1 Introduction

Upland areas belong to the most complex regions in the biosphere from both geomorphological and biological points of view. Vertical zonal sequence in these areas was investigated in most detail and proved clear relation between the biotic and soil parameters in terms of chemical composition. Flows of chemical elements are controlled by mechanical (gravitation), aerial, and water migration. While plain areas are noted for a relatively stable natural migration of chemical elements resulting in soil formation corresponding to particular soil-forming rocks, in the mountain regions migration is complicated and depends upon the rock weathering and transformation in labile geomorphic, hydrogeological, and biotic conditions (Miller 1997; Mukherjee 2001; Kabata-Pendias and Muckherjee 2007; Lacatusu et al. 2009). Investigation of biogeochemical differentiation of these areas should account of both external and internal factors. Natural factors of ecological risk (volcanic activity, flooding, earthquakes, mud streams, landslides, erosion, etc.) here are most pronounced. In case of anthropogenic activity, e.g., mining, waste storage, and tailings may cause damage in case of mismanagement or natural disaster in the area due to migration of toxic materials to settlements and arable areas (Adamo et al. 1996; Avila et al. 2008; Frau et al. 2009; Audry et al. 2010; Mukherjee et al. 2010; González-Corrochano et al. 2014). This paper presents the data on biogeochemical evaluation of contamination of the Ardon River basin by heavy metals originating from the mined polymetallic deposits of the Sadon ore-bearing region.

Our biogeochemical studies of the area were performed in 2001, 2003, and 2008 to reveal the impact of environmental and technogenic factors on chemical composition of the main components of the Pb-Zn natural biogeochemical province and its dynamics. The main objectives of investigation were: 1) to evaluate biogeochemical effects of the particular factors changing landscape structure due to enhanced mass migration and erosion of the outcropping rocks; 2) to follow transformation of chemical composition of the draining waters and flood plain soils, as well as changes in species’ composition; 3) to reveal the species indicating both natural and anthropogenic transformation of the environment; 4) to study relation between chemical composition of the solid and liquid phases of the Ardon River waters; and 5) to evaluate ecological consequences and risk of contamination of the area by heavy metals during natural and technogenic events.

2 Study area

The Ardon River is a left tributary of the Terek—the main river of the Republic of Northern Ossetia. In its upper reaches, the Ardon River drains the Sadon ore region bearing Pb-Zn deposits. Geographical coordinates of the Ardon River basin are generally cited as follows: 42° 41′ 36″ N 43° 58′ 30″ E (Alagir)-43° 14′ 18″ N 44° 18′ 52″ E (Ardon River mouth, Fig. 1). The main part of the study area lies between 800 and 1100 m above the sea level. According to its landscape and geochemical features, the Ardon basin may be divided into four sections: 1—the background area (Zaramag depression); 2—the medium mountain mining area with technogenic contamination (Misur–Unal tailings); 3—the low mountain area with local technogenic contamination; and 4—the plain area of the lower Ardon characterized by secondary redistribution and local accumulation of heavy metals (Fig. 2).

Fig. 1
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Location of North Ossetia and the study area

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Fig. 2
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The Ardon River basin with the studied geochemical sections (a–d) and location of ore deposits and the Misur MPC premises and tailings

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The generally accepted concepts suggest that ore deposition was triggered by the sublatitudinal deep-seated Sadon–Unal normal fault. The ores were formed in the Middle Jurassic period (the pre-Callovian time) when tectonic activity initiated intensive motion, heating, shearing, detachment of rocks (gabbro-diabase and andesite–dacite stocks and dikes), and hydrothermal processes. The heated solutions of the fault penetrating through the shattered Paleozoic granites mobilized rock components, including metals and sulfide sulfur. The ore-bearing solutions ascended along steep fissures and exchanged with the colder host rocks. Gradual cooling of the solutions led to the formation of base-metal filling veins and aureoles (Borisov et al. 2006).

The Sadon ore region contains about 300 outcroppings of vein deposits incorporated in the ore-bearing argillites and granites. The main ore minerals found in veins are presented by sphalerite, galena, pyrite, pyrrhotine, chalcopyrite, and such accessory minerals as arsenopyrite and marcasite. The area has been mined for Pb and Zn since 1853. Since the 1880s, the development of ore deposits has been carried out at five mines, four of them being located in the upper reaches of the Ardon River (Zakharov 1930). The mining infrastructure includes Mizur dressing works operating since that time.

Mean Pb content in ore deposits varies from 0.42 to 12.8 %. Mean concentration of Zn ranges from 1.87 to 26.2 %.

The main sources of environmental contamination are:

  • The mining solid and liquid wastes

  • Open tailing dumps

  • Wastes of the mining and processing combine

  • Deflation of dust from the surface of the tailings

  • Migration of pollutants with water

Every source has its characteristic features defining the set of heavy metals, their concentration intervals, and occurrence forms.

3 Methods

3.1 Field studies

Natural geochemical anomalies connected with the zones of dispersed ore mineralization were studied by mapping and soil and rock sampling to monitor dynamics of heavy metal concentration and lateral migration (Degtyarev and Ermakov 1998; Matveev et al. 2004). Distribution of heavy metals and other trace elements in the main landscape components within the river valley and transfer of metals to plants were studied by sampling water, plants, and soils upstream and downstream the mining and industrial areas (Karpova et al. 2007a, b) including both the hard polluted areas and surrounding territories.

En route sampling along an almost meridian 100-km long transect following the Ardon river from its headwaters of the confluence with the Terek River consisted of a series of test sites close to the following settlements: N. Zaramag (beginning), Tsey, Buron, Sadon, Mizur, N. Unal, Archon, Zintsar, Gril pond, Tamisk, Alagir, Ramonovo, Ardon, and Kardzhin (the Ardon mouth) (Fig. 2). The sampling was performed at test plots along cross-sections representing conjugated landscapes in August 2001, and was repeated in 2003 and 2008 at the plots affected by the mud stream of 2002. The cross-sections contained 42 test plots including soil vertical profiles 30–50 cm deep. Each test plot was characterized by a set of samples of the soil-forming rocks, master soil horizons, as well as by plant samples (particular species and averaged samples, Methods 1978; Motuzova 1988; Morozova 1992; Motuzova and Karpova 2013; Zyrin and Malakhov 1981). In total, 350 samples were collected. River sediments (top 10 cm 1–2 m away from the river bank) and water samples were taken where appropriate (15 samples of each type annually). The Unal tailing dump (sludge deposit) was sampled in its periphery. Samples of algae (14 samples annually) were washed in situ using river water. For microscopic study, algae samples were fixed in 70 % ethanol and 4 % formalin. Algae groups were analyzed for their genus. Their microscopic analysis was held at the Department of Mycology of the Faculty of Biology, M.V Lomonosov Moscow State University. Amphibia species were collected in the floodplain of the Ardon River at 12 test sites located from set. N. Zaramag to Kardzhin (Fig. 2).

Biogeochemical effects of the mudflow were evaluated by comparison of heavy metal concentration in soils, plants, and water before and after the indicated catastrophic event (Petrunina et al. 2006). In addition, the dairy cows’ hair and blood were sampled by veterinary specialists in the settlements of Unal and Mizur (the impact zone) and Alagir (background area) before and after the event to demonstrate consequences of the disaster in food chains. The number of the specimen for each settlement equaled 10 per sample type.

3.2 Laboratory studies

The samples were analyzed for chemical composition in the Vernadsky Institute. The air-dried plant specimen were decomposed in a mixture of HNO3 and HClO4, evaporated and diluted in HNO3, the solution was used to determine the concentration of heavy metals (HM) with the help of AAS-80 (Hitachi) or AAS-2A (KORTEC). Soil (particle size <2 mm) and other mineral samples were decomposed by means of a mixture of HNO2, HClO4, and HF acids. HM speciation in soils was determined by sequential extraction according to (Kuznetsov and Shimko 1990): 1) water soluble compounds (water-ethanol extraction); 2) exchangeable forms (1 M CH3COONa extraction, pH 7.0); 3) forms associated with carbonates and other compounds, soluble in weak acid (extraction with 1 M CH3COONa and CH3COOH buffer, pH 3.5); and 4) forms related to organic matter and some sulfides (boiling in 15 % H2O2) (Table 1). All the extracts were analyzed by means of AAS with the help of AAS-80 (Hitachi) or AAS-2A (KORTEC) using standard reference materials of soil (SRM 2709), plant mixture (SBMT-02), and hair (CRM 397).

Table 1 Scheme used for fractionation of trace elements in soils (Kuznetsov and Shimko 1990)
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Water pH, Eh, and salinity were determined in situ using pH-Eh meter MP 230 and conductometer MC 126. Heavy metals were determined in water after filtration with the help of AAS «Quant-Z.Eta» without pre-concentration. Suspension was analyzed separately.

Biochemical analysis of blood was carried out by means of technique described in Burtis et al. (2006). Metallothioneines were determined with the help of HPLC-NAM assay (Ermakov and Tyutikov 2008).

4 Results and discussion

4.1 Sources of pollution and release of metals with water

The study showed that heavy metals (Pb, Zn, Cd, Cu) found in the Ardon River and local biogeocenoses originated from dust fall-out, sewage of the Mizur Mining and Processing Combine (Misur MPC) and as well as from the natural series of Pb-Zn-ore deposits drained by tributaries of the Ardon River (Unal, Tsey, Sadon, etc.).

The sludge from the Mizur MPC is a thin suspension of rock particles (about 30 %) and the liquid waste products (about 70 %). The tail-depository surface is watered to prevent deflation. It is necessary to note that washing of the sludge leads to differentiation and chemical transformation of the composition of sediments (Degtyarev and Ermakov 1998; Degtyarev 1999).

We suggested that composition of the earlier and later deposited sludge was different, and the initial sludge contained higher concentrations of iron and manganese than the transformed one. This difference is of great importance. Transformation of the sludge started by decomposition of sulfides, first of all sphalerite, then the galena and pyrite, followed by migration of such elements as Ca, Mn, Zn, partially Fe from the precipitation tank. Concentration range of heavy metals in the mine rocks and the tailing solid and aqueous phases were almost similar. However, the minimum values of Zn, Cu, and As were higher in the solid phase of tailing dumps as compared to the mine rock debris, while in the aqueous phase of the latter the higher minimum concentration was found for Pb, Cd, and As (Table 2).

Table 2 Variation of heavy metals’ content in mine rocks and the solid (mg/kg) and liquid (mg/l) tailing phases
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Among metals, the highest concentration in the sludge was attributed to Pb (up to 9370 mg/kg) that was a characteristic feature of the pollution. In spite of addition of mortar providing alkaline pH (from 10 to 11) and metals’ precipitation in the form of carbonates (Pb and Zn), hydrocarbonates (Cu), hydroxides (Cd), some part of metals migrates with the waste water in soluble form. This is likely to take place due to formation of such ions as HMeO2 −, which is consistent with data of other investigators (Kraynov and Shvets 1992).

The medium estimate of metals’ mobilization in liquid phase of the tailing waste (kg/day) based on metal concentration in this phase and its outflow rate was as follows: 3.5 (Pb), 0.6 (Cu), 4.9 (Zn), and 0.06 (Cd).

Dump discharges to the river and sludge deflation from the dump surface in conditions of pH and Eh variation lead to sorption processes in the river alluvium and formation of anomalies in the river sediments. Compared to background concentrations in argillite and slates (typical rocks of the basin), the maximum concentration of impurities in the contaminated alluvium was 15 to 32 times as much for Pb, 4 times as much for Zn, and 40 times as much for Cd (Degtyarev 1999).

Water and alluvium sampling of the Ardon River showed also a significant spatial variation of chemical composition of the samples. In Unal village, the tail-depository water discharge into the river did not result in the marked increase of metal concentration in the river water except for As because of approximately 50 to 55 times dilution of waste solution in the river flow and transformation of the soluble metal forms into insoluble and sorption processes. Such phenomena as precipitation of metals in mixed waters were shown earlier in a special experiment with mixed water and alluvium column. For example, Pb concentration in the Ardon water 50 m downstream the tail-depository equaled to 13 μg/l while 200 m downstream it dropped to Pb background level of 0.8 μg/l. A similar situation was observed for Cu: its concentration changed from 5 to 0.2 μg/l (Degtyarev and Ermakov 1998; Degtyarev 1999).

High metal content in alluvium was found at the distance of 12 km downstream where two considerable anomalies of Pb and Cu in alluvium were found near tail-depository. One of them with maximum content of Pb (645 mg/kg) and Cu (65 mg/kg) was related to deflation of the tail-depository surface. Another anomaly was formed by the desilted wastewaters leached from the precipitation tank: maximum concentration of Pb and Cu equaled 328 and 60 mg/kg, respectively.

Both geochemical anomalies have spatial coincidence and form the corresponding biogeochemical anomaly in the adjacent meadow-alluvial soils (Fluvisols). Their relation to the tailings was proved by characteristic ratios of Pb, Cd, Zn, and Cu.

4.2 Changes in parameters of the natural and wastewaters under the impact of mudflows

Some parameters of water samples collected in the basin of the Ardon River are shown in Table 2.

The mudflow of 2002 (Evans et al. 2009) caused no drastic changes in composition of the natural and wastewaters in the Unal River basin. Salinity of the filtered water from the Ardon River was similar to that observed earlier and ranged from 60 to 90 mg/l, slightly increasing from the river head to its mouth at the confluence with the Terek River. Suspension content in the Ardon River water varied from 89 to 866 mg/l in 2001, and after the mudslide (in 2003) changed within the same range (from 92 to 715 mg/l in the Mizur-Kardzhin area). Its maximum value was determined in the mouth of the river (Kardzhin) and was caused by admixture of the suspended particles from the waters of the Terek River (Fig. 3). However, in the upper reaches of the Ardon River, the content of suspension markedly increased in 2008 (to 116–149 mg/l) as compared to that in 2001 but this was caused by technogenic activity, i.e., by erection of a new dam and river erosion of the construction materials.

Fig. 3
figure 3

Dynamics of the suspended load in the Ardon River before and after the mudflow of 2002. Years of sampling: 1—2001, 2—2003, and 3—2008

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Slight decrease was identified for the total salinity of the Ardon River water in 2008 as compared to that in 2003. According to observations in 2003, the salinity ranged from 76 mg/l (upper reaches) to 146 mg/l (the river mouth), and in 2008—from 61 to 131 mg/l, respectively. A nonlinear positive correlation was observed between the amount of suspended solids and total salinity (Fig. 4). In general, these parameters were controlled by the water supply regime (e.g., the glaciers’ melting and precipitation patterns) and the type of water-bearing rocks.

Fig. 4
figure 4

Correlation between the amount of suspension and water salinity in the Ardon River (data of 2003 and 2008)

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The redox potential of water in the river Ardon increases as a rule from its head to the mouth (Table 3). However, in 2003, its value decreased slightly from 139 to 193 mV (compared to 188–276 mV in 2001). In contrast to Eh, the pH of the Ardon River water decreases from its upper section to the lower reaches: in 2001 pH varied from 8.00 to 8.28, and after the mudflows in 2003 from 7.40 to 8.11. Similar year to year Eh and pH variations were registered for the Unaldon River: the Eh value decreased from 220 mV in 2002 to 166 mV in 2003, and the pH slightly increased from 7.15 to 7.24. For another Ardon tributary, the Mayramdon River, the tendencies were different as follows: Eh 187 mV (2002) and 193 mV (2003) and pH 8.17 (2002) and 8.07 (2003).

Table 3 Comparative parameters of natural and technogenic water in the river Ardon before (2001) and after (2003 and 2008) the mud flow event
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According to measurements, the water pH in the upper Ardon River reaches fell markedly from 8.2 to 7.7 in 2008 due to erosion and contact with the sandy rocks. This was registered in river water down to Mizur MPC but downstream the latter location the water pH, on the contrary, increased. This is apparently due to influence of the alkaline waste tailings and the type of water-bearing rocks (dolomite, limestone). Inflow of sulfurous water of the resort Tamisk decreases the pH of the Ardon River, but from Alagir to the mouth the river’s water becomes weak alkaline. This is typical for the tributaries of Ardon (Unaldon, Mayramdon).

The metals’ concentration in the water samples collected in 2003 significantly increased as compared to those of 2001. For example, the concentration of Pb in the Ardon River ranged from 0.6 to 0.8 μg/l in 2001 and from 0.6 to 1.6 μg/l in 2003 (Table 3). The same tendency was observed for Cd. The most dramatic increase in metal content was registered in 2003 in the Unaldon River water, apparently resulting from the catastrophic runoff and migration of ore material from the tail of the Holst polymetallic deposit. Later measurements of 2008 showed no further sharp increase in metal content in the water of Unaldon. However, the content of Cd in this creek remained very high (1.64 μg/l).

Enhanced Cu concentration was detected in water of the upper part of the Ardon River (6.4 μg/l) and in the tailing draining water in particular (95 μg/l). Elevated levels of Cu were observed also in the Ardon mouth apparently due to confluence with the Terek River. Lead concentration also increased (4.3 μg/l). Zinc content in the Ardon River water was low (in the range of 30 to 80 μg/l) and its concentration in the Unaldon water decreased as compared to 2003.

As a whole, the natural disaster of 2003 that occurred in the Ardon River basin caused some changes not only in morphology, but also in the nature of migration of chemical elements and in redistribution of mudflows’ material. This is particularly evident from the data obtained for the middle section of the Ardon River (the Unal hollow). The changes were less pronounced downstream the resort of Tamisk (exit to the plain); the changes showed themselves only in a slight increase in concentration of metals in water and in reduction of its pH.

Therefore, the studies conducted within 6 years after the disaster of 2003–2008 revealed no dramatic changes in concentration of metals in waters of the Ardon River and its tributaries. The higher (more alkaline) pH values of water observed after the disaster of 2002 was due to the expanded water contact with the outcropping carbonate rocks as a result of mudflow and building activities (gas pipeline laying, construction of the hydroelectric power plant).

4.3 Heavy metals in soils and deposits

The soil cover of the region is presented by different soils corresponding to natural zones, subzones, and intra-zonal features of the area, i.e., Fluvisols, Mollic Leptosols Eutric, Mollic Leptosols, Umbric Leptosols Brunic, and Mollic Leptosols Eutric (WRB 2006).

Floodplain soils of the upper and middle river sections are typically rich in rock debris. They are characterized by neutral pH values (рНw = 6.0–7.5), a high variability of organic matter content (Corg = 0.3–4.0 %), and cation exchange capacity (3–30 meq/100 g).

Concentration of heavy metals in soils of the background territory varied within the following ranges (mg/kg): Pb from 5 to 50, Zn from 100 to 360, Сu from 20 to 50, Сd from 0.02 to 0.1, and As from 2.4 to 16. Most of the soil heavy metals are strongly bound. In areas exposed to anthropogenic impact and effects, the concentration of Pb in soils increases up to 250–1900 mg/kg, that of Zn up to 450–1500 mg/kg, Cu up to 90–150 mg/kg, Cd up to 2.5–4 mg/kg, and As up to 25–40 mg/kg. High concentrations have also been found for Ag, Bi, and Cd. Maximum concentration of Pb reached two orders the baseline level. The area of Pb contamination exceeding the permissible limit in soils of the mining region in the Unal basin equaled to 40–50 ha (Fig. 5).

Fig. 5
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The anomalies of Pb in soils of the Unal hollow in 2001. MPL maximum permissible level. The topographic base prepared by Yu.M. Semenov (Matveev et al. 2004) with additions of the authors

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Despite the minor thickness of the vein deposits, the zones of dispersed ore mineralization formed considerable anomalies of Pb, Zn, Cu, As, Sb, Co and some other chemical elements (Karpova et al. 2007b). Technogenic input to the Ardon River accepting debris from the mining areas and tailings and wastes of the Mizur MPC as compared to ore-free basin of the Gizeldon stream (the Ardon river tributary) was estimated to be 9.6; 2.8; 1.4, and 1.2 times larger for Pb, Cd, Zn, and Cu correspondingly (Degtyarev 1999).

In the area of Unal village, five Pb anomalies of secondary genesis with high Pb and the related metals’ content were discovered (Matveev et al. 2004). They were located on the right bank of the Ardon River. Four of them are adjacent to the rocky floodplain of the Unaldon and Mayramdon rivers. The most extensive anomaly N3 (Fig. 5) formed a large spot in the alluvial floodplains and terraces of the Ardon River. The metal enrichment of soils of the Unal terrace was possible due to denudation of the weathered products of polymetallic ores in some deposits in the Unal river valley. Soil contamination was also related to N5 anomaly. Formation of the latter was associated with transition of the terrigenous material enriched in some metals and its accumulation by organic matter. The dust of the Unal tailings also played a definite role in pollution. Mine dumps are stored in the form of the aboveground open repository which is located in the Unal floodplain. Due to a strong wind erosion and deflation, the storage became the additional source of contamination of the surroundings. Impact of mining was easily detected in the heavy metals’ depth profile of the affected Fluvisol as compared to the background ones: the increase in the total content of Pb was 10–40 times as large, that of Cd 14–20 times as large, Zn 2.5–7 times as large, and Cu 2–3 times as large.

Repeated soil sampling was performed in the years of 2003 and 2008 after the mudflow of 2002. The sampling revealed:

  • A considerable enlargement of the secondary anomaly N3 in 2003 compared to 2001

  • The larger areas of Zn anomalies and its higher concentration in soils as compared to Pb (Pb is less mobile and concentrates on the sulfide barrier in any pH conditions)

  • The decrease of lead and other metals’ concentration in the upper horizon of meadow-alluvial soils within anomalies N2 and N5

  • Some minor changes in anomalies N1 and N4

  • A considerable change in the area of anomalies

The soils around the central part of Unal village showed a marked surface enrichment in the upper horizons (Fig. 6).

Fig. 6
figure 6

Depth distribution of metals in soil profiles. A and B, Mollic Leptosol Eutric (A 2001 and B 2003); C–D, Fluvisol (C 2001, D 2003, E 2008, respectively). Description of soil horizons (cm): Mollic Leptosol Eutric, 0–5 dark brown, fresh, silty, pulverulent, friable, with inclusions of small pebbles, abundant in roots, gradual transition to the underlying layer; 5–10 brown, dry, pulverulent fine granular, weakly consolidated, abundant roots, gradual transition to the underlying layer; 10–40 light red-brown, dry, pulverulent fine granular, consolidated, abundant fine stones, carbonate inclusions, rare roots; gradual transition zone to the next layer; 40 -… light red-brown, dry, practically unstructured, carbonate efflorescence covering granite rock debris of different size, solitary roots. Fluvisol, 0–3 black, moist, friable, crumbling with powder-like admixture, distinct transition; 3–8 dark red-brown, moist, more dense, crumbling with powder-like admixture; 8–40 dark red-brown with blue-greenish hue, moist to wet, dense, abundant pebble inclusions; 40–50 alluvium deposit with distinct blue-greenish hue, wet, water table at the depth of 50 cm

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Soil profile monitoring showed distinct increase in surface contamination and downward migration of both elements (Zn ≥ Pb, increment 10 cm). In the Mollic Leptosols Eutric (on left of the anomalies N4), lead accumulates in the lower horizon A. As a result of the mudflow of 2002, the distribution of lead changed with depth, and the concentration of the metal in the upper part of the horizon reduced as compared to 2001. Distribution of zinc before and after the mudslide did not change virtually. In the Fluvisols (anomaly N3), maximum accumulation of lead and zinc was also noted in the thin horizon A. As a resulting effect of mudflows, the distribution of metals changed and decreased with depth, indicating a strong external impact on soil-plant complex.

4.3.1 Mobile forms of heavy metals in soils

Contaminating heavy metals (HM) penetrated soils in soluble forms (physicochemical migration) and in the solid phase (mechanical transfer). Transformation of these combinations occurred under the influence of soil-forming processes. In the course of interaction with the native soil, the soluble compounds of HM lost their mobility. On the contrary, the processes of soil generation caused some changes of solid particles and partly led to increase of HM mobility. Resulting effect of transformation changes depended on specific soil-chemical properties and processes, which controlled dynamic balance in the relationship of HM forms in soils. The same properties and processes determined a buffer capacity of soils in respect to HM.

Relationship between the total concentration and the occurrence forms of different heavy metals investigated in the Ardon floodplain Fluvisols and changing of forms under a contamination impact is presented in Table 4.

Table 4 The total content and occurrence forms of heavy metals in some soils of the mountain regions of North Ossetia (mg/kg, in brackets—percentage)
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Maximal portions of Pb, Zn, and Cu (from 75 to 95 % of the total content) for all types of soil considered were present in the strongly fixed form which was not extractable.

As a result of geochemical situation developed here, the transformation of Pb and Zn forms was directed to their association with carbonates and others extracted by 1 M CH3COONa and CH3COOH buffer, pH 3.5. Maximum quantity of Pb (from 2.3 % in the background soil to 18–19 % in the contaminated soils) corresponded to the forms soluble in weak acids including the carbonates. This form was the main for Pb originating from the mine dumps and the tail-storage. Buffering properties of the investigated soils concerning transformation of Pb occurrence forms are determined by both the pool of its potentially mobile compounds in solid phase and Pb concentration in the soil solution. Even under high level of technogenic pressure, only some part (from 0.8 to 2.7 %) of the total content of the element reacted to form the exchangeable species.

Maximum quantities of Cu were associated with the organic matter (3.1–14.1 % for all types of soils considered). The tenths of percent of the total content of Cu were present in water soluble forms in both the background and technogenic soils. Possibly, this was related to soluble combinations of Cu with organic matter.

Maximum quantities of Zn in the background soil were related to organic matter or sulfides (5.2 %). Under technogenic impact transformation of Zn forms developed towards increasing the carbonate-related Zn portion (up to 13.3–15.3 %). The quota of Zn associated with the organic matter also increased in the affected soils, but not so significantly (up to 7.0–9.7 %).

Cadmium in the background soil conditions dominated in the organic-bound forms. Anthropogenic contamination increased the portion of Cd in exchangeable form (up to 10.7–16.4 %), and that associated with carbonates and other compounds soluble in weak acids (up to18.6–67.8 %). From the ecological point of view, a significant increase of mobile Cd compounds, even if its total level was low, presented the most dangerous consequence of changes in Fluvisols of the river Ardon valley areas with polymetallic mining contamination.

In technogenic floodplain soils, the total concentration of heavy metals and their mobile forms (fractions 1 and 2) were one-two orders the background values typical for background watershed and floodplain soils. Therefore, artificial carbonate binding is not sufficient to protect the floodplain from heavy metal contamination.

4.4 Heavy metals in plants and other organisms

Biogeochemical differentiation of plant associations and soil microbe complexes are of great importance for functioning of the local biogeochemical cycles and formation of anomalies.

Variation of heavy metal concentration in the averaged samples of meadow vegetation collected within the ore deposit areas and downstream the Ardon River valley is demonstrated in Fig. 7.

Fig. 7
figure 7

Distinct accumulation of Pb, Zn, Cd, and Cu in herbaceous vegetation within the mining area and the tailing dump. Settlements close to location of the sampling plots are indicated along X-axis

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Intensive accumulation of lead was observed in the particular ecogroups of plants. In the upper part of the Ardon basin outside the ore regions, the lowest level of Pb was measured in the hay of meadow grasses (<0.1–0.8 mg/kg); its concentration in xerophyte spiny grasses (Centaurea iberica) and arboreal living forms (willow, Salix sp.) was an order higher (1–2 mg/kg). Maximum enrichment in Pb was found in lithophytes, such as lichens (72.5 mg/kg). The same species-dependent Pb concentration was revealed near the Mizur MPC. With the soils containing as much Pb as 366 mg/kg, the hay contained 7.5 mg/kg, spiny grasses (C. iberica) 38.5 mg/kg, trees (alder, Alnus L.) about 7 mg/kg, and lythophytes (lichens) 755 mg/kg of Pb. The highest levels of Pb shown by edaphophytes reached 983 mg/kg (the bases of green mosses). Ether-bearing plants (Origanum vulgare) contained comparatively small quantities of Pb (7.5 mg/kg); even smaller quantities were found in spurge (Euphorbia virgata, 0.25 mg/kg). In the vicinity of the Unal tailing dump (the Ardon River terrace), where the concentration of Pb in the Mollic Leptosol Eutric reached 217 mg/kg, the steppe grasses (hay sample) contained as much Pb as 12.5 mg/kg; while Pb content in the particular species significantly varied as follows: E. virgata 5 mg/kg, C. iberica 3.5 mg/kg, and lichens 307 mg/kg. In the same region, Umbric Leptosol Dystric soil contained 471 mg/kg of Pb but Pb concentration in meadow grasses was below 0.07 mg/kg while spiny grasses accumulated 2 mg/kg of Pb (two orders higher as compared to grasses). In the lower reaches of the Ardon River, on the plain downstream the mining region where Pb concentration in Mollic Leptosol Eutric soil equaled to 85–87 mg/kg, the levels of Pb found in vegetation samples were the following: 1.5 mg/kg (hay samples) and 2.5 mg/kg (meadow grasses). The values observed in herbaceous plants were close to those detected in arborous species (alder and willow, 1.5 to 2 mg/kg).

In general, concentration of Pb in pasture and fodder plants of polymetallic biogeochemical provinces in North Ossetia reached 50–300 mg/kg, ranging from 2 to 360 mg/kg (dry matter). Copper and zinc concentrations rarely exceed the maximum permissible levels of these trace elements in fodder for farm animals.

A fulvate-humate type of humus and adaptive biochemical reactions of soil microorganisms facilitate fixation of metals. Positive correlation between concentration of Pb in soils, microorganisms, and plants and its mobile forms testifies to the moderate involvement of the metal into biogeochemical cycle. Soil microorganisms and plants show tendency to competitive interaction in the process of assimilation of mobile forms of metals in soils (Ermakov et al. 1996; Ermakov 1999).

Larvae and young amphibians were found to be highly sensitive to accumulation of metals in the environment. Lead concentration in amphibian species appeared to be 3–10 times as high as its background level (liver, mg/kg): Unal basin 1.95–6.94 (Cd), 80.1–271.4 (Cu), and 140.5–266.0 (Pb); Zintsar-Ramonovo 0.25–0.46 (Cd), 27.5–75.3 (Cu), and 54.0–57.0 (Pb). High-rate accumulation of metals in the liver of amphibians in some cases was accompanied by more active synthesis of metallothioneins. Activity of δ-aminolevulinic acid dehydratase in the blood of Rana ridibunda (but not Rana macronemis), living in polluted areas, was two times lower than that of the amphibia inhabiting water bodies in undisturbed background areas. Activity of glutathione peroxidase in the blood of various amphibian species ranged from 470 to 649 μM/min/g of hemoglobin (Unal) and from 864 to 1120 μM/min/g (Zintsar, Luar).

4.4.1 Indicative eco-geochemical capacity of algae species

Analysis of the character of Pb and Cd accumulation by algae species shown in Table 5 revealed different tendencies of metal absorption before and after the natural disaster in different areas, i.e., definite decrease of Pb and Cd concentration at burial places (Unal, Mizur) and the increased local contamination in the lakes/ponds of the Ardon River basin downstream (pond Gril, village Kardzhin).

Table 5 Heavy metals’ concentration (mg/kg, dry matter) in green algae and сharophytes before the catastrophic mudflow (2001) and 2 years after (2003)
Full size table

Thus, the natural mudflow of 2002 caused not only the geomorphological and hydrochemical changes within the Ardon River basin but also resulted in a considerable differentiation of the living matter (herbaceous plants and algae). It was discovered that HM accumulation by algal flora depended on the habitat substrate, the type of water, and hydrological regime. Ecological role of algae species in polluted Zn–Pb mining and smelting areas was also investigated by Cabala et al. (2011). In addition, algae can be successfully used for HM monitoring during natural disasters when other facilities are not available due to overloading with mud, elimination of the flood plain ox-bows, and indicator species of the terrestrial flora. Effectiveness of algae species’ usage in monitoring was related to their ability for rapid accumulation of metals in 2 weeks. For example, filamentous algae that invaded the newly formed drain in the village of N. Zaramag concentrated Cd and Pb up to the level of 4.1 and 38.1 mg/kg, respectively, and the stoneworts (Charaphyta) sampled in the affected pond of settlement Mizur also accumulated considerable amounts of these elements, i.e, 98 mg/kg (Pb) and 2.11 mg/kg (Cd). Algae species extracted the soluble forms of HM and accumulated them in biomass (like peat moss).

4.5 Biochemical analysis of cattle hair and blood

Lead concentration in cow hair in the studied areas subjected to technogenic load (Unal, Mizur) as measured in 2000 varied from 0.82 to 6.23 mg/kg (4.16 mg/kg on the average, n = 20) while in Moscow district according to our data the Pb content in cow hair ranged from 0.04 to 0.24 mg/kg (mean value 0.13 mg/kg; n = 32). In addition, the elevated level of zinc was observed in the cow tail bunch hair samples collected in N. Unal where soils and plants were enriched in this metal.

Biochemical studies of the blood of cattle living in the Ardon River basin were carried out to compare animals from the village of Mizur (the area most severely polluted with lead) and those from the city of Alagir (smaller degree of pollution). The level of hemoglobin in the animals from Mizur was somewhat higher (123–203 g/l) than that in the calves from Alagir (145–177 g/l). In both cases, this level exceeded the normal values for cattle of lowland territories (95–125 g/l), which was associated with the altitude and proved the known effect of elevation. The activity of glutathione peroxidase in both cases corresponded to the range of normal physiological conditions and showed no local difference. The activity of δ-aminolevulinic acid dehydratase in animals of both locations was low (below the normal range). In addition, the activity of this enzyme in cows and calves from Mizur was about two times lower than in the calves from Alagir. The obtained data indicated intoxication of the animals with Pb (possible blockage of hormones that control metabolism of calcium), especially in the village of Mizur.

Chemical analyses of the cattle whole blood showed a reduced concentration of Ca in the animals that live in the region of the Mizur MPC activity (33.6–41.6 mg/l) as compared to Ca level in the cow’s blood taken in Alagir (38.0–53.0 mg/l). On the whole, the Ca level in blood of the cattle from the studied province was notably lower than that in the cows from the Moscow region (43.9–50.0 mg/l). Some increase in Zn concentration (237–443 μg/l) was found in animals’ blood samples from the ore mining areas of North Ossetia in comparison with the Moscow region where the cow’s blood Zn ranged from 147 to 320 μg/l. No difference between these regions was revealed in blood concentration of Mg, Cu, and Se, 12.1–20.9 and 17.1–22.5 mg/l (Mg), 688–1200 and 650–880 μg/l (Cu), 75–117 and 80–95 μg/l (Se), respectively.

5 Conclusions

  1. 1.

    The Unal basin presents a vivid example of natural and anthropogenic transformation of Pb-Zn biogeochemical province leading to enhanced risks of ecological damage under conditions of extremely and dynamic geochemical environment. A significant local accumulation of Pb, Cd, Cu, and Zn in soils, water, and biotic components relative to background values occurred due to operation of the Misur Mining Combine.

  2. 2.

    A catastrophic natural mudflow of 2002 and the later construction of a gas pipeline and a dam for hydroelectric power station changed local landscapes and biota (plants, algae, and amphibians) and paragenetic association of chemical elements in soils characteristic for the area. Algae demonstrated high adaptive and to indicative capacity in case of both natural and artificial transformation of the environment.

  3. 3.

    Biogeochemical studies of 2001, 2003, and 2008 showed that in some cases the change of landscape structure due to mass migration and the erosion of outcropping rocks could also change the character of algae invasion.

  4. 4.

    A distinct relation between the particle size of the suspended matter in the Ardon River waters and water salinity was discovered. However, in general, the heavy metals’ concentration in waters of the river appeared to be within the acceptable hygienic standards and therefore ecologically not critical.

  5. 5.

    In accordance to biogeochemical criteria (Ermakov 1999, 2000), a number of revealed natural and technogenic provinces of primary and secondary accumulative genesis can be treated as the areas of ecological risk and crisis in terms of Pb and as the areas of risk in terms of Cd. Nevertheless, the rating of the entire Ardon River basin in terms of environmental stress or risk requires additional data on properties of the pollution sources, pathways, and volumes in the particular localities (including refuse piles), and on response reactions of living organisms inhabiting the areas with the extreme geochemical conditions, including the pathologies in animals and humans.