Sediment-Water Exchange, Spatial Variations, and Ecological Risk Assessment of Polycyclic Aromatic Hydrocarbons (PAHs) in the Songhua River, China

The sediment-water exchange, spatial variations, and ecological risk of polycyclic aromatic hydrocarbons ( PAHs) in the water and sediment of the Songhua River, China, were investigated and assessed in this paper. The fugacity fraction ( ff) was used to evaluate the sediment-water exchange of PAHs between the water and sediment. The results suggest that the values of ff decreased with an increasing number of PAH rings. The sediment acts as a secondary emission source for three- and four-ring PAHs, whereas five-ring PAHs were deposited in the sediment from the water. The high ff values of PAHs found in autumn suggest large releases of PAHs after abundant deposition in summer, and the lowest ff values of PAHs occurred in summer. The values were especially low for five-and six-ring PAHs, which exhibited considerable deposition from the water to the sediment. PAHs with low molecular weights showed strong variations, which were potentially caused by their active physical-chemical properties. Additionally, high molecular weight BaP displayed weak variations, increasing the potential risk in the sediment. The simplified qualitative method of C-water/C-sediment is useful for assessing variations in the sediment-water exchange. The relationships between C-water/C-sediment and ff were investigated by determining the Pearson correlation coefficients ( R). The results exhibited a significant negative correlation, with R = -1.000 and P = 0.000 for Flu, R = -0.993 and P = 0.007 for Phe, R = -0.998 and P = 0.002 for FlA, and R = -0.971 and P = 0.029 for BaP. The coefficients of variation indicated that five-ring PAHs were more sensitive than three- and four-ring PAHs. Thus, these low-ring PAHs can be easily exchanged between the sediment and the water. Additionally, the ecological risk of PAHs to aquatic organisms in the Songhua River is relatively low.