A phenomenon of mixing blue and brown seawater, called "Yin-Yang Sea", has existed for decades in Liang-Dong Bay. In order to determine the chemical composition and origin of pollutants in the bay, each of 5 water samples from the stream and bay were collected for chemical analyses. Meanwhile, suspension particles in the bay, the sediments on the bottom of bay and the precipitation on the bottom of the stream were also collected for chemical, X-ray diffraction and particle size analyses.The suspension particles are amorphous and are predominantly composed of Fe(OH)3. This is showen by the solubility products of water samples in the bay being higher than the ksp of Fe(OH) 3 . The deposits on the bottom of the stream also show similar results. However, the chemical concentrations of major ions, e.g. Na + , K + , Ca +2 , Mg +2 , Cland SO 4 -2 , are much higher in the bay water than of the stream water. Nevertheless, Fe +2 , Mn +2 , Cu +2 and Si +4 show positive anomalies in the lower stream where the abandoned pits are located. Thus, it is assumed that minerals in the pits, such as pyrite and marcasite, are the main source supplying iron through weathering. Iron released by minerals is carried by the stream and arrives in the bay in hours. Since the oxidation rate becomes very slow in the bay because of the effects of salts and neutral pH values, most of the particles of Fe(OH)3 form in the bay gradually.