Study on self-purification of the Second Songhua River polluted by mercury in past 20 years

The self-purifying process of the Second Songhua River polluted by mercury in the past 20 years is investigated, and the specific present models are tested in this paper. The mathematical models of mercury concentration in the sediments, and river water during the rainless period are established by fitting the data, which were collected for five times after cutting off the pollution sources. Furthermore, mercury pollution in the Second Songhua River in the following 15 years is predicted by using the models. The research results indicate that self-purifying effect of the Second Songhua River polluted by mercury is evident. It is concluded that the self-purifying rate of the surface layer sediment is higher than that in the middle and the bottom layer sediments. The self-purification process acts in different two phases after cutting off the pollution sources. In the beginning, the self-purifying rate of the Second Songhua River is higher, and it becomes lower in the present. It is anticipated that the self-purifying rate in the future would be lower than the present.