shows that the scouring effects of SS and COD_Cr are generally higher than those of NH₃-N,TN,and TP in each underlying surface.（2）The material of the underlying surface has an impact on the scouring effect.The initial scouring effect intensity of each underlying surface is:flat roof>inclined roof>main road>green space>parking lot>secondary main road.Under the conditions of light rain and moderate rain,the greater the rainfall,the stronger the initial scouring effect of each underlying surface,while under heavy rain conditions,the weaker the initial scour effect.Thirdly,according to the preliminary rainwater definition study in the study area,it is shown that:（1）The rainfall depth that needs to be controlled before the flat roof and the inclined roof meet the water quality standards for reuse is defined as initial rainwater.The initial rainwater for the flat roof under light rain,moderate rain,and heavy rain conditions is 4.5mm,3mm,and 7mm,respectively;The initial rainwater of inclined roof under light rain,moderate rain and rainstorm is 3mm,3mm and 6.5mm respectively.（2）Using 50%as the target for controlling rainwater runoff pollutants on municipal trunk roads,secondary trunk roads and parking lots,and defining initial rainwater as the required rainfall depth(D₅₀)after the correction of water quality indicators using the Environmental Quality Standards for Surface Water:The initial rainwater of municipal trunk roads under light rain,moderate rain,heavy rain and rainstorm conditions is 1.8mm,2.1mm,7.9mm and 21.3mm respectively;The initial rainwater of municipal secondary trunk roads under light rain,moderate rain,heavy rain and rainstorm conditions is 1.9mm,2.2mm,8.4mm,and 19.7mm;The initial rainwater of the parking lot under light rain,moderate rain and rainstorm conditions is 2.0mm,1.9mm and20.1mm respectively.（3）In practical engineering application,the 90 minutes before rainfall in this city can be defined as the initial rainwater of flat roof and inclined roof based on rainwater resource utilization.55min before rainfall can be defined as initial rainwater on municipal main roads,municipal secondary main roads and parking lots based on total pollutant reduction.