3.1. Main Ions and Total Dissolved Solids (TDS).
The results of the determination of the major ions and TDS are shown in Table 2
. The data of major cations and anions were plotted on the triangle diagram (Figure 2
). The cation points on the triangular diagram are located in the small area elongated in the direction of increasing the fraction content of sodium and decreasing the fraction content of alkaline–earth elements. Anion points are located in a narrow area limited by a small fractional content of chloride ions and elongated in the direction of increasing fraction contents of sulfate ions with a decrease in the fraction content of bicarbonate ions (Figure 2
At the natural background in the upper village Kholtoson (site M1), the water of the Modonkul river is fresh (bicarbonate class calcium-magnesium group). TDS varies 162–180 mg∙L−1
; the mean is 172 mg∙L−1
. The average pH is 7.3, with the range of variation 7.1–7.6. The fluoride content of F−
ions is 2.72 mg∙L−1
. The major cations are, in decreasing order, Ca2+
, and major anions are, also in decreasing order, HCO3−
, which is characterized as the natural river waters within the magmatic rocks. Among anions, bicarbonates dominate with the range of variation 100.3–109.6 mg∙L−1
; the mean value is 106.7 mg∙L−1
. The fractional content of bicarbonate is 78.5% eq. On the triangular diagram, the anion points are located in the lower area (Figure 2
). The sulfate ion concentration varies in the range 10.5–16.8 mg∙L−1
with the mean value of 15.2 mg∙L−1
(14.7% eq), Cl−
, the mean value is 6.1 mg∙L−1
(7.1% eq). Among the cations, predominantly calcium and magnesium account for 87.2% eq. The concentrations of Ca2+
vary in the range of variation 19.6–22.5 mg∙L−1
(the mean value is 20.1 mg∙L−1
) and 13.6–16.1 mg∙L−1
(the mean value is 14.1 mg∙L−1
), respectively. The concentrations of alkali metals are in the range 5.2–8.4 mg∙L−1
, the mean value is 6.3 mg∙L−1
. TDS varies from 169 to 198 mg∙L−1
. The equivalent ratio of ions is (Ca2+
= 1.24. The chemical type is HCO3–
The site M2 is located below flowing the mine water of the Kholtosonskoe deposit. At this site, the water of the river Modonkul is the result of the mixing of natural bicarbonate river water and sulfate acidic water from the adit. In this site, water is characterized as weakly acid–neutral pH (6.6–7.2), TDS varies 529–618 mg∙L−1
, mean value is 578 mg∙L−1
. The sum of major ions increases by almost 3 times. The concentration of the bicarbonate ion HCO3−
varies from 76.2 to 81.8 mg∙L−1
, the equivalent fraction is 19.3%–20.5% eq; the sulfate ion SO42−
—from 210 to 252 mg∙L−1
(74.8%–76.2% eq); Cl−
ion—from 7.8 to 8.2 mg∙L−1
(3.7%–3.8% eq), and Ca2+
—from 70.8.3 to 76.8 mg∙L−1
(34.6%–36.7% eq); Mg2+
—from 24.9 to 27.5 mg∙L−1
(21.6%–22.7% eq); Na+
—from 80.2 to 115 mg∙L−1
(39.6%–41.5% eq). The content of F−
ion increases by 24.1 times up to 64.2–66.7 mg∙L−1
. The major cations are, in decreasing order, Na+
, and the major anions are, also in decreasing order, SO42−
. The value of the equivalent ratio of ions (Ca2+
is 4.45. The chemical type was SO42−
. The chemical composition of water belongs to sulfate class sodium-calcium group (Table 2
In the triangular diagram, the cations are shifted in the direction of an increasing fraction of Na+-cation, the anions are shifted in the direction of an increasing fraction of sulfate-ion, remaining in a narrow area of the fraction of Cl− ion. The concentrations of the sulfate SO42− ion increase almost by 15 times, the bicarbonate HCO3− ion decreases by 1.3 times, the alkaline Na+ cation increases by 14.6 times, and the alkaline earth Ca2 + and Mg2 +cations increase by 1 8 to 3.6 times.
At the site M3, downstream up the city Zakamensk, the chemical composition of river water belongs to the bicarbonate-sulfate class of the calcium-sodium group. TDS varies 292–374 mg∙L−1, pH 7.06–7.15. Compared with the previous sampling site, the sum of main ions decreases by 1.7 times, the concentration of sulfate ions—by 1.4; sodium—by 4.1; calcium—by 2; magnesium—by 1.4; fluoride ions—by 9 times but only the bicarbonate ion concentration increases by 1.1 times. The chemical type is HCO3−-SO42−-Ca2+-Na+.
At the site M4, after the confluence of the tributary Incur, river water belongs to the sulfate-bicarbonate class of the calcium-sodium group. TDS varies 282–468 mg∙L−1. The water is neutral, slightly alkaline at pH 7.1–7.32. Compared with the previous sampling site, the sum of main ions increases by 1.1 times; the concentrations of calcium, magnesium, sodium, sulfate, chloride, and fluoride increase by 1.1 to 1.3 times. The concentration of bicarbonate ion decrease by 1.1 times. The chemical type is SO42−-HCO3−-Ca2+-Na+-Mg2+.
At the site M5 in the city Zakamensk, water belongs to the hydrocarbonate-sulfate class of the calcium-magnesium-sodium group. TDS varies 268–335 mg∙L−1. The water is slightly alkaline at pH 7.0–7.6. Compared with the previous sampling site, the concentrations of sulfates, fluorides, calcium decrease by 1.1–2.6 times, but the bicarbonate ion concentration increases by 1.3 times. The chemical type is HCO3−-SO42−-Ca2+-Mg2+-Na+.
The site M6 is located 1.2 to 1.5 km downstream from city Zakamensk, where the discharge of the household sewage is inflowed. In this site, water belongs to the sulfate class of the calcium group. TDS varies 758–867 mg∙L−1. The water is pH 6.8–7.4. Compared with the previous sampling site, the sum of the main ions increases by 2.5 times; the concentration of sulfates, fluorides, and calcium increase by 3.1–4.8 times. In the equivalent contents, the fraction of sulfates and calcium increases by 1.1–1.3 times, and the fraction of the other ions decreases by 1.1–1.6 times. The chemical type is SO42−-HCO3−-Ca2+-Na+.
The site M7 is located at the mouth of the river, where a fluvial terrace is formed, represented by fine-grained sands and clay materials. The water was slightly alkaline at pH 7.2–7.8. In this site, water belongs to the sulfate class of the calcium-magnesium group. TDS varies 672–727 mg∙L−1. The concentration of magnesium increases by 1.2 times and bicarbonates decrease by 1.8 times. The concentration of other ions changes no more than 1.1 times. The chemical type is SO42−-Ca2+-Mg2+.
Compared with the natural background (the upper part of the river), the TDS increases in the site M2—by 3.3, M3—by 1.9, M4—by 2.0, M5—by 1.8, M6—by 4.7, and M7—by 4.0 times. Initially, significant changes in the chemical composition occur in the site M2 after the inflow of mine water. In this site, the value of pH drops to 6.6. Downstream the pH increases up to 7.8 with the increasing TDS.
The increasing of TDS water is due to a sharp increase of the concentration of sulfate ion and the associated equivalent amounts of cations. Compared with to the natural background the concentration of sulfate ions increases in the site M2—by 15.1, M3—by 10.6, M4—by 11.8, M5—by 6.1, M6—by 24.0, M7—by 24.2 times; bicarbonate ion changes by 1.1–1.6 times; chloride content increases in the site M2—by 1.3, M6—by 2.5, M7—by 2.8 times. The concentration of Ca2+
ion increases in the site M2—by 3.6, M3—by 1.6, M4—by 2.3, M5—by 2.0, M6—by 6.2, M7—by 4.8 times. The concentration of Mg2+
increases in site M2—by 1.8, M6—by 2.5, M7—by 3.2 times, the concentration of alkali metals increases—by 14.6, M3—by 3.5, M4—by 4.9, M5—by 4.3, M6—by 11.6, M7—by 9.4 times (Figure 3
a). The F−
ion concentration increases in the site M2—by 23.8, M3 –by 2.6, M4—by 5.3, M5—by 1.9, M6—by 9.5, M7—by 9.7 times.
Compared with the natural background the equivalent fraction of sulfate ions increases in the site M2—by 5.3, M3—by 4.7, M4—by 5.1, M5—by 3.6, M6—by 4.7, M7—by 5.5 times; bicarbonate ions decrease—by 3.8, M4—by 2.7, M5—by 1.7, M6—by 2.9, M7—by 4.7 times; the fractional contents of chloride ion decrease by 1.5–3.1 times. The fractional content of Na+
cation increases—by 3.7, M4—by 2.5, M6—by 2.3, M7—by 2.1 times; the content of Mg2+
decreases by 2.1, M4—by 1.8, M6—by 1.9, M7—by 1.4 times; the content of Ca2+
ion changes in different directions, either decreasing or increasing in the range of variation 1.1–1.2 times (Figure 3
As is known, the concentration of main Ca2+
ions depends both on their abundances in the continental crust and their mobility during weathering and transport. The HCO3−
concentration characterizes by the rate of CO2
absorption from the atmosphere; therefore, in natural river waters, the equivalent ratio is (Ca2+
≈ 1 [11
]. In the case of water, r. Modonkul, as seen in Figure 4
a, when the equivalent ratio of ions (Ca2+
> 2, the chemical type of river water changes. The variation of the equivalent ratio of ions is positively correlated with TDS (Figure 4
a). In addition, along the length of river, the concentration of fluoride ions is positively correlated with the ratio (Ca2+
, as shown in Figure 4
Thus, downstream from the M1 to M7 sites, the chemical type of water changes from bicarbonate to sulfate across sulfate-bicarbonate or bicarbonate-sulfate class and from calcium-magnesium across sodium-calcium-magnesium reverse calcium-magnesium group. The river is an open dynamically changing system; the influence of inflowing tributary and mine on the chemical composition occurs along the length.
The concentration of nitrate N-NO3− varies in the range of 0.09–1.90 mg∙L−1, P-PO43− from 0.001 to 0.004 mg∙L−1. The concentration of permanganate oxidizability was 2.4–6.2 mg∙L−1; and chemical oxygen demand (COD)—8.2–22.6 mg∙L−1. The value of the five-day of biological oxygen demand (BOD5) varies from 0.5 to 3.7 mg∙L−1.
3.2. Trace Element
The results of the mean concentration of trace elements in the river Modonkul are summarized in Table 3
. In this table, the mean world concentrations of dissolved trace elements in a number of worldwide rivers are shown from review [13
Trace-element concentrations in Modonkul water span 6–7 orders of magnitude. The maximum concentrations within n 100 μg∙L−1
are observed for Sr, Mn, Zn, Ba. The concentrations within n 10 μg∙L−1
are typical for Li, Al, Fe, Cu, Ba; within n μg∙L−1
for Li, Al, Cr, Mn, Cu, Rb, Pb, U, Mo; within n 0.1 μg∙L−1
for Ti, V, Co, As, Y, Zr, Sn, Sb, Cs; within n 0.01 μg∙L−1
for Be, Sc, Ga, Th, Cd, Y, Zr, W. The remaining elements are present in concentrations not exceeding n 0.001 μg∙L−1
In the natural background (site M1), the mean dissolved concentrations of Al, Sc, V, Mn, Fe, As, Ti, Rb, Cd, Ta, Hf, W are 1.1–5.5 times less than that of the world mean values. The mean dissolved concentration of Sc is 33 times less than that of clark. The mean dissolved concentrations of Be, Cr, Ni, Cu , Zn, Sr, Y, Zr, Nb, Mo, Sb, Ba, Re, Pb, Th, U are 1.5–9 times; Pb and Zn are 15–18 times; Cs is 51 times greater than that of world mean values.
In the mouth of the river (site M7), the mean dissolved concentrations of Al, V, Fe, Ga, As, Hf, Ta, W, Th are 1.1–3.3 times, Sc—100 times less than that of the world mean values. The mean dissolved concentrations of Li, Cr, Ti, Mn, Co, Ni, Cu, Rb, Sr, Y, Zr, Mo, Sb, Ba, Re, Pb, U are 1.1–25 times; Be is 111 times; Zn is 360 times; Co, Cd, Cs are 34–55 times greater than that of the world mean values (Table 3
The downstream evolution of TDS and trace elements along the Modonkul river illustrates conservative and nonconservative behavior. Along the Modonkul, TDS shows the largest variations o4.7-fold; SO42−-ions 24.7-fold; Na+-ions 14.7-fold; F−-ions 24.7-fold. Trace elements show high enhancement for Be—by 19, Mn—by 78, Co—by 37, Zn—by 19, Cd—by 58 times. This significant enhancement of dissolved metals can be related to the occurrence of ore deposits in the river basin. While major elements show the lowest variability, trace elements show a 57-fold increase of manganese and cadmium. A steady increase (between 1.5 and 7 times) in the concentrations of Li, Al, Ti, Fe, Ni, Cu, As, Rb, Mo, Sb, Ba, W, Re, and U was observed.
Thus, trace-element concentrations in the Modonkul river span 6–7 orders of magnitude. In the dissolved load, the preferential dissolution of the different types of lithology or mineral can cause large variations in trace-element abundances in the river. A rough classification of trace element mobility in river water can be drawn. The first group comprises the highly mobile elements, with mobility more than 10–100 times greater than that of the world mean values. It consists of Be, Zn, Co, Cd, Cs. The following group of moderately mobile elements includes Li, Cr, Ti, Mn, Co, Ni, Cu, Rb, Sr, Y, Zr, Mo, Sb, Ba, Re, Pb, U. Their mobility is close to or is ~2–10 times greater than that of the world mean values. Finally, the last category, the most immobile elements, includes Al, V, Sc, Fe, Ga, As, Hf, Ta, W, Th, with mobility less than that of the world mean values. The dissolved concentrations of Al and Fe are close to or lower with the world values in the river flow. In the article [14
], it was shown that in the fraction <5 μm, Al concentrations predominantly correlate with Fe hydroxides. The dissolved concentrations of Al and Fe depend on the concentration of suspended matter; their size distribution, as well, is related to weathering regimes in the river basin. It is known that the vast majority of total Fe (>90%) is transported by suspended matter in particulate concentrations [15
]; this was observed in the river Modonkul, as it is shown in the next section.
The dissolved Mn concentration is more variable, so, at the upper river, it reaches 6.7 μg∙L−1
, but at the mouth, it rises up to 483 μg∙L−1
. This is mainly due to rapid oxidation-reduction processes. The manganese variations are consistent with redox changes within the river system. Literature data also show a large variation in dissolved manganese. For example, in the article, the largest amplitude of dissolved concentrations is observed for manganese (50 times increase with water discharge increase) and iron (8-fold variation); elements such as cadmium (8-fold) and cobalt (4-fold) show increases [16
Among the anionogenic elements F, Mo, Sb, As, V, Cr, the concentrations of F, Mo, Sb are two or more times greater than that of the world mean values for river water, which mainly is due to their enrichment from ore deposits the river basin. The dissolved concentrations of As, V, Cr are almost close or no more than 1.2–2 times that of the world mean values.
Alkali and alkaline earth trace elements are correlated to either sodium, calcium, or TDS in the river. Alkali Li, Rb noticeably migrate, their concentrations increase by a factor of 3–7 times between the upper part and mouth of the river. The dissolved concentration of alkaline-earth elements Sr, Ba increase no more than 1.6–1.9 times along the river. Their concentrations are strongly positively correlated with major elements and pH. Modokul river, being much influenced by carbonate dissolution, has high pH and alkalinity values. Like beryllium, caesium has a behavior that contrasts with that of the other alkali elements. The greater dissolved Cs concentration is related to its higher release from the widespread acidic effusives.
The concentrations of lithophilic elements Ti, Zr, Nb, Y, Hf do not practically change when moving downstream. The dissolved concentrations Zr, Y are 4–9 times greater; Ti, Nb, Hf are close to that of the world mean values.
3.3. Distribution of Heavy Metals.
and Table 5
show the dissolved and particulated contents of the heavy metals in the sampling sites.
The inflow of acid mine water of the Kholtosonskoe deposit and sludge from mine tailings to the Modonkul river worsened the pollution of an already contaminated area. The main polluting agents of the spillage were heavy metals. The pyritic wastes from mining activities arrive from the affluent Inkur river. This pollution source contained significant amounts of heavy metals (e.g., Zn, Pb, Mn, Cu, Cd) present as sulfides, and provided a low pH media that makes metallic forms readily available and, therefore, easy to propagate throughout the river basin [17
The mixing zone of natural river water with of acid mine water and tributary Incur presents oxidation-reduction processes and an increasing pH; an alkaline barrier is created. These mixing processes can lead to change in the behavior of heavy metals in dissolved and particulated loads. Dissolved ferrous iron, with increasing pH and dissolved oxygen, oxidizes and precipitates at the bottom in the form of hydroxides. In the site M2, the dissolved iron concentration is 1.15 times less than at the upper site M1. At the same time, the dissolved concentrations Mn, Zn increase by a factor of 16–16.9, but Ni, Cu, Cd—by a factor of 2–2.5. In the site M3, the dissolved concentrations Fe, Cu exceed natural background levels by a factor of 1.2-–1.5, with the exception of a 2–3 -fold increase for manganese and zinc. After the confluence of the Inkur river (site M4), the dissolved concentrations Cu, Pb, Cd increase by a factor of 2.6–3 times; Mn, Zn—33 times than that natural background levels (site M1). By contrast, the dissolved Fe concentration decreases by a factor of 1.1. It should be noted that the behavior of Fe differs from that for Mn and other metals. The amplitude variability of the dissolved Fe concentration is not exceeded by the large variation in the natural background level; probably, this is due to hydrolysis and sedimentation of hydroxides in the mixing zone (Table 4
, Figure 5
Compared with mine water, the decrease of the concentration SO42−
anions and the present alkaline barrier in the river limits the possibility of complexation oxidizing sulfides Fe, Cu, Zn, Pb, Cd. The sedimentation of Fe, Al, and Mn hydroxides and the sorption by them complexing heavy metals reduce their concentration in the river water [21
As noted above, the dissolved content of heavy metals in the site M7 exceeds the world river mean value by a factor of Fe—0.8, Mn—15, Zn—360, Cu—15, Pb—12, Cd—55 times. The behavior of heavy metals not only depends on the sources, but it is also strongly controlled by the soil and in-stream processes, particularly through aqueous (organic and inorganic) complexation and reactions with solids.
The river transports material, both in dissolved form and as a solid load (suspended matter and bottom sands). The dominant form of transport of trace elements per liter of river water depends on both the mobility of the elements in the weathering than the transport process and on the amount of solids transport annually by the river. In rivers, particles encompass a wide range of chemical and mineralogical composition (biological debris, organic substances, oxyhydroxides, clays, rock fragments). These sediments are either produced in situ or are generally derived from erosion of soils [25
The amount of suspended sediments in the upper of the river Modonkul (site M1) fluctuates from 1.6 to 26.8 mg∙L−1
; in the next site M2—from 56.3 to 158.8 mg∙L−1
. The concentration of metals associated with suspended sediments increases by a factor of 1.5 to 3.8 for Mn, Cu, Ni, Zn, Pb, and ~30 for Cd between site M2 and site M1. In the site M1, a significant fraction of heavy metals is transported in the dissolved load. Conversely, in the site M2, the most heavy metals are transported predominantly in a solid form (Table 5
, Figure 6
a,b). This observed effect can be interpreted as a dilution of pollution by the natural erosion of uncontaminated soil particles at high water discharges.
Iron, like manganese and other metals, can exist in more than one oxidation state. The solubility of such elements in aquatic systems depends on the concentration of oxygen, or, more generally, on the redox state of the fluvial system. Most often, running waters are oxygenated, but anoxic conditions can prevail locally in sites with inflowing affluent streams like Inkur, mine water from the Kholtosonskoe deposit, and urban wastewaters from Zakamensk city.
The uptake of heavy metals by surfaces is due to the presence of complexing sites similar to those complexing ions in solution (e.g., OH, COOH groups). The percentage of cation adsorption on FeOOH increases with increasing pH. Conversely, the adsorption of anions decreases with increasing pH [30
]. The association of a ligand in solution, heavy metal in solution, and a surface can lead to the formation of ternary surface complexes. Finely divided hydrous oxides of iron, aluminum, manganese, and silicon are dominant sorbents in rivers. Because the stability of manganese and iron oxides is redox-sensitive, the behavior of other heavy metals controlled by adsorption on these surfaces is expected to vary with the redox state.
In the case river Modonkul, the heavy metals concentrations in the suspended phase increase by a factor of Fe—4.7, Mn—8.2, Ni—2.3, Cu—1.7, Zn—2.7, Cd—10.5, Pb—5.3 between sites M7 and M1 (Table 5
, Figure 7
a,b). The dissolved concentration of heavy metals increases by a factor of Fe—1.7, Mn—62.7, Cu—2.9, Ni—1.5, Zn—19, Cd—4, Pb—2.5 times between the mouth and the upper part of the river. Significant discrepancy in enhancement of dissolved Mn, Zn and Cd concentrations are related in the redox state in the mouth of river. This is consistent with the findings of many authors [31
]. The variations in redox conditions affect not only the dissolved concentrations of iron and manganese but also the concentrations of elements sorbed onto iron and manganese oxides, such as zinc, copper, and cadmium in waters.
Thus, the mine waters of the Kholtosonskoe deposit and the tributary of the river Inkur are sources of ore technogenic elements into Modonkul water runoff. Their acidic waters differ from the natural water of the river. River particulates have the potential of regulating heavy metal input to aquatic systems from pollution. The pH values and dissolved oxygen cycles could change the uptake of heavy metals on suspended iron and manganese oxides, and the mechanism of removing it from the water. As a result, ore or heavy metals are removed in two ways: the river Modonkul’s water flow and sedimentation of suspended material at the bottom.