| Suzhou is a city which lacks water in water quality.After the integrated processes of water conservation,emission reduction,source control,and pollution interception,the water quality was still not very good.Sediment might be an important internal source of pollutants.To understand the total mass and distribution of conventional pollutants,and put forward the decision basis of river treatment,93 representative sections of sediments and 79 interstitial water samples were collected in the central area of Suzhou in the spring of 2019.The depth of sediments was measured,the concentration of carbon,nitrogen and phosphorus bearing pollutants in the sediments and interstitial water,and the contents of heavy metals of sediments were analyzed,and as well the pollution degree of the sediments and interstitial water were evaluated.The isothermal adsorption and release kinetic experiments of nitrogen and phosphorus on the sediments of the main rivers in Suzhou central area were used to analyze the release characteristics of nitrogen and phosphorus.Taking the ancient town rivers of Suzhou as the representative area,the variation of water parameters was predicted underwater changing,diversion water,rainfall and dredging.The depth of sediments was between 0.1?252.0 cm(average was 35.3 cm),and the total mass of sediments was approximately 4.7×106 t in the rivers of Suzhou central area.The average concentrations of total organic carbon(TOC),total nitrogen(TN),ammonia nitrogen(NH4+-N),total phosphorus(TP),and available phosphorus(AP)in the sediments were 1.9%,1443.0 mg·kg-1,125.5 mg·kg-1,1430.4 mg·kg-1,and 47.1 mg·kg-1 respectively,and the total mass were 42185.9,2754.5,262.2,2916.8 and 118.1 t respectively,45%of the sediment samples belonged to mild and moderate pollution.The average concentrations of TOC,biochemical oxygen demand(BOD5),chemical oxygen demand(COD),TN,NH4+-N,TP,and phosphorus(PO43--P)in the interstitial water were 15.9,10.8,128.8,12.4,8.2,7.3 and 1.3 mg·L-1 respectively,and the total mass of TOC,BOD5,COD,TN,NH4+-N,TP,and PO43--P in the interstitial water were 27.0,14.6,188.9,20.0,13.1,12.6 and 1.0 t respectively,26.6%of the interstitial water samples was still seriously polluted after being diluted by 10 times.Based on the distribution of on-way total mass of carbon,nitrogen and phosphorus bearing pollutants,the recommended order of dredging in the central area rivers of Suzhou was Huancheng river,Jinghang river,Ganjing river,Lindun river,Xu river,Lou river,Yuanhe river,Xiangmen river,Shantang River and Shangtang river.The average concentrations of Cd,Cu,Cr,As,Mn,Ni,Pb,and Zn in central area rivers of Suzhou were 1.3,130.7,84.7,18.0,697.7,54.2,156.7 and 357.9 mg·kg-1,respectively.The proportions of sampling points which exceeded the soil background value of Jiangsu Province were 100%,98%,44%,88%,66%,98%,95%and 100%,respectively.The geo-accumulation index showed that pollution degree of heavy metals in sediments followed by order of Cu>Zn>Pb>Cd>Ni>As>Cr>Mn.In general,Cu was in a moderate-high pollution state,Zn,Pb,and Cd were in moderate pollution states,Ni was between a slight pollution state and a moderate pollution state,and as well As,Cr,and Mn were under a slight pollution state.Based on the potential ecological risk index,the order of the potential ecological index of heavy metals was Cd>Cu>Pb>As>Ni>Zn>Cr>Mn.Generally,Cd presented a high potential ecological risk,Cu,Pb,and As presented a moderate-high potential ecological risk,and the others presented a low potential ecological risk.The comprehensive potential ecological risk index showed that more than 50%of the sediment sample were above the high-risk level,indicating that the control of heavy metals should be paid more attentioned.According to the principal component analysis,Cd,Cu,Cr,As,Ni,Pb,and Zn might be derived from man-made factors,while Mn was mainly derived from natural factors.The theoretical equilibrium release point of NH4+-N concentration in the sediments of the main rivers of Suzhou was approximately 0.1 mg·L-1,while the sediment was the sink of the NH4+-N right now.The theoretical equilibrium release point of PO43--P concentration in the sediments of the main rivers in Suzhou was approximately 0.05?0.88,0.05?0.35 and 0.05-0.95 mg·L-1,while the sediment was the source of the PO43--P right now The isothermal adsorption line of NH4+-N in the Lindun-Yuanhe(LY)river and Jinghang(JH)river sediments conformed to the Henry isotherm adsorption model,which might be chemical adsorption.The isothermal adsorption line of NH4+-N in the Ganjiang-Xiangmen(GX)river sediments conform to the Henry isotherm adsorption model,which might be both chemical and physical adsorption.PO43--P adsorption from the sediments of LY river and GX river might occur in both chemical and physical processes,while the JH river might chemical processes.The variation of NH4+-N and PO43--P release from sediments of the LY river,GX river,and JH river confirmed to second-order reaction model.The theoretical initial release rates of NH4+-N was 0.103-7.318 mg·(kg·h)-1,0.149?16.343 mg·(kg·h)-1,and 0.637-29.436 mg·(kg·h)-1,and the theoretical initial release rates of PO43--P was 0.183-4.189 mg·(kg·h)-1,0.028?0.524 mg·(kg·h)-1,and 0.027?1.538 mg·(kg·h)-1,respectively.The average concentrations of TN and TP in the ancient town rivers sediments of Suzhou were 2074 and 1765 mg·kg-1 respectively,and the average concentrations of TN and TP in the water were 2.5 and 0.18 mg·L'1 respectively.The total mass of TN and TP were 791 and 531 t respectively,and the on-way total mass was 0.2 and 12.3 t·km-1 respectively.Based on the distribution of the on-way total mass of TN and TP along the way,the recommended order of dredging in the ancient town of Suzhou was Huancheng river,the northern rivers of the ancient town,Ganjiang river and the southern rivers of the ancient town.Under the rainfall scenario,the initial runoff pollutant concentration was high,which would lead to a decline in water quality.The total mass of TN in the water was reduced by 0.2 t underwater changing and diversion water,and the total mass of TN in the water was further reduced by 4.58 and 2.19 t respectively after dredging. |
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