Effects of pumping station discharge on water quality in the central urban reach of Suzhou Creek, Shanghai

Taking three water quality automatic monitoring stations （Beixinjing,Mengqingyuan,and Wenzhou Road） in the central urban area of Suzhou River,as well as adjacent pumping stations （Jianhe,Daguangfu,and Chengdu North）,as the research objects,based on the water quality automatic monitoring data from 2019 to 2023,this study analyzes the impact and changes of pumping station discharge on the water quality of receiving water bodies during rainy days,with a focus on two indicators:ammonia nitrogen （NH₃-N） and potassium permanganate (COD_Mn).Through statistical analysis of 315 rainy pumping station discharge events over the past 5 years,it was found that the water quality impact caused by discharge usually recovers to pre-discharge levels within 3 days;During this period,there may be short-term deterioration of Class V water quality,but in most cases （85.7%）,it can be restored to Class V water within 1～2 days.Research has shown that there are significant differences in the effects of pumping stations discharging into the river on NH₃-N and COD_Mn.The concentration of NH₃-N is significantly affected by the impact of river discharge,with an average increase of up to 194%,resulting in a decrease in water quality category from Class II to Class IV;However,the concentration of COD_Mnonly slightly increased,with an average increase of 26%,and the water quality category remained at Class III.There is no significant linear correlation between the discharge volume of the 3 pumping stations and the peak concentration of NH₃-N,but there is a general increase in NH₃-N concentration when the discharge volume is high.The impact mechanism of pumping station discharge on water quality is complex,and the amount of discharge is not a single determining factor,but is also related to various factors such as the number of sunny days,rainfall,rainfall patterns,and duration of discharge.