Analysis of accumulation formation of sediment contamination in reservoirs after decades of running: a case study of nitrogen accumulation in Biliuhe Reservoir.

Sediment contamination is an important influencing factor for reservoir water quality. Investigations have shown that reservoirs are facing the risk of sediment contamination after running for several decades in China. This paper proposes that the accumulation of sediment contaminant is resulted from the difference between the input and output of contaminant. Further, an accumulation model of reservoir sediment nitrogen is established based on this theory. The calculation result of Biliuhe Reservoir shows that inflow rate of total nitrogen into the reservoir is 4521.47 t/a, the outflow rate is 1033.97 t/a, nitrogen removal by denitrification is 1465.81 t/a, and the accumulation rate is 1841.68 t/a. The accumulation rate of total nitrogen is 77.84 t/a in water, 924.42 t/a in suspended solids, and 839.42 t/a in sediment. The accumulation of nitrogen resulted in the total nitrogen concentration in water increasing from 1.71 mg/L in 1995 to 3.78 mg/L in 2013, and that in sediment increasing from 779.10 mg/kg in 1993 to 2725.00 mg/kg in 2013. It is concluded that sediment contamination has the characteristics of significant accumulation trend, complex forms, and high security risks, which has been a hidden security risk for reservoirs after decades of running. Heterogeneity of the reservoir and complicated influencing factors of sediment contaminant accumulation should be concerned next.