Single Particle Characterization of Inorganic Suspension in Lake

Lake Baikal is situated in southeastern Siberia, close to the. Mongolian border. ... Baikal species is the tiny crustacean Epischura baicalensis, whic...
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Environ. Sci. Technol. 1997, 31, 1525-1533

Single Particle Characterization of Inorganic Suspension in Lake Baikal, Siberia WENDY JAMBERS AND RENE ´ VAN GRIEKEN* Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerpen, Belgium

Automated and manual electron probe X-ray microanalyses were used to characterize the chemical composition and morphology of individual suspended particles, collected at different tributaries of Lake Baikal and in the central part of southern Lake Baikal. The data sets were reduced using hierarchical clustering, and the results were compared by means of selection rules. All the samples are dominated by natural aluminosilicates and Si-rich particles. But small amounts of anthropogenic aluminosilicates (i.e., fly ash); Ferich, Al-rich, Cr-rich, and Zn-rich particles; and Cl-coated fibers were also detected. The contamination is especially severe in the southern basin, which can easily be reached by atmospheric pollution from the industrial centers near Irkutsk and Ulan-Ude and which is also influenced by the large Baikalsk cellulose factory. At the northern basin, pollution is only detected at the inflow of the Tiya River, where the dumping of debris from the construction of the Baikal-Amour railroad results in high concentrations of Ferich particles.

Study Area Lake Baikal is situated in southeastern Siberia, close to the Mongolian border. With an average depth of 900 m, it is the deepest freshwater basin on Earth. It is 635 km long and on average 48 km wide and holds 23 000 km3 of barely polluted water, which is 20% of the world’s total freshwater content and 80% of the freshwater of the former Soviet Union. The cold climate of Siberia is responsible for a thorough mixing of the Baikal water and for the occurrence of rich life even at the deepest parts of the lake. One of the 1500 endemic Baikal species is the tiny crustacean Epischura baicalensis, which strains bacteria and algae from the water, making the center of the lake exceptionally pure (3). The lake has 336 tributaries that drain an area of 557 000 km2 and only one outflowing river, the Angara. The major tributaries are the Upper-Angara, the Barguzin, and the Selenga. This Selenga River supplies half the water flowing into the lake and is loaded with sediments and human and industrial wastes from three Mongolian cities and from UlanUde (3). The Barguzin and the Upper-Angara also supply pollution to the lake, but in much smaller quantities than the Selenga. In 1987, Lake Baikal was officially protected by the Soviet Government, and only recently (December 1996), it has also been inscribed on UNESCO’s World Heritage List. However, the cellulose factory at Baikalsk is still discharging thousands of tons of minerals, suspension, bacteria, and organic byproducts (including difficultly biodegradable chlorinate organic compounds) into the southern basin, while the northern basin has been polluted by the construction of the Baikal-Amur railroad, which caused erosion of the northern shore and resulted in overloading of the rivers with waste. An additional threat to the lake is the industrial sites in the Selenga and the Angara Valleys, whose pollution products can easily reach the lake through the air (3).

Introduction

Experimental Section

Although single particle analyses are a valuable complement to the more conventional bulk techniques, they are scarcely used for environmental applications (1, 2). During the last few years, the analysis of individual aerosol particles has gained interest. But, although particles are also the major material carriers in water, hardly any research has been done on the chemical characterization of single particles in suspension and in sediments. Barely any environmental data are available from the former Soviet Union. Recently most parts of Russia have become accessible to Western scientists, who, in cooperation with Russian scientific institutes, have started to develop environmental research in this large country. Siberia has many remote and pristine areas, without any industry and with a very low population density. But it also has regions with huge industrial sites where the local pollution is enormous. Lake Baikal is a good example of this duality. The center of the lake is still very pure, but in the southern part and near the shore pollution is starting to threaten this unique environment. In this study, automated electron probe X-ray microanalysis (EPXMA) combined with a recently improved data reduction method was used to determine the chemical and morphological characteristics of individual micrometer-sized suspension particles, collected in the river mouths of 13 tributaries of Lake Baikal and at eight different depths in the central part of southern Lake Baikal.

Sampling. Surface water samples from 23 tributaries of Lake Baikal were collected during the sampling campaign on board the R/V Mercury from August 23 to September 3, 1990. The suspension was filtered on preweighed 100-mm nuclear polycarbonate filters (Russian brand) with pores of 0.45 µm. After filtering, the samples were dried for 1 day at 60 °C and stored in air-tight Petri dishes. For single particle analysis, the loading of the filters should be relatively low. Particles are not allowed to touch because then they will be analyzed as one. Unfortunately, these samples were also used for flame atomic absorption measurements for which a higher loading is preferred (4). For this reason, the loading of some filters was a bit too high for single particle analysis, which results in some overlapping particles. The filtered volume and the weighed suspended matter (representative for the loading) of the filters that were selected for single particle analysis are represented in Table 1, while their sampling locations are shown in Figure 1. Figure 1 also contains the sampling site of December 12, 1995, in the central part of southern Lake Baikal (indicated with A). At the coordinates N 51°41′ and E 105°, i.e., the middle of the trajectory Listvjanka-Tankhoj, water was collected with a Niskin bottle at eight different depths, starting at the surface and with depth intervals of 200 m. Different volumes of water (represented in Table 2) were filtered on 47-mm polycarbonate Nuclepore filters with 0.4-µm pore size (Nuclepore, Pleasanton, CA), resulting in adjusted particle loadings for single particle analysis. To prepare the samples for EPXMA, part of the filter was mounted with double-sided tape on a 25 mm diameter plastic

* Corresponding author fax: +32 3 820 23 76; e-mail address: [email protected].

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 1997 American Chemical Society

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TABLE 1. Filtered Volume of Water and Weighed and Calculated Suspended Matter at Outflow of Different Rivers into Lake Baikal, for 13 Sampling Locations Selected from the 1990 Campaign sampling location 1 3 5 6 8 12 13 16 17 18 20 21 23

Selenga Solzan Tompuda Tiya Barguzin Khara-Murin Utulik Pereyemnaya Mishikha Buguldeyka Goloustnaya Rel Sneznaya

filtered weighed calcd vol of suspended matter suspended matter water (mL) content (mg/L) content (mg/L) 1 750 5 000 9 800 3 600 2 900 5 000 6 000 6 500 6 000 2 800 10 000 10 000 6 750

18.11 0.79 0.22 1.15 4.60 0.60 1.01 0.09 0.36 1.48 0.07 0.26

3.60 0.24 0.02 0.13 4.30 0.08 0.21 0.03 0.07 0.80 0.005 0.02 0.048

FIGURE 1. Map of Lake Baikal with the different sampling locations. plate, which fits into the sample holder. Electrical and thermal charging during analysis was prevented by coating the samples with a 50-nm carbon layer. Instrumentation. EPXMA measurements have been performed with a JEOL JXA-733 Superprobe (JEOL, Tokyo, Japan) connected to a Tracor Northern TN-2000 X-ray analyzing system (Tracor Northern, Middleton, WI) and a 486 personal computer. All measurements were performed with an acceleration voltage of 25 kV and a beam current of 1 nA. Under these conditions and with a collection time of 20 s, energy dispersive X-ray measurements yield a detection limit of about 1% or lower for most elements (5). To ensure the statistical relevancy of the results, 250 particles were analyzed per sample. To enable the analysis of a large number of particles in a short time, EPXMA was automated with the homemade 733 particle recognition and characterization program (5). An X-ray recording time of 20 s and a magnification of 1000 were used to characterize particles with a spherical diameter larger than 0.4 µm. The image resolution at this magnification is 0.2 µm. Manual EPXMA was used to study the homogeneity of the individual particles and to determine the relation between particle composition, origin, and shape. During 60 s, spectra

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TABLE 2. Filtered Volume of Water and Weighed and Calculated Suspended Matter at Different Depths at Location A in Central Part of Southern Lake Baikal, Sampled on December 12, 1995 sampling depth (m)

filtered vol of water (mL)

weighed suspended matter content (mg/L)

calcd suspended matter content (mg/L)

0 200 400 600 800 1 000 1 200 1 400

1 810 2 000 1 405 5 595 1 585 890 290 2 120

1.93 0.12 0.30 0.30 0.10 0.20 0.14 0.14

0.14 0.16 0.07 0.09 0.04 0.20 0.22 0.04

were accumulated using either selected area or spot mode. To enable comparison with the results of automated analysis, these spectra were also deconvoluted using the fast filter algorithm, and the peak intensities were normalized. Data Treatment. The automated analysis of 250 particles in each sample resulted in a huge data matrix, which was reduced for interpretation by means of a geochemically relevant clustering, i.e., a hierarchical clustering based on Euclidean distances, with the Ward’s error sum classification as similarity criterion (5, 6). This clustering was performed using the integrated data analysis system (IDAS) developed at the University of Antwerp by Bondarenko et al. (7). This software includes the consistent Akaike information criterion (CAIC). A minimum in the CAIC corresponds to an accurate number of groups (7, 8). Comparison of the results of the different samples is possible by performing a secondary clustering of the groups resulting from the first clustering (5). However, this combined clustering procedure is very time consuming, and an alternative data reduction is proposed using selection rules, based on the mean relative X-ray intensities of characteristic particle types (9, 10). Because the same rules are used for all data, no extra clustering is needed. The results obtained using this method are in good agreement with those obtained by the combined clustering procedure (10). However, these selection rules can only be used when some knowledge about the chemical composition of the data is available. Because in this study no prior knowledge about the chemical composition of the data was available, the data reduction per sample was done using hierarchical clustering. However, the resulting groups supply information about the chemical composition of the data. This information was combined with the mean relative X-ray intensities of characteristic particle types in riverine suspension (10) to obtain selection rules relevant for Lake Baikal suspension. These selection rules are represented in Table 3 and were used to compare the results of the different samples by identifying all groups resulting from the hierarchical clustering procedures. Using this approach, data can be reduced without any prior knowledge of the chemical composition, and the time-consuming secondary clustering is avoided by using the selection rules as an equal alternative.

Results and Discussion The different particle types found at the inflow of the rivers in Lake Baikal and in the center of southern Lake Baikal are given, together with their abundances, in the Figures 2a,b and 3. They will be discussed in detail in the next paragraphs. Aluminosilicates. The aluminosilicate particle types dominate in most samples and are characterized by high relative X-ray intensities for Al, Si, Fe, K, and sometimes Ca or Ti. According to their composition, they are divided into six subgroups: pure aluminosilicate with only Al and Si; aluminosilicate with Al, Si, and small amounts of K, Fe and/

TABLE 3. Selection Rules Used To Compare Groups Resulting from Hierarchical Clustering of Different Samples particle type

selection rules based on relative X-ray intensities

aluminosilicates A1 A2 A3 A4 A5 A6 B C D E F

pure aluminosilicates aluminosilicates Ca-rich aluminosilicates K-rich aluminosilicates Fe-rich aluminosilicates Ti-rich aluminosilicates

Al + Si > 90 Al + Si + K + Fe > 90 Al + Si + K + Fe + Ca > 90 Al + Si + K + Fe > 90 Al + Si + K + Fe > 90 Al + Si + K + Fe + Ti > 90 Si > 70 Ca > 50 Fe > 50 Ti > 50 Al > 50

S-rich Fe-S-rich Ba-S-rich Ca-S-rich

S > 70 S > 40 S > 40 S > 40 Mn > 50 Cr > 50 Zn > 50 Cl > 50 net X-ray counts < 1000

Si-rich Ca-rich Fe-rich Ti-rich Al-rich

and 15 < Si < 85 and 15 < Si < 85 and 15 < Ca < 50 and K > Al and Fe > Al and Ti > Al and Al < 5

S-rich G1 G2 G3 G4 H I J K O

Mn-rich Cr-rich Zn-rich Cl-rich organic

or Ca (in the rest of the discussion this will be called the aluminosilicate subgroup); and Ca-rich, K-rich, Fe-rich, and Ti-rich aluminosilicates with high abundances for these specific elements. The main source of these particles is the erosion of rocks and soils, but they are also produced during high-temperature combustion processes. These fly ash particles cannot be distinguished from mineral aluminosilicates during automated EPXMA, but they usually have a typically spherical morphology that can easily be recognized during manual analysis. In the samples collected at the inflow of the rivers, none were found, and analyses of aerosols collected above the northern and central basin of Baikal confirm the limited contribution of these fly ash particles (11). However during manual analysis of the first four depth samples, collected in the southern basin, small amounts of fly ash particles were detected at all depths. Also, in the air collected in the vicinity of the Baikalsk cellulose plant, a major part of the aluminosilicates was identified as fly ash (11). Thus, in water samples collected in the southern basin, fly ash will contribute to the aluminosilicate fraction, though only in limited amounts. Generally, the pure aluminosilicate (A1) and aluminosilicate subgroup (A2) mainly consist of clay minerals like kaolinite, illite, and montmorillonite and micas like muscovite and biotite (12-14). The first group is present in all depth samples with abundances ranging from 5 to 33%. For the river inflow samples, this particle type is only found at two locations in the southern basin and at the Rel River. The second group, however, is present in the majority of the samples, and with abundances ranging from 7 to 54%, it is the most important subgroup. Ca-rich aluminosilicates (A3) are present in most river inflow samples, but they were only found in two depth samples. This particle type probably originates from the several Ca-containing aluminosilicate minerals, like anorthite, Ca-montmorillonite, epidotes, and zeolites, which are present in suspended matter of natural waters (10, 14). But because this particle type had strongly enhanced levels of abundance in air masses collected above the more polluted southern basin of Lake Baikal (11), an anthropogenic source cannot be excluded. This subgroup has high contributions for the rivers Tompuda (26%) and Sneznaya (18%). For the Tompuda River, this enhancement can be explained by the possible flocculation of aluminosilicates with Ca-rich particles, which are

and Fe > 20 and Ba > 20 and Ca > 20

present in a relatively high abundance. However, in the sample collected at the Sneznaya River, no Ca-rich particles were detected. This river is located in the southern basin close to the cellulose plant, which could point to an anthropogenic source. But because no Ca-rich aluminosilicate particles were detected at the rivers Khara-Murin and Solzan, this anthropogenic contribution will be limited. Small abundances of K-rich aluminosilicates (A4) are found in a majority of the samples. Only at the Rel River is the abundance high (18%). This particle type can be characterized as K-containing aluminosilicate minerals like orthoclase and microcline (12, 14). The Fe-rich aluminosilicates subgroup (A5) is generally characterized by minerals like chlorite and Fe-smectite (10). However aluminosilicates, and in particular clay minerals, are known to adsorb trace elements (14, 15). Thus, aluminosilicates that have absorbed large amounts of Fe can also contribute to this particle type. These Fe-rich aluminosilicates are detected in all samples, but only at the sampling locations 13, 16, and 20 and at the depth samples at 0 and 200 m were abundances higher than 20% found. These enhancements are probably due to local variations of the minerals. This is the only particle type for which a decreasing tendency with depth could be observed. Small amounts of Ti-rich aluminosilicates (A6) are only detected in the samples collected at the Selenga River. Because no mineral is known that corresponds to this particle type (12, 13) and rutile frequently occurs in granite, gneiss, and mica schist (13), these are most likely naturally formed aggregates. The aluminosilicate particle types dominate, with abundances larger than 50%, in the majority of the samples. Only in four samples do the Fe-rich or Si-rich particles dominate. This Si-rich particle type represents, together with the aluminosilicate groups, the major fraction of the suspended matter (abundances between 50 and 100%). Si-Rich Particles (B). This particle type contains particles with a relative X-ray intensity for Si that is higher than 70%. Sometimes also small amounts of Fe and/or Ca are detected. These are most likely diatoms or fragments of sponges that have adsorbed iron oxides and calcium carbonates (10, 16). Si-rich particles are detected in all samples with abundances ranging from 12 to 70%. Especially in the sample collected at the Mishikha River, a large amount of Si-rich particles was found (up to 70%). Smaller enhancements were found at the

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FIGURE 2. Relative abundances of the different particle types detected in suspension samples collected at different tributaries of Lake Baikal with (a) the total lake and (b) an enlargement of the southern basin.

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FIGURE 3. Relative abundances of the different particle types detected in suspension collected at (a) 0, (b) 200, (c) 400, (d) 600, (e) 800, (f) 1000, (g) 1200, and (h) 1400 m depth at location A in the central part of southern Lake Baikal. rivers Buguldeyka, Rel, and Solzan, which all contain about 40% of Si-rich particles. For the depth samples, higher concentrations were found at 0 (32%), 200 (33%), and 1200 m (51%). These variations in concentration are most probably due to local changes in mineral soils. Si-rich particles can have a mineral or a biogenic source. By means of the X-ray intensities, no distinction can be made between these two types of particles; hence, they will both be classified into the same group. The quartz deposits found in Siberia are among the largest in the world, which results in a large mineral contribution to the Si-rich particle type. However, biogenic particles produced by diatoms, radiolarian, and silicoflagellates will also contribute, especially during

spring and summer when there is a high primary production. During manual analysis, the morphology of the Si-particles is used to determine their source. This is demonstrated in the Figures 4 and 5, which show diatoms and a quartz particle, respectively. A third possible source for Si-rich particles is the combustion of coal in power plants (17). This anthropogenic source will especially contribute to the southern basin of the lake. But since large fluctuations in the abundances are observed over the total lake, mineralogic and biogenic sources are of greater importance. Ca-Rich Particles (C). Small abundances of this particle type were detected in all depth samples and in eight river inflow samples, with the highest abundances at the Tompuda

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FIGURE 4. Secondary electron image of two Si-rich diatom skeletons.

FIGURE 5. Secondary electron image of a quartz particle. (10%) and the Goloustnaya (7%). In these particles, high amounts of Ca and sometimes small contributions for Si, P, and Fe were detected, which results in a characterization as calcium carbonate. This calcite can originate from the weathering of limestone, but it can also be biogenic. In freshwater, however, opal (amorphous SiO2) production dominates a much smaller biogenic calcite production. But this production includes the formation of exoskeletons of higher animals, like crustacea (including Epischura baicalensis

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(18)) and mollusks (14). In the summer, millions of Epischura are detected in the water of Lake Baikal, and although they are so large (a mature animal is 0.57-1.5 mm (18)) that they are rejected from automated analysis by manual inspection of the analyzing area, parts of their shell can still be included. Fe-Rich Particles (D). This particle type may consist of mineralogical particles, like hematite, goethite, and siderite, and anthropogenic particles produced by the ferrous metallurgy. Since Fe-rich minerals and iron ore are commonly observed in the Baikal region (12), a natural source is most likely. However, during manual analysis of the depth samples, some spherical Fe-rich particles were detected, which are produced during ferrous metallurgical processes (11). The nearest heavy industry in this region is found around Irkutsk and Ulan-Ude (19). Particles for the Ulan-Ude region can reach the lake though the Selenga River, while anthropogenic particles from both regions can also be transported by air. Because the predominant wind direction over Lake Baikal is southwest (20), the contribution of the Irkutsk region is dominant for the deposition of anthropogenic particles in the southern basin. Small concentrations of this particle type are found in the majority of the river inflow samples. However, at the Tiya River an abundance of 41% was found. This enhancement can be explained by the large amount of debris that was dumped into this river during the construction of the BaikalAmur railroad (3). Fe-rich particles are also found in all depth samples, and their abundances range from 4 to 10%. Only at 1000 m was an unexplainable high abundance of 43% detected. Ti-Rich Particles (E). Small concentrations of this particle type were detected in all depth samples and in six river inflow samples. These particles can be anthropogenic alloy produced by paints, soil dispersion, and asphalt production (10, 14), but since small amounts of Ti-rich particles are found in most surface waters (14), a mineral source (like rutile) is more likely. Al-Rich Particles (F). This particle type was only detected in four depth samples, with abundances ranging from 0.4 to 2%. These particles most likely originate from the huge aluminium reduction plant at Shelekov, ca. 12 km southwest of Irkutsk, which is known to emit alumina dust (21). This pollution source is confirmed by the enhanced concentration of Al-rich particles detected above the southern basin when the wind was originating from the Irkutsk area (11). S-Rich Particles. The S-rich particles (G1) are found in small concentrations in three samples and are most probably from biogenic origin. Fe-S-rich particles (G2) are found in three samples, with abundances ranging from 0.8 to 2.4%. These particles could be of mineral origin, i.e., pyrite, marcasite, or melanterite, but because these minerals are not very stable in oxidized zones, it is more likely that they are pyrite framboids formed by phytoplankton through concentration of iron hydroxides and organic matter (22). Ba-S-rich particles (G3) are found in two samples and can have a mineralogic origin, barite, but barium sulfate is also used in the manufacturing of rubber and paper as a filler and a weighting agent and as dye during the producing of paints (12). The comparable concentrations of barium found in sediment cores taken between 0 and 50 m and between 50 and 100 m (23) indicate the dominance of the mineral source. Ca-S-rich particles (G4) were only detected at the inflow of the Goloustnaya. A possible source for this calcium sulfate is gypsum sedimented in salt deposits, which are part of a large saliferous belt reaching from the upper part of the Angara to the upper part of the Lena (24). However, gypsum particles were also detected in air masses above Lake Baikal. The relative high and constant occurrence of these particles in the northern and middle basin (13% and 14%) indicate a natural source like the Gobi Desert in Mongolia (11). Southern

cyclones, which frequently occur in the Baikal region during summer (25), can easily transport gypsum particles that were formed by weathering of different rocks in this desert (11). But because the amount of Ca-S-rich particles detected at the Goloustnaya is relatively high (12%) and none were detected at other locations, the salt deposit is the most probable source. Heavy Metal-Rich Particles. Low abundances of particles containing heavy metals were detected in the majority of the depth samples. Mn-rich particles (H) are detected at five depths, with abundances ranging from 0.8 to 3.2%. These particles will be pyrolusite, present in most crystalline rocks (13). However Mn-rich particles are also produced by the steel industry, but since none were found in air collected above the lake, this contribution will be limited. Also small concentrations of Cr-rich (I) and Zn-rich particles (J) were detected. Their anthropogenic origin is confirmed by their presence in air masses, collected above Lake Baikal and originating from the industrial zones at Irkutsk and Ulan-Ude (11). Small abundances of Pb-rich particles, produced by industry and automobile exhaust, were detected in air collected above the lake (11). Since none were found in suspension, these particles must have been dissolved when entering the lake. Cl-Rich Particles (K). This particle type was detected in three river inflow samples and in two depth samples. In all samples the abundances were low (1-2%), except for the sample collected at the Goloustnaya, where 13% was found. The particles detected at the Goloustnaya and Rel only contain Cl and K, which points to the mineral sylvite sedimented in the salt deposits found in this area, but an organic origin cannot be excluded. The particles detected at the inflow of the Sneznaya and in the center of the southern basin also contain Si, S, and Fe. Manual analysis of sample 12 (in this sample no Cl-rich particles were observed, but their abundance is in general rather smalls, so it is possible that they are hidden behind large groups during automated analysis) and sample 23 reveals that these Cl-rich particles have a fibrous structure on which small aluminosilicate particles are adsorbed. They are most likely cellulose fibers discharged by the Baikalsk cellulose plant. The low abundances of these fibers are comparable to the low concentration of organic matter detected at the discharge point of the factory (26). It can thus be concluded that this pulp factory is responsible for the input of some chlorated organic particles into the lake. Organic Particles. During automated measurements with conventional EPXMA, only the ticker and more dense organic particles, like some biogenic material, will have a backscatter signal that can exceed the threshold value set for the organic Nuclepore filter and are thus detected. Additionally, the fixed beryllium window of the energy dispersive X-ray detector absorbs low Z-elements, and the detected pure organic particles will thus be characterized by a spectrum with no interpretable peaks. These dense organic particles were detected in one river inflow sample and in seven depth samples. Since the abundances for the depth samples are relatively high, between 3 and 8%, in comparison with the 0.4% detected at the Pereymnaya, it can be concluded that the dense organic material most probably originates from biogenic activity in the lake. Calculated Suspended Mass. The suspended matter content (particles/mL) can be calculated from the area on which the 250 particles were detected and dividing the amount of particles per filter by the filtered volume. When the diameters of the particles and a mean density for individual natural particles of 2 g/cm3 (27) are included, it is also possible to calculate the mass of the detected particle fraction. However, these calculations will only give a rough estimation

FIGURE 6. Secondary electron image of a K-Cl-coated organic fiber and the results of spot analyses on different locations on this particle. of the mass, because (a) only particles that are detected during automated analysis, i.e., the particles with an average diameter between 0.4 and 10 µm with a high enough backscatter signal, are included; (b) a homogeneous loading of the filter is presumed; (c) for the calculation of the particle volumes, a spherical shape is assumed; and (d) a mean density of natural particles is used (Buffle et al. (27) report that most natural particles have densities between 1 and 3 g/cm3, so a mean density of 2 g/cm3 is used). Tables 1 and 2 show the weighed and calculated suspended mass of all samples. For the riverine samples, the calculated mass represents on average 30% of the total mass, while for the depth samples this is 65%. The remaining mass can partially be explained by the undetected organic fraction that varies between 2 and 40% at the delta of the Selenga River and between 12% at the surface and 3% at 1300 m in the central part of the southern basin (28). However, for the riverine samples, the largest part of the observed mass differences will be due to the presence of numerous particles with a diameter larger than 10 µm, which were excluded from automated analysis by manual inspection of the analyzing area, and to the inhomogeneous loading of these samples. For fairly homogeneously loaded samples, like the depth samples, the calculated mass gives a good estimate of the total inorganic mass.

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spherical particles are frequently noticed after high-temperature combustion processes (10) and could be identified as aluminosilicates (i.e., fly ash) or as heavy metal-rich particles. Figure 7 shows a typical aggregate of fly ash particles detected in the surface water sample. These fly ash particles were detected at all studied depths (i.e., down to 600 m), which proves the good mixing of the cold Baikal water and the maintenance of the typical structure of fly ash particles for extended periods of time in an aquatic environment (30).

FIGURE 7. Secondary electron image of an aggregated fly ash particle. These mass data also show that the Selenga is certainly the most important tributary to the lake. The low concentrations found for Tompuda, Pereyemnaya, Mishikha, Goloustnaya, and Rel are typical for mountain rivers flowing through rocky valleys (4). For the depth samples, comparable concentrations were found at all depths, which confirms the thorough mixing of the cold Baikal water. Study of the Internal Particle Homogeneity. Manual analyses have been performed on eight river inflow samples, i.e., samples collected at the locations 3-16, and 20, and 21. This study was partially focused on the larger particles, i.e., with a diameter larger than 10 µm. These large particles were rejected from automated analysis, because at a magnification of 1000 they are mostly divided over two or more analyzing fields and will thus not be detected as one. This rejection is performed by a manual inspection of the whole selected analyzing area. If large particles are detected in this area, another area of the filter is selected for automated analysis. However, these large particles can reveal extra information about the samples, and some are thus analyzed during the homogeneity study. The majority of these large particles are naturally formed aggregates, but in the samples collected close to the Baikalsk cellulose plant (i.e., samples 3 and 12), large fibers were also detected. The spectrum, collected during a line scan over a fiber, has a noisy background, which points to the presence of organic material (29). The secondary electron image and the results of spot analyses on this fiber are shown in Figure 6. At spot f, no elements were detected, which indicates ‘pure’ organic matter. At the other spots, comparable ratios for the relative peak intensities of Cl and K were obtained, which refers to a coating of the fiber. For the other elements, no comparable ratios could be calculated, which indicates that these peaks most likely originate from small aluminosilicate and Si-rich particles that are adsorbed onto the fiber. Four depth samples (i.e., at 0, 200, 400, and 600 m) were used for manual analysis. Because the large aggregates were not so abundant in these samples, more attention was given to the smaller (i.e.,