Impacts of inland pollution input on coastal water quality of the Bohai Sea

Highlights: • About one third of the Bohai Sea area belongs to unclean seawater in 2017. • The N, P, petroleum and heavy metals were main pollutants. • The water quality of the Bohai Sea was great affected by inland pollutant inputs. • The river and sewage outlet inland pollutants mainly derived from. • River transportation and sewage outlet discharge are two main routes of inland pollutants inputs Inland pollutants input is a key impact factor for the coastal water quality of the Bohai Sea. The coastal and inland water pollutant inputs were analyzed by using monitoring data of recent years from the State Oceanic Administration. The results showed that more than 56% of the Bohai Sea area was unclean seawater in 2012, although the water quality improved gradually after that time. In 2017, about one-third of the Bohai Sea area still had unclean seawater. Inorganic nitrogen, reactive phosphate, and petroleum hydrocarbons are the main pollutants in the seawater. A total of approximately 840,000 t of pollutants was transported to the sea each year by major rivers during 2010–2017. Significant correlations (p < 0.05) were found between the third-grade level seawater area and the pollutants of COD_cr, petroleum, NO₃⁻–N, NH₄⁺–N, NO₂⁻–N, Cu, and Pb and between the inferior fourth-grade level seawater area and the pollutants of petroleum, NO₂⁻–N, Pb, and NO₃⁻–N. The standard discharge rate of terrestrial-source sewage outlets was no more than 50%. The low standard discharge rate of the major terrestrial-source sewage pollutants of COD_cr, NH₄⁺–N, TP, BOD₅, and SS caused more than 80% of the monitored sea areas adjacent to the selected key sewage outlets to not meet the water quality requirements of the marine functional area. The results suggest that implementing a coastal water management plan is necessary to reduce the heavy ecological burdens on the coastal zone of the Bohai Sea.