Study On The Spatial-temporal Distribution Characteristic Of Nonpoint Source Pollution In Mountainous City

With the fast development of urbanization, urban nonpoint source pollution causedby washing-off of rainfall-runoff has become a serious threat to water quality ofreceiving water bodies. There are rare studies coincident with each other for therandomness of urban nonpoint source pollution occurring and numerous influencingfactors. The generation of nonpoint source pollution is more complex in mountainouscities with varied terrain. It is significant to understand the spatial-temporal distributionof urban stormwater runoff pollution, first flush effect, compositions of pollutants andconstruction of models for rainfall-runoff simulation in mountainous cities. TakingChongqing as a case study, representative underlying surfaces are selected as studyobjects, rainfall-runoff from different landuse types are observed and monitoring resultsare also analyzed. The main results are as follows:①Results of temporal distribution of pollutant concentrations in mountainous cityshowed that there was great difference for the variation process of pollutantsconcentrations during rainfall events. The pollutants concentrations decreasedmonotonically and the main reduction occurred in the initial period of rainfall events（e.g.,10-20min after the beginning of runoff） from small catchment with simple landusetypes. However, the highest pollutants concentrations didn’t appear until the middleperiod of rainfall events when the catchment with complex layout of landuse. The peakof pollutants concentrations preceded the peak of runoff flow rate in integrated basiswith a smaller scale （except Cd）, while the contrary phenomenon was observed whenthe basin had a larger scale. The peak of runoff flow rate was significantly correlatedwith rainfall intensity and rainfall duration in larger scale basin, but the significantcorrelation was found between peak of runoff volume and total rainfall/rainfall durationwhen the area of urban basin was small.②The simulation of rainfall-runoff in mountainous city showed that parametervalues were different from plain cities. In urban basin, hydrological parameter values（e.g., runoff initial loss volume in pervious and impervious surfaces and manningcoefficient in pervious surfaces） were smaller than plain city, while water qualityparameter values （e.g., washing-off index and coefficient） were higher in SWMMmodel. For underlying surfaces with simple landuse types, the temporal distribution ofpollutant concentrations belonged to exponential washoff model and the value of R2were0.75-0.88,0.81-0.91and0.73-0.94for urban traffic road, roofs and residential road, respectively. The absolute values of washing-off index were higher inmountainous city, which implied a faster reduction rate of pollutant concentrations inrainfall-runoff.③The analysis of EMCs showed that EMCs of TSS and COD （597and408mg/L,respectively） from urban traffic roads （UTRs） were higher than those from residentialroads （RRs）, commercial areas （CAs）, concrete roofs （CRs）, tile roofs （TRs）, andcampus catchment areas （CCAs）. The EMCs of TN from RRS, CAs and UTRs weresimilar to each other （7.1-8.6mg/L）, which were higher than that of the other threelanduse types. Nitrogen in stormwater was predominantly dissolved （73-82%）, withDIN （Dissolved Inorganic Nitrogen） as the main form （63-82%of TN）. The EMCs ofTP and NH₃-N from UTRs and CAs were respectively2.35-5times and3times ofclass-III standard values specified in Environmental Quality Standards for SurfaceWater （GB3838-2002）, while that from CCA and TRs could meet class-III standardvalues. The EMCs of Pb and Cd were much higher than the class-III standard values,and that of Cu and Zn could meet class-III standard values.④The analysis of pollutant load producing coefficients （PLPCs） revealed thaturban traffic road were the main pollution contributor. PLPCs of TSS, COD, TP, TN,NH₃-N, NO₃-N, Fe, Cu, Zn, Pb and Cd from urban traffic road were589,404,1.0,8.5,4.4,2.1,11.1,0.124,0.6,0.63and0.05t/（km²*y） respectively, which were higher thanother landuse types. Compared with plain cities, PLPCs of urban traffic road was higherthan plain cities premised on similar annual rainfall volume.⑤The study results of first flush effect based on M（V） method showed that50-80%of pollutant mass was transported in the first40%of runoff for catchment withsimple landuse type with small slope （e.g.,≤2.5%） when rainfall intensity is9.1±4.0mm/h and42%-58%of pollution load was carried by the initial20%of total runoffwhen the road slope was as high as30%with rainfall intensity is8.5mm/h. So, the first2.6-3.8mm rainwater should be controlled for road with bigger slopes and3.2mm forroad with smaller slopes from the point of view of pollution load controlling.Limitations existing in traditional method could be overcome by the optimalsegmentation mode, such as the difference among estimation standards, inability tocapture initial volume needing to be controlled directly, and failure in detecting the flusheffect if middle or end pollutant concentrations were high, and so on, but runoff volumeneed to be controlled should be decided based on the primary pollutant. Compared withplain cities, the probabilities of first flush effect occurrence for simple landuse types inmountainous cities could be as high as100%, which was higher than plain cities. Thebigger the slope was, the stronger the first flush effect was. ⑥The peak of runoff rate and total runoff volume could be reduced highly bygreen roof compared to impervious roofs with the same area. Meanwhile, green roofshad good ability in neutralizing acid deposition and NH3-N detention, but poorefficiency in controlling nitrogen and phosphors. Compared to rain water quality, greenroof runoff had higher concentrations of dissolved nitrogen and phosphors. It was thegreat reduction of runoff volume that improved pollution load reduction in green roofsystem. The water quality of stormwater runoff from green roofs varies significantlywith seasons, which was better in summer and worse in spring and autumn. Overall, theconcentrations of TN and NO₃-N in runoff from green roofs decreased gradually whenthey were operated for a long term, while that of TP and PO₄-P showed fluctuations.Analysis of Pearson correlations among meteorological factors indicated that the highertemperature and longer drying period, the more decrease of concentration of NH3-N.The TP and NH3-N of runoff in green roofs came from rain water primarily.