Precise Emission Reduction of Total Nitrogen for Jurisdictions in Laizhou Bay, China: A Simulation-Optimization Approach Linking Land-Sea Management

This study aims to provide a quantitative basis for the precision emission reduction of land-based total nitrogen (TN) pollutants in Laizhou Bay (LZB), China. The total maximum allocated load (TMAL) of TN pollutants within jurisdictions around LZB was calculated using the stepwise simulation-optimization model after the division of source units, simulation of the concentration response matrix, setting of the additional constraint conditions for stepwise source units, and calculation of TMAL. The load reduction allocation system for jurisdictions can be constructed on the basis of the TMAL and the load monitored or statistics. The index system for TN emission reduction included indices for two groups: a fixed index of marine emission reduction rate for jurisdictions, which was 11.93% on average; and threshold indices of pollution prevention rate at sources, the increment rate of pollution removal by municipal sewage treatment systems, and watershed retention of jurisdictions, which were 7.43%, 1.58%, and 1.09% on average, respectively. Efficiency of pollutant reduction can be evaluated using a 3D hydrodynamic-water quality model. Simulation results showed that the precise reduction indicators achieved the seawater quality improvement target of LZB 5 years earlier than the equal proportion reduction indicators currently used in LZB Basin.