Risk assessment of sectional water quality based on deterioration rate of water quality indicators: A case study of the main canal of the Middle Route of South-to-North Water Diversion Project

The Middle Route of South-to-North Water Diversion Project of China (MRSNWDPC) is the most important interbasin water diversion project. This study screened out 8 water quality indicators, including pH, ammonia nitrogen (NH3-N), fecal coliform (F. coli), dissolved oxygen (DO), permanganate index (PI), five-day biochemical oxygen demand (BOD5), fluoride (F-), sulfate (SO42-), which were from monthly monitoring data of 29 sections between January 2016 and December 2018, and analyzed the probability distribution characteristics of water quality deterioration rate (WQDR) samples, compared the advantages of three kinds of probability density function parameter optimization method, then the probability density model of each water quality indicator is constructed. By defining the section water quality risk and setting the threshold, the risk level of harmful water quality events (" degradation "or" standard exceedance ") in the next one month was calculated based on the real-time monitoring data. The main conclusions include: (1) The probability density of WQDR samples in the MRSNWDPC basically corresponded to the p-III distribution, and the optimum line method can be used as the parameter optimization method of the distribution function. (2) The water quality risk assessment method proposed in this paper could calculate sectional risk based on real-time monitoring data of water quality and play an early warning role in the occurrence of harmful water quality events in the next one month. (3) From 2016 to 2018, water quality deterioration existed in the MRSNWDPC, and the probability of pH, DO and BOD5 with higher risk level was higher than other indicators. However, further research is needed to confirm whether a stable trend of water quality deterioration exists. This study redefines the concept of risk of sectional water quality from the perspective of the probability of occurrence of harmful events and sets the threshold value of risk level in combination with the water body function, so as to predict the future water quality risk level in the next month, which plays a good warning role in the water quality management of the MRSNWDPC and can help relevant management departments to make response plans in advance. It is worth mentioning that the method proposed in this paper has been applied to the water quality monitoring and early warning integrated management platform of the main canal of the MRSNWDPC.