Drilling fluid is one of the primary contaminants produced in the offshore drilling operation, which would pollute the water column if it is directly discharged into the ocean. In view of the insufficient far-field research on drilling fluid's discharge in the Chinese offshore area, a field study of the influence on the marine environment is conducted based on an oil field in South China Sea. At a constant rate, the water based fluid is pumped into the seawater vertically from a pipe on the

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... om of the platform. A relative large monitoring range at distances up to 2000 meters from the discharge source is determined in the light of the results from the previous study and the environmental impact assessment report. The water quality and the sediment solids are measured by the field monitoring under different stations and water depths. To obtain the particle size, the drilling fluid to be discharged is sampled and taken back to the laboratory for a statistics test. The single index method is introduced to assess the environmental quality in this field and the suspended solids, total mercury and cadmium are chosen as the evaluation factors. The monitoring results show that, the influence of the drilling fluid's discharge is very small on the water quality for the surrounding sea areas. The concentration distribution of the suspended solids presents a trend of spreading outward from the discharge source and the monitoring results can be attributed to the natural fluctuation of the background value. There are only small deviations between different water depths for the suspended solids and mercury, while the amount of the cadmium is relatively great in the surface layer of the water column. In this monitoring field, none of the concentrations of the total mercury and cadmium exceed the standard value, which meet the national primary standard of the seawater quality. The results of this study contribute to the understanding of the drilling fluid's dispersion in South China Sea and the field data can provide references for the marine environmental prediction and evaluation.