Lacustrine, wastewater, interstitial and fluvial water quality in the Southern Lake Baikal region

The quality of surface water is an important and urgent environmental issue (Babić et al. 2019). Coastal water resources are habitually exposed to indiscriminate anthropogenic pollution (Adeniji et al. 2019). Due to their negative consequences to the public health, recreational waters require continuous monitoring for disease-causing organisms. As the human population and associated coastal development continue to grow, monitoring water quality for sewage pollution is essential (Abaya et al. 2018). With such an intense release of human waste into water bodies, it is not surprising that global models of wastewater pollution predict large-scale contamination of surface waters (Font et al. 2019). Whereas the idea of a threat of a sanitary crisis to human population has been well established, until recently, the impact of various types of pollution on ecosystems themselves has not been sufficiently addressed by ecologists (Wear et al. 2021).

The genus Enterococcus is in the intertidal interstitial water in concentrations that could affect water quality and public health (Sanchez Dominguez et al. 2015). It is, therefore, recommended that interstitial water monitoring should be included as a measure of predictive microbiological safety of recreational water on sandy beaches. The safety of recreational waters is an important and timely issue specially when dealing with public health and sustainable water management, particularly in a context of climate changes. The number of hazardous microorganisms and forms present in recreational waters is large and the regulatory agencies approach is guided by fecal contamination events. The limitation of traditional FIB, the regulatory approach and the potential effects of climate change on pathogenic organisms and human infectious diseases need an improvement of the indicator systems (Rodrigues & Cunha 2017). In addition to classical filtration and cultivation methods to detect FIB, marker genes have also been used to determine the source of fecal contamination in water bodies (Brooks et al. 2020; Sangkaew et al. 2021).

Pollution of surface waters with nutrients is a widespread problem, calling for regional assessments of water quality conditions (Clune et al. 2020). The global character of this problem is underlined by recent reports on quality of water in many areas around the world (Kucuksezgin et al. 2019; Shi et al. 2020; Dantas et al. 2021; Wang et al. 2021). For instance, a recent analysis of the ecological state of the coastal zone of Izmir Bay in Turkey (Kucuksezgin et al. 2019) reports on microbial pollution (enterococci and E. coli) in the bay area and the impact of human waste from the coastal population on the quality of water in the rivers flowing into the bay. Another example is ecological survey of water quality in the water bodies located within an urbanized area in the Velhas River basin in Brazil (Dantas et al. 2021) which shows the presence of nutrients and coliforms in the water samples due to inefficient treatment of wastewater.

The term ‘splash zone’, which is commonly used in the scientific literature on marine coastal zoning, refers to the part of the littoral zone that is subject to wetting by breaking waves (Lincoln et al. 1982). Such a zone, with distinct gradients in abiotic conditions, permanently exists in Lake Baikal and occupies the area from the shoreline to the slope base. The splash zone (as part of the near-shore zone) of most Eurasian lakes has not been extensively studied until recently and remains a white spot for limnologists (Zohary & Gasith 2014; Timoshkin 2018). However, it was demonstrated that this zone is a rapid and reliable environmental indicator capable of reflecting the responses of ecosystems to anthropogenic stress (Timoshkin et al. 2012a).

Several studies reported massive overgrowth of filamentous algae in the southern basin of Lake Baikal (Kravtsova et al. 2014; Timoshkin et al. 2016, 2018). This outgrowth is believed to result from the constant supply of phosphates and nitrates to the ecosystem and may indicate the pollution of the lake. Recent reports also show an elevated amount of nutrients and FIB in the coastal water of the Listvyanka settlement (Malnik et al. 2019a) as well as the presence of sanitary-indicative bacteria in interstitial water. Detailed studies of this problem have been carried out in the southern basin of Lake Baikal (Gorshkova et al. 2020). The outstanding questions are (i) to determine the specific places on the coast of Lake Baikal that are subject to pollution and (ii) to find the solution that will stop pollution of these places on the coast of the southern basin of the lake.

The main aim of this study was to perform a comprehensive evaluation of the quality of the surface, bottom and interstitial water in rivers, lacustrine water and wastewater in the Southern Lake Baikal region. Another aim of the study was to assess ‘the quality of the interstitial waters’ of the lake's splash zone.¹ This zone is prone to fecal and hydrochemical contamination and therefore represents a sensitive and effective indicator of pollution of coastal areas.

Lake Baikal, in the northeastern part of Eurasia, comprises one of the most unique freshwater ecosystems on Earth (Figure 1(a)). Estimated to be over 25 million years old, Lake Baikal is the world's deepest (maximum depth of 1,637 m) and largest freshwater basin, and it contains up to 20% of all freshwater globally (Galaziy 1993).

Settlements in the southern Lake Baikal area with populations of ∼20,000 people were chosen as sampling sites (Figure 1). Settlements that possessed centralized wastewater purification systems (i.e., Sludyanka and Baikalsk) and those lacking such systems (i.e., Bolshoe Goloustnoe and Listvyanka) were included in the survey. In some of these settlements, there has been a significant increase in the recreational load during summer, mainly due to tourism. The Listvyanka settlement is a tourist destination located within Listvennichnyi Bay on the western shore of southern Lake Baikal, and it lies to the east of the Angara River head (Figure 1). The settlement stretches from the Rogatka Cape to the Listvennichnyi Cape over 5 km (Suturin et al. 2016). The coastal zone of the open Listvennichnyi Bay is characterized by rocky beaches. There are a total of six streams and rivers in the valleys of Listvyanka, and among these, the Krestovka River has the highest water levels. The other tributaries include the Bolshaya Cheremshanka, Malaya Cheremshanka, Kamenushka, and Sennaya rivers, and the Bannyi stream. Water from the Bolshaya Cheremshanka, Malaya Cheremshanka, Kamenushka and Krestovka rivers was subjected for sanitary-microbiological (definition of E. coli, fecal enterococci) and chemical analyses (definition of nutrients) (Figure 1(d); Table 1).

To detect pollution (or its absence), water samples collected upstream of the settlements (about 2 km from the mouth) as well as in the mouths of the rivers were analyzed (e.g., the Listvyanka and Bolshoe Goloustnoe settlements, and Sludyanka).

Samples of surface water (n = 22), interstitial water (holes were made in the swash zone about 1 m above the water's edge; n = 9) and bottom water (n = 4) from the coastal zone of the lake, as well as wastewater samples (n = 2), were collected. For sampling, sterile 150 mL Janet syringes were used. Water samples were taken as described previously (Water. Sampling for Microbiological Analysis 2012). Briefly, samples of surface water were taken from a depth of 10–30 cm from the water surface, whereas samples of bottom water were taken from a depth of 30–50 cm from the bottom. For bottom water sampling, the syringes were attached to a long stick. The samples were then poured into sterile glass 250 mL vials, which were placed into a cooler bag with ice packs and analyzed in the chemical and microbiological laboratories of LIN SB RAS (Irkutsk, Russia). A detailed description of the term ‘swash zone’ is given by Timoshkin et al. (2012a). In general, detritus-free areas were chosen on the shores for the purpose of collecting samples. Interstitial water samples for microbiological and hydrochemical analyses were collected from the same hole (hole depth varied from 0.15 to 0.7 m) immediately after the holes were made by sterile stainless-steel shovel. This is mainly lake water that has passed through the soil, to a lesser extent this is river water flowing through the soil, finally, this can also be moisture accumulated in the ground after rain, which was not able to penetrate deeper to the groundwater level. The samples of the waste water upon treatment were obtained from the outlet pipe of the water treatment facility in Sludyanka town. Sterile gloves and sterile bottles were used for sampling.

All microbiological assays were performed in two replicates. Chemical analyses were performed once, but in every location, the standard analytical error was taken into account. All sampling and analyses were performed in June of 2016 (from 10th to 14th) and in August of 2016 (23th).

Detection and counting of the study groups of bacteria (E. coli and fecal enterococci) and interpretation of the results were conducted according to the 2012 standards published by the United States Environmental Protection Agency (Recreational Water Quality Criteria 2012). The following sample volumes were used for filtration: 50–100 mL for surface samples, 50 mL for bottom samples and 5–20 mL for both wastewater and interstitial samples. Filtration was performed by using 0.45 μm nitrocellulose filters.

E. coli were identified following filtration and sample incubation at 44 °С for 24 h using the selective agar HiCrome (Himedia production, No. M1571). Fecal enterococci were determined by membrane filtration and sample concentration using Slanetz and Bartley Medium (Himedia production, No. M612) incubated at 37 °С for 24 h, as well as Bile Esculine Azide Agar (Himedia production, No. М493) incubated at 44 °С for 2 h (Murray 2003). Colonies that acquired a black color were counted as fecal enterococci (Enterococcus faecius and E. fecalis).

For chemical analyses, the water samples were filtered through acetate cellulose membrane filters with a pore diameter of 0.45 μm. Cations were determined by atomic absorption spectroscopy (calcium [Ca²⁺] and magnesium [Mg²⁺]) and the flame emission method (sodium [Na⁺] and potassium [K⁺]) on an Atomic Absorption Spectrophotometer 30 (AAS-30; Carl Zeiss Jena, Germany) with 5–7% error (Fomin 2000). Anion concentrations (bicarbonate [HCO₃⁻], chloride [Cl⁻], sulfate [SO₄²⁻] and nitrate [NO₃⁻]) were measured by high-performance liquid chromatography on a chromatograph Milichrom A-02 (EcoNova, Russia) with 7–10% error (Khodzher et al. 2016). The sum of ions (∑i) was calculated as HCO₃⁻ + SO₄²⁻ + Cl⁻ + Ca²⁺ + Mg²⁺ + Na⁺ + K⁺. Dissolved oxygen (DO) concentrations were measured on site (in situ) by the Winkler test with 3% error. Nutrient analyses (phosphate-P, ammonium-N and nitrite-N) were performed with a spectrophotometer KFK-3 (Zagorsky Optical-Mechanical Plant, Russia) with 10–20% error. Mineral phosphorus concentrations (soluble reactive phosphate) were estimated with ammonium molybdate, ammonium nitrogen concentrations were measured with indofenol, and nitrite concentrations were estimated with the Griess reagent (Boeva 2009).

The temperature and conductivity at the sampling sites were measured with the use of a portable device (Horiba, Japan) and the pH was measured with the use of pH meter (Hanna, Germany).

Confidence intervals (95%) for the average values in microbiological and chemical parameters were estimated using the bootstrap method (1,000 replicas) (Davison & Hinkley 1997). The results of bootstrap statistics were used to determine the significance of differences between the average values of parameters in sampling points of different zones (ecotopes). The calculations were performed using the «boot» (Canty & Ripley 2017) and «sfsmisc» (Maechler 2017) packages for the R programming language.

The principal component analysis (PCA) method was used to determine the similarities and differences between the samples based on the summation of microbiological and chemical parameters. Before PCA analysis all data were transformed by logarithmic scaling (natural logarithm) to eliminate the physical dimensions. PCA results were visualized using the «gplot2» package (Wickham 2009).

Pairwise correlations between all microbiological and chemical parameters were estimated with Spearman's r correlation coefficient. P-values for the correlation coefficients were calculated using Spearman's «W» statistics and corrected for the false discovery rate in multiple comparisons using the Benjamini–Hochberg equation.

At the mouths of the rivers flowing into Lake Baikal near Listvyanka, such as Bolshaya Cheremshanka, Malaya Cheremshanka and Kamenushka, the counts of E. coli (Figure 2(a)) were 12, 35 and 24 times higher than the US EPA recommended standard (235 colony forming units (CFUs) per 100 mL), respectively (Recreational Water Quality Criteria 2012). Similarly, enterococci counts (Figure 2(a)) were 16, 64 and 47 times higher than the standard value (61 CFU/100 mL) in the Bolshaya Cheremshanka, Malaya Cheremshanka and Kamenushka rivers, respectively (Recreational Water Quality Criteria 2012; Figure 2(a)). In the wastewater discharged into the Pokhabikha River, the E. coli and enterococci counts exceeded the standard values by 26 and 31 times, respectively (Figure 2(a)).

In the other rivers flowing into the southern part of the lake (i.e., the Bolshaya Goloustnaya, Solzan and Bolshaya Osinovka), the indicator microorganism concentrations were within the sanitary standards (Figure 1(b) and 1(e); Table 2). At the sampling site in Lake Baikal near the mouth of the Malaya Cheremshanka River, the number of enterococci exceeded the US EPA recommended standard (Recreational Water Quality Criteria 2012) by 12 times, and the number of E. coli exceeded the standard by 9 times (Figure 2(a)).

Analysis of interstitial water revealed abundant FIB in the samples collected in the holes near the mouths of the Krestovka River (E. coli = 3,800 CFU/100 mL; enterococci = 25 CFU/100 mL) and the Bolshaya Cheremshanka River (E. coli = 2,120 CFU/100 mL; enterococci = 800 CFU/100 mL) (Figure 2(a)). We also found high amounts of FIB near the Kultuk settlement, at the mouth of the Solzan River, and near the Bolshoe Goloustnoe settlement (Table 2). Interestingly, the obtained data are consistent with the results of the water quality analysis that was carried out on the southern coast of Lake Michigan (Nevers et al. 2020). The authors showed that the concentration of E. coli in interstitial water samples was significantly higher when compared to that of sediment and overlying water samples. The similarity of these data might indicate a common pattern in how these territories are affected by anthropogenic activities.

Analysis of the near-bottom water revealed that FIB concentration near the Kultuk settlement, Bolshoe Goloustnoe settlement and Baikalsk town met the US EPA standards (Recreational Water Quality Criteria 2012; Table 2). Analysis of the surface water revealed that FIB concentrations near the Kultuk settlement exceeded both national and international standards (Inspection 2000; Recreational Water Quality Criteria 2012). The quality of surface water near the Solzan, Baikalsk and Bolshoe Goloustnoye settlements complied with existing standards (Inspection 2000; Recreational Water Quality Criteria 2012; Table 2).

In addition, the sample from an effluent pond in Baikalsk also contained FIB (E. coli = 222 CFU/100 mL; enterococci = 72 CFU/100 mL). The concentration of enterococci in the water sample obtained in the river part upstream of the settlement differed significantly from those obtained in the estuaries and interstitial water in the lower parts of the rivers (P = 0.005 and 0.007, respectively; Table 3). Even greater difference (P = 0.002 and 0.003, respectively) was observed for the concentration of E. coli in the parts of the rivers upstream and downstream of the settlements. At the same time, we observed no significant difference between concentrations of E. coli in the sample of water obtained in the lake and those obtained in the river part upstream of the settlement, downstream of the settlement and interstitial water (P = 0.07, 0.091 and 0.17, respectively). Likewise, no significant difference was observed in the concentration of enterococci for the aforementioned sampling sites (P = 0.08, 0.22 and 0.195, respectively).

Chemical analyses of the samples from the four rivers connected to the Listvyanka settlement area revealed that the water samples collected upstream of the settlement had low concentrations of salts (∑_i − 50–150 mg/L) and nutrients. The quantity and composition of nutrients carried by rivers play an important role in maintaining the ecosystem of downstream rivers and marginal seas (Wang 2020). Water mineralization (sum of major ions) in the mouths of the Krestovka and Kamenushka rivers were not different from the upstream sites (Table 3). By the sum of ions, the maximum differences were found between samples taken from (i) the river upstream of the settlement and interstitial waters (P = 0.001), (ii) the estuarine river and interstitial waters (P = 0.003) and (iii) lake and interstitial waters (P = 0). The largest sum of ions was observed in interstitial water in comparison with lake and river waters (Table 3). The chlorine concentration in samples taken above the settlement was significantly lower than in samples taken at river mouths, interstitial and lake water (Table 3). In our opinion, this is due to the influence of anthropogenic activity. K⁺ ions concentration in the sample of water taken upstream of the settlement was significantly lower than those taken in lake and interstitial waters (Table 3). In our opinion, this is also associated with the influence of anthropogenic activities.

It is generally known that phosphate-P and nitrate-N, even at low, but constant concentrations, can stimulate the growth of aquatic vegetation (e.g., genus Spirogyra) which is atypical for this habitat (Timoshkin et al. 2016). High concentrations of chloride ion and ammonium-N may indicate their anthropogenic input, since chlorine and ammonium are contained in human waste products, such as urine (Alemayehu et al. 2020). Therefore, the determination of the concentration of these ions is very important.

The concentrations of phosphate-P and ammonium-N were elevated in the river mouths compared with areas upstream of the settlements, whereas the concentrations of nitrate-N and nitrite-N did not significantly differ (Table 3). Phosphate-P concentrations in water samples obtained upstream of the settlement and lake were significantly lower than those in the sample of interstitial water as well as samples taken at the estuary (P = 0.05 and 0.009, respectively). The concentration of ammonium-N in water samples taken upstream of the settlement was significantly lower than in those taken at the river mouths and interstitial samples (P = 0.015 and 0.028, respectively). Interestingly, similar results were previously obtained for four rivers in Listvyanka settlement (Onischuk et al. 2019). The ammonium-N levels in the parts of the rivers located upstream of the residential buildings differed from those in the downstream parts of the rivers downstream. The authors linked this observation with the flow of municipal washing water into the rivers.

For all the samples tested, the highest nitrate-N concentration was found in interstitial water. The concentration of nitrite-N in samples taken upstream of the settlement was significantly lower than in that of interstitial water (P = 0.019). Concentrations of nutrients in the rivers that flow through other settlements in the southern part of Lake Baikal were within standard values (Table 2). Importantly, our data are in agreement with the results of the recent report on ecological status of the tributaries and shores of Lake Baikal (Gagarinova et al. 2021). The authors show that the maximal values of phosphate-P, nitrate-N, ammonium-N and nitrite-N in the Goloustnaya River for the study period from 2015 to 2019 were 0.009, 0.200, 0.250 and 0.084, respectively. We also found high chloride concentrations (22–44 mg/L) in the samples of interstitial water taken from the holes located near rivers. These values considerably exceeded those obtained in the rivers of the Listvyanka settlement (<12.6 mg/L) or the coastal waters of the lake in this area (<0.6 mg/L).

In the Pokhabikha River, water in the area that is downstream of the point where the effluent from Sludyanka is discharged upon treatment is of poor quality (Figures 2 and 3). High concentrations of ammonium-N (up to 0.43 mg/L) and nitrate-N (up to 0.82 mg/L) levels were detected in the area of the river upstream of the river mouth. This may be due to (i) dense development near the river in the area upstream of the discharge site, as well as (ii) excessive grazing in that area. The concentration of ammonium-N levels in interstitial water near the mouth of the Pokhabikha River was significantly higher than those in the samples of lake water as well as samples taken upstream of the sewage discharge site (Table 3). The shore in this area is mostly sandy, but it also contains organic material deposited by the river. The concentration of DO in the interstitial water sample was below 1 mg/L. A plausible explanation for the high ammonium-N concentration in the interstitial water is its accumulation in the coastal soil where it may remain unoxidized for a long time, due to the high density of the sandy soil (Wang & Alva 2000; Sieczka & Koda 2016).

A high concentration of ammonium-N (0.31 mg/L) and low amount of DO (3 mg/L) were also detected in the interstitial water sample obtained from the sandy beach next to the Kultuk settlement which might indicate the presence of the buried vegetation in the coastal zone. The ammonium which is formed during the decomposition of vegetation can persist for a long time under low oxygen conditions (Garcia-Robledo et al. 2008; Tomberg et al. 2012). High concentrations of nitrate-N and phosphate-P (up to 5 and 1.13 mg/L, respectively) were observed in (i) an effluent pond in Baikalsk, which is likely associated with the oxidation of ammonium to nitrate and with an excess of phosphate in municipal wastewater and (ii) the interstitial water sample obtained on the lakeshore in the area that is influenced by pond drainage (Table 2), which may be associated with seepage of wastewater through interstitial and groundwater.

Low concentration of nutrients was detected in the bottom water samples. Concentrations of nitrite-N did not exceed 0.002 mg/L, those of nitrate-N were below 0.03 mg/L and those of ammonium-N and phosphate-P were below 0.01 mg/L (Table 2). Such low values can be explained by (i) dilution of nutrients in Baikal water and (ii) consumption of the nutrients by aquatic vegetation (Afanasiev & Verbolov 1977; Kreiling et al. 2011; Kulikova et al. 2018, 2021).

Despite the inflow of contamination with the waters of the tributaries and interstitial waters, we did not detect high concentrations of chemical elements in the samples of surface water. The concentration of ammonium-N, phosphate-P and nitrate-N in interstitial water and river mouth was significantly higher than in lake water (Table 3), which can be associated with geochemical barriers in the form of soils and alluvial deposits. In addition, we measured several abiotic parameters (pH, T) of the water samples (Supplementary Material, Table S4).

Timoshkin et al. (2018) reported on the poor water quality (i.e., fecal contamination and high nutrient concentrations) of the coastal zone of the southern Baikal region. Several studies have previously addressed the presence of nutrients and FIB in the stream waters entering Lake Baikal, as well as in the splash zone of the lake (Kravtsova et al. 2014; Suturin et al. 2016; Malnik et al. 2019a; Onischuk et al. 2019). However, there is a lack of current comprehensive data on the sanitary bacteriological and hydrochemical conditions of the tributaries and coastal zone of the southern part of Lake Baikal. The results of this study were compared with published bacteriological and hydrochemical data from Lake Baikal to detail the changes in lake water quality between 2011 and 2016 (Kravtsova et al. 2014). In a number of countries, similar studies have also been carried out (Jiang et al. 2016; Jin et al. 2017; Vadde et al. 2018). Long-term changes (2003–2014) of the water temperature, dissolved oxygen (DO), pH, total nitrogen (TN), ammonium-N, total phosphorous (TP) and chemical oxygen demand (COD_Mn, where Mn refers to the chemiluminescence method for COD analysis) were examined at eight sampling stations along the Liujiang River (Jiang et al. 2016). Water quality parameters showed considerable spatial and temporal variability. Pollution levels were generally higher in the lower Liujiang than in the upper and middle parts of the river because of the impacts of urban sewage.

We can associate the changes in the coastal water quality of Lake Baikal with the increased concentrations of FIB and nutrients observed in the rivers flowing into the lake. Two model areas of the southern Baikal region, namely the Listvyanka settlement (west-facing coast) and the town of Sludyanka (east-facing coast), were chosen to test this hypothesis.

Our data show that the samples with elevated concentrations of E. coli and enterococci also contain elevated concentrations of nutrients such as phosphate-P, ammonium-N and nitrite-N (Figure 2). We can, therefore, assume that pollution by non-oxidized organic compounds takes place in these sampling sites. Nitrate-N commonly appears when organics are completely oxidized (Xia et al. 2018). For example, ammonium-N at sampling point 37 was oxidized to nitrate-N in a pond aerator (Figure 3), which indicates that the organic oxidation process has passed. Most of the points on the resulting graph are located closely to each other, which indicate the proximity of the values of certain concentrations. Samples with a high concentration of the determined parameters are located at points to the right of the central axis. A group of points with a high sum of ions, chlorine, nitrate-N is located in the lower right rectangular region, whereas points with high concentrations of FIB, ammonium-N and phosphate-P can be seen in the upper right rectangular region. The potassium concentration was high in all the samples located to the right of the central axis. The graph shows that samples 5, 19, 22, 23 and 24 from river estuaries did not differ much from lake water samples. And some of the estuarine river samples 2, 12 and 8 were close to the wastewater sample in Sludyanka No. 18. Samples 3, 9, 14, 25 and 26 in their characteristics are similar to that of the sample from the pond aerator No. 37 (originally untreated municipal wastewater). In general, in the sampling points with interstitial water, we can observe a large spread in their position on the graph. We also performed a correlation analysis of the studied indicators and found that the concentration of enterococci and Escherichia coli correlates well with each other (Table 4). The concentration of enterococci correlates well with those of phosphate-P, nitrate-N, nitrite-N, ammonium-N and chlorine, whereas the concentration of E. coli correlates well with those of ammonium-N, nitrite-N, potassium and chlorine. Interestingly, a similar correlation of FIB with ammonium-N and nitrite-N concentrations was described in the recent water quality analysis of the samples obtained from four rivers and a lake in the Wujiang District in China (Sekar et al. 2021). We speculate as well that the strong correlation between concentrations of FIB and chlorides is due to anthropogenic influence on sampling sites, since chlorides are contained in biological fluids excreted from the body (e.g., urine). The concentration of phosphate-P correlates well with those of ammonium-N, nitrate-N, nitrite-N and chlorine. Ammonium-N concentration correlates well with those of nitrate-N and nitrite-N and chlorine. Furthermore, nitrate-N concentration shows correlation with that of nitrite-N, potassium and chlorine. Finally, the concentration of potassium correlates well with that of chlorine (Table 4).

Many households in the Listvyanka settlement lack connections to centralized sewage facilities. Most of the local population use on-plot latrines in their yards, while hotels have cesspits that often leak. The settlement is located in the water protection zone, in which wastewater discharge is supposed to meet specific requirements stipulated by Sanitary Rules and Regulations 2.1.5.980-00 (Inspection 2000). According to these requirements, urban wastewater must not be discharged within settlements. A small portion of the urban wastewater is drawn aside from the Listvyanka settlement to the Angara riverhead, and the rest of the sewage is passively filtered through the soil, where it then appears in the coastal zone of Lake Baikal or flows there via tributary waters. The paradox is that urban wastewater, which is a minor component of all local wastewater, is drawn aside from the settlement, and there are no other treatment facilities. Therefore, no one is penalized for improperly discharging wastewater because there is no pipe and sewage is not discharged directly into the lake. Domestic wastewater penetrates into the soil and into the groundwater. It then gets transported into the tributaries or coastal zone of Lake Baikal. A peak in the wastewater load occurs during winter and early spring, when small rivers are only supplied by groundwater.

Investigations on Lake Baikal tributaries in the Listvyanka settlement revealed the following water quality patterns: (i) upper parts of the rivers (outside of the settlements) are clean and (ii) water quality in the lower parts of the rivers (including river mouths) is poor (Figure 2). Water quality of other analyzed tributaries (i.e., Bolshaya Osinovka, Solzan and the Goloustnaya rivers) met both the national and the US EPA standards (Table 2).

High concentrations of FIB and nutrients were detected in the interstitial waters of the splash zone near the river mouths in Listvyanka settlement (Figure 2(a)–2(c)) and in Sludyanka town (Figure 2(a)–2(c)). FIB were also detected in the surface water of Lake Baikal near the river mouths (Figure 2(a)). Furthermore, microbiological analyses revealed persistent fecal pollution of the interstitial and surface waters of Lake Baikal around coastal settlements (i.e., Listvyanka settlement and Sludyanka). Neither national nor international water quality standards were met when the samples of the ‘purified’ wastewater from Sludyanka were analyzed (Figure 2). Thus, the sewage and wastewater treatment facilities in Sludyanka do not seem to adequately filter wastewater contaminants, which results in the discharge of wastewater enriched in nutrients and FIB into the coastal zone of Lake Baikal. A unified concept of the elaboration of a water disposal system has been developed stage-by-stage for a large area in the central ecological zone of Baikal region with the use of a program–goal-oriented method (Pupyrev et al. 2020). The authors propose to construct (i) wastewater treatment facilities in the Kultuk settlement; (ii) a pipeline system for transport of wastewater from nearby settlements around Lake Baikal to the treatment facilities and (iii) a pipeline system to transport the treated wastewater to Bystraya river which flows directly into Irkut river and thereby bypasses Lake Baikal. Undoubtedly, this idea is quite interesting and if it is implemented into reality, the ecological situation in the region of the southern basin of the lake might improve.

The interstitial water in Bolshoe Goloustniy is contaminated by FIB, which may be related to the building out of the coastal territory of the lake with hotels and cottages without a wastewater treatment system, as well as to the grazing of large livestock in the coastal zone of the lake. However, the quality of surface and bottom water in the settlement met US EPA standards, which may indicate a good buffering capacity of this type of soil in the splash zone.

The quality of all three types of water in Baikalsk and Solzan settlements was good (with the exception of interstitial water in Solzan), which may be related to the small number of tourists in the area, as well as to the cessation of the Baikalsk pulp and paper mill. All three types of water are polluted in Kultuk, where significant anthropogenic loading occurs.

Previous studies of the Listvyanka area have shown that the concentration of E. coli in the water of the inflow tributaries (in their lower reaches) and interstitial waters reached up to 8,300 CFU/100 mL and 1,280 CFU/100 mL, respectively (Kravtsova et al. 2014; Suturin et al. 2016). In the present study, these areas had E. coli counts of up to 8,200 CFU/100 mL in the water of inflowing tributaries and 3,770 CFU/100 mL in interstitial waters (Figure 2(a)). The difference in bacterial counts between these studies may have been due to the different sampling times as the samples from the earlier study (Kravtsova et al. 2014) were collected in August of 2011, whereas our samples were obtained in June of 2016. We speculate that the ecological situation in this region has deteriorated over this period of time.

A similar study was conducted in the lacustrine environment of Lake Wigry, one of the largest and deepest lakes in Poland (Aleksander-Kwaterczak & Zdechlik 2016). Elevated amounts of the trace metals and nutrients were detected in the sample of the sediment and interstitial water obtained in the Czarna Han'cza River estuary and are believed to be the result from increasing industrial wastewater, domestic sewage discharge and runoff from agricultural soil. The presence of nutrients in the water of inflow tributaries (in their lower reaches) and in the interstitial waters in the Listvyanka settlement area has been previously reported (Kravtsova et al. 2014). Another study has shown that when passing through the settlements, the rivers are more or less enriched with all chemical elements. The greatest enrichment was observed for chlorine – an average of 7 times (Suturin et al. 2016). Our studies showed that chlorine concentrations in B. and M. Cheremshanka differed by 10 and 50 times, respectively. This may indicate that anthropogenic chlorine is being introduced into the rivers. These data also indicate that the river and lake waters around Listvyanka settlement are subjected to continuous fecal and hydrochemical pollution. Taken together, these data reveal that the concentrations of FIB and chemical compounds in the waters of the lower reaches of many tributaries, interstitial waters and anthropogenic wastewater are very high in comparison with the normative standards of Russia and the US EPA (Inspection 2000; Recreational Water Quality Criteria 2012).

Counts of FIB in interstitial waters of the splash zone in the southern Baikal region were performed in 2010–2011 (Timoshkin et al. 2012b) and again in 2016 (Suturin et al. 2016). The concentrations of thermotolerant coliform bacteria ranged from 0 to 4,950 CFU/100 mL in these studies. These data are not consistent with our observations on the pollution of interstitial waters of Lake Baikal at points with high anthropogenic loads. The concentrations of E. coli in our research ranged from 0 to 16,300 CFU/100 mL. First, this may be related to the use of different sampling points, and second, to the increase in anthropogenic loading on coastal areas over the last 6 years (Suturin et al. 2016; Malnik et al. 2019a). Sampling procedures used in these studies were similar. At the Pokhabikha River, the wastewater was neither disinfected (microbiological data) nor devoid of macronutrients, such as nitrate-N, nitrite-N, ammonium-N and phosphate-P, after purification. This situation is very problematic in that the river water contains E. coli in the areas where it flows into Lake Baikal, and in the lake, such contamination can be found at distances of 1.5–2 km from the Pokhabikha River mouth (Figure 2(a)). Therefore, recreation in this area has become unsafe, and the risk of intestinal infections to those coming into contact with the contaminated water, especially children, is very high. Additionally, the increased concentrations of macronutrients, especially nitrogen and phosphorus, discharged into the lake can trigger an increase in the algal biomass, including Spirogyra sp., which is atypical for Lake Baikal and can result in the replacement of endemic algae species (Kravtsova et al. 2014; Timoshkin et al. 2015a, 2016; Kobanova et al. 2016). These new algae can accumulate at the bottom of the lake, and storms may then transport the algae to the shoreline, where it rots, as has happened in northern Lake Baikal (Timoshkin et al. 2015b).

The situation in Listvyanka is similar to that found in the Pokhabikha River. High phosphorus and nitrogen concentrations in the rivers of the settlement can be associated with the increase in tourists during summer and consequential increase in the wastewater sewage discharged from hotels and other activities such as washing clothes and dishes and bathing (Malnik et al. 2019b). The enormous exceedance of the standards in rivers for FIB increases the risk of intestinal infections in people present on the shore of Lake Baikal. This problem is compounded because the popular beaches for locals and tourists occur near the mouths of the contaminated rivers, especially near the mouth of the Bolshaya Cheremshanka River. The increase in the number of tourists and residents in the southern Baikal region has led to an increase of human waste products, which are mostly disposed through on-plot latrines. Such human waste likely contaminates ground water and penetrates into Lake Baikal as interstitial water. To demonstrate the possibility of human waste contamination through interstitial water, we measured FIB not only in the rivers but also in the interstitial waters of the lake. The analyses of the interstitial water samples obtained on the beaches of the Listvyanka settlement near to the river mouths revealed the accumulation of chemical compounds (phosphate-P, ammonium-N, nitrite-N and nitrate-N) at high concentrations. Nitrification processes in such conditions are very unlikely to occur, which implies that water enriched with ammonium-N flows directly into the lake.

Interstitial waters near the mouths of the Krestovka and Bolshaya Cheremshanka rivers are also contaminated with FIB (Figure 2(a)) derived from the toilets of hotels and private households. However, during the tourist season, the natural purifying system cannot cope with the huge anthropogenic loads. As a result, the concentrations of FIB fail to meet the US EPA standards (Recreational Water Quality Criteria 2012; Figure 2(a)).

There are many articles about eutrophication (Longley et al. 2019; Vincone-Leite & Casenave 2019; Wang et al. 2019) and lake pollution (El-Rayis et al. 2019; Egessa et al. 2020; Akiner & Akiner 2021). Lake Baikal is such a huge lacustrine system that human settlements seem to represent an insignificant influence, but as our study has shown these settlements do have significant effects on the littoral zone of the lake.

We propose to take the following measures to reduce and prevent further pollution of the coastal and splash zones of Lake Baikal: (i) construction of the effective water treatment facilities, where appropriate (e.g., Kultuk, Sludyanka and Listvyanka settlements); (ii) use and construction of isolated septic tanks in the settlements around Lake Baikal and (iii) use of phosphate-free washing powders and detergents in the area around Lake Baikal.

The study has shown that contaminants in river waters flowing into Lake Baikal, which were derived from the surrounding residential areas, have negatively affected the water quality of the lake. Samples from areas located upstream of the Listvyanka settlement were uncontaminated, while in the river mouths within the boundaries of a built-up area, pollutant levels were very high. Interstitial waters were also found to be contaminated near the lakeshore. The possibility of human waste contamination through the interstitial water pathway has been demonstrated.

The data from the Pokhabikha River (Sludyanka town area) also revealed fecal and nutrient contamination, despite the presence of a centralized sewage system, which suggests that the current system does not purify wastewater well as the river waters contain very high levels of phosphate-P, nitrate-N, ammonium-N and FIB (Supplementary Material, Table S3).

The ecological conditions around southern Lake Baikal should be recognized as critical, and actions should be taken to remediate the influence of untreated wastewater contamination. First, the wastewater treatment situation in Sludyanka must be improved. An efficient wastewater system should also be constructed in Listvyanka as the influx of tourists has increased from year-to-year (Kobanova et al. 2016), and the ecological state of the coastal zone near the settlement is very poor (Kravtsova et al. 2014; Timoshkin et al. 2016). This is, in part, due to the mass proliferation of non-typical for Baikal Spirogyra algae and subsequent degradation of water quality in the lake. Additionally, our findings suggest that the coastal zone of Lake Baikal is under the persistent threat of eutrophication. The results of this study demonstrate that eutrophication should be considered the main driver of the ecological crisis, which is currently taking place in the shallow waters of Lake Baikal.

A comprehensive data analysis, including information on the splash zone (along with the analysis of interstitial water) contributes to a more accurate assessment of the ecological situation around Lake Baikal. In particular, it allows us to determine the quality of coastal or river water in any specific location around Lake Baikal. Thus, the applicability of the splash zone for assessing the state of the ecosystem of the coastal zone of the lake within its southern basin seems both necessary and appropriate. Based on the obtained data, we conclude that the sanitary-microbiological and hydrochemical indicators of the quality of interstitial waters should be included in the standard monitoring procedures for large lakes around the world.

The authors are thankful to Mr AG Lukhnev, Mrs YuM Zvereva and Mrs OV Medvezhonkova for their help in sampling. This work was financially supported by the Japan Society for the Promotion of Science (JSPS) through Grant-in-Aid for Scientific Research (B) (KAKENHI Grant Nos. 15H05112) and the State Assignments no. 0279-2021-0007 ‘Comprehensive studies of the coastal zone of Lake Baikal: long-term dynamics of communities under the influence of various environmental factors and biodiversity; causes and consequences of negative environmental processes’ and no. 0279-2021-0014 ‘Investigation of the role of atmospheric deposition on aquatic and terrestrial ecosystems of Lake Baikal basin, identification of sources of atmospheric pollution'. The authors would like to thank Dr EP Chebykin for editing the drawings.

The authors declare that there is no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.