Spatiotemporal distribution characteristics and ecological risk assessment of nitrogen, phosphorus and heavy metals in the surface sediments of Qianwu Reservoir

To understand the environmental quality of the surface sediments of Qianwu Reservoir in wet and dry seasons (August and October) of 2019, nitrogen and phosphorus contents and 8 heavy metals were monitored at 6 sampling points in the reservoir area, and the ecological risk assessment and traceability analysis were carried out by using the comprehensive pollution index method, the geo cumulative index method and the potential ecological risk index method. The results showed that the average values of TN and TP contents in the surface sediments of Qianwu Reservoir in wet and dry seasons were 2 010 and 433, 1 873 and 308 mg/kg, respectively. It presented the temporal and spatial distribution characteristics of high in the middle and low in the north and south ends. The comprehensive pollution index evaluation results showed that the overall TN of Qianwu Reservoir was heavily polluted, and TP was mildly polluted. The average contents of Hg, As, Cr, Mn, Ni, Cu, Zn, and Pb in the surface sediments of Qianwu Reservoir were 0.22, 10.64, 30.97, 293.25, 17.51, 21.01, 102.65, and 55.32 mg/kg, respectively. In terms of temporal and spatial distribution, except for Mn and Zn, the pollution of other heavy metals was heavier in wet seasons than in dry seasons. Affected by the topography of the reservoir and seasonal water transfer, except for Mn and Pb, the concentration levels of other heavy metals showed that the area from the head to the middle of the reservoir (points Q3 and Q4) during the wet season, and the area from the middle to the tail of the reservoir (Q2 and Q3) was high during the dry season. The evaluation results of the geoaccumulation index method and the potential ecological risk index method both showed that Hg was the main ecological risk contributing factor, with a contribution rate of 75.8%. The overall potential ecological risk level of the reservoir area was medium risk.