Water Quality in Marginal Seas off China in the Last Two Decades

Table 1

Seawater Quality Standard of China (GB3097-1997, excerpt; unit: mg L⁻¹).

Item	Clean water (Category I)	Relatively clean water (Category II)	Slightly polluted water; (Category III)	Medium polluted water (Category IV)	Heavily polluted water (inferior to Category IV)
Dissolved oxygen	>6	>5	>4	>3	<3
Chemical oxygen demand (COD)	≤2	≤3	≤4	≤5	≥5
Biochemical oxygen demand (BOD5)	≤1	≤3	≤4	≤5	≥5
Inorganic nitrogen (as N)	≤0.20	≤0.30	≤0.40	≤0.50	≥0.50
Phosphate (as P)	≤0.015	≤0.030	≤0.045		≥0.045
Mercury	≤0.00005	≤0.0002	≤0.0005		≥0.0005
Cadmium	≤0.001	≤0.005	≤0.010		≥0.010
Lead	≤0.001	≤0.005	≤0.010	≤0.050	≥0.050
Chromium (VI)	≤0.005	≤0.010	≤0.020	≤0.050	≥0.050
Total chromium	≤0.05	≤0.10	≤0.20	≤0.50	≥0.50
Arsenic	≤0.020	≤0.030	≤0.050		≥0.050
Copper	≤0.005	≤0.010	≤0.050		≥0.050
Zinc	≤0.020	≤0.050	≤0.10	≤0.50	≥0.50
Oil	≤0.05	≤0.30	≤0.50		≥0.50