Geochemistry of Water and Bottom Sediments in Mountain Rivers of the North-Eastern Caucasus (Russia and Azerbaijan)

Abstract

This study provides a comprehensive assessment of the geoecological status of selected mountain rivers in the North-Eastern Caucasus—specifically, the Sunzha, Sulak, Ulluchay, Karachay, and Atachay—through an analysis of chemical element concentrations, including heavy metals (HMs), in surface water, suspended particulate matter (SPM), and bottom sediments. The elemental composition was determined using inductively coupled plasma mass spectrometry (ICP-MS) on a PlasmaQuant MS Elite instrument (Analytik Jena, Germany), enabling high-precision quantification of 70 chemical elements. Element concentrations in surface water were compared against regulatory limits (e.g., maximum permissible concentrations (MPCs)) defined in international and national guidelines; concentrations in SPM were assessed relative to global average riverine values; and those in bottom sediments were evaluated with reference to average upper continental crust abundances (Clarke values). To trace potential sources of heavy metals entering the riverine systems, enrichment factors (EFs) were calculated for bottom sediments. The results indicate that surface water, suspended particulate matter, and bottom sediments in the investigated rivers exhibit enrichment in numerous chemical elements to levels exceeding their respective reference values (MPCs, global river means, or crustal Clarke values). Significant regional variations in abiotic parameters were observed. Water temperature ranges were 4.6–28 °C (Russian rivers) and 6.9–13.6 °C (Azerbaijan rivers). The pH of Russian rivers was circumneutral to mildly alkaline (7.12–8.83), whereas Azerbaijani rivers were distinctly alkaline, with values reaching 9.88. Reducing conditions in sediments (Eh as low as −206 mV) were prevalent at several stations across both regions. This enrichment reflects an overall unfavorable geoecological status of the studied river systems. Elevated concentrations of several rare earth elements (REEs), observed across multiple sampling locations, suggest a substantial lithogenic contribution linked to the geological structure of the catchments, including the composition of the drained rocks and the presence of ore-bearing formations. Furthermore, localized increases in the concentrations of key heavy metals—such as copper, zinc, cadmium, arsenic, and mercury—point to anthropogenic inputs, most likely associated with mining operations, industrial activities, or other human-induced sources.

1. Introduction

The problem of pollution of clean freshwater is a global issue for all humanity, affecting not only river basins and landscapes but also the health of billions of people, food security, and the economy [1,2,3].

Current estimates from the United Nations [4] indicate that over two billion people lack access to safe drinking water, while 3.4 billion are deprived of adequate sanitation. Consequently, approximately 1.4 million annual deaths are attributed to waterborne infectious diseases, such as cholera, diarrhea, and typhoid. Ouattara et al. [3] further stress that nearly half the global population is affected by freshwater pollution, with about 250 million water-related disease cases per year linked to the ingestion of water contaminated with pathogenic bacteria, viruses, or parasites, resulting in significant mortality. According to Mekonnen and Hoekstra [5], roughly two-thirds of the world’s population experiences severe water scarcity for at least one month per year.

Ensuring the availability and sustainable management of water and sanitation for all is a principal objective of the UN Sustainable Development Goals (SDG 6). Established under the 2030 Agenda in 2015, this goal calls for universal and equitable access to safe drinking water, adequate sanitation, and hygiene, alongside a commitment to reducing pollution [6,7,8]. The severity of water pollution is particularly pronounced in developing nations. Cassardo and Jones [9] note that contamination degrades both surface and groundwater quality, exacerbated in these regions by the direct discharge of approximately 95% of untreated urban wastewater into surface water bodies.

River systems and their catchments, which serve as focal points for human settlement, agriculture, and economic activity [10,11], are subjected to significant anthropogenic pressure compounded by natural processes, leading to elevated pollutant concentrations. Key anthropogenic pollution sources include industrial effluents (containing heavy metals and chemicals) [12], agricultural runoff laden with pesticides and fertilizers [13], and untreated municipal sewage [14]. Notable examples of severely impacted systems include the Ganges River in India [15,16], the Citarum River in Indonesia [17], and the Mekong River Delta in Vietnam [18], where pollutant levels frequently exceed safe thresholds by orders of magnitude. Natural mechanisms also contribute to water quality degradation [19,20,21]; for instance, erosion facilitates pollutant transport, and the weathering of certain lithologies can release heavy metals and other contaminants into aquatic systems. Furthermore, atmospheric circulation patterns can influence the spatial distribution of pollutants [22,23,24]. Therefore, identifying pollution sources and delineating their spatial distribution are research priorities of critical importance.

The river basins of small and medium-sized rivers in the North-Eastern Caucasus remain largely uninvestigated. While fragmentary data exist for some medium-sized rivers [25], small rivers are particularly understudied. Research has predominantly focused on tributaries of major river systems, such as the Terek, whereas independent rivers discharging directly into the Caspian Sea have received scant attention. A further methodological shortcoming in existing studies of North-Eastern Caucasian rivers is the lack of a holistic, basin-wide approach to characterizing the entire catchment area. This knowledge gap is concerning given that these rivers are vital sources of water for Russia, Azerbaijan, and Iran. Consequently, despite their significance, systematic research on the mountain rivers of the North-Eastern Caucasus remains incomplete.

This scarcity of data is primarily attributable to logistical challenges associated with field campaigns in these basins and the subsequent complexities of sample analysis. For the sustainable development of the North-Eastern Caucasus—a region where river systems supply water to millions of people and irrigate vast agricultural lands—these research deficiencies are critically important.

A comparable gap exists in understanding the geochemical features and processes within these river basins. The mountain rivers of the Caspian Sea basin are ecologically and economically vital; they sustain regional biodiversity and landscapes by delivering freshwater and nutrients, while also supporting water supply, irrigation, and hydropower generation [26,27,28]. Prevailing research has predominantly centered on a few major rivers and their catchments (e.g., the Terek and Sulak), often overlooking spatial and temporal heterogeneity, as well as critical interactions between the water column and bottom sediments.

The flow regimes of mountain rivers, including those in the North-Eastern Caucasus and Azerbaijan, are strongly affected by climate change, manifested in global warming, the retreat of glaciers and snow cover in mountainous areas, and declining precipitation [29,30]. At the same time, anthropogenic pressures on river basins are intensifying [31,32], driven by population growth in catchment areas, rising freshwater demand, and the discharge of wastewater from industrial and agricultural activities. These processes collectively result in the contamination of rivers with multi-source pollutants, declining water quality across aquatic systems, and the degradation of small river ecosystems [33].

Trace element composition, including potentially hazardous heavy metals, represents a key indicator of river water quality, as exceedances of maximum allowable concentrations can exert adverse effects on aquatic biota. Variations in river water chemistry are driven by inputs of meltwater and precipitation via slope runoff, as well as by direct discharges of domestic, agricultural, and industrial effluents. In addition to these external inputs, in-channel processes such as riverbed erosion constitute an important internal source of trace elements, particularly in mountain systems. As a result, trace elements, including heavy metals, are typically present in both dissolved and suspended particulate matter (SPM) fractions of river waters.

A key aspect of water quality is trace element composition. Their presence in river systems stems from external sources like meltwater runoff and industrial discharges, as well as internal processes such as riverbed erosion. These elements partition between dissolved and suspended particulate matter (SPM) phases, with sediments acting as a major sink—and potential secondary source—for pollutants [34,35]. Intense hydrodynamics in mountain rivers frequently remobilize sediments, releasing stored contaminants back into the water column [36,37].

Chemical parameters of river water, such as concentrations of dissolved and particulate trace elements, reflect the geoecological state of the system at the time of sampling. In contrast, sediments provide an integrated archive of the chemoecological conditions of the river ecosystem over longer timescales [38,39]. Accordingly, a comprehensive evaluation of the geoecological status of mountain rivers requires simultaneous investigation of all components within the “water–suspended matter–sediment” system.

Furthermore, it should be noted that for the majority of small and medium-sized river basins, the chemical composition of water and bottom sediments is not routinely monitored, primarily due to the high costs associated with field campaigns and subsequent laboratory analyses. The studies that do exist are currently considered insufficient for a comprehensive assessment of all contaminants. Furthermore, the data obtained are often difficult to compare due to the use of diverse analytical equipment. The acquisition of long-term data series is also problematic, as permanent monitoring studies are not conducted on these rivers. Remote sensing methods, including satellite and UAV imagery, are not currently regarded as a direct solution, as the methodologies for deriving water pollution data from these platforms remain underdeveloped. Nevertheless, the necessity of such research is evident, and the determination of the chemical composition of water and bottom sediments is considered an urgent task.

This work is considered pioneering because a new network of sampling stations was established for future monitoring in previously unstudied locations, providing coverage across the investigated river basins. The present study was designed to address this knowledge gap through a comprehensive analysis of the geochemical composition of water and bottom sediments in key mountain rivers.

The aim of this research was to conduct a comparative assessment of the geo-ecological state of abiotic components (water, bottom sediments) in the ecosystems of small and medium-sized rivers in the North-Eastern Caucasus of Russia and Azerbaijan. This involved determining the concentrations and identifying the sources of a range of trace elements, including heavy metals, to inform recommendations for the sustainable management of natural resources in the region under conditions of climate change and increasing anthropogenic pressure.

The paper is structured into five sections. The importance of water pollution research is outlined in the Section 1, along with the rationale for focusing on water, suspended solids, and bottom sediments. A description of the study area and the methodologies employed for field and laboratory work are provided in the Section 2. The results of the study are presented in the Section 3, which are then described and analyzed in the Section 4. The conclusions drawn from the work are summarized in the final, Section 5.

2. Materials and Methods

2.1. Study Area

The study focused on five mountain rivers: the Sunzha, Sulak, and Ulluchay in the Russian Federation, and the Karachay and Atachay in Azerbaijan (Figure 1).

The Sunzha River, a major tributary of the Terek River, ultimately drains into the Caspian Sea. Its basin extends across four administrative regions of the Russian Federation—the Chechen Republic, Ingushetia, North Ossetia–Alania, and Dagestan—while its right-hand tributaries, the upper reaches of the Assa and Argun, partially lie within Georgia. The basin ranges in elevation from approximately 28 m at the confluence with the Terek to 4492.6 m at Mount Tebulosmta on the Russia–Georgia border, yielding a total relief of about 4.5 km [40].

Digital elevation model (SRTM 30 m) analysis indicates maximum, minimum, and mean basin elevations of 4464 m, 27 m, and 1037 m, respectively [41]. The river is 278 km long, drains an area of about 12,000 km², and has a mean discharge of 82.9 m³/s. The annual suspended sediment yield is estimated at 12.2 Mt, with an average suspended load of 3800 g/m³ [42,43]. Recent DEM-based assessments estimate the basin area at 12,138 km² [41].

The Sulak River also flows into the Caspian Sea and drains the Republic of Dagestan (Russia). Its basin spans the foothills and slopes of the Greater Caucasus as well as the Caspian Lowland. The river is fed by a mixed regime dominated by snowmelt and originates at the confluence of the Andi Koysu and Avar Koysu rivers. Hydrologically, the Sulak exhibits typical mountain flow in its upper reaches, semi-mountain flow across 16% of its length in the foothills, and lowland flow across the Caspian Plain. At its mouth, the river forms a delta of about 44 km². The mean annual discharge is 4 km³ [44]. The Sulak is 169 km long, with a drainage basin of 15,200 km².

Based on Copernicus DEM (30 m), maximum, minimum, and mean basin elevations are 4455 m, −28 m, and 1950 m, respectively [41]. Mean annual discharge, measured 123 km from the mouth, is 176 m³/s. Suspended sediment concentrations average 450 g/m³, with peak values up to 45,000 g/m³. The Sulak hosts a cascade of hydroelectric power plants [45,46,47]. DEM-based analysis gives a basin area of 13,991 km² [41].

The Ulluchay River discharges into the Caspian Sea near the village of Segeler and flows entirely within the Republic of Dagestan, Russian Federation. Its source is located in springs at the northwestern end of the Kokmadag Ridge. The river is 111 km long and drains a catchment area of 1440 km². According to Copernicus 30 m DEM data [41], the Ulluchay basin has a maximum elevation of 2956 m, a minimum elevation of −28 m, and a mean elevation of 1554 m. Hydrologically, the river exhibits a flood-dominated regime during the warm season and a low-flow regime in winter. The mean annual discharge is 4.83 m³/s, with a peak discharge of 366 m³/s. The river flows through a V-shaped valley, with progressively increasing flow and sediment transport capacity downstream. A stepped longitudinal profile is particularly evident in the upper reaches, reflecting alternating layers of poorly erodible clayey shales and more easily erodible sandstones (siltstones and argillites). Extensive water abstraction for irrigation frequently results in complete flow cessation in the lower reaches [45,48,49,50]. DEM-based analysis [41] estimates the total catchment area at 13,991 km².

The Karachay River also drains into the Caspian Sea and is located within the Republic of Azerbaijan. Its catchment occupies the northeastern slopes of the Greater Caucasus. The river originates from the northern slopes of the Main Caucasus Ridge near Mount Babadag at an elevation of 3100 m a.s.l. Copernicus 30 m DEM analysis [41] indicates that the basin’s maximum, minimum, and mean elevations are 3644 m, −26 m, and 1567 m, respectively. The Karachay River is characterized by high discharge and an unstable hydrological regime. Average discharge near the village of Alych is 2.35 m³/s, with maximum flow observed from May to July and minimum flow during the winter months [45,51]. DEM-based estimates of the basin area amount to 1324 km² [41].

The Atachay River flows into the Caspian Sea and is located within the territory of the Republic of Azerbaijan. It originates from Mount Dyubar at an elevation of 1870 m. The river has a total length of 45 km and a catchment area of 347 km². According to Tabunshchik et al. [41], the Atachay River basin is characterized by maximum, minimum, and mean elevations of 2203 m, −28 m, and 779 m, respectively, based on the Copernicus 30 m DEM. Idrisov and Guseynova [52] report a catchment area of 360 km² for the Atachay River. The river is primarily fed by rainfall, and its water is used for irrigation of agricultural lands [53,54]. According to Tabunshchik et al. [41], the catchment area of the Sunzha River is estimated at 404 km² based on the Copernicus 30 m DEM and 374 km² based on the ALOS DEM.

2.2. Sampling Strategy

Sampling sites for each river were selected during the preparatory stage using satellite imagery (Figure 2 and Figure 3). Detailed descriptions of each sampling site and coordinates are provided in the Appendix A (

Table A1. Characteristics of sampling stations on the Sunzha, Sulak and Ulluchay rivers (Russia).

Site. No.	Date of Sampling	Water Basin	Location Coordinates	Observations of the Area	Field Observations of Water	Field Observations of Sediments/Soils
1	12 August 2024	Sulak River	43.259126° N 47.537988° E	The sampling site closest to the confluence with the Caspian Sea	Turbid water; weak flow	Silt-gray—viscous—no additional features—odorless
2	12 August 2024	Sulak River	43.3657051° N 47.1130054° E	Upstream section of the river with a steep (eroded) bank slope	Turbid water; moderate flow	Silt-gray—less viscous—no additional features—odorless
3	13 August 2024	The Andean Koisu (Sulak river basin)	42.7838265° N 46.7084910° E	Station located downstream of the confluence of rivers and the reservoir	Highly turbid water; strong flow	Silt with fine sand admixture—gray—medium plasticity—no additional features—odorless
4	13 August 2024	The Andean Koisu (Sulak river basin)	42.7470082° N 46.8311485° E	Water sampled from the discharge canal of the Irganay Hydropower Plant	Clear water; sample collected immediately downstream of the dam	Gravel (coarse and fine) with coarse sand admixture—mixed color—heterogeneous texture—no additional features—odorless
5	13 August 2024	Karakoysu (Sulak River basin)	42.5511654° N 46.9678067° E	Sample collected from a silty section, as coarse gravel dominates along the banks	Turbid water; strong flow	Silt with fine-grained sand admixture—gray—medium plasticity—no additional features—odorless
6	13 August 2024	The Andean Koisu (Sulak river basin)	42.542857° N 46.961278° E	Sample collected from a silty section	Water less turbid than at site 5; moderate flow	Silt with fine sand and coarse clastic admixture—gray—medium plasticity—surface layer covered with algal film—odorless
7	13 August 2024	The Andean Koisu (Sulak river basin)	42.566422° N 46.961170° E	Sample collected from a silty section	Turbid water; weak flow	Sand, fine-grained, with silt admixture—light gray—fine texture—no additional features—odorless
15	18 August 2024	Sulak River	43.265222° N 46.89490° E	Sample collected from a silty section of the Sulak River floodplain, in the area of the bank-protection dam	Clear water; weak flow Abundant fish and tadpoles observed	Silt with fine-grained sand admixture—gray—medium plasticity—abundant fine and medium gravel—odorless
16	18 August 2024	Chiryurt reservoir (Sulak River basin)	43.129265° N 46.846214° E	Sample collected from a silty section	clear water; no flow	Silt-gray—liquid—odor of decomposed organic matter
17	18 August 2024	Chirkeyskoye reservoir (Sulak River basin)	42.998112° N 46.896593° E	Located near a recreation facility, with runoff from the highway	Turbid water; considerable litter present	Sand, fine-grained, with admixture of silt and coarse clastic material—gray—fine texture—abundant vegetation and marl—odorless
18	19 August 2024	Kazikumukhskoye Koisu (Sulak River basin)	42.501137° N 47.062029° E	Near the village of Gergebil, with vertical rocky cliffs	Turbid water; strong flow	Silt with fine-grained gray sand admixture—viscous—odorless
19	19 August 2024	Gatsailinsky reservoir (Sulak River basin)	42.505932° N 46.900056° E	Sample collected from a silty section	Turbid water; weak flow	Silt-gray—viscous—odorless
20	19 August 2024	Avar Koisu (Sulak River basin)	42.185360° N 46.346697° E	Confluence of the Djirmut and Khanzor rivers, forming the Avar Koisu	Turbid water; moderate flow	Silt with fine-grained sand admixture—gray—medium plasticity—presence of gravel—odorless
8	14 August 2024	The Ulluchay River	42.2038221° N 48.0178070° E	Wide river section; settlement and fish-processing plant located nearby	Turbid water; moderate flow	Silt with sand admixture—gray—liquid—odorless
9	14 August 2024	The Ulluchay River	42.0359673° N 47.7814836° E	Large boulders along the banks	Turbid water; moderate flow	Silt with sand and decomposed leaf admixture (between stones)—gray—medium plasticity—odorless
10	15 August 2024	The Ulluchay River	41.971074° N 47.439094° E	Near the village of Kunki; livestock farm in the vicinity; coarse clastic material along the banks	Clear water	Silt with fine-grained sand admixture—gray—with oxidized surface layer—odorless
11	15 August 2024	The Ulluchay River	41.996778° N 47.582291° E	Near the village of Itsari	Clear water; rapid flow; large boulders along the banks	Sand, fine-grained, with silt admixture and clastic material—odorless
12	16 August 2024	Jivus (Ulluchay River basin)	42.104276° N 47.763029° E	2 km upstream from the confluence with the Ulluchay River; coarse gravel along the banks; sample collected from a silty section	Turbid water; strong flow	Silt with fine-grained sand admixture and coarse gravel—gray—viscous—odorless
13	16 August 2024	The Ulluchay River	42.286798° N 48.133469° E	Mouth of the Ulluchay River, at its confluence with the Caspian Sea	Turbid water; abundant surface litter; weak flow	Silt with fine-grained sand admixture—gray—viscous—with mollusk shells—odorless
14	16 August 2024	Bugan (Ulluchay River basin)	42.124791° N 47.613141° E	Sample collected from a silty section, with sandstone outcrops	Turbid water; strong flow	Sand-gray—with silt admixture—odorless
21	23 August 2024	The Sunzha River	43.318015° N 45.136283° E	Republic of Ingushetia, village of Barsuki	Clear water; moderate flow; eutrophic backwater with abundant vegetation	Silt-gray to dark gray—liquid—with abundant decomposed organic matter and strong odor—numerous stones present
22	23 August 2024	The Sunzha River	43.318015° N 45.146283° E	Vicinity of the village of Sernovodsk	Clear water; moderate flow	Silt with fine-grained sand admixture—gray—viscous—odorless—abundant small stones
23	23 August 2024	The Assa River (Sunzha River basin)	43.251638° N 45.428024° E	Vicinity of the village of Novy Sharoy	Turbid water; moderate flow	Silt with fine-grained sand admixture—gray—viscous—with H2S odor—coarse gravel present
24	23 August 2024	The Sunzha River	43.255000° N 45.42086111° E	Vicinity of the village of Zakan-Yurt	Turbid water; moderate flow	Silt with fine-grained sand admixture—viscous—numerous stones and fragments of plants—odorless
25	24 August 2024	The Sunzha River	43.2524094° N 45.5121152° E	Vicinity of the village of Alkhan-Kala, near the road bridge	Turbid water; weak flow	Silt with fine-grained sand admixture—gray—with oxidized surface layer—faint odor of decomposed organic matter
26	24 August 2024	The Sunzha River	43.250392° N 45.575238° E	Vicinity of the village of Alkhan-Yurt, adjacent to riverbed widening works	Turbid water; weak flow	Silt-viscous—minor sand admixture—surface layer enriched with oxidized organic matter—faint H2S odor—silted banks
27	26 August 2024	The Sunzha River	43.440629° N 46.134110° E	Vicinity of the village of Braguny, at the confluence of the Sunzha and Terek rivers	Highly turbid water; weak flow	Silt-viscous—clayey—gray—odorless
28	26 August 2024	The Sunzha River	43.365294° N 46.065576° E	Vicinity of the village of Kundukhovo, near the town of Gudermes, at the confluence with the Belka River	Turbid water; moderate flow; plume of more turbid water from the Belka River	Silt with fine-grained sand admixture—gray—with washed material from brown clay banks
29	26 August 2024	Argun River (Sunzha River basin)	43.315776° N 45.891950° E	Region of Argun	Dark turbid water; strong flow	Fine-grained gray sand with silt admixture, abundant decomposed plant matter, slight odor, pebbles present.
30	26 August 2024	The Sunzha River	43.339914° N 45.755358° E	City Area of Staraia Sunzha village, suburb of Grozny city	Turbid water; weak flow	Viscous silt with fine sand admixture, abundant leaves, slight H2S odor.

and

Table A2. Characteristics of Sampling Stations on the Karachay and Atachay Rivers (Republic of Azerbaijan).

Site No.	Date of Sampling	Water Basin	Location Coordinates	Observations of the Area	Field Observations of Water	Field Observations of Sediments/Soils
1	17 October 2024	Karachay River	41.111389° N, 48.327778° E	Sampling site located upstream of the village of Garhun.	Water turbid, odorless.	Fine gravel with silt admixture.
2	17 October 2024	Karachay River	41.144722° N, 48.359444° E	Sampling site located downstream of the villages of Garhun and Ryuk, before the river enters the canyon.	Water turbid, odorless; flow turbulent.	Silty-sandy, odorless, dark color, nearly black.
3	18 October 2024	Karachay River	41.244722° N, 48.470833° E	Area upstream of the village of Digyakh, where the river divides into several narrow channels; sampling was conducted in the widest section.	Water turbid with a high concentration of suspended matter, gray-brown in color; flow rapid.	Silty-sandy, odorless, dark color, nearly black.
4	18 October 2024	Karachay River	41.294722° N, 48.521944° E	In the vicinity of the village of Armaki (Ermeki), the river divides into several narrow channels; sampling was conducted in the widest channel.	Water turbid, high suspended solids, gray-brown color. Flow rapid.	Silty-sandy, odorless. Color dark, nearly black.
5	18 October 2024	Karachay River	41.328056° N, 48.560000° E	Near the village of Nugadi, upstream of the road bridge.	Water turbid, high suspended solids, gray-brown color. Flow rapid.	Silty-sandy, odorless. Color dark, nearly black.
6	18 October 2024	Karachay River	41.361389° N, 48.654444° E	Near the village of Karachay, upstream of the road bridge.	Water turbid, brown color, high suspended solids. Flow very rapid.	Odorless, compact clay with sand.
7	18 October 2024	Karachay River	41.378556° N, 48.755833° E	In the vicinity of the village of Garachy-Zeid, the river splits into several narrow channels; sampling was conducted in the widest channel.	Water highly turbid, high suspended solids, gray-brown color. Flow rapid.	Silty-sandy, odorless. Color dark, nearly black.
13	20 October 2024	Karachay River	41.461389° N, 48.975000° E	The closest point upstream of the river mouth.	Water turbid, high suspended solids. Flow moderate.	Compact clay with sand.
14	20 October 2024	Karachay River	41.444722° N, 48.892778° E	Sampling conducted under the bridge, downstream of the village of Karakashly; vegetation present along the banks.	Water turbid, brown color, high suspended solids. Flow moderate.	Compact clay with sand.
15	20 October 2024	Karachay River	41.411389° N, 48.823056° E	Area near the village of Khulyovlyu.	Water highly turbid, dark brown, with a large amount of suspended solids. Flow velocity moderate.	Dense clay with sand.
8	19 October 2024	Atachay River	41.094722° N, 49.157778° E	The river channel is located in a ravine, with saline-type soils along both banks.	Water highly turbid, brown, with a large amount of suspended solids.	Dense brown clay with sand inclusions, odorless.
9	19 October 2024	Atachay River	41.078056° N, 49.143333° E	Sampling conducted near the bridge and road, upstream of the settlement of Kolani.	Water turbid, brown, with a large amount of suspended solids. Flow velocity moderate.	Dense brown clay with sand inclusions, odorless.
10	19 October 2024	Atachay River	41.094722° N, 49.175833° E	Sampling in the estuarine section, before the river discharges into the sea.	Water highly turbid, brown, with a large amount of suspended solids. Flow velocity moderate.	Silt and clay, brown, with sand inclusions, H2S odor.
11	19 October 2024	Atachay River	41.094722° N, 49.148333° E	An automobile highway runs nearby, with agricultural fields on both sides of the channel.	Water highly turbid, brown, with a large amount of suspended solids. Flow velocity moderate.	Clay and silt, light brown, with H2S odor.
12	19 October 2024	Atachay River	41.044722° N, 49.125556° E	Under the bridge, one bank is reinforced with a concrete wall. Slightly downstream, an apparent waste dump is present, with trash being burned.	Water highly turbid, brown, with a large amount of suspended solids. Flow velocity moderate.	Water-saturated clay, light brown, with H2S odor.
16	21 October 2024	Atachay River	40.961389° N, 49.029444° E	The river enters a deep canyon. Slightly upstream, a large tributary flows into the river.	Water is highly turbid, dark brown in color, with a large amount of suspended solids. Flow velocity is fast.	Dense clay with minor sand admixture, no odor.
17	21 October 2024	Atachay River	40.928056° N, 49.036667° E	Sampling conducted upstream of the confluence with the large tributary.	Water is highly turbid, dark brown in color, with a large amount of suspended solids. Flow velocity is fast.	Soft soil with silty texture, two cores (58 mm diameter) collected from the 0–5 cm surface layer, no odor.
18	21 October 2024	Atachay River	40.911389° N, 48.979444° E	Sampling in the settlement of Bakshishly; vehicles cross the river at this point, and livestock graze along the banks.	Water is highly turbid, dark brown in color, with a large amount of suspended solids. Flow velocity is fast.	Soft silty soil with characteristic silty odor.
19	21 October 2024	Atachay River	40.878056° N, 48.934167° E	Sampling downstream of the settlement of Altyagach. The riverbanks are heavily littered.	Water is highly turbid, dark brown in color, with a large amount of suspended solids. Flow velocity is fast.	Soft silty soil with characteristic silty odor.
20	21 October 2024	Atachay River	40.861389° N, 48.906389° E	Sampling upstream of the settlement of Altyagach.	Water is highly turbid, dark brown in color, with a large amount of suspended solids. Flow velocity is fast.	Dense clay with minor sand admixture, no odor.

).

During the preparatory phase of the study, using a series of cartographic materials—high-resolution satellite images, topographic maps and digital elevation models [41], geological maps [55], river network modeling maps [41], and land-use and anthropogenic impact maps [32]—preliminary sampling points were selected (for example, Figure 2 and Figure 3). These points allowed us to take into account terrain features, the presence of tributaries, and the location of settlements and industrial facilities before the start of the expeditionary work, and to obtain a list of points and their coordinates. During fieldwork, the selected points were marked on the ground and, if necessary, slightly adjusted.

Coastal sediment samples (upper layer 0–5 cm) were collected at the same stations where river water was sampled (Figure 2 and Figure 3) using a hand sampler in accordance with [56]. Salinity was monitored at the river mouth, which corresponded to fresh water in all samples. In the studied fast-flowing watercourses, river sediment samples were collected in areas with established dynamic equilibrium between suspended particles and bottom sediments, where there is no runoff of the latter. The sampling depth did not exceed 1.5 m. Stations were selected based on the objectives of assessing the impact of natural and anthropogenic factors on the accumulation of pollutants in coastal sediments (at the mouths of tributaries, in the area of reservoirs, large settlements, where the river flows into the sea, etc.).

2.3. Analytical Procedures

Surface water samples for elemental analysis and total suspended solids (TSS) determination were collected from the river surface using a plastic bucket and transferred into 5 L polyethylene containers. Bottom sediments were sampled with an acrylic corer (5 cm height), retrieving the upper 0–5 cm layer. Immediately after sampling, in situ measurements of temperature, pH, and Eh were performed using a Nitron-pH-150 MA pH-meter/thermometer (LLC SPE “Biomer”, Krasnoobsk, Russia) [57]. Geographic coordinates of the sampling sites and hydrochemical parameters of water and sediments are provided in Appendix A.

Pre-treatment of water samples was carried out on the day of collection in accordance with [58]. Dissolved and particulate fractions were separated by vacuum filtration through pre-weighed cellulose nitrate membrane filters (FMNC-0.45, pore size 0.45 µm, JSC “Vladisart”, Vladimir, Russia). Filters containing suspended matter were air-dried, sealed in labeled zip-lock bags, and transported to the IBSS laboratory for further analysis. For dissolved elements, 1 L of filtrate was acidified with concentrated suprapure HNO₃ (LLC “Component-Reaktiv”, Moscow, Russia) to pH < 2 and transported to the IBSS Core Facility “Spectrometry and Chromatography.” Sediment samples were weighed immediately after collection in a field laboratory, sealed in polyethylene bags, labeled, and transported for processing.

In the laboratory of the IBSS, the concentration of total suspended solids on dried filters was determined gravimetrically in accordance with [58]. Trace elements from bottom sediment and suspended particulate matter samples were extracted by acid digestion following [59]. The concentrations of 70 elements (Li, Be, B, Na, Mg, Al, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th, and U) were quantified in acid extracts and acidified water filtrates using inductively coupled plasma mass spectrometry (ICP-MS), following the procedure outlined in GOST R 56219-2014 [60]. Analyses were carried out on a PlasmaQuant MS Elite spectrometer (Analytik Jena, Jena, Germany) at the Shared Research and Education Center for Spectrometry and Chromatography, IBSS.

Instrument calibration was carried out with multi-element standard solutions (IV-ICPMS-71A, IV-ICPMS-71B, and IV-ICPMS-71C; Inorganic Ventures, Christiansburg, VA, USA). Calibration curves were constructed from serial dilutions of these standards to encompass the full concentration range of the target elements. Each analyte was measured in at least seven replicates per sample. The acquisition time for individual m/z ratios was adjusted according to detector signal intensity and ranged from 0.01 to 0.1 s. The relative measurement error for all elements did not exceed 10%.

Due to restrictions on transporting water samples to the Russian Federation, additional analyses of river water from Azerbaijan were conducted at the Central Laboratory of the United Water Supply Service for Large Cities, State Water Management Agency of Azerbaijan. In these samples, the concentrations of 28 elements (Al, As, V, B, P, Fe, Mn, Ga, Sn, Sb, Ti, Zn, Ni, Co, Cr, Ba, Mo, Cd, Ag, Se, Pb, Be, Na, Li, K, Cu, Si, and Sr) were determined using ICP-OES, in accordance with [61].

A total of 100 water samples were collected, comprising 50 samples for the analysis of dissolved trace elements and 50 samples for suspended particulate-bound fractions, along with 50 surface sediment samples (0–5 cm). In total, 15,960 determinations of trace element concentrations, including heavy metals, were performed for water and sediments from mountain rivers in Dagestan, Chechnya, and Azerbaijan. This dataset included 8400 measurements of dissolved and particulate-bound forms in water, 4200 measurements in bottom sediments from rivers in Dagestan and Chechnya, 560 measurements of dissolved forms and 1400 of suspended particulate-associated elements in water, and 1400 measurements in bottom sediments from rivers in Azerbaijan.

To assess the geoecological status of river waters, the measured concentrations were compared with maximum permissible concentrations (MPCs) established by Russian sanitary regulations for surface waters used for drinking and recreational purposes (MPCdr) [62], as well as with fishery water quality standards (MPCfw) regulated by Order No. 552 of the Ministry of Agriculture of the Russian Federation (13 December 2016). In addition, reference was made to the national drinking water standards of Azerbaijan (MPCazs) [63].

For bottom sediments, no legally defined MPCs exist in the Russian Federation. Therefore, geochemical reference levels such as average crustal abundances (Clarke values) in the upper continental crust [64] are commonly applied. At the international level, the “Dutch Target and Intervention Values” (commonly referred to as the “Dutch Lists”) [65] are widely acknowledged and used in scientific and regulatory practice. Unlike Russian standards, the Dutch guidelines establish threshold values not only for dissolved forms of trace elements in water (MPCdl) but also for total concentrations (MPCtot), which include both dissolved and particulate-bound fractions. They also provide benchmark values for contaminants in bottom sediments (MPCsed). The regulatory benchmarks applied in this study are summarized in

Table 1. Maximum allowable concentrations of selected elements in surface waters and bottom sediments as specified by various regulatory frameworks.

Element	Li	Be	B	Na	Mg	Al	Si	P	K	Ca	Ti
MPCdr, µg·L−1	30	0.2	500	200,000	50,000	200	25,000	0.1	–	–	100
MPCfw, µg·L−1	80	0.3	100	120,000	40,000	40	–	0.01	50,000	180,000	60
MPCAZS, µg·L−1	2000	12	2400	200,000	–	200	10,000	0.1	12,000	–	20
MPCdl, µg·L−1	–	0.2	650	–	–	–	–	–	–	–	–
MPCtot, µg·L−1	–	0.2	–	–	–	–	–	–	–	–	–
MPCsed, mg·kg−1	–	1.2	–	–	–	–	–	–	–	–	–
Element	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	As	Se
MPCdr, µg·L−1	100	50	100	300	100	20	1000	5000	–	10	10
MPCfw, µg·L−1	1	70	10	100	10	10	1	10	–	50	2
MPCAZS, µg·L−1	100	50	500	300	100	70	1000	5000	5	10	40
MPCdl, µg·L−1	4.3	8.7	–	–	2.8	5.1	1.5	9.4	–	25	5.3
MPCtot, µg·L−1	5.1	84	–	–	3.1	6.3	3.8	40	–	32	5.4
MPCsed, mg·kg−1	56	380	–	–	19	44	73	620	–	55	2.9
Element	Br	Rb	Sr	Zr	Nb	Mo	Ag	Cd	Sn	Sb	Te
MPCdr, µg·L−1	200	–	7000	–	10	70	50	1	2000	5	10
MPCfw, µg·L−1	–	100	400	70	–	1	–	5	112	–	3
MPCAZS, µg·L−1	–	100	7000	–	10	250	50	1	5	20	10
MPCdl, µg·L−1	–	–	–	–	–	290	0.08	0.4	18	6.5	–
MPCtot, µg·L−1	–	–	–	–	–	300	–	2	220	7.2	–
MPCsed, mg·kg−1	–	–	–	–	–	200	5.5	12	–	15	–
Element	I	Cs	Ba	Sm	Eu	W	Hg	Tl	Pb	Bi	U
MPCdr, µg·L−1	125	–	700	–	–	50	0.5	0.1	10	100	15
MPCfw, µg·L−1	400	1000	740	–	–	0.8	0.01	–	6	–	–
MPCAZS, µg·L−1	125	–	1300	24	300	50	0.5	0.1	10	100	30
MPCdl, µg·L−1	–	–	220	–	–	–	0.2	1.6	11	–	1
MPCtot, µg·L−1	–	–	230	–	–	–	1.2	1.7	220	–	–
MPCsed, mg·kg−1	–	–	300	–	–	–	10	2.6	530	–	–

.

The accumulation coefficient (AC) was employed as an indicator of the capacity of suspended particulate matter (SPM) and bottom sediments (BS) in the studied mountain rivers of Russia and Azerbaijan to concentrate trace elements. The AC was calculated according to [10] as:

$$AC={\displaystyle \frac{C_{el\left(BS\right)or \left(SPM\right)}·1000, \mathrm{m}\mathrm{g}/\mathrm{k}\mathrm{g}}{C_{el\left(water\right)}, \mathrm{\mu }\mathrm{g}/\mathrm{L}}},$$

(1)

where C_el(BS) and C_el(SPM) represent the concentrations of the element in bottom sediments and suspended particulate matter, respectively, and C_el(water) denotes the concentration of the same element in river water (Russia and Azerbaijan).

To identify potential sources of trace elements in bottom sediments, EF were also calculated following Barbieri [66] and Starodymova et al. [67]. EF values reflect the degree of enrichment of an element in sediments relative to its natural geochemical background in the Earth’s crust. The EF was computed as:

$$EF={\displaystyle \frac{{(\frac{C_{el}}{C_{ref}})}_{sample}}{({\frac{C_{el}}{C_{ref}})}_{UCC}}}$$

(2)

where C_el and C_ref are the concentrations of the element of interest and the reference element in the sediment sample (subscript “sample”), while the denominator represents their corresponding average abundances (Clarke values) in the upper continental crust (subscript “UCC”).

In this study, iron (Fe) was selected as the reference element due to its wide geochemical distribution in the lithosphere. Clarke values for the investigated elements were taken from Vinogradov [64]. Generally, EF values below 3 indicate that the trace elements in bottom sediments are predominantly of lithogenic origin [67].

2.4. Statistical Analysis

The data were processed using the Microsoft Excel 2010 and Statistica 13.5.0.17 software packages. The standard error of the mean value was determined using Excel parquet analysis with a confidence level of 0.95.

The method of principal components, allowing the most efficient analysis of multivariate systems, was used to analyze multivariate data. During the principal component analysis (PCA), the data were pre-normalized. The PCA was performed using the Varimax rotation method.

This method was applied to determine (relatively quantitatively) the main sources of trace elements at various sampling stations. The sampling stations were grouped according to the content and composition of elements using PCA and discriminant analysis.

3. Results

The physicochemical parameters of river water and sediments obtained during field measurements are summarized in

Table 2. Physicochemical characteristics of river water and sediments in the studied mountain rivers.

Site. No.	Date of Sampling	Coordinates	t Water (°C)	pH Water	pH Bottom Sediments	Eh Bottom Sediments (mV)
Sulak River with tributaries and reservoirs (Russian Federation)
1	12 August 2024	43.259126° N, 47.537988° E	28	8.52	7.33	−154
2	12 August 2024	43.3657051° N, 47.1130054° E	23	7.83	6.93	−126
3	13 August 2024	42.7838265° N, 46.7084910° E	17	7.63	7.8	+192
4	13 August 2024	42.7470082° N, 46.8311485° E	17	7.54	Not defined	Not defined
5	13 August 2024	42.5511654° N, 46.9678067° E	20	8.13	8.03	325
6	13 August 2024	42.542857° N, 46.961278° E	20	7.83	7.62	+138
7	13 August 2024	42.566422° N, 46.961170° E	20	8.1	8.14	+143
15	18 August 2024	43.265222° N, 46.894010° E	23	8.49	8.05	+180
16	18 August 2024	43.129265° N, 46.846214° E	19	7.68	6.81	−206
17	18 August 2024	42.998112° N, 46.896593° E	25	8.83	7.77	+148
18	19 August 2024	42.501137° N, 47.062029° E	19	7.12	6.64	+134
19	19 August 2024	42.505932° N, 46.900056° E	24	7.56	6.65	+186
20	19 August 2024	42.185360° N, 46.346697° E	16	7.37	7.23	+158
Ulluchay River with tributaries and reservoirs (Russian Federation)
8	14 August 2024	42.2038221° N, 48.0178070° E	27	8.14	7.69	+110
9	14 August 2024	42.0359673° N, 47.7814836° E	17	8.61	7.81	+168
10	15 August 2024	41.971074° N, 47.439094° E	16	8.4	8.14	+118
11	15 August 2024	41.996778° N, 47.582291° E	16	8.41	7.85	+185
12	16 August 2024	42.104276° N, 47.763029° E	18	7.72	7.67	+166
13	16 August 2024	42.286798° N, 48.133469° E	25	7.67	7.43	+151
14	16 August 2024	42.124791° N, 47.613141° E	16	8.05	7.84	+199
Sunzha River with tributaries and reservoirs (Russian Federation)
21	23 August 2024	43.318015° N, 45.136283° E	20	7.05	6.65	−40
22	23 August 2024	43.318015° N, 45.146283° E	24	7.67	7.16	−179
23	23 August 2024	43.251638° N, 45.428024° E	21	7.32	6.9	+49
24	23 August 2024	43.255000° N, 45.4208611° E	22	7.42	6.65	−37
25	24 August 2024	43.2524094° N, 45.5121152° E	21	7.59	6.88	−161
26	24 August 2024	43.250392° N, 45.575238° E	23	7.4	6.71	−182
27	26 August 2024	43.440629° N, 46.134110° E	24	7.33	7.12	+135
28	26 August 2024	43.365294° N, 46.065576° E	21	7.44	7.44	+94
29	26 August 2024	43.315776° N, 45.891950° E	19	7.38	7.04	−80
30	26 August 2024	43.339914° N, 45.755358° E	21	7.72	7.12	+70
Karachay River with tributaries and reservoirs (Azerbaijan)
1	17 October 2024	41.111389° N, 48.327778° E	10	8.5	6.0	+190
2	17 October 2024	41.144722° N, 48.359444° E	12	8.6	6.5	+290
3	18 October 2024	41.244722° N, 48.470833° E	11	8.15	7.46	+294
4	18 October 2024	41.294722° N, 48.521944° E	11	9.3	9.46	+286
5	18 October 2024	41.328056° N, 48.560000° E	11	8.98	8.25	+259
6	18 October 2024	41.361389° N, 48.654444° E	13	7.62	7.84	+283
7	18 October 2024	41.378556° N, 48.755833° E	12.7	7.04	6.08	+106
13	20 October 2024	41.461389° N, 48.975000° E	11.7	8.33	8.45	−64.1
14	20 October 2024	41.444722° N, 48.892778° E	10.9	9.29	9.86	+173
15	20 October 2024	41.411389° N, 48.823056° E	11.5	9.88	9.52	+152
Atachay River with tributaries and reservoirs (Azerbaijan)
8	19 October 2024	41.094722° N, 49.157778° E	13.3	9.88	9.52	+152
9	19 October 2024	41.078056° N, 49.143333° E	13.5	8.79	9.01	+274
10	19 October 2024	41.094722° N, 49.175833° E	12	9.24	8.2	+206
11	19 October 2024	41.094722° N, 49.148333° E	13.6	9.3	8.3	+498
12	19 October 2024	41.044722° N, 49.125556° E	12.7	8.8	8.5	+184
16	21 October 2024	40.961389° N, 49.029444° E	9.8	9.0	8.3	+82
17	21 October 2024	40.928056° N, 49.036667° E	10	8.9	8.7	+72
18	21 October 2024	40.911389° N, 48.979444° E	7.5	8.9	8.9	−158
19	21 October 2024	40.878056° N, 48.934167° E	6.9	8.4	8.1	+63
20	21 October 2024	40.861389° N, 48.906389° E	4.6	9.5	9.1	+159

.

The pH values in water of rivers in the Russian Federation ranged between 7.1 and 8.8, falling within the drinking water quality standards adopted in the Russian Federation [68]. By contrast, in the rivers of Azerbaijan, elevated pH values were observed at several sampling sites—stations 14 and 15 of the Karachay River and stations 8, 10, 11, and 20 of the Atachay River (Table 2)—reaching up to 9.9. Such values classify the water as alkaline, which is undesirable for continuous human consumption and poses potential risks when used for drinking purposes.

As presented in Table 2, negative redox potential (Eh) values to −206 mV, were detected in river sediments across both Russia and Azerbaijan. Such strongly reducing conditions can arise not only from localized hydrodynamic stagnation (e.g., site 1, Sulak River; Table A1) but also from inherent geochemical features of the catchments. In the Sunzha River, for example, negative Eh values were measured at stations 22, 24, 25, and 26, located near the town of Sernovodsk (Table A1), where hydrogen sulfide–rich mineral springs exert a notable influence. In all cases, reducing conditions in sediments facilitate the remobilization of contaminants into the overlying water column, thereby enhancing the risk of secondary pollution in aquatic ecosystems [35].

Elemental concentrations in river water, suspended particulate matter (SPM), and bottom sediments (BS) across the study regions are presented in

Table 3. Elemental concentrations (the range of values in the numerator and the mean with standard error in the denominator) in river water, suspended particulate matter, and bottom sediments of the study regions, compared with global average concentrations in river waters [69,70,71], SPM [69,72,73], and average crustal abundances (Clarke values) in the continental crust [64,74].

Element	Water			Suspended Matter			Bottom Sediments
	Concentration Range in Districts 1–3 (Russia), µg·L−1	Concentration Range in Districts 4–5 (Azerbaijan), µg·L1	Average Concentration in the Rivers of the World, µg·L−1	Concentration Range in Districts 1–3 (Russia), mg·kg−1	Concentration Range in Districts 4–5 (Azerbaijan), mg·kg−1	The Average Concentration In The Rivers Of The World, mg·kg−1	Concentration Range in Districts 1–3 (Russia), mg·kg−1	Concentration Range in Districts 4–5 (Azerbaijan), mg·kg−1	Clark in the Earth’s Crust, mg·kg−1
Major elements
Na	3581–78,000 16,208.5 ± 2629.4	5500–198,000 61,725.0 ± 14,192.8	5100	104–4133 674.6 ± 173.4	73–7403 712.0 ± 356.7	7100	134–591 280.1 ± 18.4	198–1964 644.1 ± 81.8	25,000
Mg	39–245 101.7 ± 7.4	–	3800	1317–4488 2538.8 ± 141.3	<200–2749 1286.0 ± 213.0	12,600	1104–5855 3781.9 ± 184.3	2376–10,068 5102.3 ± 491.3	18,700
Al	12.8–26.8 16.5 ± 0.6	2.8–28.0 8.5 ± 1.4	32	4071–11,322 6489.4 ± 267.5	7810–60,098 27,637.9 ± 3368.6	87,200	2807–14,838 6329.4 ± 513.9	6260–17,481 11,465.6 ± 823.7	80,500
Si	–	324–794 468.1 ± 24.6	5420	–	–	254,000	–	–	295,000
P	13–330 43.5 ± 12.0	<10–25 3.7 ± 1.7	10	4574–117,039 19,628.4 ± 4719.0	1684–20,367 7324.9 ± 1225.8	2010	4979–28,343 8555.9 ± 720.9	6478–16,052 10,612.8 ± 614.3	930
K	163–3993 1416.6 ± 180.7	800–19,400 6005.0 ± 924.5	1350	534–2286 1406.7 ± 79.0	52–9567 3089.6 ± 737.1	16,900	373–3092 1202.9 ± 116.0	920–4788 2430.5 ± 264.6	25,000
Ca	34,477–170,223 82,897.8 ± 5625.8	–	14,600	882–20,478 6420.4 ± 884.6	5135–63,589 23,869.0 ± 3755.3	25,900	1494–77,464 15,488.2 ± 2755.3	10,804–86,304 50,075.5 ± 4765.4	29,600
Ti	<0.2–2.2 1.0 ± 0.1	6.1–12.0 9.9 ± 0.4	0.49	9.5–181 37.8 ± 6.9	4.4–136.6 25.6 ± 8.0	4400	13–226 47.9 ± 9.8	16–65 33.4 ± 3.2	4500
Mn	0.3–1716 63.1 ± 57.1	<1–24 4.6 ± 1.2	34	90–514 216.7 ± 16.2	273–7590 1322.7 ± 373.2	1679	64–932 290.9 ± 28.0	268–5704 1534.1 ± 453.6	1000
Fe	25–265 142.0 ± 10.1	1.9–71.0 11.4 ± 3.6	66	10,881–29,709 18,172.5 ± 992.2	1546–10,109 5508.8 ± 626.6	58,100	3287–17,688 9220.3 ± 562.7	7571–38,740 18,547.9 ± 2038.9	46,500
Trace elements
Li	2.0–11.3 5.0 ± 0.4	8.3–33.0 19.9 ± 2.3	1.84	<4–62 20.9 ± 2.6	10–2776 221.3 ± 149.9	8.5	<0.5–21.9 8.6 ± 0.9	6.1–24.0 12.3 ± 1.1	32
Be	<0.01	<1	0.009	<0.05–0.55 0.35 ± 0.02	0.54–1.30 0.82 ± 0.04	1.8	0.10–0.79 0.32 ± 0.03	0.29–0.97 0.50 ± 0.04	3.8
B	33–609 129.3 ± 21.4	43–565 269.4 ± 48.6	10.2	1.4–38.5 11.0 ± 1.3	10–1994 214.7 ± 127.9	70	2.4–24.9 8.8 ± 0.9	5.9–32.1 16.7 ± 1.9	12
V	<0.1–4.1 0.4 ± 0.2	8.4–21.0 14.8 ± 0.7	0.71	9.7–21.2 15.3 ± 0.5	4.2–78.3 40.7 ± 5.3	129	7.2–29.8 14.3 ± 0.9	3.7–24.5 9.2 ± 1.2	90
Cr	<0.3–1.8 0.7 ± 0.1	1.3–4.3 2.2 ± 0.2	0.7	13–186 35.5 ± 6.7	4.5–33.8 22.9 ± 1.6	130	10.2–33.1 18.4 ± 1.0	7.3–16.3 12.1 ± 0.7	83
Co	<0.03–0.42 0.05 ± 0.02	<1	0.15	7.1–16.5 10.1 ± 0.4	6–224 29.8 ± 14.5	22.5	1.9–14.9 6.2 ± 0.4	6.6–15.1 9.5 ± 0.6	18
Ni	<0.9–2.8 0.8 ± 0.2	<1	0.8	16.2–34.6 23.1 ± 0.9	16–552 58.0 ± 26.4	74.5	9–70 23.9 ± 2.0	19–42 26.3 ± 1.5	58
Cu	<0.3–4.6 0.8 ± 0.2	<5–8.9 5.4 ± 0.7	1.48	7.2–35.0 14.6 ± 1.2	15–47 31.1 ± 1.9	75.9	5–48 12.4 ± 1.6	12–44 23.4 ± 2.2	47
Zn	1.5–20.0 5.0 ± 0.7	1.6–3.7 2.2 ± 0.1	0.6	26–277 71.5 ± 10.0	19–4329 416.0 ± 261.7	208	14–128 36.6 ± 4.0	23–74 38.7 ± 2.8	83
As	<2.3	1.1–8.1 4.4 ± 0.4	0.6	3.5–16.6 6.5 ± 0.5	<0.8–3.8 1.5 ± 0.3	36.3	2.1–14.4 6.4 ± 0.5	<0.9–12.9 5.2 ± 1.0	1.7
Se	<5.3	<1–7.2 3.6 ± 0.4	0.07	<10	<0.4–1.1	–	<0.3–0.8 0.04 ± 0.03	<0.5	0.05
Br	<18–135.6 34.1 ± 7.5	–	20	11–581 73.4 ± 23.2	<70	21.5	12.3–44.4 22.5 ± 1.4	<19	2.1
Sr	129–986 453.3 ± 36.7	419–1317 829.8 ± 75.0	60	18–309 71.1 ± 11.5	40–398 144.1 ± 21.0	187	21–411 82.3 ± 13.3	77–371 204.7 ± 23.2	340
Y	<0.006	–	0.04	3.2–7.5 5.5 ± 0.2	3.7–18.6 5.6 ± 0.8	21.9	1.2–9.2 4.1 ± 0.3	4.4–10.0 7.0 ± 0.4	29
Zr	<0.05–0.13 0.07 ± 0.01	–	0.04	0.4–10.0	1–1980 152.9 ± 108.4	260	0.1–5.6 1.7 ± 0.2	0.7–4.4 2.2 ± 0.3	170
Nb	<0.01	–	0.002	0.01–0.64 0.15 ± 0.03	0.04–0.21 0.12 ± 0.01	13.5	0.0004–0.300 0.044 ± 0.012	0.009–0.050 0.026 ± 0.003	21
Mo	0.16–1.67 0.60 ± 0.07	1.3–9.4 5.8 ± 0.6	0.42	0.4–7.6 1.2 ± 0.3	0.2–1.9 0.5 ± 0.1	2.98	0.3–8.5 0.8 ± 0.3	0.2–1.1 0.6 ± 0.1	1.1
Ag	<0.1	6.3–8.2 7.3 ± 0.1	0.3	0.05–58.37 8.0 ± 2.8	<0.01–4.11 0.36 ± 0.23	13	<0.004–0.423 0.023 ± 0.014	<0.006–35.48 1.80 ± 1.77	0.07
Pd	<0.08–0.26 0.03 ± 0.01	–	0.03	0.2–1.2 0.44 ± 0.04	0.1–61.5 4.9 ± 3.4	0.4	0.07–0.53 0.20 ± 0.02	0.09–0.22 0.15 ± 0.01	0.009
Sn	0.2–5.9 0.9 ± 0.2	16–123 40.6 ± 6.5	0.5	0.1–8.0 1.1 ± 0.3	0.2–10.0 1.6 ± 0.6	4.57	0.05–0.47 0.13 ± 0.01	0.6–3.0 1.5 ± 0.1	2.5
Sb	0.06–0.40 0.14 ± 0.02	<5	0.07	0.01–2.29 0.27 ± 0.09	0.01–0.71 0.09 ± 0.04	2.19	0.01–0.11 0.04 ± 0.01	0.03–0.26 0.06 ± 0.01	0.5
Te	<0.22–0.41 0.03 ± 0.02	–	–	<0.02–0.18 0.02 ± 0.01	<0.02–2.5 1.7 ± 1.2	0.54	<0.02–0.10 0.04 ± 0.01	0.07–1.00 0.22 ± 0.06	0.001
I	3.4–16.6 6.6 ± 0.6	–	7	2.0–166 24.7 ± 7.4	<0.1–2.9 0.8 ± 0.2	70	0.9–18.0 4.6 ± 0.8	<0.1–158 30.1 ± 8.9	0.4
Cs	<0.03	–	0.011	1.8–5.7 3.6 ± 0.2	2.5–7.1 4.1 ± 0.3	6.25	0.5–3.5 1.2 ± 0.1	1.2–3.9 2.1 ± 0.2	3.7
Ba	2.1–30.1 18.3 ± 1.3	12–37 20.5 ± 1.8	23	19.1–132 51.0 ± 4.3	68–6500 676.5 ± 325.1	522	11–662 95.6 ± 23.7	88–303 174.6 ± 16.9	650
Ta	<0.03	–	0.001	<0.005	0.011–0.063 0.026 ± 0.003	1.27	<0.0002	0.013–0.024 0.018 ± 0.001	2.5
W	<0.001–0.040 0.007 ± 0.002	–	0.1	<0.001–0.263 0.025 ± 0.011	<0.002–0.100 0.017 ± 0.005	1.99	<0.001–0.196 0.012 ± 0.007	0.01–0.50 0.06 ± 0.02	1.3
Os	<0.07	–	–	0.002–0.067 0.011 ± 0.003	<0.001–0.018 0.002 ± 0.001	–	0.002–0.007 0.003 ± 0.001	<0.001–0.012 0.003 ± 0.001	0.0002
Ir	<0.001	–	–	<0.01	<0.001–0.073 0.007 ± 0.005	–	<0.0003	<0.0005	0.00065
Pt	<0.001	–	0.1	<0.05	<0.001–0.207 0.020 ± 0.012	–	<0.002–0.014 0.001 ± 0.001	<0.001	0.005
Au	<0.2	–	2	<0.010–0.015 0.001 ± 0.001	0.03–0.17 0.08 ± 0.01	0.004	<0.001–0.040 0.001 ± 0.001	0.036–0.052 0.046 ± 0.001	0.009
Hg	<0.5	–	0.07	0.02–0.57 0.09 ± 0.02	0.008–0.039 0.024 ± 0.002	0.08	0.016–0.051 0.025 ± 0.001	<0.004–0.050 0.010 ± 0.002	0.08
Pb	<0.1	<5–5.8 2.5 ± 0.6	0.08	4.8–31.0 9.4 ± 1.0	6.1–95.8 17.9 ± 5.0	61.1	2.8–16.1 7.6 ± 0.6	7.7–19.7 12.7 ± 0.08	16
Bi	<0.05	–	–	<0.02–1.40 0.23 ± 0.04	0.07–1.63 0.27 ± 0.10	0.85	0.03–1.89 0.16 ± 0.06	0.09–0.35 0.16 ± 0.02	0.004
Th	<0.026–0.041 0.016 ± 0.003	–	0.041	2.1–4.5 3.4 ± 0.1	2.2–5.0 3.2 ± 0.2	12.1	0.8–3.6 2.2 ± 0.1	1.3–5.2 3.0 ± 0.3	14
U	<0.01–4.1 0.5 ± 0.2	–	0.37	0.11–1.00 0.32 ± 0.03	0.1–6.0 0.7 ± 0.3	3.3	0.12–1.41 0.28 ± 0.04	0.09–0.40 0.23 ± 0.02	2.5
Rare and rare-earth elements
Sc	<0.1–1.1 0.19 ± 0.04	–	1.2	1.8–4.2 2.8 ± 0.1	3.0–6.8 4.6 ± 0.2	18.2	1.1–4.5 2.4 ± 0.1	2.0–5.8 3.2 ± 0.2	11
Ga	0.1–1.7 0.9 ± 0.1	<5	0.03	6.1–19.4 10.4 ± 0.5	6–398 31.2 ± 19.3	18.1	2.2–24.4 6.3 ± 0.8	7.5–23.0 13.3 ± 1.0	19
Ge	<0.08	–	0.007	7.7–15.9 11.5 ± 0.4	1.1–3.3 2.2 ± 0.1	1.23	1.6–14.4 6.5 ± 0.5	1.2–3.9 2.1 ± 0.2	1.4
Rb	0.1–4.0 0.8 ± 0.1	–	1.63	9.6–37.3 23.8 ± 1.1	21–56 39.1 ± 2.3	78.5	3.1–18.4 8.4 ± 0.7	7.5–31.4 17.9 ± 1.7	150
Ru	<0.03	–	–	<0.003–0.086 0.023 ± 0.003	<0.002–0.016 0.003 ± 0.001	–	<0.001–0.073 0.014 ± 0.003	<0.003–0.030 0.010 ± 0.002	0.004
Rh	<0.02	–	0.09	<0.002–0.055 0.012 ± 0.002	<0.001	–	0.001–0.047	<0.001	0.005
Cd	<0.1	<1	0.08	<0.002–3.93 0.20 ± 0.13	<0.01–3580 248.5 ± 188.5	1.55	<0.002–0.52 0.05 ± 0.02	0.1–5.9 0.62 ± 0.28	0.13
In	<0.03–0.03		–	<0.001–0.021 0.007 ± 0.001	<0.001–0.020 0.007 ± 0.002	–	<0.001–0.012 0.003 ± 0.001	<0.001–0.032 0.009 ± 0.001	0.07
La	<0.01	–	0.12	3.8–10.8 7.4 ± 0.4	2–288 33.9 ± 18.6	37.4	2.5–8.5 5.3 ± 0.3	2.8–18.6 9.4 ± 1.3	29
Ce	<0.01	–	0.26	7.1–22.6 12.4 ± 0.8	6.6–29.6 15.5 ± 1.5	73.6	5.2–18.1 11.5 ± 0.6	7.5–25.7 13.6 ± 1.2	70
Pr	<0.017–0.019	–	0.04	1.2–2.9 2.0 ± 0.1	1.0–3.4 1.8 ± 0.2	8	0.7–2.4 1.5 ± 0.1	1.1–5.4 2.7 ± 0.3	7
Nd	<0.030–0.039	–	0.15	5.1–12.6 8.8 ± 0.4	4.4–13.7 7.7 ± 0.6	32.2	2.7–9.5 6.3 ± 0.3	5.3–28.1 13.2 ± 1.6	30
Sm	<0.02	–	0.036	1.2–2.6 2.0 ± 0.1	1.4–2.6 1.8 ± 0.1	6.12	0.6–2.4 1.5 ± 0.1	1.6–4.4 2.7 ± 0.2	7
Eu	<0.02	–	0.01	0.25–0.60 0.47 ± 0.02	0.30–1.13 0.46 ± 0.04	1.29	0.13–0.78 0.35 ± 0.02	0.41–0.99 0.63 ± 0.03	1.2
Gd	<0.04	–	0.04	1.3–2.6 2.1 ± 0.1	1.3–2.5 1.8 ± 0.1	5.25	0.5–2.5 1.5 ± 0.1	1.8–4.1 2.8 ± 0.1	7
Tb	<0.02	–	0.006	0.03–0.35 0.25 ± 0.01	0.14–0.27 0.20 ± 0.01	0.82	0.06–0.36 0.19 ± 0.01	0.21–0.47 0.32 ± 0.01	1
Dy	<0.04	–	0.03	0.8–1.7 1.3 ± 0.1	0.8–2.5 1.3 ± 0.1	4.25	0.3–1.9 0.9 ± 0.1	1.2–2.6 1.8 ± 0.1	4.6
Ho	<0.01	–	0.007	<0.01–0.28 0.18 ± 0.01	0.13–0.63 0.21 ± 0.03	0.88	0.05–0.33 0.15 ± 0.01	0.18–0.42 0.29 ± 0.02	1.3
Er	0.012–0.046 0.028 ± 0.002	–	0.02	0.29–0.75 0.52 ± 0.02	0.4–2.6 0.6 ± 0.1	2.23	0.12–0.88 0.39 ± 0.03	0.4–1.1 0.7 ± 0.1	3.1
Tm	<0.013–0.014	–	0.003	<0.002–0.062 0.029 ± 0.004	0.03–0.48 0.08 ± 0.03	0.38	0.002–0.090 0.030 ± 0.003	0.04–0.12 0.08 ± 0.01	0.5
Yb	<0.11	–	0.02	<0.01–0.52 0.30 ± 0.02	0.2–4.2 0.6 ± 0.2	2.11	0.08–0.61 0.25 ± 0.02	0.27–0.72 0.47 ± 0.04	0.3
Lu	<0.001	–	0.024	<0.001–0.050 0.022 ± 0.003	0.02–0.83 0.10 ± 0.05	0.35	<0.001–0.078 0.023 ± 0.003	0.03–0.10 0.06 ± 0.01	0.8
Hf	0.10–0.21 0.17 ± 0.01	–	0.006	0.03–0.62 0.23 ± 0.02	0.1–31.3 2.8 ± 1.8	4.04	0.02–0.20 0.08 ± 0.01	0.04–0.26 0.13 ± 0.02	1
Re	<0.1	–	0.0004	<0.0001–0.006	<0.001	–	<0.0001–0.005	<0.0003	0.0007
Tl	<0.05	–	0.007	<0.004–0.20 0.10 ± 0.01	0.06–0.22 0.13 ± 0.01	0.53	0.01–0.18 0.05 ± 0.01	0.04–0.14 0.08 ± 0.01	1

. The data in Table 3 are grouped by geographical location: sites 1–3, located in Russia, and sites 4–5, located in Azerbaijan. For comparative purposes, global mean concentrations of elements in river water [69,70,71] and SPM [69,72,73], as well as Clarke values in the continental crust [64,74], were adopted as reference values.

The calculated AC are provided in

Table 4. Range of AC of trace elements, including heavy metals, in SPM and BS of mountain rivers in Russia and Azerbaijan.

Element	Suspended Matter			Bottom Sediments
	AC Range in Districts 1–3 (Russia)	AC Range in Districts 4–5 (Azerbaijan)	Range of AC in Suspended Matter (rivers of Russia and Azerbaijan)	AC Range in Districts 1–3 (Russia)	AC Range in Districts 4–5 (Azerbaijan)	AC Range in Bottom Sediments (Rivers of the Russian Federation and Azerbaijan)
Li	5.5·103	1.2·103–8.4·104	n × 103–n × 104	2.5·102–2.2·104	7.3·102	n × 102–n × 104
Be	5.5·104	5.0·102–1.3·103	n × 102–n × 104	7.9·104	3.0·102–1.0·103	n × 102–n × 104
B	6.3·101	2.3·102–3.5·103	n × 101–n × 103	7.2·101	5.6·101–1.4·102	n × 101–n × 102
Na	5.3·101	3.7·101	n × 101	7.6·100–3.7·101	9.9·100–3.6·101	n × 100–n × 101
Mg	3.4·104	WND	n × 104	7.2·104	WND	n × 104
Al	3.2·105–4.2·106	2.8·105–2.1·106	n × 105–n × 106	2.3·103–5.5·105	6.2·105–2.2·106	n × 103–n × 106
P	3.5·105	8.1·105	n × 105	1.5·104–3.8·105	6.4·105	n × 104–n × 105
K	5.7·102–3.3·103	6.5·101–5.0·102	n × 101–n × 103	9.3·101–2.3·103	2.5·102–1.2·103	n × 101–n × 103
Ca	2.5·101–1.2·102	WND	n × 101–n × 102	8.8·100–4.3·101	WND	n × 100–n × 101
Sc	3.8·103–1.8·104	WND	n × 103–n × 104	1.0·103–1.1·104	WND	n × 103–n × 104
Ti	8.2·104	7.2·102–1.1·104	n × 102–n × 104	5.9·103–6.5·104	5.4·103	n × 103–n × 104
V	5.2·103–9.7·104	5.0·102–3.7·103	n × 102–n × 104	1.8·103–7.2·104	4.4·102–1.2·103	n × 102–n × 104
Cr	4.3·104–1.0·105	7.9·103	n × 103–n × 105	5.7·103–3.4·104	5.6·103	n × 103–n × 104
Mn	2.9·102–3.0·105	3.2·105	n × 102–n × 105	3.7·101–2.1·105	2.7·105	n × 101–n × 105
Fe	4.4·105	8.1·105	n × 105	1.2·104–1.3·105	5.5·105–4.0·106	n × 105–n × 106
Co	3.9·104–2.4·105	6.0·103–2.2·105	n × 103–n × 105	4.5·103–6.3·104	6.6·103–1.5·104	n × 103–n × 104
Ni	1.6·104	1.6·104–5.5·105	n × 104–n × 105	3.2·103–1.0·104	4.2·104	n × 103–n × 104
Cu	7.6·103–2.4·104	5.3·103	n × 103–n × 104	1.4·103–1.7·104	4.9·103	n × 103–n × 104
Zn	1.7·104	1.1·104–1.1·106	n × 104–n × 106	7.0·102–9.3·103	2.0·104	n × 102–n × 104
Ga	6.1·104	1.2·103–8.0·104	n × 103–n × 104	1.3·103–2.2·104	4.6·103	n × 103–n × 104
Ge	9.6·104–2.0·105	WND	n × 104–n × 105	2.0·104	WND	n × 104
As	7.2·103	7.3·102	n × 102–n × 103	9.1·102	8.1·102–1.6·103	n × 102–n × 103
Se	1.8·103	4.0·102	n × 102–n × 103	5.6·101	6.9·101–5.0·102	n × 101–n × 102
Br	6.1·102–4.2·103	WND	n × 102–n × 103	9.0·101–6.8·102	WND	n × 101–n × 102
Rb	9.3·103–9.6·104	WND	n × 103–n × 104	7.8·102–3.1·104	WND	n × 102–n × 104
Sr	3.1·102	WND	n × 102	2.1·101–1.6·102	WND	n × 101–n × 102
Y	5.3·105–1.2·106	WND	n × 105–n × 106	2.0·105	WND	n × 105
Zr	8.0·103–7.7·104	WND	n × 103–n × 104	7.7·102–2.0·103	WND	n × 102–n × 103
Nb	4.5·103	WND	n × 103	4.0·101	WND	n × 101
Mo	1.0·102–2.9·103	2.0·102	n × 102–n × 103	1.8·102–1.9·103	1.5·102	n × 102–n × 103
Ru	1.0·102–2.8·103	WND	n × 102–n × 103	3.3·101	WND	n × 101
Rh	4.6·103	WND	n × 103	5.0·101	WND	n × 101
Pd	5.0·102–5.8·105	WND	n × 102–n × 105	8.8·102	WND	n × 102
Ag	2.0·101–3.9·104	1.6·100–5.0·102	n × 100–n × 104	4.0·101	1.0·100–4.3·103	n × 100–n × 103
Cd	3.3·101–7.0·102	1.0·101–3.6·106	n × 101–n × 106	2.0·101	1.0·102–5.9·103	n × 101–n × 103
In	3.3·101–7.0·102	WND	n × 101–n × 102	3.3·101	WND	n × 101
Sn	5.0·102–1.3·103	8.1·101	n × 101–n × 102	1.7·101–5.0·102	3.8·101	n × 101–n × 102
Sb	1.7·102–5.7·103	2.0·100–1.4·102	n × 100–n × 103	2.5·101–1.7·102	6.0·100–5.2·101	1.6·100–5.0·102
Te	9.0·101–4.5·102	WND	n × 101–n × 102	9.0·101	WND	n × 101
I	5.9·102–1.0·104	WND	n × 102–n × 104	5.4·101–2.6·102	WND	n × 101–n × 102
Cs	6.0·104–1.9·105	WND	n × 104–n × 105	1.6·104	WND	n × 104
Ba	9.0·103	5.7·103–1.8·105	n × 103–n × 105	3.7·102–5.2·103	8.2·103	n × 102–n × 103
La	3.8·104–1.1·105	WND	n × 104–n × 105	2.5·104	WND	n × 104
Ce	7.1·104–2.3·105	WND	n × 104–n × 105	5.2·104	WND	n × 104
Pr	7.1·104–1.5·105	WND	n × 104–n × 105	4.1·104	WND	n × 104
Nd	3.2·105	WND	n × 105	9.0·104	WND	n × 104
Sm	6.0·104–1.3·105	WND	n × 104–n × 105	3.0·104	WND	n × 104
Eu	3.2·104	WND	n × 104	6.5·103	WND	n × 103
Gd	6.5·104	WND	n × 104	1.3·104	WND	n × 104
Tb	7.1·103–1.5·104	WND	n × 103–n × 104	3.0·103	WND	n × 103
Dy	4.3·104	WND	n × 104	7.5·103	WND	n × 103
Ho	1.0·103–2.8·104	WND	n × 103–n × 104	5.0·103	WND	n × 103
Er	2.5·104	WND	n × 104	8.3·103	WND	n × 103
Tm	1.5·102–4.4·103	WND	n × 102–n × 103	1.5·102	WND	n × 102
Yb	9.1·100–4.7·103	WND	n × 100–n × 103	7.3·102	WND	n × 102
Lu	1.0·103–5.0·104	WND	n × 103–n × 104	1.0·103	WND	n × 103
Hf	1.0·102–5.0·103	WND	n × 102–n × 103	9.5·101–2.0·102	WND	n × 101–n × 102
Ta	1.6·102	WND	n × 102	6.7·100	WND	n × 100
W	7.5·103	WND	n × 103	2.5·101–1.0·103	WND	n × 101–n × 103
Re	6.0·100	WND	n × 100	1.0·100	WND	n × 100
Os	2.9·101–8.6·102	WND	n × 101–n × 102	2.8·101	WND	n × 101
Ir	1.0·104	WND	n × 104	3.0·102	WND	n × 102
Pt	5.0·104	WND	n × 104	2.0·103	WND	n × 103
Au	7.5·101	WND	n × 101	5.0·101	WND	n × 101
Hg	4.0·101–1.1·103	WND	n × 101–n × 103	3.2·102	WND	n × 102
Tl	8.0·101–4.0·103	WND	n × 101–n × 103	2.0·102	WND	n × 102
Pb	4.8·104–3.1·105	1.2·103–1. 7·104	n × 103–n × 105	2.8·104	3.4·103	n × 103–n × 104
Bi	4.0·102–2.8·104	WND	n × 102–n × 104	6.0·102	WND	n × 102
Th	8.1·104–1.1·105	WND	n × 104–n × 105	3.1·104	WND	n × 104
U	2.4·102–1.1·104	WND	n × 102–n × 104	2.9·101–1.2·104	WND	n × 101–n × 104

Note: WND—Was not determined.

.

As shown in Table 4, AC for trace elements in SPM and BS of the studied rivers varied from n × 10⁰ times n × 10⁶ times. In Russian rivers, the highest AC values in SPM (up to n × 10⁶ times) were recorded for Al and Y. Elevated AC values were also observed for the following groups of elements: (n × 10⁵)—P, Cr, Fe, Mn, Co, Ge, Pd, Cs, La, Ce, Pb, Nd, Sm, Pr, Th and (n·10⁴)—Be, Mg, Ti, V, Ni, Zn, Ga, Rb, Zr, Ag, I, Tb, Dy, Ho, Er, Lu, Ir, Pt, Bi, U.

The lowest AC values in SPM were observed for Yb and Re (n × 10⁰ times), and for B, Na, Ca, Cd, Os, Hg, and Tl (n·10¹). For all other elements, AC values generally ranged between n × 10²–n × 10³.

In Azerbaijani rivers, the ranking of AC values in SPM (Table 4) demonstrated a similar pattern. The highest ACs (n × 10⁶) were associated with Al, Zn, and Cd, while substantial values (n × 10⁵) were obtained for P, V, Fe, Co, Ni, and Ba. Moderately elevated ACs (n × 10⁴) were characteristic of Li, Ti, Ga, and Pb. The lowest values were recorded for Ag and Sb (n × 10⁰), and for Na, K, and Sn (n × 10¹).

According to Egorov [35], AC values also reflect the tendency of elements to transfer from the dissolved phase to solid phases and accumulate in bottom sediments. For elements with AC SPM > 10⁵, nearly the entire pool is bound to particles and subsequently deposited into sediments. Conversely, low AC SPM values indicate a predominance of dissolved forms in the aquatic environment.

Our calculations showed that, in Russian mountain rivers, AC BS values were generally one order of magnitude lower than the corresponding AC SPM, ranging from (n × 10⁰) to (n × 10⁵). A similar pattern was observed in Azerbaijani rivers, although exceptionally high ACBS values of up to (n × 10⁶) were calculated for Al and Fe.

Overall, our results show that in mountain rivers of both Russia and Azerbaijan, high AC values (n × 10⁴)–(n × 10⁶) were consistently associated with Al and several toxic metals (Pb, Zn, Cd, Ni), independent of hydrochemical background and discharge variability. The systematically lower AC values in BS compared to SPM highlight the distinct geochemical partitioning and accumulation pathways between suspended and deposited phases.

EF for bottom sediments relative to the studied elements are presented in

Table 5. EF of bottom sediments in the studied rivers, calculated using Fe as the reference element.

Element	EF		Element	EF		Element	EF
	Districts 1–3 (Russia)	Districts 4–5 (Azerbaijan)		Districts 1–3 (Russia)	Districts 4–5 (Azerbaijan)		Districts 1–3 (Russia)	Districts 4–5 (Azerbaijan)
Li	0.2–5.1	0.8–1.8	Br	15–510	0	Nd	0.4–1.4	0.3–3.6
Be	0.03–0.42	0.2–0.7	Rb	0.2–0.7	0.1–0.9	Sm	0.4–1.2	0.4–2.6
B	0.3–5.9	1.1–10.5	Sr	0.1–1.8	0.3–5.9	Eu	0.4–2.0	0.6–3.4
Na	0.01–0.30	0.02–0.25	Y	0.3–1.0	0.3–1.8	Gd	0.4–1.3	0.5–2.5
Mg	0.2–0.5	0.4–1.2	Zr	0.01–0.14	0.01–0.11	Tb	0.1–1.1	0.5–2.0
Al	0.1–0.4	0.2–1.2	Nb	0.002–0.071	0.001–0.009	Dy	0.4–1.2	0.5–2.6
P	17–232	20–48	Mo	0.9–29.5	0.9–2.0	Ho	0.1–0.7	0.3–1.6
K	0.1–0.3	0.1–0.7	Ru	1.7–91.7	2.9–38.8	Er	0.2–0.8	0.2–1.7
Ca	0.1–3.0	0.4–17.0	Rh	1.2–47.4	0	Tm	0.1–0.5	0.1–1.2
Sc	0.4–1.1	0.5–1.6	Pd	44–276	14–133	Yb	1.0–5.6	1.3–12.8
Ti	0.01–0.09	0.01–0.05	Ag	1.9–1450	0.24–2137	Lu	0.03–0.23	0.1–0.6
V	0.3–0.8	0.1–1.4	Cd	0.2–55.8	1.2–229.6	Hf	0.1–1.1	0.1–1.0
Cr	0.5–4.1	0.2–0.9	In	0.1–0.9	0.1–0.7	Ta	0	0.01–0.04
Mn	0.2–1.1	0.7–29.3	Sn	0.1–5.9	0.6–7.3	W	0.01–0.46	0.01–0.47
Fe	1	1	Sb	0.1–8.4	0.1–1.0	Os	29–615	24.4–70.9
Co	0.8–2.8	0.9–2.7	Te	69–761	158–5822	Au	1.0–3.7	6.4–31.7
Ni	0.5–2.6	0.8–2.2	I	16–713	46–474	Hg	0.7–13.2	0.1–2.6
Cu	0.4–1.6	0.7–3.4	Cs	1.1–4.3	0.8–3.0	Tl	0.1–0.9	0.1–0.5
Zn	1.0–6.2	0.9–2.9	Ba	0.1–0.9	0.2–2.5	Pb	0.9–3.6	1.3–3.9
Ga	0.9–4.4	0.7–7.1	La	0.3–1.3	0.1–3.1	Bi	52–793	59–232
Ge	12.4–28.7	3.0–5.6	Ce	0.2–1.1	0.2–2.2	Th	0.3–1.0	0.2–1.4
As	6.3–18.0	2.5–11.8	Pr	0.3–1.4	0.2–3.1	U	0.1–1.7	0.04–0.90

Note: Values greater than 3 are shown in bold.

.

It was established (Table 5) that the ranges of EF for most trace elements in bottom sediments were broadly comparable between Russian and Azerbaijani rivers. EF values below 3, relative to Clarke concentrations in the Earth’s crust, generally indicate a predominantly lithogenic origin of trace elements in the mountain rivers of the Caucasus (Table 5). By contrast, EF values exceeding 3 are typically interpreted as evidence of substantial anthropogenic enrichment when compared with the average crustal composition [74,75,76,77,78].

In the present case, however, the interpretation of elevated EF values requires careful consideration. The studied mountain rivers drain the geological province of the Greater Caucasus Range, an area rich in mineral resources, including ore-bearing formations. This geological setting likely exerts a strong influence on the geochemical background of riverine sediments, complicating the distinction between natural lithogenic contributions and potential anthropogenic inputs.

4. Discussion

4.1. Dissolved Trace Elements in Mountain River Waters

Our results show that the concentrations of dissolved trace elements in mountain rivers of Chechnya, Dagestan, and Azerbaijan were substantially elevated, in many cases exceeding global river water averages by more than threefold (Figure 4).

High concentrations of Li, B, Na, P, Ti, V, Cu, Zn, Sr, Mo, and Sn were characteristic of rivers in both study regions. However, in Azerbaijan, mean and maximum concentrations of most of these elements (except P and Zn) were consistently higher than in Russian rivers. This likely reflects both natural differences in the geochemical background and variations in anthropogenic pressure across the catchments.

In rivers of the Russian Federation (sites 1–3; Table 3, Figure 1), relatively high concentrations of Ca, Mn, Fe, Ga, Br, Pd, and U were identified. In contrast, rivers in Azerbaijan (sites 4–5; Table 3, Figure 1) exhibited elevated levels of K, Cr, As, Se, Ag, and Pb. Importantly, in both regions the concentrations of Cu, Mn, Sr, B, V, and Mo exceeded MPCs established for surface waters (Table 1). In addition, exceedances of MPCs for Fe and Zn were recorded in sites 1–3, whereas Na, Li, Se, Sn, and Pb were above MPCs in sites 4–5. These results indicate an unfavorable geoecological status of the investigated rivers in relation to these elements.

To determine the structure of relationships between dissolved elements in water, a factor analysis (principal component analysis (PCA)) was performed. The results are shown in Figure 5. On each graph representing data on rivers in Russia, one association of chemical elements with negative loadings of factor 1 is highlighted This reflects the overall geochemical composition of river waters in the study areas. The concentrations of elements not included in the associations likely vary under the influence of local factors (lithogenic and anthropogenic) at the water sampling sites. Analysis of the results for Azerbaijani rivers is complicated by the limited set of elements and the less precise method for determining their concentrations.

4.2. Trace Elements in Suspended Matter

The concentration of total suspended matter (TSM) in the investigated rivers varied widely, spanning nearly three orders of magnitude—from as low as 3 mg/L to as high as 1200 mg/L, as recorded in the Sulak River basin (Russia). Mean TSM concentrations were 147 mg/L in Russian rivers (sites 1–3, Figure 1) and 190 mg/L in Azerbaijani rivers (sites 4–5, Figure 1). Maximum TSM levels in Azerbaijani rivers reached 600 mg/L, which is approximately half the maximum concentration observed in the Sulak River (Figure 6).

A consistent downstream increase in TSM concentrations was identified, reflecting the mobilization of particles from headwater areas and their progressive accumulation in lower reaches. The role of hydraulic infrastructure (reservoirs, hydropower plants) was also evident, as such structures disrupt natural flow regimes and act as sinks for suspended matter and the trace elements it transports.

Figure 7 presents the elements whose concentrations in suspended matter exceeded global river averages by more than threefold.

Overall, suspended matter in Azerbaijani rivers showed stronger enrichment in trace elements, both in terms of absolute concentrations (Table 3) and in the diversity of significantly enriched elements. This enrichment primarily reflects the geochemical composition of bedrock in the headwater regions and along river channels. The Greater Caucasus hosts extensive ore deposits, including iron, manganese, polymetallic, tungsten–molybdenum, and lead–zinc ores. These deposits also contain nickel, cobalt, gold, silver, indium, cadmium, bismuth, and other elements [79]. Anthropogenic influences are also likely to contribute, given the presence of hydropower plants (e.g., on the Sunzha River), as well as settlements, construction activity, and road infrastructure along river valleys.

The study area is characterized by a complex geological framework that exerts a direct control on the geochemical properties of river waters. Dagestan, in particular, is rich in mineral resources, including fuel and energy deposits (oil, gas, coal, oil shales, peat); nonferrous and rare metals (copper, lead, zinc, strontium, cobalt, mercury); precious metals (gold, silver, platinum); non-metallic resources (carbonate, sulfate, zeolite-bearing and siliceous rocks, clays, quartz sand, vein quartz, shell deposits, sulfur, phosphorites, etc.); and diverse groundwater reserves (fresh, mineral, thermal, industrial) [80,81].

According to Kurbanov and Dashtiev [82,83], three major mineral resource zones are identified within the Republic of Dagestan, two of which—the Metlyuta–Dzhurmut and Nagornaya zones—coincide with the catchments of the rivers studied in this work. The Metlyuta–Dzhurmut zone is primarily associated with volcanogenic massive sulfide and polymetallic ore deposits, whereas the Nagornaya zone is characterized by extensive celestine mineralization. Dagestan also hosts the largest strontium-bearing province in Russia [84]. Within the Sulak River basin, one of the country’s largest strontium (celestine) deposits, the Siniye Vody site, is located, which represents a significant source of geochemical input to the fluvial system.

In the Sunzha River basin, mineral resources are primarily composed of non-metallic deposits [85,86], similar to the river basins examined in Azerbaijan [87,88]. The region also contains rock outcrops and geological exposures that contribute chemical inputs to the rivers via surface runoff. As reported by Cherkashin and Gazaliev [88], the North-Eastern Caucasus contains sulfide minerals of iron, copper, lead, and zinc. Upon exposure to atmospheric oxygen and groundwater, these sulfides oxidize to form sulfates, facilitating the release and downstream transport of heavy metal ions such as Fe, Cu, Zn, and other elements associated with ore deposits.

Chemical constituents in river water, whether dissolved or particle-bound, significantly influence aquatic organisms. However, most water quality regulations primarily address dissolved fractions, reflecting water usage for domestic and industrial purposes, which typically involves the removal of suspended matter. Suspended particles nevertheless affect aquatic life by settling on surfaces, being ingested with food, or entering organisms during respiration. Consequently, organisms are exposed not only to particulate matter but also to the sorbed chemical compounds it carries, some of which may be toxic. Therefore, the “Dutch Standards” framework regulates total concentrations of trace elements, encompassing both dissolved and particulate-bound forms.

Assessment of total trace element concentrations in the studied rivers revealed exceedances of MPCtot for several heavy metals, including Be, V, Co, Ni, and Cu, in both regions. In the Azerbaijani rivers, elevated levels of Zn, Cd, and Ba were also observed (Figure 8), indicating compromised geoecological water quality concerning these elements.

Factor analysis for suspended matter elements revealed the presence of two associations of chemical elements—with negative and positive loadings of factor 1 in each river studied (Figure 9). This can be explained by the fact that the total suspended matter of the rivers has a lithogenic and biogenic component.

4.3. Bottom Sediments

The chemical composition of sediments in mountain rivers primarily reflects the geochemical properties of the channel bed and the lithological characteristics of the drained catchments. Consequently, comparisons with Clarke values in the Earth’s crust provide a useful geochemical benchmark. Substantial exceedances of these reference values may indicate either anthropogenic inputs or the presence of ore-bearing formations in the underlying geology.

Figure 10 illustrates the trace elements whose concentrations in bottom sediments exceeded the crustal average (Clarke values) by more than threefold.

When benchmarked against the Dutch Target and Intervention Guidelines for sediments, Ni and Ba exceeded permissible limits in the Russian study area, while Ba and Ag were elevated in Azerbaijani rivers.

EF—calculated relative to crustal Clarke values (Table 5)—indicate that most trace elements are primarily lithogenic in origin (EF < 3), consistent across both regions. Nevertheless, certain elements in each region exceeded the lithogenic threshold (EF > 3), with enrichment ranging from 1.1 to 1941 times (Table 5). Such anomalies likely stem from the geochemical characteristics of mountain catchment soils, which substantially imprint on sediment composition.

Elements exhibiting both lithogenic and anthropogenic contributions in Russian rivers include Li, B, Cr, Fe, Zn, Ga, Mo, Ru, Ag, Cd, Sn, Sb, Cs, Yb, Au, Hg, Pb, and Rh. In Azerbaijani rivers, these are B, Ca, Mn, Fe, Cu, Ga, Ge, As, Sr, Ru, Ag, Cd, Nd, Eu, Yb, and Pb.

Elements with a notably anthropogenic signal (i.e., elevated EF in sediments) in the Russian sector (Table 5) include P, Ge, As, Br, Pd, Te, I, Os, and Bi. For Azerbaijani rivers, these include P, Pd, Te, I, Os, Au, and Bi. Several of these (e.g., Te, Os, Pd) are rare elements whose high occurrence here likely reflects natural occurrence in polymetallic ore bodies of the Greater Caucasus rather than only anthropogenic inputs [79].

Overall, sediments from both regions are enriched in similar suites of elements—Ag, Te, I, Bi, Cd, Pd, Os, P—reflecting their shared geological context. Notable contrasts include lower Br and Rh but higher Au concentration in Azerbaijani sediments.

Based on widely used sediment contamination frameworks [74,75,76,77,78], several elements in Russian rivers reach “extremely severe” enrichment levels (P, Br, Pd, Ag, Cd, Te, I, Os, Bi), while in Azerbaijani rivers, elements such as Pd, Ag, Cd, Te, I, Os, and Bi fall into this highest contamination category. These results signal poor ecological status of bottom sediments regarding these elements.

In synthesis, while lithogenic sources are the main contributors of trace elements and heavy metals to the river ecosystems, anthropogenic impact is clearly discernible.

The predominantly lithogenic origin of the elements in the bottom sediment is confirmed by the results of factor analysis. As with the analysis of water data, the elements in the bottom sediments of each river form one association in the region of negative loadings of factor 1 (Figure 11).

Comparison of element concentrations between suspended matter and bottom sediments (Figure 12) showed that most trace element concentrations are higher in suspended particulate matter (SPM) than in sediments, often by multiple orders of magnitude.

For certain elements, maximum concentrations in suspended matter were hundreds to thousands of times higher than in sediments. This pattern likely reflects inputs of enriched allochthonous material from catchments, combined with downstream transport of highly concentrated suspended particles. These results highlight the critical role of suspended matter in concentrating trace elements in the studied river ecosystems [35], serving as the primary source of trace elements to bottom sediments in these mountain rivers.

4.4. Spatial Distribution of Elements in River Ecosystems

Although all five studied rivers belong to the mountain river systems of the Northeastern Caucasus, the spatial distribution of chemical elements within them is highly heterogeneous, with each river exhibiting distinct geochemical signatures.

To identify critical river reaches affected by heavy metal contamination and to trace potential sources of these pollutants, we analyzed the spatial distribution of elements across different components of river ecosystems (water, suspended particulate matter, and bottom sediments).

It is established [89] that elevated pH and Eh values correlate with increased concentrations of Pb, Zn, Ba, and Cu. Given that the primary differentiating parameter between the rivers of Russia and Azerbaijan was the pH level, it is plausible that in the rivers of Azerbaijan at sampling stations with alkaline conditions (pH 8–9), this factor influenced the content of the aforementioned elements. Furthermore, it is noted [89] that the pH of natural waters is also governed by the geology of the catchment basin.

The Greater Caucasus Range is a major folded mountain structure formed as a result of the collision between the Eurasian and Arabian lithospheric plates. Its formation is associated with the Alpine stage of tectogenesis, which continues actively to this day. The complex geodynamic setting, involving subduction, collision, and intense orogeny, has led to the formation of diverse tectonic zones, deep-focus magmatism, metamorphism, and the widespread development of fault tectonics. It is this dynamic and multifaceted geological history that has predetermined the exceptional mineral wealth of the region. The geological structure of the Greater Caucasus is characterized by zonation. Its structure includes the axial zone (Main Range), lateral ranges, and foredeeps. The axial zone (Main Range) is composed of Precambrian and Paleozoic crystalline schists, gneisses, and granites, intruded by Mesozoic and Cenozoic intrusions. This zone is the center of the mountain system and the source of many ore elements. The lateral ranges (Skalistyy, Pastbishchnyy, etc.) are composed mainly of Mesozoic and Cenozoic sedimentary strata (limestones, dolomites, argillites, sandstones). The foredeeps are filled with powerful thicknesses of Cenozoic deposits, which are associated with hydrocarbon deposits. At the same time, it should be noted that the river basins we have considered, which drain into the Caspian Sea, differ in their geological structure. The lower and middle parts of the basins are located predominantly in areas with Quaternary and Paleogene-Neogene deposits, while most of the upper reaches of the river basins are composed of Cretaceous and Jurassic deposits. It should also be said that the basins of the rivers in question are located on the northeastern slope of the Caucasus and do not have large, distinct deposits of metallic minerals. There are isolated deposits of oil, gas, and construction and mineral raw materials. Generally, the river basins under study primarily host deposits of combustible and non-metallic minerals. Particularly notable among the combustible mineral reserves is the Sunzha River basin. The most contrasting geological conditions are found in the Sulak River basin, where the middle and upper parts of the watershed fall within several metallogenic provinces. In the upper reaches of the Avarskoye Koisu River, there is a copper-lead-zinc ore district. In the upper reaches of the Sulak River basin, there is a lead-zinc metallogenic zone, and in the middle part of the basin, a strontium-magnesium metallogenic zone. In the Atachay River basin, a copper-lead-zinc metallogenic zone is located in the upper reaches. Furthermore, detailed characteristics of element content for the rivers and sampling stations in question are presented for the first time [90].

4.5. Rivers Sunzha, Sulak, and Ulluchay (Russia)

The geochemical characteristics of these Russian rivers include elevated Fe and Sr concentrations in water and consistently high I levels in suspended matter. Suspended loads of the Sunzha and Sulak Rivers are also enriched in Mo, whereas suspended matter from the Ulluchay and Sunzha Rivers shows notable Bi enrichment. These patterns were observed consistently across all sampling stations, suggesting a strong link to the intrinsic geochemical conditions of the catchments. Figure 13 highlights sampling sites where localized exceedances of element concentrations were recorded in water, suspended matter, and sediments.

Concentrations were compared against established benchmarks: MPCs for water, global river suspended matter averages, and Clarke values for the Earth’s crust in the case of bottom sediments.

It is important to note that elevated element concentrations were detected at different stations and in different ecosystem components, but most frequently in suspended matter and sediments. This indicates that trace elements are primarily delivered via solid-phase transport and exhibit limited mobility once incorporated into suspended particles or sediment deposits. At several sites, however, clear signatures of anthropogenic input were evident.

For instance, at station 17 (Figure 13, Sulak River, Chirkey Reservoir), sediments exhibited high concentrations of Ni, Cu, Cd, and As. This pattern is most likely associated with road runoff from a nearby highway. The impact is exacerbated by the presence of the reservoir, which reduces flow velocities and hinders the downstream transport of suspended material. Consequently, contaminants delivered to the system are prone to deposition and accumulation in bottom sediments.

The spatial distribution of sampling stations was analyzed using principal component analysis (PCA), which allows for the most effective analysis of multidimensional systems. As a result, leading factors PC1 and PC2 were identified for each studied component of the aquatic ecosystem: water, suspended matter, and bottom sediments (Figure 14). The sampling stations along the PC1/PC2 axes form a single cluster. This pattern indicates relatively similar conditions for the accumulation and distribution of elements at different stations. Some stations are significantly distant from the main association, indicating the influence of local factors at a specific station.

4.6. Karachay and Atachay Rivers (Azerbaijan)

These rivers differ markedly from those in the Russian sector, with water, SPM, and bottom sediments enriched in various elements. The water of both rivers contains elevated concentrations of V, Mo, Ag, Sn, and Sr. In addition, the Karachay River is enriched in Ti and Cr, while the Atachay River contains high levels of Li, B, Na, K, and Cu. In SPM, elevated concentrations are characteristic of Be, B, Al, V, and Au; in addition, K, Ba, and Pb are particularly enriched in the Karachay River, whereas Ca and Ce are elevated in the Atachay River. The bottom sediments of the Atachay River are distinguished from those of the other studied rivers by high concentrations of a wide range of elements (B, Na, Al, Ca, Ga, Rb, Sr, Zr, Ag, Sn, I, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Tm, Yb, Lu, Hf, Au), including a substantial number of rare earth elements. This unusually rich geochemical composition is likely attributable to the lithological and geochemical characteristics of the rocks in the catchment area. In contrast, the bottom sediments of the Karachay River show a more limited elemental diversity, although they display distinctive features such as elevated concentrations of Al, P, Fe, Cu, Ga, As, Ag, Sn, I, and Au. These geochemical signatures in water, SPM, and sediments of the Karachay and Atachay Rivers were consistently observed across nearly all sampling stations and, therefore, can be considered characteristic features of these river systems.

Among localized geochemical features of the Azerbaijani rivers, it is important to highlight stations 1 and 20, located closest to the river headwaters (Figure 15).

In both cases, the concentrations of most analyzed elements in SPM were two orders of magnitude higher than at the other stations. This can be explained by the input of coarse lithogenic fragments highly enriched in trace elements that are not transported further downstream but settle near the source zones. At the same time, the total SPM concentrations at these stations were 32 and 48 mg/L, respectively (Figure 15), which are relatively low values for mountain rivers. Consequently, the fluxes of trace elements associated with SPM are minor and cannot significantly affect the overall ecological status of the rivers.

The above conclusion is supported by the results of factor analysis performed using the PCA. Stations 1 and 2 are located at a significant distance from the main associations in the analysis of element concentrations in suspended matter and bottom sediments (Figure 16). It should be noted that, unlike the Russian rivers, stations belonging to the Atachay and Karachay rivers are represented by two separate clusters for each component analyzed. This indicates that these two rivers differ significantly in the chemical composition of their water, suspended matter, and bottom sediments.

Thus, the spatial distribution of trace elements, including heavy metals, in the mountain rivers of Russia differs substantially from that in the rivers of Azerbaijan. In the SPM of the Sunzha, Sulak, and Ulluchay rivers, high concentrations of Bi and Mo are observed, which are associated with the geochemical conditions of their catchments. Anthropogenic inputs of heavy metals into these river systems are linked to the construction of reservoirs along their courses and the presence of various infrastructure facilities in close proximity to the rivers (for example, at station 17 of the Sulak River—Chirkey Reservoir, as well as a nearby highway).

In contrast, the waters of the Azerbaijani rivers are enriched in V, Mo, Ag, Sn, Sr, Ti, Cr, Li, B, Na, K, and Cu. The Atachay River sediments are notable for their enrichment in a wide spectrum of elements (B, Na, Al, Ca, Ga, Rb, Sr, Zr, Ag, Sn, I, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Tm, Yb, Lu, Hf, Au), including many rare earth elements. Such an unusually diverse geochemical composition is most likely attributable to the mineralogical and lithological characteristics of the regional bedrock.

In recent years, publications have emerged related to the river basins under study [55] or to adjacent river basins [25,91]. However, they do not allow for a comprehensive assessment of all processes due to a lack of data, a gap which this scientific work aims to fill. At the same time, there is a large number of publications with global coverage, which, however, do not provide a complete picture of the specific river basins under investigation [92,93,94,95].

It is very difficult to compare our data with the results of other studies for our region of interest and for other regions. As emphasized earlier, the data obtained for these river basins are largely the first of their kind, and there is simply nothing to compare them with. The study’s limitations include a number of factors: the limited number and access to sampling points, the sampling methodology, post-processing of samples, analysis using the specific equipment employed, and the comparability of the obtained data with data from other instruments and equipment.

5. Conclusions

Elevated concentrations of dissolved forms of a number of trace elements (Li, B, Na, P, Ti, V, Cu, Zn, Sr, Mo, Sn) have been identified in the river basins of the North-Eastern Caucasus (Sunzha, Sulak, Ulluchay, Karachay, and Atachay). Systematic exceedances of maximum permissible concentrations (MPCs) for a group of heavy metals were revealed: in the studied rivers of Russia and Azerbaijan for Be, V, Co, Ni, Cu; with additional exceedances for Zn, Cd, and Ba in the watercourses of the Azerbaijani sector. A shift in the hydrogen index (pH) towards the alkaline range (up to 9.88) was recorded in the rivers of Azerbaijan, rendering the water quality non-compliant with hygienic standards for drinking water use.

Suspended solids and bottom sediments act as the primary reservoir and concentrator for Al, Pb, Zn, Cd, and Ni. The accumulation of elements in bottom sediments proceeds less intensively than in suspended form. At a number of sampling stations in the rivers, the negative values of the redox potential (Eh) in the bottom sediments pose a risk of secondary water pollution. A comparison with reference values (Dutch Target) revealed exceedances of MPCs for Ni and Ba in the bottom sediments of rivers in the Russian part, and for Ba and Ag in the Azerbaijani part.

The calculation of enrichment factors (EF) indicates the predominance of a lithogenic source for the bulk of trace elements (EF < 3) in the rivers of both Russia and Azerbaijan, which is corroborated by elevated background concentrations of rare earth elements. Concurrently, a suite of elements indicative of anthropogenic impact was identified: for the rivers of Russia—P, Ge, As, Br, Pd, Te, I, Os, Bi; for the rivers of Azerbaijan—P, Pd, Te, I, Os, Au, Bi. Local anomalies of key heavy metals (Cu, Zn, Cd, As, Hg) also point to the presence of point sources of anthropogenic pollution.