Trends analysis and targets study of the water quality in Tianjin coastal waters

Mastering the seawater quality variation trends and formulating scientific and reasonable water quality targets are helpful to the accurate implementation of the total amount control of pollutant discharge in key sea areas and formulate effective pollutant control policies. The water quality variation trends analysis model and the water quality target determination method were established by using the Generalized Additive Models (GAM), based on the nutrient concentration and precipitation data of Tianjin coastal waters for the 2007-2018 periods. The water quality targets of Tianjin coastal waters were put forward on the basis of evaluating the variation trend of inorganic nitrogen and reactive phosphate concentrations at 12 stations in Tianjin coastal waters, and the rationality and accessibility of the water quality targets were analyzed. The results showed that the concentration of inorganic nitrogen in the coastal waters of Tianjin generally showed a downward trend, with a decreasing ratio of 13.19% and a 95% confidence interval ranging from −30.37% to 3.96% in 2013-2018, compared with 2007-2012. The concentration of reactive phosphorus generally showed an upward trend, with an upward ratio of 7.01% and a 95% confidence interval ranging from −11.43% to 25.45%, which had not recovered to the average level from 2007 to 2012. It was proposed that the proportion of Grade Ⅰ-Ⅱ water quality of inorganic nitrogen and reactive phosphorus in Tianjin coastal waters would reach 75% in 2025. It was suggested to implement water quality zoning management, and the Tianjin coastal waters were divided into seven zones. Other measures such as further strengthening the prevention and control of agricultural non-point sources pollution, enhancing the upstream and downstream collaborative governance and inter provincial joint prevention and treatment of water pollution were also proposed to continually improve the water quality of the coastal waters of Tianjin.