Administrative responsibility and land-sea synergistic regulating indicators for nutrient pollution: Linking terrigenous emission to water quality target in Laizhou Bay, China

Globally, many bays have been subjected to water quality (WQ) deterioration, due to excessive discharge of terrigenous total nitrogen (TN). In China, several projects have been implemented to control TN loads; however, due to the lack of linkage between a bay’s polluted segments and its basin’s jurisdictions, the WQ improvement efficiency usually is lower than expected. In this study, we focus on the basin of Laizhou Bay (LZB), China. A series of differentiated TN reduction indicators for 13 cities in the basin of LZB were developed on the base of the total maximum allocated loads (TMALs), which meets the WQ standard of LZB. Four land-sea synchronous investigations were conducted to discern seasonally polluted segments of LZB in 2017. The seriously nitrogen-polluted segments located at southwest and southeast of LZB and the largest area accounted for about 70% of LZB. Due to the heterogeneity of the terrigenous TN load and TMALs of the 13 jurisdictions in the basin, the responsibility apportionment rate for the nitrogen-polluted segments showed a spatial disparity, the city of Weifang with the highest value of 32%, followed by Zibo (28%), Ji’nan (20%), and the other 10 cities (19%). Accordingly, the TN load reduction rate among the 13 jurisdictions varied from 1% to 99%, with an average of 62%. The average values for reduction rate of TN generation, increase rate of TN removal by wastewater treatment, increase rate of TN retention in soil matrix, and TN attenuation in riverway were 44% (21%-69%), 53% (0–88%), 31% (0–61%), and 63% (15–84%), respectively. The three-dimensional seawater quality model simulation results showed that the “precise reduction indicator scheme” was six years earlier than the “equal percent reduction indicator scheme” in each jurisdiction to achieve the WQ standard. Seven cities need to cut emissions more than the current rate. The precise indicators throughout the land-based pollutant entire migration pathway will help implement the TN reduction plan more efficiently.