Dynamics of Trace Element Composition of Bratsk Reservoir Water in Different Periods of Anthropogenic Impact (Baikal Region, Russia)

Bratsk water reservoir is a part of the unified Baikal-Angara water system, the world's largest reservoir of surface drinking water. This study intends to assess the water quality of Bratsk reservoir by examining the spatial and temporal variability of trace element concentrations over periods of changing by the anthropogenic impact. The water samples were collected along the length of the reservoir in 2003, 2008, 2012, and 2018 and analyzed for 12 trace elements (Al, Cr, Mn, Fe, Cu, Zn, As, Cd, Cs, Pb, U, and Hg). The results show that in the zone of maximum anthropogenic impact the concentrations of these elements increased as follows: Al 781.0 µg L-1; Cr 1.43 µg L-1; Fe 1052.4 µg L-1; Cu 3.34 µg L-1; Zn 15.52 µg L-1; Cd 0.691 µg L-1; Cs 0.070 µg L-1; U 1.74 µg L-1; Hg 0.0505 µg L-1. The trace element concentrations in Bratsk reservoir water did not exceed levels set by the international drinking water quality standards. At the same time, the uniqueness of the reservoir requires the use of stricter criteria for water quality control. The trace element concentrations in the water from the source of the Angara river (the only run-off from Lake Baikal) used as reference levels indicate higher levels of metal contamination and pollution indices in Bratsk reservoir water. Over different periods of observation, the contamination factor (CF) showed very high contamination for Al, Mn, Cd, and Cs. PLI indices for 2003 and 2018 classify the majority of water samples as polluted. The NPI values were indicative of severe water contamination by Mn and Pb (all years), Cu (2009), Fe and Cd (2003), Al, Cr, Zn, Cs, and Hg (2003, 2009, 2018). The obtained results showed the importance of using water quality criteria with a view to strengthening the requirements of environmental protection measures in the Baikal region.