Single Particle Characterization ofInorganic Suspension inLake Baikal, SiberiaW E N D Y J A M B E R S A N DR E N E V A N G R I E K E N *

Department of Chemistry, University of Antwerp (UIA),Universiteitsplein 1, B-2610 Antwerpen, Belgium

Automated and manual electron probe X-ray microanalyseswere used to characterize the chemical composition andmorphology of individual suspended particles, collectedat different tributaries of Lake Baikal and in the central partof southern Lake Baikal. The data sets were reducedusing hierarchical clustering, and the results were comparedby means of selection rules. All the samples are dominatedby natural aluminosilicates and Si-rich particles. But smallamounts of anthropogenic aluminosilicates (i.e., fly ash); Fe-rich, Al-rich, Cr-rich, and Zn-rich particles; and Cl-coatedfibers were also detected. The contamination is especiallysevere in the southern basin, which can easily be reachedby atmospheric pollution from the industrial centers nearIrkutsk and Ulan-Ude and which is also influenced by thelarge Baikalsk cellulose factory. At the northern basin, pol-lution is only detected at the inflow of the Tiya River, wherethe dumping of debris from the construction of theBaikal-Amour railroad results in high concentrations of Fe-rich particles.

IntroductionAlthough single particle analyses are a valuable complementto the more conventional bulk techniques, they are scarcelyused for environmental applications (1, 2). During the lastfew years, the analysis of individual aerosol particles hasgained interest. But, although particles are also the majormaterial carriers in water, hardly any research has been doneon the chemical characterization of single particles insuspension and in sediments.

Barely any environmental data are available from theformer Soviet Union. Recently most parts of Russia havebecome accessible to Western scientists, who, in cooperationwith Russian scientific institutes, have started to developenvironmental research in this large country. Siberia hasmany remote and pristine areas, without any industry andwith a very low population density. But it also has regionswith huge industrial sites where the local pollution isenormous. Lake Baikal is a good example of this duality. Thecenter of the lake is still very pure, but in the southern partand near the shore pollution is starting to threaten this uniqueenvironment.

In this study, automated electron probe X-ray microanal-ysis (EPXMA) combined with a recently improved datareduction method was used to determine the chemical andmorphological characteristics of individual micrometer-sizedsuspension particles, collected in the river mouths of 13tributaries of Lake Baikal and at eight different depths in thecentral part of southern Lake Baikal.

Study AreaLake Baikal is situated in southeastern Siberia, close to theMongolian border. With an average depth of 900 m, it is thedeepest freshwater basin on Earth. It is 635 km long and onaverage 48 km wide and holds 23 000 km3 of barely pollutedwater, which is 20% of the world’s total freshwater contentand 80% of the freshwater of the former Soviet Union. Thecold climate of Siberia is responsible for a thorough mixingof the Baikal water and for the occurrence of rich life evenat the deepest parts of the lake. One of the 1500 endemicBaikal species is the tiny crustacean Epischura baicalensis,which strains bacteria and algae from the water, making thecenter of the lake exceptionally pure (3).

The lake has 336 tributaries that drain an area of 557 000km2 and only one outflowing river, the Angara. The majortributaries are the Upper-Angara, the Barguzin, and theSelenga. This Selenga River supplies half the water flowinginto the lake and is loaded with sediments and human andindustrial wastes from three Mongolian cities and from Ulan-Ude (3). The Barguzin and the Upper-Angara also supplypollution to the lake, but in much smaller quantities than theSelenga.

In 1987, Lake Baikal was officially protected by the SovietGovernment, and only recently (December 1996), it has alsobeen inscribed on UNESCO’s World Heritage List. However,the cellulose factory at Baikalsk is still discharging thousandsof tons of minerals, suspension, bacteria, and organicbyproducts (including difficultly biodegradable chlorinateorganic compounds) into the southern basin, while thenorthern basin has been polluted by the construction of theBaikal-Amur railroad, which caused erosion of the northernshore and resulted in overloading of the rivers with waste. Anadditional threat to the lake is the industrial sites in the Selengaand the Angara Valleys, whose pollution products can easilyreach the lake through the air (3).

Experimental SectionSampling. Surface water samples from 23 tributaries of LakeBaikal were collected during the sampling campaign on boardthe R/V Mercury from August 23 to September 3, 1990. Thesuspension was filtered on preweighed 100-mm nuclearpolycarbonate filters (Russian brand) with pores of 0.45 µm.After filtering, the samples were dried for 1 day at 60 °C andstored in air-tight Petri dishes. For single particle analysis,the loading of the filters should be relatively low. Particlesare not allowed to touch because then they will be analyzedas one. Unfortunately, these samples were also used for flameatomic absorption measurements for which a higher loadingis preferred (4). For this reason, the loading of some filterswas a bit too high for single particle analysis, which resultsin some overlapping particles. The filtered volume and theweighed suspended matter (representative for the loading)of the filters that were selected for single particle analysis arerepresented in Table 1, while their sampling locations areshown in Figure 1.

Figure 1 also contains the sampling site of December 12,1995, in the central part of southern Lake Baikal (indicatedwith A). At the coordinates N 51°41′ and E 105°, i.e., themiddle of the trajectory Listvjanka-Tankhoj, water wascollected with a Niskin bottle at eight different depths, startingat the surface and with depth intervals of 200 m. Differentvolumes of water (represented in Table 2) were filtered on47-mm polycarbonate Nuclepore filters with 0.4-µm pore size(Nuclepore, Pleasanton, CA), resulting in adjusted particleloadings for single particle analysis.

To prepare the samples for EPXMA, part of the filter wasmounted with double-sided tape on a 25 mm diameter plastic

* Corresponding author fax: +32 3 820 23 76; e-mail address:vgrieken@uia.ua.ac.be.

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plate, which fits into the sample holder. Electrical and thermalcharging during analysis was prevented by coating the sampleswith a 50-nm carbon layer.

Instrumentation. EPXMA measurements have beenperformed with a JEOL JXA-733 Superprobe (JEOL, Tokyo,Japan) connected to a Tracor Northern TN-2000 X-rayanalyzing system (Tracor Northern, Middleton, WI) and a486 personal computer. All measurements were performedwith an acceleration voltage of 25 kV and a beam current of1 nA. Under these conditions and with a collection time of20 s, energy dispersive X-ray measurements yield a detectionlimit of about 1% or lower for most elements (5).

To ensure the statistical relevancy of the results, 250particles were analyzed per sample. To enable the analysisof a large number of particles in a short time, EPXMA wasautomated with the homemade 733 particle recognition andcharacterization program (5). An X-ray recording time of 20s and a magnification of 1000 were used to characterizeparticles with a spherical diameter larger than 0.4 µm. Theimage resolution at this magnification is 0.2 µm.

Manual EPXMA was used to study the homogeneity of theindividual particles and to determine the relation betweenparticle composition, origin, and shape. During 60 s, spectra

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 filteralgorithm, and the peak intensities were normalized.

Data Treatment. The automated analysis of 250 particlesin each sample resulted in a huge data matrix, which wasreduced for interpretation by means of a geochemicallyrelevant clustering, i.e., a hierarchical clustering based onEuclidean distances, with the Ward’s error sum classificationas similarity criterion (5, 6). This clustering was performedusing the integrated data analysis system (IDAS) developedat the University of Antwerp by Bondarenko et al. (7). Thissoftware includes the consistent Akaike information criterion(CAIC). A minimum in the CAIC corresponds to an accuratenumber of groups (7, 8).

Comparison of the results of the different samples ispossible by performing a secondary clustering of the groupsresulting from the first clustering (5). However, this combinedclustering procedure is very time consuming, and an alterna-tive data reduction is proposed using selection rules, basedon the mean relative X-ray intensities of characteristic particletypes (9, 10). Because the same rules are used for all data,no extra clustering is needed. The results obtained usingthis method are in good agreement with those obtained bythe combined clustering procedure (10). However, theseselection rules can only be used when some knowledge aboutthe chemical composition of the data is available.

Because in this study no prior knowledge about thechemical composition of the data was available, the datareduction per sample was done using hierarchical clustering.However, the resulting groups supply information about thechemical composition of the data. This information wascombined with the mean relative X-ray intensities of char-acteristic particle types in riverine suspension (10) to obtainselection rules relevant for Lake Baikal suspension. Theseselection rules are represented in Table 3 and were used tocompare the results of the different samples by identifyingall groups resulting from the hierarchical clustering proce-dures. Using this approach, data can be reduced withoutany prior knowledge of the chemical composition, and thetime-consuming secondary clustering is avoided by usingthe selection rules as an equal alternative.

Results and DiscussionThe different particle types found at the inflow of the riversin Lake Baikal and in the center of southern Lake Baikal aregiven, together with their abundances, in the Figures 2a,band 3. They will be discussed in detail in the next paragraphs.

Aluminosilicates. The aluminosilicate particle typesdominate in most samples and are characterized by highrelative X-ray intensities for Al, Si, Fe, K, and sometimes Caor Ti. According to their composition, they are divided intosix subgroups: pure aluminosilicate with only Al and Si;aluminosilicate with Al, Si, and small amounts of K, Fe and/

TABLE 1. Filtered Volume of Water and Weighed andCalculated Suspended Matter at Outflow of Different Riversinto Lake Baikal, for 13 Sampling Locations Selected from the1990 Campaign

samplinglocation

filteredvol of

water (mL)

weighedsuspended matter

content (mg/L)

calcdsuspended matter

content (mg/L)

1 Selenga 1 750 18.11 3.603 Solzan 5 000 0.79 0.245 Tompuda 9 800 0.22 0.026 Tiya 3 600 1.15 0.138 Barguzin 2 900 4.60 4.30

12 Khara-Murin 5 000 0.60 0.0813 Utulik 6 000 1.01 0.2116 Pereyemnaya 6 500 0.09 0.0317 Mishikha 6 000 0.36 0.0718 Buguldeyka 2 800 1.48 0.8020 Goloustnaya 10 000 0.00521 Rel 10 000 0.07 0.0223 Sneznaya 6 750 0.26 0.048

FIGURE 1. Map of Lake Baikal with the different sampling locations.

TABLE 2. Filtered Volume of Water and Weighed andCalculated Suspended Matter at Different Depths at LocationA in Central Part of Southern Lake Baikal, Sampled onDecember 12, 1995

samplingdepth (m)

filtered volof water (mL)

weighedsuspended matter

content (mg/L)

calcdsuspended matter

content (mg/L)

0 1 810 1.93 0.14200 2 000 0.12 0.16400 1 405 0.30 0.07600 5 595 0.30 0.09800 1 585 0.10 0.04

1 000 890 0.20 0.201 200 290 0.14 0.221 400 2 120 0.14 0.04

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or Ca (in the rest of the discussion this will be called thealuminosilicate subgroup); and Ca-rich, K-rich, Fe-rich, andTi-rich aluminosilicates with high abundances for thesespecific elements.

The main source of these particles is the erosion of rocksand soils, but they are also produced during high-temperaturecombustion processes. These fly ash particles cannot bedistinguished from mineral aluminosilicates during auto-mated EPXMA, but they usually have a typically sphericalmorphology that can easily be recognized during manualanalysis. In the samples collected at the inflow of the rivers,none were found, and analyses of aerosols collected abovethe northern and central basin of Baikal confirm the limitedcontribution of these fly ash particles (11). However duringmanual analysis of the first four depth samples, collected inthe southern basin, small amounts of fly ash particles weredetected at all depths. Also, in the air collected in the vicinityof the Baikalsk cellulose plant, a major part of the alumi-nosilicates was identified as fly ash (11). Thus, in watersamples collected in the southern basin, fly ash will contributeto the aluminosilicate fraction, though only in limitedamounts.

Generally, the pure aluminosilicate (A1) and alumino-silicate subgroup (A2) mainly consist of clay minerals likekaolinite, illite, and montmorillonite and micas like muscoviteand biotite (12-14). The first group is present in all depthsamples with abundances ranging from 5 to 33%. For theriver inflow samples, this particle type is only found at twolocations in the southern basin and at the Rel River. Thesecond group, however, is present in the majority of thesamples, and with abundances ranging from 7 to 54%, it isthe most important subgroup.

Ca-rich aluminosilicates (A3) are present in most riverinflow samples, but they were only found in two depthsamples. This particle type probably originates from theseveral Ca-containing aluminosilicate minerals, like anorthite,Ca-montmorillonite, epidotes, and zeolites, which are presentin suspended matter of natural waters (10, 14). But becausethis particle type had strongly enhanced levels of abundancein air masses collected above the more polluted southernbasin of Lake Baikal (11), an anthropogenic source cannot beexcluded. This subgroup has high contributions for the riversTompuda (26%) and Sneznaya (18%). For the Tompuda River,this enhancement can be explained by the possible floc-culation of aluminosilicates with Ca-rich particles, which are

present in a relatively high abundance. However, in thesample collected at the Sneznaya River, no Ca-rich particleswere detected. This river is located in the southern basinclose to the cellulose plant, which could point to ananthropogenic source. But because no Ca-rich aluminosili-cate particles were detected at the rivers Khara-Murin andSolzan, this anthropogenic contribution will be limited.

Small abundances of K-rich aluminosilicates (A4) are foundin a majority of the samples. Only at the Rel River is theabundance high (18%). This particle type can be characterizedas K-containing aluminosilicate minerals like orthoclase andmicrocline (12, 14).

The Fe-rich aluminosilicates subgroup (A5) is generallycharacterized by minerals like chlorite and Fe-smectite (10).However aluminosilicates, and in particular clay minerals,are known to adsorb trace elements (14, 15). Thus, alumi-nosilicates that have absorbed large amounts of Fe can alsocontribute to this particle type. These Fe-rich aluminosilicatesare detected in all samples, but only at the sampling locations13, 16, and 20 and at the depth samples at 0 and 200 m wereabundances higher than 20% found. These enhancementsare probably due to local variations of the minerals. This isthe only particle type for which a decreasing tendency withdepth could be observed.

Small amounts of Ti-rich aluminosilicates (A6) are onlydetected in the samples collected at the Selenga River.Because no mineral is known that corresponds to this particletype (12, 13) and rutile frequently occurs in granite, gneiss,and mica schist (13), these are most likely naturally formedaggregates.

The aluminosilicate particle types dominate, with abun-dances larger than 50%, in the majority of the samples. Onlyin four samples do the Fe-rich or Si-rich particles dominate.This Si-rich particle type represents, together with thealuminosilicate groups, the major fraction of the suspendedmatter (abundances between 50 and 100%).

Si-Rich Particles (B). This particle type contains particleswith 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 thathave adsorbed iron oxides and calcium carbonates (10, 16).Si-rich particles are detected in all samples with abundancesranging from 12 to 70%. Especially in the sample collectedat the Mishikha River, a large amount of Si-rich particles wasfound (up to 70%). Smaller enhancements were found at the

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

aluminosilicatesA1 pure aluminosilicates Al + Si > 90 and 15 < Si < 85A2 aluminosilicates Al + Si + K + Fe > 90 and 15 < Si < 85A3 Ca-rich aluminosilicates Al + Si + K + Fe + Ca > 90 and 15 < Ca < 50A4 K-rich aluminosilicates Al + Si + K + Fe > 90 and K > AlA5 Fe-rich aluminosilicates Al + Si + K + Fe > 90 and Fe > AlA6 Ti-rich aluminosilicates Al + Si + K + Fe + Ti > 90 and Ti > AlB Si-rich Si > 70 and Al < 5C Ca-rich Ca > 50D Fe-rich Fe > 50E Ti-rich Ti > 50F Al-rich Al > 50

S-richG1 S-rich S > 70G2 Fe-S-rich S > 40 and Fe > 20G3 Ba-S-rich S > 40 and Ba > 20G4 Ca-S-rich S > 40 and Ca > 20H Mn-rich Mn > 50I Cr-rich Cr > 50J Zn-rich Zn > 50K Cl-rich Cl > 50O organic net X-ray counts < 1000

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

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rivers Buguldeyka, Rel, and Solzan, which all contain about40% of Si-rich particles. For the depth samples, higherconcentrations were found at 0 (32%), 200 (33%), and 1200m (51%). These variations in concentration are most probablydue 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 madebetween these two types of particles; hence, they will bothbe classified into the same group. The quartz deposits foundin Siberia are among the largest in the world, which resultsin 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-particlesis used to determine their source. This is demonstrated inthe Figures 4 and 5, which show diatoms and a quartz particle,respectively. A third possible source for Si-rich particles isthe combustion of coal in power plants (17). This anthro-pogenic source will especially contribute to the southern basinof the lake. But since large fluctuations in the abundancesare observed over the total lake, mineralogic and biogenicsources are of greater importance.

Ca-Rich Particles (C). Small abundances of this particletype were detected in all depth samples and in eight riverinflow samples, with the highest abundances at the Tompuda

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.

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(10%) and the Goloustnaya (7%). In these particles, highamounts of Ca and sometimes small contributions for Si, P,and Fe were detected, which results in a characterization ascalcium carbonate. This calcite can originate from theweathering of limestone, but it can also be biogenic. Infreshwater, however, opal (amorphous SiO2) productiondominates a much smaller biogenic calcite production. Butthis production includes the formation of exoskeletons ofhigher animals, like crustacea (including Epischura baicalensis

(18)) and mollusks (14). In the summer, millions of Epischuraare detected in the water of Lake Baikal, and although theyare so large (a mature animal is 0.57-1.5 mm (18)) that theyare rejected from automated analysis by manual inspectionof the analyzing area, parts of their shell can still be included.

Fe-Rich Particles (D). This particle type may consist ofmineralogical particles, like hematite, goethite, and siderite,and anthropogenic particles produced by the ferrous metal-lurgy. Since Fe-rich minerals and iron ore are commonlyobserved in the Baikal region (12), a natural source is mostlikely. However, during manual analysis of the depth samples,some spherical Fe-rich particles were detected, which areproduced during ferrous metallurgical processes (11). Thenearest heavy industry in this region is found around Irkutskand Ulan-Ude (19). Particles for the Ulan-Ude region canreach the lake though the Selenga River, while anthropogenicparticles from both regions can also be transported by air.Because the predominant wind direction over Lake Baikal issouthwest (20), the contribution of the Irkutsk region isdominant for the deposition of anthropogenic particles inthe southern basin.

Small concentrations of this particle type are found in themajority of the river inflow samples. However, at the TiyaRiver an abundance of 41% was found. This enhancementcan be explained by the large amount of debris that wasdumped into this river during the construction of the Baikal-Amur railroad (3). Fe-rich particles are also found in all depthsamples, and their abundances range from 4 to 10%. Onlyat 1000 m was an unexplainable high abundance of 43%detected.

Ti-Rich Particles (E). Small concentrations of this particletype were detected in all depth samples and in six river inflowsamples. These particles can be anthropogenic alloy pro-duced by paints, soil dispersion, and asphalt production (10,14), but since small amounts of Ti-rich particles are found inmost surface waters (14), a mineral source (like rutile) is morelikely.

Al-Rich Particles (F). This particle type was only detectedin four depth samples, with abundances ranging from 0.4 to2%. These particles most likely originate from the hugealuminium reduction plant at Shelekov, ca. 12 km southwestof Irkutsk, which is known to emit alumina dust (21). Thispollution source is confirmed by the enhanced concentrationof Al-rich particles detected above the southern basin whenthe wind was originating from the Irkutsk area (11).

S-Rich Particles. The S-rich particles (G1) are found insmall concentrations in three samples and are most probablyfrom biogenic origin.

Fe-S-rich particles (G2) are found in three samples, withabundances ranging from 0.8 to 2.4%. These particles couldbe of mineral origin, i.e., pyrite, marcasite, or melanterite,but because these minerals are not very stable in oxidizedzones, it is more likely that they are pyrite framboids formedby phytoplankton through concentration of iron hydroxidesand organic matter (22).

Ba-S-rich particles (G3) are found in two samples and canhave a mineralogic origin, barite, but barium sulfate is alsoused in the manufacturing of rubber and paper as a filler anda weighting agent and as dye during the producing of paints(12). The comparable concentrations of barium found insediment cores taken between 0 and 50 m and between 50and 100 m (23) indicate the dominance of the mineral source.

Ca-S-rich particles (G4) were only detected at the inflowof the Goloustnaya. A possible source for this calcium sulfateis gypsum sedimented in salt deposits, which are part of alarge saliferous belt reaching from the upper part of the Angarato the upper part of the Lena (24). However, gypsum particleswere also detected in air masses above Lake Baikal. Therelative high and constant occurrence of these particles inthe northern and middle basin (13% and 14%) indicate anatural source like the Gobi Desert in Mongolia (11). Southern

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

FIGURE 5. Secondary electron image of a quartz particle.

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cyclones, which frequently occur in the Baikal region duringsummer (25), can easily transport gypsum particles that wereformed by weathering of different rocks in this desert (11).But because the amount of Ca-S-rich particles detected atthe Goloustnaya is relatively high (12%) and none weredetected at other locations, the salt deposit is the mostprobable source.

Heavy Metal-Rich Particles. Low abundances of particlescontaining heavy metals were detected in the majority of thedepth samples. Mn-rich particles (H) are detected at fivedepths, with abundances ranging from 0.8 to 3.2%. Theseparticles will be pyrolusite, present in most crystalline rocks(13). However Mn-rich particles are also produced by thesteel industry, but since none were found in air collectedabove the lake, this contribution will be limited.

Also small concentrations of Cr-rich (I) and Zn-richparticles (J) were detected. Their anthropogenic origin isconfirmed by their presence in air masses, collected aboveLake Baikal and originating from the industrial zones at Irkutskand Ulan-Ude (11).

Small abundances of Pb-rich particles, produced byindustry and automobile exhaust, were detected in aircollected above the lake (11). Since none were found insuspension, these particles must have been dissolved whenentering the lake.

Cl-Rich Particles (K). This particle type was detected inthree river inflow samples and in two depth samples. In allsamples the abundances were low (1-2%), except for thesample collected at the Goloustnaya, where 13% was found.The particles detected at the Goloustnaya and Rel only containCl and K, which points to the mineral sylvite sedimented inthe salt deposits found in this area, but an organic origincannot be excluded. The particles detected at the inflow ofthe Sneznaya and in the center of the southern basin alsocontain Si, S, and Fe. Manual analysis of sample 12 (in thissample no Cl-rich particles were observed, but their abun-dance is in general rather smalls, so it is possible that theyare hidden behind large groups during automated analysis)and sample 23 reveals that these Cl-rich particles have afibrous structure on which small aluminosilicate particlesare adsorbed. They are most likely cellulose fibers dischargedby the Baikalsk cellulose plant. The low abundances of thesefibers are comparable to the low concentration of organicmatter detected at the discharge point of the factory (26). Itcan thus be concluded that this pulp factory is responsiblefor the input of some chlorated organic particles into thelake.

Organic Particles. During automated measurements withconventional EPXMA, only the ticker and more dense organicparticles, like some biogenic material, will have a backscattersignal that can exceed the threshold value set for the organicNuclepore filter and are thus detected. Additionally, the fixedberyllium window of the energy dispersive X-ray detectorabsorbs low Z-elements, and the detected pure organicparticles will thus be characterized by a spectrum with nointerpretable peaks.

These dense organic particles were detected in one riverinflow sample and in seven depth samples. Since theabundances for the depth samples are relatively high, between3 and 8%, in comparison with the 0.4% detected at thePereymnaya, it can be concluded that the dense organicmaterial most probably originates from biogenic activity inthe lake.

Calculated Suspended Mass. The suspended mattercontent (particles/mL) can be calculated from the area onwhich the 250 particles were detected and dividing the amountof particles per filter by the filtered volume. When thediameters of the particles and a mean density for individualnatural particles of 2 g/cm3 (27) are included, it is also possibleto calculate the mass of the detected particle fraction.However, these calculations will only give a rough estimation

of the mass, because (a) only particles that are detected duringautomated analysis, i.e., the particles with an average diameterbetween 0.4 and 10 µm with a high enough backscatter signal,are included; (b) a homogeneous loading of the filter ispresumed; (c) for the calculation of the particle volumes, aspherical shape is assumed; and (d) a mean density of naturalparticles is used (Buffle et al. (27) report that most naturalparticles have densities between 1 and 3 g/cm3, so a meandensity of 2 g/cm3 is used).

Tables 1 and 2 show the weighed and calculated suspendedmass of all samples. For the riverine samples, the calculatedmass represents on average 30% of the total mass, while forthe depth samples this is 65%. The remaining mass canpartially be explained by the undetected organic fraction thatvaries between 2 and 40% at the delta of the Selenga Riverand between 12% at the surface and 3% at 1300 m in thecentral part of the southern basin (28). However, for theriverine samples, the largest part of the observed massdifferences will be due to the presence of numerous particleswith a diameter larger than 10 µm, which were excluded fromautomated analysis by manual inspection of the analyzingarea, and to the inhomogeneous loading of these samples.For fairly homogeneously loaded samples, like the depthsamples, the calculated mass gives a good estimate of thetotal inorganic mass.

FIGURE 6. Secondary electron image of a K-Cl-coated organic fiberand the results of spot analyses on different locations on this particle.

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These mass data also show that the Selenga is certainlythe most important tributary to the lake. The low concen-trations found for Tompuda, Pereyemnaya, Mishikha, Goloust-naya, and Rel are typical for mountain rivers flowing throughrocky valleys (4). For the depth samples, comparableconcentrations were found at all depths, which confirms thethorough mixing of the cold Baikal water.

Study of the Internal Particle Homogeneity. Manualanalyses 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 wererejected from automated analysis, because at a magnificationof 1000 they are mostly divided over two or more analyzingfields and will thus not be detected as one. This rejection isperformed by a manual inspection of the whole selectedanalyzing 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 informationabout the samples, and some are thus analyzed during thehomogeneity study.

The majority of these large particles are naturally formedaggregates, but in the samples collected close to the Baikalskcellulose plant (i.e., samples 3 and 12), large fibers were alsodetected. The spectrum, collected during a line scan over afiber, has a noisy background, which points to the presenceof organic material (29). The secondary electron image andthe results of spot analyses on this fiber are shown in Figure6. At spot f, no elements were detected, which indicates ‘pure’organic matter. At the other spots, comparable ratios for therelative peak intensities of Cl and K were obtained, whichrefers to a coating of the fiber. For the other elements, nocomparable ratios could be calculated, which indicates thatthese peaks most likely originate from small aluminosilicateand Si-rich particles that are adsorbed onto the fiber.

Four depth samples (i.e., at 0, 200, 400, and 600 m) wereused for manual analysis. Because the large aggregates werenot so abundant in these samples, more attention was givento the smaller (i.e., <1 µm) and more spherical particles. These

spherical particles are frequently noticed after high-temper-ature combustion processes (10) and could be identified asaluminosilicates (i.e., fly ash) or as heavy metal-rich particles.Figure 7 shows a typical aggregate of fly ash particles detectedin the surface water sample. These fly ash particles weredetected at all studied depths (i.e., down to 600 m), whichproves the good mixing of the cold Baikal water and themaintenance of the typical structure of fly ash particles forextended periods of time in an aquatic environment (30).

Summary. Suspended matter collected at the inflow ofthe tributaries of Lake Baikal and in the central part of southernLake Baikal is dominated by natural aluminosilicates andSi-rich particles. At the Goloustnaya River, 25% of the particles(i.e., 12% of gypsum and 13% of sylvite) originate from saltdeposits that are abundant on the western shore of the lake.However, traces of pollution of the lake are also visible,especially in the southern basin where small amounts ofanthropogenic aluminosilicate (fly ash), Fe-rich, Al-rich, Cr-rich, and Zn-rich particles and Cl-coated fibers were detected.The source of these anthropogenic particles is not only thelarge pulp mill in Baikalsk on the southern shore of LakeBaikal but also the industry near Irkutsk and Ulan-Ude. Heavymetal-rich particles and fly ashes discharged in the air atIrkutsk can easy reach the lake, because the predominantwind direction over Lake Baikal is southwest. Also in thenorthern basin, pollution is starting to influence the lake.The Tiya River contributes large concentrations of Fe-richparticles to the lake, which most likely originate from thedebris dumped into the river during the construction of theBaikal-Amur railroad.

Although the pollution of Lake Baikal is still limited,anthropogenic particles are present and further study will benecessary to monitor the influence of pollution on the uniqueenvironment of the lake.

AcknowledgmentsWe would like to thank Vladimir Potemkin and TatjanaPotemkina from the Limnological Institute, Irkutsk, andVladimir Shevchenko from the Institute of Oceanology,Moscow, for supplying us with the samples and all necessarygeological information. This study was partially supportedby INTAS (Contract 93-0182) and the Belgian StatesPrimeMinister’s ServicesServices for Scientific, Technical andCultural Affairs (Contract IN/RU/001).

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FIGURE 7. Secondary electron image of an aggregated fly ashparticle.

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Received for review September 16, 1996. Revised manuscriptreceived November 30, 1996. Accepted January 3, 1997.X

ES9608003

X Abstract published in Advance ACS Abstracts, March 1, 1997.

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