苏州古城区域河道碳氮磷类污染物的分布特征

Distribution Characteristics of Carbon, Nitrogen, and Phosphorus Bearing Pollutants in the Ancient Town Rivers of Suzhou

苏州是水质型缺水城市,节水、减排、控源和截污等工程实施后,水质问题依然严峻.为了解苏州古城区域河道中碳氮磷类污染物的总量及分布特征,提出河道疏浚决策依据,于2019年春季在苏州古城区域采集了20个代表性断面的河道底泥和水体样品,测定了河道底泥的深度,分析了河道底泥和水体样品中碳氮磷类污染指标的含量,评价了河道底泥和水体的污染程度,并预测了换水、引水、降雨和疏浚情境下水质的变化.结果表明,苏州古城区域河道底泥深度在22~1 025 mm之间(均值为266 mm),底泥总质量约为5.2×10^(5)t.底泥中总有机碳、总氮、氨氮、总磷和有效磷平均含量分别为3.4%、2 074 mg·kg^(-1)、140.2 mg·kg^(-1)、1 765 mg·kg^(-1)和57.2 mg·kg^(-1),属中度污染,总磷含量超标点超过90%,环城河污染程度最高,建议优先疏浚.水体中总有机碳、生化需氧量、化学需氧量、总氮、氨氮、凯氏氮、总磷和磷酸盐平均浓度分别为7.8、0.6、13.1、2.5、0.643、1.3、0.18和0.09 mg·L^(-1),属重度污染,为劣V类地表水,总氮浓度严重超标.基于沿程碳氮磷类污染物总量的分布情况,苏州古城区域河道疏浚推荐顺序为环城河、古城北部河道、干将河和古城南部河道.降雨情景下,初期径流污染物浓度高,将导致河道水质急剧下降;换水和引水情境下水中总氮总量均减少0.2 t,完全疏浚后水中总氮总量分别进一步减少4.58 t和2.19 t.底泥磷以外源输入为主,可受纳部分水体中的磷,故疏浚后,水体中总磷总量可能增加.

Suzhou is a water-deficient city with water quality issues.Despite water conservation measures,emission reductions,source control,and pollution interception,water quality remains poor.To understand the total mass and distribution characteristics of carbon,nitrogen,and phosphorus bearing pollutants and inform decisions regarding river dredging,sediment and water samples were collected from 20 representative sections in the town’s rivers during the spring of 2019.The depths of the sediments were measured along with the concentrations of carbon,nitrogen,and phosphorus bearing pollutants in the sediments and water,and the pollution degree was evaluated.Variations in various parameters were predicted for change water,diversion water,rainfall,and dredging.The results show that the sediment depths ranged between 22 and 1 025 mm (average=266 mm),and the total mass of sediment was approximately 5.2×10^(5)t in the ancient town rivers of Suzhou.The average proportions and concentrations of TOC,TN,NH_(4)^(+)-N,TP,and AP in the sediments were 3.4%,2 074 mg·kg^(-1),140.2 mg·kg^(-1),1 765 mg·kg^(-1),and 57.2 mg·kg^(-1),respectively,indicating a moderate level of pollution.The concentration of TP in the sediments at 90%of the sampling points exceeds the national standard.Huancheng River was found to have the highest concentration of TP,suggesting that dredging shuld be targeted here first.In the water samples,the average concentrations of TOC,BOD5,COD,TN,NH_(4)^(+)-N,Kjeldahl nitrogen,TP,and PO43--P were 7.8,0.6,13.1,2.5,0.643,1.3,0.18,and 0.09 mg·L^(-1),respectively,indicating a severe level of pollution.Overall,water quality in these rivers falls below Class V surface water,and the concentration of TN seriously exceeds the national standard.Based on the patterns of total carbon mass and nitrogen and phosphorus bearing pollutants,the recommended order of dredging in Suzhou is the 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 concentrations of pollutants in runoff were high,which leads to a decline in water quality.The total mass of TN in the water was reduced by 0.2 t under the change water and diversion water scenarios,and was further reduced by 4.58 t and 2.19 t,respectively,after dredging.Phosphorus bearing pollutants in the sediment were mainly imported from other sources,meaning that the total mass of TP in the water may increase following dredging activities.