## 3 Results and Discussion  
PCBs in the Upper Water Layer of the Pelagic Zone and Deep Layers of the Water Column of Lake Baikal  
The concentrations of PCBs in the water of Lake Baikal were characterised by seasonal and interannual variability (Figure 2). In May 2015, we detected the maximum total concentrations of PCB congeners (\(\Sigma\)PCBs) ranging from 1.4 to 6.6 ng/L. There were 24 to 34 congeners of PCBs dominated by homologues of penta-dichlorobiphenyls as well as 7 indicator congeners 28, 52, 101, 118, 153, 138, and 180. The total concentrations of indicator PCB congeners (\(\sum\)PCBs) ranged from 0.43 to 2.0 ng/L, and the contribution from \(\sum\)PCBs to the total concentration of all detected congeners ranged from 28 to 35%. In September 2019, the concentration of \(\sum\)PCBs also increased at some stations (19 and 21) to 0.24-2.8 ng/L with an increase in the number of the detected congeners to 27. Indicator congeners were detected in the concentration range from 0.17 to 1.3 ng/L with a contribution to \(\sum\)PCBs of up to 40-70%. The minimum concentrations of \(\sum\)PCBs in Baikal water ranged from 0.11 to 0.82 ng/L, and the number of the detected congeners were recorded in September 2016, May 2018 and 2019, and in the spring and summer of 2020 and 2021. The concentrations of \(\sum\)PCBs in these seasons ranged from \(\leq\)0.11 to 0.47 ng/L. The average concentration and statistically significant range of detected \(\sum\)PCB concentrations in Baikal water were 0.30 and 0.26-0.34 ng/L, respectively. Notably, the indicated level of the detected \(\sum\)PCB concentrations was comparable with the concentrations of PCBs in Lake Baikal identified previously in 1992 and 1993 (0.08-1.9 ng/L) and estimated as background [49; 51].  
The global atmospheric transport is the dominant source of PCBs in Baikal water [11; 49; 50; 51]. An increase in the concentrations of \(\sum\)PCBs in May 2015 and September 2019 was likely due to the influx of PCBs from additional sources, which a change in the ratio of indicator congeners indicated. In spring 2015, the proportion of indicator congeners with a high degree of chlorination (138, 153, and 180) increased in water samples up to 30-37%. In autumn 2019, the proportion of pentachlorinated congeners 101 and 118 increased, and the proportion of 'light' tri- and tetrachlorinated congeners 28 and 52 decreased compared to the monitoring data during spring 2015. During the minimum concentrations of PCBs in the water of the pelagic zone of Lake Baikal, tri-, tetra-, and pentachlorinated homologues (28, 52, 101, and 118) predominated in the composition of indicator congeners, whose total concentrations reached 80-90% in the fraction of indicator PCBs.  
Seasonal changes in the PCB concentrations in the upper water layer (5 m) of the pelagic zone in May 2015 and September 2019 were not tracked throughout the water column to depths of 800-1600 m (depending on the depth of the lake's basin). In May 2015, a high level of \(\sum\)PCBs in the upper water layer (5 m) of the southern basin of the lake, up to 3.1 ng/L, was recorded at some stations. However, in deep water layers of the water column (50-1200 m), the \(\sum\)PCB concentrations remained in a narrow range from 0.20 to  
Figure 2: Average concentrations of \(\sum\)PCBs in the pelagic zone \(\sum\) and deep layers of Lake Baikal determined during the monitoring of PCBs in Baikal water.  
0.51 ng/L. An increase in the \(\Sigma_{7}\)PCB concentrations in the upper water layer at stations 19 and 21 to 1.6-2.2 ng/L in autumn 2019 was also not accompanied by an increase in the \(\Sigma_{7}\)PCB concentrations at depths of 50 to 800 m, which remained at a minimum level from 0.31 to 0.69 ng/L.