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

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 1025 mm (average=266 mm), and the total mass of sediment was approximately 5.2×105 t in the ancient town rivers of Suzhou. The average proportions and concentrations of TOC, TN, NH4+-N, TP, and AP in the sediments were 3.4%, 2074 mg·kg-1, 140.2 mg·kg-1, 1765 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, NH4+-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.