Geochemical behaviour of major and trace elements in suspended particulate material of the Irtysh river, the main tributary of the Ob river, Siberia

Abstract

In July 2001, samples of surface suspended particulate material (SPM) of the Irtysh river in its middle and lower reaches (from Omsk City to the confluence with the Ob river) and its main tributaries were collected (18 stations along 1834 km). The SPM samples were analyzed for major and trace element composition. The results show that the geochemistry of Irtysh river SPM is related to landscape and geochemical peculiarities of the river basin on one hand and to industrial activities within the drainage area on the other hand. In the upper basin polymetallic and cinnabar deposits and phosphorite deposits with high As content are widespread. The open-cut mining and developed oil-refining, power plants and other industries lead to the contamination of the environment by heavy metals and other contaminants. The territory of the West Siberian lowland, especially the Ob-Irtysh interfluve, is characterized by the occurrence of swamps and peat-bogs. Tributaries of the Irtysh river originating in this region, have a brown color and the chemical composition of the SPM is specific for stagnant water. In the first 500–700 km downstream from Omsk City the Irtysh river has the typical Al–Si-rich suspended matter composition. After the inflow of the tributaries with brown water the SPM composition is significantly changed: an increase of POC, Fe, P, Ca, Sr, Ba and As concentrations and a strong decrease of the lithogenic elements Al, Mg, K, Na, Ti, Zr can be observed. The data show that Fe-organic components (Fe-humic amorphous compounds, which contribute ca. 75–85% to the total Fe) play a very important role in SPM of the tributaries with brown water and in the Irtysh river in its lower reaches. Among the trace metals significant enrichments relative to the average for global river SPM could only be observed for As and Cd (coefficient of enrichment up to 16 for As and 3–3.5 for Cd). It can be shown that the enrichment of As in the SPM is related to natural processes, i.e. the weathering of phosphate containing deposits with high As concentrations in the upper Irtysh basin and the high As–P affinity in the swamp peaty soil. Dissolved P and As are absorbed by amorphous organic C/Fe oxyhydroxide components which act as carriers during the transport to the main stream of the Irtysh river. The role of anthropogenic factors is probably insignificant for As. In contrast, the enrichment of Cd is mainly related to anthropogenic input. The threefold enrichment of Cd in the SPM just below Omsk City and its continuous decrease down to background level at a distance of 500–700 km downstream points quite definitely to the municipal and industrial sewage of Omsk City as the main source of Cd in the SPM of the Irtysh river.

Introduction

During recent years the region of the Russian Arctic has attracted the attention of scientists not only in connection with expected climate changes but also with the developing and processing of new oil and gas deposits and the probable subsequent increase of anthropogenic impact on the environment. The rivers are the most important pathways for dissolved and particulate material, including contaminants, from the land to the sea.

The first analyses of major and trace elements in dissolved and particulate forms in the main rivers of the former Soviet Union, including the Russian Arctic, were started at the beginning of the 1950s during the last century in the framework of the USSR Hydrometeorological Survey (Konovalov, 1959, Konovalov et al., 1968). Similar research was carried out by specialists of the Geological Institute of the USSR Academy of Sciences (Glagoleva, 1959, Nesterova, 1960, Kontorovich, 1968, for Siberia). However, adequate methods of sampling, treatment and analyses of water and suspended particulate matter (SPM) had not been developed at that time. The use of metallic water samplers, paper filters and analytical chemical–spectral methods of low sensitivity did not allow obtaining reliable data, especially for dissolved heavy metals (HM).

Monitoring of HM levels in river water and other environments of the Russian Arctic was performed in the 80–90s of last century by the State System for Observation and Control of Environmental Pollution of the USSR State Committee for Hydrometeorology. An “Atlas of Environmental Pollution of the aquatoria and coastal seas of the Russian Arctic” was prepared by the Regional Center for Monitoring of the Arctic of the Russian Federal Survey on Hydrometeorology and Monitoring of the Environment (Melnikov, 1999) under the umbrella of the Arctic Monitoring and Assessment Program (AMAP).

Concentrations of Fe, Mn, Cu, Zn, Ni, Co, Cd, Pb, Cr and Sn in unfiltered surface waters and SPM of the Pechora, Ob, Yenisey, Khatanga, Lena, Indigirka and Kolyma rivers were presented in this atlas. However, the interpretation of these data is problematic, because it has to be expected that parts of the particulate elements were transferred into dissolved form due to acidification of the water samples.

Geochemical studies of the Russian arctic rivers (Lena, Ob, Yenisey) were carried out in the framework of the “Scientific Program on Arctic and Siberian Aquatorium (SPASIBA)” from 1989 to 1995. Modern methods of sampling and analyses were applied in 3 expeditions to the estuaries of the Ob and Yenisey rivers and to the delta of the Lena river; reliable data for some trace elements (Fe, Cu, Zn, Ni, Pb, Cd, Hg, As) in river and estuarine waters were obtained (Martin et al., 1993, Gordeev and Sidorov, 1993, Kravtsov et al., 1994, Dai and Martin, 1995, Gordeev and Shevchenko, 1995, Guieu et al., 1996, Cossa et al., 1996, Cauwet and Sidorov, 1996, Nolting et al., 1996). The results of these studies clearly documented that previous data on dissolved HM in the arctic rivers were largely overestimated and that the actual concentrations of HM in dissolved and particulate forms in the lower courses of the 3 studied rivers were comparable to the average concentration in global river discharge or even lower.

In the following years the geochemistry of major and trace metals in the Siberian rivers (Lena, Yana, Khatanga, Yenisey) was considered in details within the studies of the Russian-German projects “The Laptev Sea System” and “Siberian river-Runoff (SIRRO)” (Rachold et al., 1996, Rachold, 1999, Rachold and Hubberten, 1999, Lara et al., 1998, Lukashin et al., 1999, Beeskow and Rachold, 2003). It could be shown that HM concentrations in the Lena river SPM do not exceed the World river average. The elevated content of As, Bi and Sb in the Yana river SPM could be related to granitic intrusions forming the source of Au and Sn ore-deposits in the Yana basin. High Co, Cu, Fe, Ni and V concentrations in the SPM of the Khatanga river are a result of basaltic rocks of the Siberian Traps in its basin (Rachold, 1999). In summary, it could be concluded that the chemical composition of the SPM of these Siberian rivers is controlled primarily by the types of rocks and sediments in the basins.

At the same time scientific groups from the USA paid significant attention to the geochemical features of the Siberian rivers (Moran and Woods, 1997, Guay and Falkner, 1998, Huh et al., 1998, and other). Moran and Woods (1997) determined the concentrations of Cd, Cr, Cu, Ni and Pb in filtered water, SPM and bottom sediments of the middle Ob and a few samples of the lower Irtysh river, the Ob's major tributary (Fig. 1). Dissolved Cd, Cr, Cu and Ni concentrations were similar to or slightly higher than those of other Russian arctic and World rivers and estuaries. The comparison between trace metal ratios in crustal material and suspended and bottom sediments suggested that the source of Cr, Cu and Ni was continental weathering. Particulate Cd and Pb concentrations were elevated relative to their crustal abundance, and the authors suggested an additional source for these two elements, although the nature of the source could not be determined.

Shvartsev et al. (1996) stated that within the last few decades a dramatic decrease of river water quality that led to significant changes in the ecosystem took place in the Ob river basin. In 1997 the International Research Center on Physics of Environment and Ecology of the Siberian Branch of the Russian Academy of Sciences established a complex ecological research program in the Ob basin. Three river expeditions and 2 workshops “Ecology of the Poymas of Siberian rivers and Arctic” (1999, 2000) were organized and scientific reports including the results of the HM determinations in the middle reaches of the Ob river and some of its tributaries were published (Shvartsev et al., 1999, Sorokovikova et al., 1999, Leonova et al., 2000). Shvartsev et al. (1999) collected 20 water samples along the stream of the middle Ob river below its tributary the Tom river and analyzed 9 HM in dissolved form. Sorokovikova et al. (1999) obtained 53 samples of the Tom river and the middle Ob river and 9 HM in filtered water samples were determined by ICP–MS. However, in both studies the interpretation of the results is very limited. Shvartsev et al. (1999) note an increase of Zn and Mn from south to north along the Ob river. They consider that this increase is a result of the association of metals with organic acids, the concentrations of which increase to the north due to the inflow of tributaries with water of swamp genesis. Sorokovikova et al. (1999), on the other hand, did not identify any definite tendency in concentration changes along the rivers but report an unsystematic scatter of concentrations. Leonova et al. (2000) collected water samples of the Ob river (below Novosibirsk City) and the Tom river (from Tomsk City down to the mouth of the river). The analyses showed that the Tom river is much more contaminated by HM than is the Ob river. Especially high concentrations of Hg were determined in the lower course of the Tom river: Hg diss.=0.045–1.5 μg/l and Hg part.=8–32 μg/g in the Tom river (n=5); Hg diss. <0.02 μg/l and Hg part.=0.4–1.7 μg/g in the Ob river (n=4).

As a result of the combined action of strong anthropogenic impact and specific natural geochemical anomalies (geochemical provinces), the concentrations of many HM in the upper Irtysh river and its tributaries in the territory of Kazakhstan appear to be extremely high (Panin and Sibirkina, 2000, Panin, 2002). The analyses of large volume water samples performed in 1984–1996 show that the average concentrations of dissolvable metals in the upper Irtysh basin are (in μg/l): Mn: 33.1±4.2 (n=832), Zn: 80.5±20.8 (n=1289), Cu: 51.9±12.1 (n=1277), Pb: 14.4±2.2 (n=916), Cd 2.64±0.46 (n=788), Cr: 5.2±0.6 (n=687), Co: 1.85±0.25 (n=1006), Mo: 10.0±1.1 (n=934). Unfortunately, no information about sampling, filtration and other treatment of the water samples was presented in the papers except for a reference to “Unified Methods of Water Analyses in the USSR” (1978) and a note on the analytical methods, which were ICP–OES and GF–AAS (for Cd determinations) (Panin, 2002). Thus, the reliability of these data remains questionable, especially because the term “dissolvable form” is usually related to the sum of the dissolved content and a part of the leachable fraction of the SPM.

During the third expedition of the program “Ecology of the Poymas of Siberian rivers and Arctic” 3 small groups of geochemists including a group of two co-authors of the present paper (VVG and IEV) participated in sampling. The preliminary results of this expedition to the middle and lower courses of the Irtysh river (July 2001) and some additional data were published in the report “Ecologo-geochemical investigation in the Ob river basin”. Sazonova and Shvartsev (2002) analyzed dissolved major cations and anions, nutrients, organic C and 10 dissolved trace elements. The total mineralization of the Irtysh river water decreased from 250 mg/l below Omsk City to 115 mg/l near the confluence with the Ob river with an average of 205 mg/l. The authors state that low mineralization of the southern tributaries and technogenic influence of Omsk City are the main reasons for this tendency. The highest concentrations of dissolved Zn and Cu were detected below Omsk City (20–37 μg/L Zn and 4–6 μg/l Cu) and dissolved Pb concentrations of up to 9.4 μg/l were determined near Tobolsk City. The other elements (Cd, Li, Sr, Br, F, Hg) did not exceed the maximum allowed concentrations (MAC).

Kovalskaya et al. (2002) analyzed 26 major and trace elements in 40 SPM samples of the Irtysh river and its tributaries. river water was filtered through paper filters “blue line” with a pore size of 2.5 μm and the samples were analyzed by XRF with synchrotron radiation. The results were reported in μg/l units but SPM concentrations were not provided. The authors could not detect any trends in the elemental composition of SPM along the Irtysh river between Omsk City and Khanty-Mansiysk City and the comparison between the right and left tributaries did not show significant differences.

Preliminary results for 10 dissolved metals (Fe, Mn, Cu, Zn, Ni, Co, Pb, Cd, Cr and Hg) and for particulate major and trace elements, which were obtained by the authors of the present paper in the middle and lower courses of the Irtysh river, are reported in the above mentioned report (“Ecologo-geochemical investigation in the Ob river basin”) and in Gordeev and Vlasova, 2002, Gordeev et al., 2002. The results and conclusions of this earlier study will be shown in the following in context with the results of the present paper. However, it should be noted here that the consistency of the analytical results obtained by the 3 groups involved in the program “Ecology of the Poymas of Siberian rivers and Arctic” was quite poor.

Summarizing, this short review of available data on the geochemistry and the level of contamination of Russian arctic rivers and estuaries by HM demonstrates that the results of different authors exhibit significant inconsistencies. The concentrations along the main stream of the rivers appear to vary unsystematically. Many features of the geochemical behavior of the trace elements in dissolved and particulate form in river discharge are still studied insufficiently. Thus, a critical review and new assessments of HM concentrations and their fluxes in the Russian arctic rivers was published recently (Gordeev, 2001).

This study concentrates on the geochemistry of the suspended particulate matter (SPM) of the Irtysh river, the main tributary of the Ob river, East Siberia, Russia. The main objectives are to characterize the behaviour of major and trace elements in the SPM and to identify natural and anthropogenic sources of the particulate load. Sampling was conducted in July 2001 and extended from Omsk City down to the confluence with the Ob river along a distance of 1834 km. As discussed above, the upper Irtysh river waters are polluted by HM in dissolvable form but SPM data are not available (Panin, 2002). Information on HM in the middle and lower courses of the Irtysh river is very scarce. In particular, the role of the tributaries with brown acid waters of swamp genesis in the transformation of the chemical composition of water and SPM is unknown.

The Irtysh river is the largest western tributary of the Ob river (Fig. 1). Its water and suspended sediment discharges are 99.8 km³/a and 15.2×10⁶ t/a, respectively (Lisitzina, 1974). The upper stream of the Irtysh river is located in the middle and high mountains of the Altay. The most widespread rocks are granites, clayey sandstones and limestones. To the north, in the lower mountains of the Kazakhstan hills, Paleozoic deposits consisting of quartzite, limestones, sandstones and shales are more common.

Below Omsk City the Irtysh river flows through the lowlands of the East Siberian Plain, which comprise numerous peat swamps and lakes interrupted by ice ridges, hills and thermokarst trenches. The western part of the basin between the Irtysh and Ob rivers is occupied by a huge swamp system (approximately 800×350 km). The level of swampiness reaches 50–80% and reed overgrowths are the dominating type of vegetation in these swamps (Panin, 1972).

Numerous deposits of polymetals are concentrated in the Irtysh river basin, especially in its southern part in the territory of Kazakhstan, and a large-scale extractive industry is developed. Today the bare surfaces of mine sites and their dumps and tailings deposits outflows from metallurgical, chemical, engineering industries and power plants and of the construction industry and agriculture results in significant HM pollution of the environment (Panin and Sibirkina, 2000, Panin, 2002).