The results reveal that only minimum Cl- concentration in sediment pore water in the Bratsk reservoir was similar to its average content in the overlying water (Table 2). In pore water samples, minimum concentrations of - НСО3 are lower and those of SO42-, Са2+, Na+ and K+ are higher in relation to the overlying water. Maximum concentrations of major ions in the pore water were found to be higher as compared with the overlying water as follows: НСО3-- in 7 times, K+, Са2+, Mg2+- in 11-26 times, SO42-, Cl-, Na+ - 46-101 times. Therefore, the results indicate that variations in major ion concentrations between the pore and overlying water of the Bratsk reservoir are much greater than those obtained for the Ivan’kovskoe, Vyshnevolotsky reservoirs and Lake Baikal.
In contrast to surface water, pore waters typically have low dynamic characteristics due to slow water exchange. It helped identifying the factors affecting the accumulation of chemical elements in the sediment pore water. Variations in ion concentrations between the overlying and pore water samples, combined with element concentrations in sediment pore water along the depth profiles indicate that, the chemical constituents of pore water in the Bratsk reservoir are affected by a number of factors discussed below.
Overlying water of reservoir
By the studies, held earlier for different water reservoirs, it has been demonstrated, that the interaction with the overlying water affects the pore water chemistry. In the Caspian Sea, Cl- concentrations determined in the pore and overlying waters were found to be similar and be dependent on locations [34]. In the freshwater-saltwater transition zone, exemplified by the Yenisei River - Kara Sea profile, the pore water of the uppermost sediment layers exhibit a regular increase in the Cl- concentration along the river mouth-open sea gradient [35]. Since Lake Baikal, provides the major portion of the surface water to the Angara cascade reservoirs, the comparison of the Bratsk reservoir with the lake seems to be most appropriate. Earlier studies of bottom sediments from the pelagic zone of Lake Baikal, characterized by regular sedimentation rates [32], demonstrated that similar to the lake waters, low-mineralized pore waters of Lake Baikal were dominated by calcium bicarbonate.
In the Bratsk Reservoir, at initial stages of sedimentation, the poorly mineralized overlying water directly affected the pore water chemistry. However, our results show significant variations in chemical compositions of pore and overlying water with regard to major ions after 50 years of reservoir operation (Table 2). With regard to major ion composition, only the pore water, taken from upper part of B-4 sediment core with the mineralization of 169 mg L-1 was similar to the overlying water. This similarity in the ionic composition suggests the intensified exchange between the pore and overlying water.
Since the Bratsk reservoir is a reservoir with high technogenic impact, we analyzed a possible effect of highly mineralized wastewater of Usolie-Sibirskoe industrial zone, entering the reservoir, on the ion concentrations in the pore water. Taken together, the results gained from this study (Table 3, Fig. 2) confirm earlier conclusions [17] stating that the technogenic migration of Cl-, SO42-, Na+, Ca2+, and Mg2+ was mostly associated with their transport along the left bank of the reservoir. The levels of these ions decreased in the overlying water, reaching the mean values at a distance of 5 km downstream from the wastewater discharge system. The increase of the above ion concentrations in the overlying water close to Usolie-Sibirskoe town (Fig. 2) and their further decrease suggest that the anthropogenic factors have no profound effect on the accumulation of the major ions in the sediment pore water from the locations under study. At the same time, we cannot completely exclude the impact of wastewater on the concentration of major ions, primarily Cl- and Na+, in the pore water due to chlor-alkali industrial activities taking place in Usolie-Sibirskoe town in the period from 1970 to 1998.
Sediments of reservoir
The terrigenous material, entering the reservoir due to the destruction of rocks and abrasions of shores, and transported with water flow, plays an important role in the genesis of pore water. As was shown earlier, the pore water chemistry is greatly influenced by the chemical and mineralogical composition of bottom sediments [36].
The most common rocks in the catchment area are carbonates and sulfates, including dolomites, limestones (calcite), gypsum and anhydrites [37]. In addition, terrigenous material leaching during sedimentation leads to pore water saturation with Ca2+, Mg2+, HCO3- and SO42-.
Within the channel part, the highest Ca2+, Mg2+ and SO42- concentrations were recorded in the sediment pore water samples from B-1 and B-3 locations, marked by high sedimentation rates. As follows from the regional geology (Fig.1), the main component of bottom sediments from these locations is terrigenous material coming from the abrasion of shores, composed of easily eroded gypsum-carbonate rocks. Easily soluble halite crystals present in gypsum-bearing deposits saturate the pore water from these sediment cores with chloride and sodium ions (Figs.3, 4). At locations B-1, B-2, B-3, the sediment composition is greatly influenced by the Angara River and its tributaries - the Irkut, Kitoy, Belaya rivers, carrying the minerals, formed from breaking apart of sedimentary (calcite, halite, clay minerals) and igneous (quartz, feldspars, mica) rocks [38]. Though the pore water samples at B-1, B-2 and B-3 locations were produced in a single geographic zone, SO42-, Ca2+ concentrations and the pore water mineralization at B-2 were lower, which might result from a higher proportion of quartz, feldspars and clay minerals (alumosilicates and silicates) at B-2 in relation to B-1 and B-3 sites.
In the studied pore water samples, the major ions showed maxima in the bays of Osa, Unga and Tal'kino (Figs. 3, 4). At locations Z-1 and Z-2, the sediments have a greater share of the material transported from the catchment areas of the Reservoir’s tributaries - the Unga, Zalarinka and Osa rivers. At the confluence of the Bratsk Reservoir, gypsum-bearing rocks of the Upper Lena Formation occurring in the catchment area (Fig. 1) play a dominant role in SO4-Ca type water formation with mineralization up to 1112 mg L-1 in the Unga River, to 1127 mg L-1 in the Zalarinka River and up to 414 mg L-1 in the Osa River.
The mineralization of the pore water at locations B-4, B-5, B-6, B-7, B-8 and Z-4 was much lower in relation to B-1, B-2, B-3, Z-1, Z-2, Z-3 sampling sites. It, first of all, can be explained by lower sedimentation rates and, therefore, better conditions for circulation between low-mineralized overlying and pore water at B-4, B-5, B-6, B-7, B-8 and Z-4. Similar to sampling sites with higher sedimentation rates, the saturation of pore water with the major ions here is mainly due to dissolution of terrigenous material supplied into the sedimentation basin. The increase in major ion concentrations along the sediment depth profile, observed at all locations, suggests a longer interaction between the solid and liquid phases of bottom sediments.
In samples, taken from both bottom sediments and subaqueous soils, a change in pore water mineralization is symbate. The pore water samples from soils and bottom sediments at Z-1, Z-2, Z-3 locations had higher levels of major ions (Figs. 3, 4) in relation to Z-4, B-7 and B-8, therefore indicating that bottom sediments generally inherited the composition of primary material of soil-forming rocks. In bays, this tendency was even more evident, as the hydrodynamic conditions of a bay favored the deposition of a larger portion of weathering products within its water area. Accumulation of bottom sediments at Z-1 and Z-2 locations was greatly influenced by the sulfate karst formation, in which displacements occurred along the clay interlayer formed at the contact between gypsum-anhydrite rocks and layers of limestones and gypsum-bearing dolomites [39]. The karst processes are particularly intense in the Osa bay [40]. In the Bratsk reservoir, the intense karst-landslide deformations under water level changes is one of main factors triggering the accumulation of the terrigenous sulfate formation in bottom sediments.
The leaching processes are studied using scatter plots, showing the enrichment of water with chemical elements due to the water-rock interaction [41]. The dominant reaction, occurring in the pore water- bottom sediment system, is demonstrated by a scatter plot between (Ca2+ + Mg2+) vs (SO42- + HCO3-) (Fig. 5 a) showing the dissolution of calcite, dolomite, anhydrite and gypsum. Most pore water samples occur along the 1:1 equiline (Fig. 5a). Moreover, if ion exchange is the dominant process, the data points of the plot tend to shift to the right due to the excess of HCO3- + SO42- over Ca2+ + Mg2+. However, if the dominant process is the reverse ion exchange, the points are shifted to the left [42].
Figure 5b shows that the dissolution of carbonate rocks was not the major process that influenced the pore water chemistry within the study area. Similar to groundwater [43], in the Ca2+ vs SO42- scatter plot (Fig.5c), the majority of points followed close to 1:1 equiline (Ca2+ = SO42-), indicating that dissolutions of gypsum and anhydrite were involved into geochemical processes. The points, corresponding to pore water samples, which occur above 1:1 equiline, show that dissolutions of these minerals should occur, while excess of calcium indicates an additional geochemical process [44]. In the Bratsk reservoir, this additional process included, first of all, dissolution of carbonates followed by the cation exchange, when the alkali and alkaline earth metals Na+, K+ and Mg2+ passed into the absorbing complex of the sediment and displaced Ca2+ from this complex, leading to the increase of calcium levels in pore water.
Groundwater
As shown above, the increase in major ion concentrations, primarily SO42- and Ca2+, in the pore water in relation to overlying water can be attributed to leaching processes. However, the increase in Cl- and Na+ concentrations in the pore water samples at B-1, B-3 and Z-2 locations, could not only be explained by the occurrence of halite in the terrigenous material as NaCl was readily dissolved in the Bratsk reservoir due to a huge amount of overlying water. The water migration coefficients of Cl- and Na+ in well-oxidized water of the Bratsk reservoir is very high [45]; therefore, these ions should be transported over long distances. Groundwater discharge, which is regarded as a synonym of pore water in water-saturated sediments [46, 47] could be regarded as an additional source of major ions in the pore water. Moreover, submarine groundwater discharge is an important source of nutrients to coastal areas [46, 48].
Compared to the Angara River and lateral inflows, groundwater discharge in the Bratsk reservoir is insignificant (not over 0.5 l / (s·km2) [49]. The studies of hydrochemical characteristics in the Ust-Ilimsk and Boguchansk reservoirs, which are also parts of the Angara cascade, revealed that at several locations, the subaqueous groundwater discharge leads to the increased concentrations of major ions in the bottom water in relation to the surface one [50, 51]. A lack of significant variations in the chemistry of surface and bottom waters within the Bratsk Reservoir (Fig.2) shows that the groundwater chemical composition has a minor impact on the overlying water chemistry. In the study area, there are also several tectonic dislocations, including tectonic faults, reaching the surface, and zones of tectonic fracturing [52]. In the view of Krivtsov and Sigee [53], certain combinations of meteorological and hydrological parameters could cause the groundwater percolation to the reservoir’s ecosystem even through a low-permeable layer of clay deposits. Hence, percolation of more saline groundwater may be expected to affect the ionic composition of the sediment pore water. The groundwater impacts will depend on the concentration gradient and the groundwater volume.
Before filling the Bratsk reservoir, fresh, hydrocarbonate groundwater was widespread in the study area. At deeper aquifers, the groundwater composition changed into brackish sulfate [54]. After filling the reservoir, the depth to the fresh- brackish water interface was established at a smaller depth due to changes in conditions maintaining exchange between the overlying and groundwater [25]. Changes in the groundwater regime changed the pressure water discharge: in the variable backwater effect zone, the pressure water was discharged as saline springs [49], while close to the Unga Bay, sodium chloride water with mineralization of 6.2 g / l occurred [54]. Therefore, at some locations, the concentrations of major ions in the pore water may increase due to large amounts of groundwater infiltration. This is primarily true to the southern part of the study area (from Usolie-Sibirskoe town to the Unga Bay) with sites of dome-like groundwater occurrence on the left shore of the reservoir [54]. Sodium chloride water is the most common type in the center of the dome while brackish sulfate-type water occurs mainly in dome’s wings. The groundwater primarily flows upwards thus leading to Cl-SO4-Na-Ca chemical composition of pore water at B-1 and B-3 locations and SO4-Na-Ca water type at Z-2 site (Table 4). Such a phenomenon was found for Lake Baikal as well: the hydraulic groundwater-pore water interaction was discovered in the Selenga shallow water [55]. At the same time, similar to the Bratsk reservoir, the subaqueous groundwater discharge appears not to affect the mineralization of the bottom water.
The concentrations of SO42-, Cl- and Na+ in the pore water decreased toward the top of the sediment (Fig.3), therefore suggesting the groundwater penetration. The bottom sediment layers at B-5, Z-1, Z-2, Z-4 sampling sites, contained the soils demonstrating higher permeability and an ability to accumulate many chemical elements as opposed to bottom sediments. Low dynamic characteristics of pore water and low filtration coefficients of clayey rock formations, dominant in the upper part of the sediment cores, are the main factors, responsible for accumulation of chemical elements brought with groundwater into the lower sediment section.
Early diagenetic processes
The early diagenetic transformations of Lake Baikal sediments were studied based on the distribution of major ions in sediment pore water [56]. It was found, that at sites with relatively calm sedimentation conditions (no faults, and groundwater infiltration), the pore water of the upper (oxidized) sediment layer contained much lower НСО3- concentrations than the bottom water. In the reducing sediments, НСО3- concentration increased due to the decay of the buried organic matter. On the contrary, the SO42-conncentration showed the maximum in the oxidized layer, decreasing along the sediment depth profile by an order of magnitude due to the bacterial sulfate reduction. In the Baikal region, a similar distribution of НСО3- and SO42- was found in organogenic sediments of small lakes [57]. As opposed to lakes with a longer existence period, early diagenetic transformations of sediments from the Angara cascade reservoirs, remains an insufficiently studied geochemical sector. However, the data obtained for the Bratsk reservoir show early diagenetic transformations of organic and mineral substances brought to the bottom of the Bratsk reservoir during sedimentation. In the Bratsk reservoir, one of the early diagenesis indicators is the change in the hydrochemical composition type of the overlying water. The Ca-HCO3 type, which was dominant in the overlying water, occurred in the pore water samples taken only from the first and second layers at B-2 location and upper layer at B-4 site (Table 4). The pore water by SO4 and HCO3-SO4 types, with exchangeable cations, mainly Ca, became more common in the Bratsk reservoir (Table 4).
Negative Eh values in pore water samples of the Bratsk Reservoir (Figs. 3, 4) indicate that the bottom sediments undergo reduced diagenetic transformations whose intensity depends on the amount of organic matter [58]. It should be noted that large amounts of organic materials were brought during the reservoir filling when 166 thousand hectares of agricultural and 135.2 thousand of forest were flooded [23]. Under reducing conditions, the most mobile labile organic matter compounds are transferred to the solution due to microbiological transformations leading to an increase of HCO3- contents in the pore water along the sediment depth profile.
Color stratification along the sediment depth profile is an indicator of sulfate reduction activity [59]. In the Bratsk reservoir, the redox early diagenetic processes are shown by the presence of brown-orange surface sediments, which are then replaced by dark gray sediments, indicating sulfate reduction and the formation of hydrogen sulfide derivatives. It was found that the concentrations of SO42- in pore water samples increased as the sediment depth increases thus suggesting that the sulfate reduction processes were less intensive. Yudovich and Ketris [60] showed that the poorly decomposed organic matter in anaerobic environments does not favor the processes of sulfate reduction and requires a longer presence of organic matter in anaerobic conditions. On the other hand, in such relatively young reservoir like the Bratsk, the dominant processes determining the pore water chemical composition are dissolution of detrital material and subaqueous groundwater discharge.
At initial stages of the Bratsk reservoir’s operation, the subaqueous soils contained fine – grained calcite [22], a mineral formed from diagenetic transformations. The product of the molar concentrations of Ca2+ and SO42- which is over 2.37·10-5 favors the formation of CaSO4 in the solution [45]. Currently, these values in the pore water of Bratsk reservoir range from 3.5·10-5 to 1389.3·10-5, indicating the formation of authigenic minerals.
Table 4. Pore water classification, Bratsk reservoir.
Sampling site
|
Type of water a
|
Sampling site
|
Type of water
|
Sampling site
|
Type of water
|
В-1
|
0-8 cm
|
|
B-4
|
0-10 cm
|
|
Z-1
|
0-13 cm
|
|
8-19 cm
|
|
10-20 cm
|
|
13-26 cm
|
|
19-30 cm
|
|
В-5
|
0-17 cm
|
|
26-46 cm
|
|
B-2
|
0-8cm
|
|
17-34 cm
|
|
Z-2
|
0-16 cm
|
|
8-19 cm
|
|
B-6
|
0-8 cm
|
|
16-32 cm
|
|
19-30 cm
|
|
8-17 cm
|
|
32-42 cm
|
|
30-41 cm
|
|
В-7
|
0-7 cm
|
|
Z-3
|
0-20
cm
|
|
B-3
|
0-8 cm
|
|
7-16 cm
|
|
20-45 cm
|
|
8-21 cm
|
|
В-8
|
0-6 cm
|
|
45-70 cm
|
|
21-42 cm
|
|
6-14 cm
|
|
Z-4
|
0-8cm
|
|
42-64 cm
|
|
14-32 cm
|
|
8-16 cm
|
|
64-86 cm
|
|
16-24 cm
|
|
a – M – mineralization (mgL-1); the number next to the ion mean the ion content (% mEq L-1) of the total number of cations and anions corresponding to 100%. The type of water is determined by ions with the content more than 12%.