Trace Element Composition of the Dissolved Matter Runoff of the Russian Arctic Rivers

By inergency On Feb 14, 2024

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1. Introduction

The chemical composition of dissolved matter from river runoff, which is one of the main inputs that affects the ocean’s geochemical balance, with the exception of cyclic salts, is formed as a result of the weathering of rocks in land catchment areas. Currently, an extensive database on the basic salt composition of the waters of the world’s largest rivers was established, and fairly reliable estimates of the ion fluxes in the ocean were obtained [1,2,3,4]. Similar reports on dissolved trace elements [5,6] were compiled from a much smaller volume of factual material and should be considered as purely preliminary estimates.

It can be assumed that the petrographic differences in the lithogenic basis of watersheds decrease as their areas increase, due to which the specific chemical composition of river runoff from higher-order watersheds is formed to a greater extent under the influence of climatic factors. In this regard, it is important to expand the database based on the concentrations of dissolved trace elements in the river waters of various climatic zones.

For a long time, the authors systematically studied the abundance of dissolved trace elements in the waters of the outlet sections (mouth reaches) of large, medium, and small rivers of the Russian Arctic using modern, highly sensitive analytical methods. The objective of this work is to systematize and generalize the results of these studies [7,8,9,10] (including the unpublished data from A.V. Savenko) in conjunction with data from other literature sources [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28] and to estimate the mean concentrations of dissolved trace elements in the river runoff from the White, Pechora, Kara, Laptev, and East Siberian sea watersheds.

2. Materials and Methods

Information about the location of the considered rivers, the long-term average water runoff in the outlet sections, observation periods, the phases of the hydrological regime during sampling, and the number of analyzed water samples are presented in Figure 1 and Table 1. The total number of river water samples was 217, 109, 535, 112, and 98 for the White, Pechora, Kara, Laptev, and East Siberian sea watersheds, respectively. At the same time, at least 5 water samples were collected in each river outlet section during periodic hydrological and hydrochemical surveys, which covered different phases of the hydrological regime for the majority of rivers. The mean concentrations of dissolved trace elements in the outlets of large and medium rivers or a group of small rivers were calculated using all available information on these water bodies: research data from the authors and literature sources. The averaged composition of the runoff from the Arctic sea watersheds was obtained while taking into account the ratio of the volumes of the long-term average water runoff of the studied rivers. This ensured that the mean concentrations estimates are reasonably representative.

The authors carried out natural observations and an analysis of water samples as follows. Water samples were taken with a plastic bathometer and immediately after boarding, were filtered through an acetate–cellulose membrane filter with a pore diameter of 0.45 μm into 3 containers, hermetically sealed and placed in sealed plastic bags:

Plastic flasks measuring 100 mL with the addition of 1 mL of chloroform to determine the content of mineral phosphorus and silicon by standard colorimetric methods with ammonium molybdate;
Similar flasks measuring 30 mL without a preservative for measuring the fluoride content by direct potentiometry with a fluoride ion-selective electrode in the presence of acetate saline buffer;
Polypropylene tubes measuring 10 mL with 0.25 mL of 5 N nitric acid of ultrapure grade previously added under laboratory conditions to determine the concentrations of all other trace elements using inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7500 ce instrument.

The relative measurement error was ±3%. The accuracy of the analyses was assessed using the international river water standards SLRS-4 and SLRS-5, for which the discrepancy between the measured and certified concentrations of the studied elements did not exceed 20%.

Most of the literature data over the past 20–25 years were obtained using a similar sample preparation procedure and analytical measurements. In the 1990s, the most common method for the determination of heavy metals and other trace cations was atomic absorption with atomization in a graphite cuvette, the results of which showed close agreement with those of ICP-MS.

3. Results and Discussion

The results of the calculations of the mean concentrations of dissolved trace elements in the river waters of the Russian Arctic watersheds in comparison with estimates of the average composition of the global river runoff are given in Table 2 and Table 3. Due to the rather strong spatial–temporal variability of dissolved trace element concentrations in the river waters and a relatively small number of measurements for most of them, discrepancies in the average values of 2–3 times are usually not taken into account, and only differences of more than half an order of magnitude (>5 times) are considered significant.

Considering the data on the mouth reaches of the rivers of the White and Pechora sea watersheds, it can be argued that the concentrations of most of the trace elements dissolved in their waters have similar values. The mean concentrations of P, Si, Li, Rb, Be, Mn, Fe, Co, Ni, Cu, Cd, Pb, Ga, Y, rare earth elements, B, Ge, As, Sb, Se, and Mo differ by less than two times the average. Concentrations of Cs, Sr, Ba, Tl, Al, Ti, Zr, Hf, Th, U, V, and Cr in the Pechora Sea watershed are 2–3 times lower, and the Zn concentration is 2.8 times higher compared to the White Sea watershed. Significant differences are found only for W, the content of which in the Pechora River waters is eight times less than that in the rivers of the White Sea watershed. In general, the concentrations of dissolved trace elements in the Pechora River are slightly lower than the mean values of the rivers of the White Sea watershed (Figure 2a), which can be associated with the presence of continuous permafrost in the Pechora River watershed (7% [34]), leading to a decrease in the intensity of the processes of chemical element mobilization. At the same time, based on the similarity of the trace element composition of river waters, the White and Pechora sea watersheds can be generalized into a conjoint watershed of the European territory of the Russian Arctic.

In the watersheds of the Asian territory of the Russian Arctic (Figure 2b,c), the concentrations of many dissolved trace elements (Li, Rb, Cs, Be, Sr, Ba, Mn, Fe, Zn, Cd, Pb, and B) are lower than those in the watersheds of the corresponding European territory. These elements are characterized by a tendency to decrease in concentration from west to east, i.e., with increasing climate severity and the prevalence of continuous permafrost. This trend is not seen for hydrolysate elements (Al, Y, rare earth elements, Zr, Hf, Th, and U) and Tl due to their increased concentrations in the Lena River waters of the Laptev Sea watershed (Figure 2b), and it is also not clearly observed for Co, Ni, Cu, Ga, Ti, and anionogenic elements (F, P, Si, Ge, V, As, Sb, Cr, Se, Mo, and W), the content of which is not systematically varied, differing in the studied watersheds by no more than 5–7 times the average. Along with this, the concentrations of dissolved trace elements in the river waters of the easternmost watershed of the East Siberian Sea (the Kolyma River) are generally 3.1 times lower than for the White and Pechora sea watersheds, and 1.8 times lower compared to the Kara and Laptev sea watersheds (Figure 3):

C_ESS = 0.32C_WPS, r = 0.82,

(1)

C_ESS = 0.54C_KLS, r = 0.94.

(2)

A comparison of the mean chemical composition of the waters of the mouth reaches of rivers of the Russian Arctic sea watersheds and the global river runoff shows a fairly close correspondence between the concentrations of most trace elements (Figure 2, Table 2 and Table 3). The largest and systematic discrepancies were found for W, Cs, Zn, and Cd.

The W and Cs content in river waters carried to all seas of the Russian Arctic is significantly lower than estimates [6] for the global river runoff. Since the concentrations of W and Cs in river waters were rarely determined using modern, high-sensitivity analytical methods, it can be assumed that the average content of these elements in the global river runoff is overstated; however, an alternative explanation is also possible and is related to the overall lower content of dissolved trace elements in the river runoff from the Russian Arctic sea watersheds.

Another systematic discrepancy was noted for Zn and Cd. The mean Zn concentrations in the river waters of different seas of the Russian Arctic watersheds are in the range of 0.9–11.5 μg/L, and the minimum values (0.9–2.2 μg/L) refer to the watersheds of its Asian territory, which are characterized by the low intensity of weathering processes and experience the least anthropogenic impact. According to [6], the average Zn content in the global river runoff is equal to 0.6 µg/L, which is 7 and 19 times lower than the estimate for the river runoff leading into the White and Pechora seas. The reason for this discrepancy is not clear. It is possible that the estimate [6] is low, since the mean Zn concentration in the global river runoff is noticeably lower than that of Cu, which is detected extremely rarely in river waters (usually the opposite relationship occurs). In addition, other estimates of the mean Zn concentration in the global river runoff (20–30 μg/L [35,36]) are an order of magnitude higher than the value suggested in [6]. For Cd, an element with similar chemical and geochemical properties to Zn, its average concentration in rivers of the world, on the contrary, is much higher than in the runoff from the Russian Arctic sea watersheds, and the discrepancy increases from west to east, reaching a maximum for the East Siberian Sea watershed.

Many authors believe that anthropogenic sources have a strong influence on the concentrations of Zn and Cd in terrestrial surface waters. From this point of view, the decrease in Zn and Cd concentrations in the river runoff from west to east of the Russian Arctic territory has a logical explanation since the intensity of anthropogenic processes and associated anthropogenic pollution decreases in the same direction; however, this assumption is contradicted by the weak variability of Pb concentrations in all studied watersheds of the Russian Arctic, which is consistent with the world average value given in [6].

Thus, the data presented in this review show a fairly close correspondence between the mean concentrations of dissolved trace elements in the river runoff from the Russian Arctic sea watersheds and those in the river waters of the world. Significant discrepancies were established only for W, Cs, Zn, and Cd.

4. Conclusions

The concentrations of dissolved trace elements (P, Si, Li, Rb, Be, Sr, Ba, Mn, Fe, Co, Ni, Cu, Tl, Pb, Al, Ga, Y, Ti, Zr, Hf, Th, U, rare earth elements, F, B, Ge, V, As, Sb, Cr, Se, and Mo) in the river runoff from the Russian Arctic sea watersheds are generally consistent with estimates of their average content based on the global river runoff. Significant systematic differences in the mean chemical composition of river waters in the Russian Arctic sea watersheds and that of the river waters of the world (up to an order of magnitude) are observed only for dissolved W, Cs, Zn, and Cd.

Correlation relationships between the mean concentrations of dissolved trace elements in the waters of the considered Arctic watersheds show a tendency to decrease in the direction from west to east. The concentrations of dissolved trace elements in the river waters of the easternmost watershed of the East Siberian Sea are generally 1.8 times lower than those of the Kara and Laptev sea watersheds, and 3.1 times lower compared to those of the White and Pechora sea watersheds.

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