Spatial differences and formation conditions of beach mudding and blackening in Sanya Bay

The beach mudding and blackening in Sanya Bay, Hainan Island, South China has been a problem for a long time, which seriously affected the sustainable development of local tourism resort and caused widespread public concern. Through hydrodynamic numerical simulation, on-site in-situ observation, and sample analysis, we conducted a systematic research on the spatial differences and formation conditions of beach mudding and blackening to remediate the beach. The numerical simulation of tidal currents showed that the currents in Sanya Bay are mainly east-west reciprocating. Both the central and outer parts of the bay are dominated by ebb tides, and the maximum flow velocity can reach 1.0 m/s, whereas the fluctuating tidal velocity inside the bay is less than 0.1 m/s, where the maximum residual current is as low to 0.02 m/s. In some areas, such as Tianya Haijiao and the south side of Luhuitou, there are areas with high flow velocity, and the residual current is offshore movement in Tianya Haijiao. In addition, field survey and lab analysis showed that the degree of blackening in the east and west beaches of Sanya Bay is relatively higher than that in the central part. The entire bay can be divided into eastern stripped blackening zone, central normal zone, and western patchy blackening zone. The average content of ilmenite from east to west zone is 0.35%, 0.58%, and 2.08%, and the average content of organic carbon is 0.20%, 0.10%, and 0.07%, the average total nitrogen content is 0.011%, 0.006%, and 0.004%, respectively. The blackening of the west beach is mainly due to the mechanical transportation, sorting, and enrichment of non-ferrous mineral ilmenite. The material comes from the Maling coastal ilmenite placer, where the wave energy is concentrated. Under the strong waves, ilmenite grains could reach and blacken the surrounding beaches under the reciprocating movement of east-west tidal currents. The organic matter is attached to fine-grained sand to form sapropel under the works of anaerobic bacteria, which makes the eastern beaches of Sanya Bay black and stiff. Pollutants are mainly from the local domestic sewage discharge and the resuspension of submarine sludge; the low self-purification ability of sea water in this coastal area is an important factor in aggravating beach blackening.