HEAVY METAL CONTAMINATION IN SEDIMENTS OF LAKENAHUEL HUAPI, NAHUEL HUAPI NATIONAL PARK, NORTHERN

PATAGONIA, ARGENTINA

SERGIO RIBEIRO GUEVARA1∗, JULIETA MASSAFERRO2,GUSTAVO VILLAROSA2, MARÍA ARRIBÉRE1 and ANDREA RIZZO3

1 Laboratorio de Análisis por Activación Neutrónica, Centro Atómico Bariloche and InstitutoBalseiro, 8400 Bariloche, Argentina, Comisión Nacional de Energía Atómica and Universidad

Nacional de Cuyo; 2 CONICET, Argentina; 3 Laboratorio de Análisis por Activación Neutrónica,Centro Atómico Bariloche, Argentina, Comisión Nacional de Energía Atómica and CONICET

(∗ author for correspondence, e-mail: ribeiro@cab.cnea.gov.ar, fax: 54 2944 445299)

(Received 4 April 2000; accepted 23 June 2001)

Abstract. Trace metals (Sb, As, Br, Cs, Co, Cr, Ag and Hg), rare earth elements (Ce, Eu, La, Lu,Sm, Tb and Yb), and Sc. were analysed in suspended load and in two sediment cores from LakeNahuel Huapi, Nahuel Huapi National Park, Patagonia, Argentina, by using Instrumental NeutronActivation Analysis. The core activity profiles of 210Pb and 137Cs were measured to estimate theage of the sediments. Silver was enriched in the upper layers of both cores, and correlated with thestrong growth of the population of Bariloche City in the last 50 yr. Concentrations are higher in thecore sampled near Bariloche, and indicate some transport in the water body. Silver concentrationsmeasured in the suspended load were 4.58±0.36 and 3.46±0.40 µg g−1 in the southern marginof the lake, near San Carlos de Bariloche City, whereas in the northern margin the concentrationswere 1.68±0.18 and 1.88±0.23 µg g−1. Concentration ratio for silver in suspended load and uppercore layers were equal in both sampling points. These facts suggest that Ag contents correspond toinputs associated with human activities. Bromine concentrations show a strong increase above baseline values, similar to silver. In contrast, caesium, chromium, and cobalt concentrations do not differfrom base line values. The metalloids antimony and arsenic show little variation with regard to baseline concentration. Noticeable, the mercury contents found in the suspended load.

Keywords: 137Cs dating, heavy metals, Instrumental Neutron Activation Analysis, Lake NahuelHuapi, 210Pb dating, sediment contamination

1. Introduction

Lake sediments contain valuable historic information and can be considered asenvironmental archives that can be read employing different analytical techniques.In this work we use Instrumental Neutron Activation Analysis (INAA) to assessthe history of input of heavy metals and other pollutants, into Lake Nahuel Huapi,Nahuel Huapi National Park, Patagonia, Argentina. The study of elemental con-centration profiles of sediment cores sampled from a lake reveals the historicaldevelopment of the element input to the ecosystem (Hardaway, 1998). Compar-ing measured concentrations with base line values allows us to identify historical

Water, Air, and Soil Pollution 137: 21–44, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

22 S. RIBEIRO GUEVARA ET AL.

changes in the lake environment, and the dating of the sediment core makes itpossible to correlate these changes with natural or anthropogenic events occurringin the lake surroundings. Core dating was performed by measuring 210Pb and 137Csspecific activity profiles using low background gamma ray spectrometry. Sedi-ment core analyses were complemented with analyses of the suspended load todetermine current heavy metal input to the lake ecosystem.

Simple geochemical tools were used to identify the origin of the sedimentaryclastic contributions (Condie, 1993; Cullers, 1994). Rare earth elements (REE;La, Ce, Sm, Eu, Tb, Yb and Lu) normalised by North America Shale Compositepatterns (NASC; Haskin et al., 1968) were used as tracers of the provenance ofthe materials. Sc contents, also an immobile element (Cullers et al., 1988; Cullers,1995), was likewise studied and used as a reference.

Lake Nahuel Huapi is located in the Nahuel Huapi National Park, where norelevant industrial or extensive agricultural activities exist. Potential sources ofcontaminants could be found only in human settlements around the lake. Moreover,for metals like mercury, global atmospheric transport and deposition may con-tribute to increase concentrations in lake sediments (US Environmental ProtectionAgency, 1997). San Carlos de Bariloche is the most important city in the region,and it is located in the south margin of lake Nahuel Huapi (see Figure 1), witha population of 100 000 inhabitants. The main activity of the region is tourism,and effluents of the many photography laboratories of Bariloche City could be apotential source of contamination. These effluents have high silver contents andare discharged without any treatment, with only partial silver recovery in somecases. Liquid effluents of the San Carlos de Bariloche sewage treatment plant aredirectly released to the lake as shown in Figure 1. This plant has been in operationsince February 1996; before, San Carlos de Bariloche City wastewater has beendischarged into the lake without treatment. Contamination of waters by heavymetals is associated with various human activities (Deely, 1992; Hatje, 1997),including sewage discharges (Bachtiar, 1996). In a previous work (Kestelman etal., 1997) chromium enrichment in riverbed sediments have been reported at themouth of Ñireco River. This river flows through San Carlos de Bariloche Cityand into Lake Nahuel Huapi (see Figure 1). In that work the authors ascribed thechromium excess to anthropogenic inputs from San Carlos de Bariloche. Sincewe use a multi element technique, we will study not only elements associatedtypically with human activities, but also such elements that indicate natural fluc-tuations. A previous work on heavy metal contents in water and plankton fromLake Nahuel Huapi, showed increased concentrations, which could be attributed toanthropogenic influences (Markert, 1997).

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 23

Figure 1. Map of the study area, including sampling points.

2. Methodology

Lake Nahuel Huapi (557 km2 total area, 464 m maximum depth, 764 masl) is loc-ated in the Nahuel Huapi National Park, Patagonia, Argentina (40◦50′S, 71◦30′W).It is a warm monomictic lake with a thermal stratification during summer. Althoughit can be considered as an ultraoligotrophic system, it shows some mesotrophicareas located near Bariloche City and Villa La Angostura, due to waste discharges(see Figure 1). There is a strong vegetation gradient from dense rainforest (mainlyNothofagus dombeyii and Austrocedrus chilensis) in the West to dry grassland (eco-

24 S. RIBEIRO GUEVARA ET AL.

tonal forest) in the East. The West to East precipitation gradient is also steep, from2000 to 700 mm (June, 1996 to June, 1997); in Bariloche City precipitation was1200 mm (Pedrozo et al., 1999).

A 70 cm sediment core, named NHS core, was sampled in Lake Nahuel Huapi,near the point of discharge of the liquid effluents of the San Carlos de Barilochesewage treatment plant into the lake (see Figure 1). The core was taken at 42 mdepth by using a messenger activated gravity type corer. Two 1 cm-layers weresampled from the upper end of the core, and thereafter the core was sampled every2 cm. Another sediment core was sampled in the northern part of the lake, nearPuerto Cisnes (see Figure 1), named NHN core. Core NHN was 72 cm long andtaken at a depth of 40 m. Core NHN was sub-sampled every 2 cm.

Suspended load was collected by using Trypton type sediment traps; one trapwas placed near the NHS coring site (see Figure 1), at a depth of 60 m (NHSsamples), and the other near the NHN coring site (see Figure 1), at a depth of 82 m(NHN samples). Sampling of the suspended load occurred:

Sample NHS1 20 August, 1998 to 18 November, 1998

Sample NHS2 18 November, 1998 to 14 January, 1999

Sample NHN1 23 July, 1998 to 20 August, 1998

Sample NHN2 20 August, 1998 to 18 November, 1998

Sediment samples were analysed by INAA. The samples were irradiated in theRA-6 nuclear reactor, Centro Atómico Bariloche, inside heat sealed nylon vials,with a final disk shape 1 to 2 mm high and 1 cm in diameter. The samples werefreeze dried until constant weight. Storage and drying of sediments could involvechanges in the elemental contents. Dried samples can be stored for long periodswithout relevant looses (Bamba, 1997). Lyophilization is considered to be the mostcareful drying method to reduce risks of losing the more volatile elements (Bamba,1997). NHN core samples were dried in oven at 80 ◦C until constant weight.Relevant mercury losses are expected when drying samples at this temperature.For this reason mercury was not analysed in these samples. Sample mass of coresediments was about 150 mg, while sample mass of suspended load was lower(80 mg). Core sediment and suspended load samples were irradiated in the reactorcore (φth

∼=6×1012 n cm−2 s−1, φepi∼=2×1011 n cm−2 s−1 and φf

∼=2×1012 n cm−2

s−1). NHS core sediments were irradiated for 8 hr, NHN core samples for 24 hrand suspended load samples for 27.5 hr. After decay times of 7 d, 20 d and 1 yr,gamma spectra of the NHS core sediment samples were collected respectively for1, 4, and 40 hr. Three spectra were taken on NHN samples: one after 9 d decay timeand 2 hr counting time; the other two after 20 to 40 d decay time; with 4 and 50 hrcounting time (this spectrum was taken with high dead counting time, and it wasused only for mercury and silver determinations). For the suspended load, decaytimes of 10 and 30 d, with respective counting times of 2 and 24 hr, were chosen.An intrinsic HPGe N type detector, 12.3 % relative efficiency, and a 4096-channel

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 25

analyser were used, and spectra were analysed by using the GAMANAL routineincluded in the GANAAS package, distributed by IAEA. The absolute parametricmethod was used to determine concentrations, using nuclear constants taken fromtables (Tuli, 1995; Firestone et al., 1996; Mughabghab et al., 1981). Thermal andepithermal neutron fluxes were determined by using the (n,γ ) reactions of the pairCo-Au (using high purity wires of pure Co and 0.112% Au-Al), and the neutronfast flux was determined by using the 58Ni(n,p)58Co reaction (σ = 111±3 barns).Corrections for spectral interference were performed. The elements determinedwere those related to human activities, namely, Sb, As, Br, Cs, Co, Cr, Hg, and Ag,and activated geochemical tracers and major elements.

Organic matter content of core sediments was determined by loss on ignition(LOI), calculated after freeze drying the samples, at 550 ◦C for 8 hr.

The 210Pb total activity core profile, together with the 226Ra activity, were meas-ured using low background gamma ray spectrometry (Appleby et al., 1986; Joshi,1987; Schelske et al., 1994; Ribeiro Guevara et al., 1999). 210Pb excess specificactivity was determined with these measurements, and the age of the core sedi-ment layers and sedimentation rates were computed using the CRS model (Ap-pleby, 1978; Joshi, 1991; Robbins et al., 1993; Binford, 1990). The 137Cs activitycore profile of both cores was also measured through low background gamma rayspectrometry, as an independent age marker (Ribeiro Guevara et al., accepted forpublication in the Journal of Radioanalytical and Nuclear Chemistry).

3. Results

Figures 2 and 3 show 210Pb excess specific activity profiles. The 226Ra specificactivity, which represents the supported 210Pb activity, coincides with the total210Pb of layer 2–4 cm (2.75 g cm−2) of NHS core, and layer 6–8 cm (1.66 g cm−2)of NHN core, and all subsequent layers. Therefore, the unsupported, or excess,210Pb activity is significant only in the upper layers of both core profiles. Underthese conditions, the dating of the remainder layers of the sediment core was notpossible. However, with the data available from the upper layers, an estimation ofthe mean sedimentation rate was obtained. The mean sedimentation rate estimatedfor the last 50 yr in core NHS, is 59 mg cm−2 yr−1 (0.043 cm yr−1) (RibeiroGuevara et al., 1999), hence the estimated accumulation period for layers 0–1 cm(0–1.12 g cm−2) and 1–2 cm (1.12–2.75 g cm−2) are 1975–1998 and 1952–1975,respectively. The mean sedimentation rate estimated in the last 32 years is 22 mgcm−2 yr−1 (0.123 cm yr−1) for core NHN, hence the estimated accumulation periodfor layers 0–2 and 2–4 cm are 1982–1998 and 1966–1982, respectively. The coreactivity profile for 137Cs was also measured as an independent age marker; thevalues obtained are also shown in Figures 2 and 3. The onset of 137Cs inputs into thebiosphere coincides with the widespread atmospheric testing of nuclear weapons,which started in 1954, and the 1959–1960 and 1963–1964 intervals are reported

26 S. RIBEIRO GUEVARA ET AL.

Figure 2. Specific activity profile of 210Pb excess and 137Cs in the Puerto Cisnes lake core (NHNcore). The sediment accumulation periods of the upper layers were estimated by 210Pb dating.

as periods of maximum 137Cs fallout in the Northern Hemisphere (Ritchie et al.,1973; Joshi, 1989). The Chernobyl accident (1986) is also clearly registered in137Cs core profiles in the Northern Hemisphere (von Gunten et al., 1993). But themain 137Cs fallout contribution in this region were originated in the South Pacificnuclear tests, from 1966 to 1974, and the Chernobyl accident was not registered(Ribeiro Guevara, accepted for publication in the Journal of Radioanalytical andNuclear Chemistry). The low resolution of core NHN and NHS sub-sampling,combined with the low sedimentation rate, does not allow distinguishing the 137Csfallout peaks in the activity profile. However, these profiles are consistent with

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 27

Figure 3. Specific activity profile of 210Pb excess and 137Cs in the Bariloche City lake core (NHScore). The sediment accumulation periods of the upper layers were estimated by 210Pb dating.

the 210Pb sedimentation rate estimation, regarding the 137Cs fallout time evolutionin the study region (Ribeiro Guevara, accepted for publication in the Journal ofRadioanalytical and Nuclear Chemistry).

The NHS and NHN core profiles of organic matter contents are shown in Fig-ure 4. The results of the elemental analyses are shown in Tables III and V, withconcentration profiles of the two sediment cores shown in Figures 5 to 9. Standard

28 S. RIBEIRO GUEVARA ET AL.

Figure 4. Organic matter contents of NHS and NHN core sediments. Determinations performed byloss on ignition.

Reference Material (Buffalo River Sediment), supplied by the National Instituteof Standards and Technology, USA, was also analysed to check the quality ofanalysis. Results are shown in Table I.

4. Discussion

A sound analysis of sediment-bound elements associated with human activities hasto be based on well-chosen base line values. The definition of base line values isrelated to the evaluation of possible sediment inputs associated with human activ-ities in Lake Nahuel Huapi. Bariloche City is the biggest human settlement in thestudy area, placed in the southern part of the lake, and the effluents of sewagetreatment plant are the potential biggest source of pollutants. For this reason, themain sampling point, named NHS, of a sedimentary core and suspended load wasplaced near the emission point of effluents into the lake (see Figure 1). The othersampling point, named NHN, was selected in the northern lakeshore, in a similarenvironment, but far from the emission point (see Figure 1), to get a reference andcompare suspended load and upper core layer contents with NHS samples. Since

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 29

Figure 5. NHS sedimentary core. Geochemical tracers; Rare Earth element concentration profiles. La(Light Rare Earth element), Eu (Middle Rare Earth element), and Lu (Heavy Rare Earth element).

base line values have to reflect undisturbed conditions, without including variationsassociated with sample characteristics, we choose to study the deep layers of theNHS core.

Our choice of base line values can be justified in six definite facts:

(i) Relevant anthropogenic inputs have surely been negligible in the Nahuel Huapiregion until after the first decades of this century.

(ii) The sedimentation rate of NHS core obtained through a measurement of 210Pb,agrees with determinations performed with 137Cs originated in fall-out, andestablishes ages older than 90 yr for core sediment layers below a depth of4 cm.

(iii) As shown in Table II, the mean concentration values for Sb, As, Br, Cs, Co, Crand Hg in NHS core, in the 4–10 cm layers (3 determinations) and 10–40 cmlayers (15 determinations), demonstrate that there are no significant differencesamong these mean values in all cases (it can be seen also in Figure 7), and meanstandard deviations are smaller than the analytical uncertainties, showing lowvariability of element contents in each core segment.

30 S. RIBEIRO GUEVARA ET AL.

Figure 6. NHS sedimentary core. Concentration profiles of geochemical tracers Sc, Ta and Th, andmajor elements Fe and Na.

(iv) Within a detection limit of 0.3 µg g−1 in the 4–10 cm range, silver was notdetected in these core layers.

(v) Geochemical tracer concentrations of NHS core show the same behaviour inthe 0–40 cm range (see Figures 5 and 6), indicating that there were no relevantnatural variations in the geochemical fluxes of these elements during a periodof a few hundred years.

(vi) Findings (iii), (iv) and (v) are consistent with assumption (i) and measurementsdescribed in (ii).

Based on these facts, the average concentration values in the 4–10 cm NHS coresegment were chosen as our base line (BL). Mobilisation of the elements studiedafter deposition due to diagenetic processes could have occurred, changing thevertical distribution in the sediment. Moreover, organic matter contents of NHScore sediments are low, and they do not show relevant variations along the coreprofile (see Figure 4), except in the two upper layers of the profile, where smalldeviations from the mean value occur. Then, we do not expect relevant diageneticprocesses associated to organic matter along the NHS core. Rare Earth concen-

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 31

Figure 7. NHS sedimentary core. Concentration profiles of Sb, As, Br, Co, Cs and Cr.

tration profiles, represented in Figure 5 by La, a light rare earth element, Eu, aintermediate rare earth element, and Lu, a heavy rare earth element, are constantalong the NHS core (see Figure 5). The concentration profiles of the geochemicaltracers Th and Ta (Cullers et al., 1988) show little vertical variations in the NHScore, while major elements Fe and Na, and geochemical tracer Sc are constantwith depth (see Figure 6). For this reason, we do not assume relevant diageneticchanges. Also, the deeper layers of the NHS core used to define base line values,do not show relevant vertical concentration variations of the elements studied (seeFigures 7 and 8). Only an increase of Cr concentration in the deepest layers, anda small positive peak in Co contents around depth 38 g cm−2, can be observed.This constant behaviour in layers deeper than 5 g cm−2, is also reflected in the lowstandard deviation of the average concentration values shown in Table II.

The radioactive isotopes 210Pb and 137Cs behave different after sediment depos-ition. 210Pb behaves chemically in the same way as common lead. Also anthro-pogenic 137Cs, which become incorporated to sediments after radioactive fallout,moves within the sediment column after deposition. It can be found in layers depos-ited before 1940 (see Figure 2), and in some cases 137Cs dating is not possible be-cause of this reason (Longmore, 1983). Studies of sediment cores on diagenesis and

32 S. RIBEIRO GUEVARA ET AL.

Figure 8. NHN sedimentary core. Geochemical tracers; Rare Earth element and Sc concentrationprofiles. La (Light Rare Earth element), Eu (Middle Rare Earth element), and Lu (Heavy Rare Earthelement).

mobilisation of metals, including Pb, showed that 210Pb dating shows a good cor-relation with 137Cs dating, after correction for Pb mobilisation (Hardaway, 1998).The results obtained in both cores analysed show a good agreement between bothdating methods, within the limitations related to the low sedimentation rate andcore subsampling, and confirm that no relevant diagenetic processes are observedin upper layers of NHS core (see Figure 3). Concentration profiles of the threeupper layers of NHS core show only relevant variations for bromine (increasesstrongly in the last layers, see Figure 8) and silver (this element was measured onlyin the two upper layers). Also, arsenic contents are slightly higher in the two upperlayers in contrast to all other elements, which remain constant (see Figures 5, 6 and7).

The REE and scandium concentrations were measured to trace vertical vari-ations due to different geological inputs in the sediments, and in the suspendedload. Table III shows REE and Sc, Figures 5 and 6 the concentration profiles of theNHS sediment core. Figure 8 gives the elemental distribution found in the NHNcore, and the normalised REE patterns of the sediments are illustrated in Figure 10.

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 33

Figure 9. NHN sedimentary core. Concentration profiles of Sb, As, Br, Co, Cs and Cr.

The concentration profiles of the geochemical tracers of the NHS core do not showrelevant variations with depth. The REE patterns in NHS core sediments are similarfor all samples, and there is a good agreement between the REE pattern of the BLand the 10–40 cm layers, as well as with results obtained for river bed sediments(see Figure 10) sampled from the southern part of Lake Nahuel Huapi in a previouswork (Kestelman et al., 1997). The shift of the river bed REE pattern found in theprevious work is due to a different grain size composition of the samples: the lessthan 63 µm grain size fraction, while in the present work we analyse the totalsample, which causes a dilution effect. There is also a good agreement betweenmean absolute concentration values of BL and those corresponding to the 10–40 cm layers (see Table III and Figure 11). Suspended load REE patterns of NHSsamples are also similar to those of the BL, and the absolute values are also similar(see Figure 10 and Table III). Except for a higher dispersion in the heavy REEconcentrations, the same is valid for the REE patterns of the upper layers of thecore. Scandium concentration behaviour is consistent with REE concentrations inall cases. These results lead us to the conclusion that no significant alteration e.g.due to diagenetic processes has occurred in the sediment of the southern margin ofLake Nahuel Huapi. For this reason, one may suggest that concentration variations

34 S. RIBEIRO GUEVARA ET AL.

TABLE I

Standard Reference Material Buffalo River Sediment,NIST, measurement results and certified values

Element Measurement Certified value

Antimony 3.89±0.41 3.79±0.15

Arsenic 20.70±1.5 23.40±0.7

Cerium 73.10±6.4 72a

Caesium 6.08±0.46 6a

Cobalt 14.72±0.60 14.00±0.56

Chromium 140.80±8.3 135.00±6

Scandium 12.32±0.51 12a

Europium 1.425±0.084 1.3a

Ytterbium 3.37±0.43 2.8a

Lanthanum 33.40±1.2 29a

Lutetium 0.465±0.044 0.6a

Mercury 1.60±0.30 1.44±0.07

Samarium 6.45±0.44 6.7a

a Informed value.

in these sediments are not associated with natural geological processes, it may berelated to anthropogenic inputs.

Concentration profiles of geochemical tracers in the NHN upper core layersshow only slight variations (see Figure 8), and REE patterns are similar throughoutthe sediment profile (see Figure 10). The REE patterns of NHN suspended loadalso do not differ from NHS patterns, and the absolute concentration values arealso similar (except for sample NHN2 whose REE concentration is lower).

Organic matter is the another major constituent in sediments. The organic mattervertical sediment profile of the NHS core show only little deviation from the meanvalue, 2.5%, in the two upper most layers, but in both cases it is less than 20%(see Figure 4). The organic matter behaviour of NHN core is different: the deeperlayers show similar concentration as the mean NHS value (2.5%), but increaseparticularly in two upper layers (see Figure 4).

Elemental enrichment factors (EF factor, see Equation (1)), normalised to scan-dium concentrations (element strongly associated with the clastic fraction), werecomputed in order to find elemental enrichments above base line values. Table IVshows the computed enrichment factors. Silver is not shown in Table IV because itwas not detected in the sediments layers selected for base line determination.

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 35

Figure 10. Rare Earth concentrations normalized with NASC (Haskin et al., 1968). Base line values,NHS core sediment, suspended load concentrations, and mean composition of the Nahuel HuapiSouth margin sediments (Kestelman et al., 1997).

36 S. RIBEIRO GUEVARA ET AL.

TABLE II

Base line values determination. Mean concentration values in 4–10 cm and10–40 cm layers of the sediment core (µg g−1)

Element Mean values of 4–10 cm Mean values of 10–40 cm

layers concentrations layers concentrations

Mean SDa AUb Mean SDa AUb

Antimony 0.287 0.020 0.048 0.256 0.025 0.046

Arsenic 4.89 0.05 0.51 4.82 0.17 0.42

Bromine 3.08 0.21 0.41 2.35 0.16 0.29

Caesium 3.17 0.01 0.24 2.94 0.03 0.23

Cobalt 18.53 0.06 0.76 20.37 0.06 0.76

Chromium 62.00 0.20 4.10 71.10 0.90 3.80

Mercury 0.261c 0.033 0.074 0.50d – 0.21

Silver <0.30 NMe

a Standard deviation (SD) of the mean.b Minimum analytical uncertainty (AU) in the range of interest.c Only two determinations were obtained in the calculation range: 4–6 cmlayer, 0.37±0.12 µg g−1 and 8–10 cm layer, 0.151±0.074 µg g−1, and thedetection limit for layer 6–8 cm was 0.7 µg g−1.d only one measurement was available in the calculation range (10–12 cmlayer range), while the detection limit of the other determination was higherthan the measured value.e NM = Not measured.

Enrichment factor:

EF = [element]sample/[Sc]sample

[element]baseline/[Sc]baseline(1)

According to this formula, we arranged the following classification:

(i) Caesium, chromium, and cobalt. Concentration of these elements in the upperNHS core layers remains constant (see Figure 7), and, like concentration ofsuspended load, is similar to base line values (see Table V). EF values indicatethat these elements have the same behaviour as scandium. Therefore, we con-clude that caesium, chromium and cobalt are associated to the clastic fractionof the sediments.

(ii) Antimony and arsenic. The concentration of these elements in the upper layersof NHS core are constant (see Figure 7), and similar to BL values (see Table V).The EF values corresponding to the 0–1 and 1–2 cm NHS core layers areslightly higher than 1 but similar, while EF values of 2–4 cm layer are around 1(see Table IV). However, concentrations in suspended load samples are higher

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 37

TAB

LE

III

Geo

chem

ical

trac

erco

ncen

trat

ions

ofth

esu

spen

ded

load

and

NH

Sco

rese

dim

ents

ofL

ake

Nah

uelH

uapi

mea

sure

dby

INA

A(µ

gg−

1)

Sam

ple

Lan

than

umC

eriu

mS

amar

ium

Eur

opiu

mTe

rbiu

mY

tter

bium

Lut

etiu

mS

cand

ium

NH

S1

18.7

5±0.

9040

.9±3

.04.

49±0

.38

1.15

5±0.

067

0.66

5±0.

051

2.75

±0.3

20.

414±

0.04

516

.28±

0.68

NH

S2

21.6

0±1.

146

.6±4

.04.

82±0

.36

1.33

3±0.

072

0.72

8±0.

067

2.65

±0.2

90.

420±

0.04

218

.97±

0.71

NH

N1

17.7

2±0.

8641

.2±4

.34.

25±0

.35

1.13

5±0.

070

0.66

4±0.

055

2.54

±0.2

70.

451±

0.05

216

.26±

0.68

NH

N2

13.4

3±0.

8029

.3±3

.03.

15±0

.31

0.82

2±0.

050

0.51

2±0.

042

1.90

±0.2

30.

261±

0.02

912

.16±

0.59

0–1

cm18

.21±

0.67

43.3

±4.1

4.47

±0.3

01.

278±

0.07

70.

791±

0.09

92.

39±0

.26

0.40

4±0.

049

18.9

8±0.

87

1–2

cm18

.09±

0.69

39.3

±3.7

4.45

±0.2

81.

217±

0.07

10.

660±

0.07

52.

75±0

.40

0.34

8±0.

078

20.8

6±0.

96

2–4

cm18

.60±

0.65

41.9

±4.0

4.52

±0.2

91.

215±

0.07

10.

736±

0.05

62.

47±0

.33

0.36

7±0.

056

21.0

6±0.

97

4–10

cma

19.4

0±0.

6544

.7±4

.04.

54±0

.29

1.25

7±0.

073

0.69

6±0.

054

2.67

±0.2

80.

391±

0.03

621

.39±

0.88

10–4

0cm

a19

.30±

0.68

43.4

±3.5

4.34

±0.2

81.

278±

0.07

70.

669±

0.07

12.

55±0

.24

0.36

0±0.

027

21.9

1±0.

83

aM

ean

valu

eof

the

indi

vidu

alIN

AA

dete

rmin

atio

nin

the

rang

ein

dica

ted.

The

unce

rtai

ntie

sre

port

edar

eth

em

inim

uman

alyt

ical

unce

rtai

ntie

sin

the

rang

eof

inte

rest

.

38 S. RIBEIRO GUEVARA ET AL.

TAB

LE

IV

Hea

vym

etal

and

othe

rpo

llut

ant

elem

ent

enri

chm

ent

fact

ors

(Equ

atio

n(1

))fo

rsu

spen

ded

load

and

core

sedi

men

tsof

Lak

eN

ahue

lHua

pi

Sam

ple

Ant

imon

yA

rsen

icB

rom

ine

Cae

sium

Cob

alt

Chr

omiu

mM

ercu

ry

(µg

g−1)

Sus

pend

edlo

adN

HS

12.

45.

017

.71.

41.

01.

24.

3

Sus

pend

edlo

adN

HS

21.

9–

8.9

1.2

0.99

1.3

5.1

Sus

pend

edlo

adN

HN

12.

83.

022

.91.

21.

41.

58.

6

Sus

pend

edlo

adN

HN

23.

5–

20.3

1.1

1.2

1.1

9.9

Cor

eN

HS

0–1

cm1.

31.

42.

41.

00.

961.

0–

Cor

eN

HS

1–2

cm1.

61.

41.

00.

870.

950.

812.

3

Cor

eN

HS

2–4

cm1.

11.

11.

00.

941.

00.

941.

9

Cor

eN

HN

0–2

cm1.

52.

87.

60.

931.

10.

70–

Cor

eN

HN

2–4

cm1.

72.

23.

71

0.93

0.62

–

Cor

eN

HN

4–6

cm1.

41.

42.

10.

820.

930.

65–

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 39

than base line values, and above 1 (see Table V). But no significant differ-ences in antimony and arsenic contents were observed in the suspended loadsampled from both sites. Antimony concentration of NHN core sediments (seeFigure 9), show EF values higher than 1 (see Table V). This is also reflected inhigher EF values of the NHN suspended load when compared with NHS (seeTable IV). The arsenic in NHN core sediments increases in the upper layers(see Figure 9), but remains similar to the profile of organic matter (see Fig-ure 4). Since arsenic can be associated with organic matter, the variation in theupper layers can be ascribed to organic matter. Therefore, it is not associatedwith antrophogenic inputs For these two elements, there are not conclusivefacts to associate the concentration changes with antrophogenic inputs.

(iii) Bromine and mercury. These elements show concentration values above theBL in the upper layers of NHS core and in the suspended load (see Table V).Bromine in suspended load reaches very high EF values (8.9 to 22.9; seeTable IV). Bromine can be associated with organic matter, but only a smallpart of the bromine increment in the upper layers of NHN core can be explainedby organic matter variations (see Tables IV and V, and Figures 4 and 9), andthe organic matter profile of NHS core does not show relevant variations (seeFigure 4). From the data we can conclude that bromine variations in suspendedload and upper layers are not related with variations of main sediment con-stituents, but rather with changes of external inputs to the sediments. Mercuryconcentration in the suspended load have high EF values with values at theNHN sampling site about twice as much as at the NHS sampling site (EF= 8.6 and EF = 9.9 in the northern lake margin and EF = 4.3 and EF = 5.1in the southern margin, respectively). This spatial variation could be partiallyexplained by the higher organic matter content of NHN core sediments, sincemercury can be associated with organic matter. Mercury concentrations of the1–2 and 2–4 cm layers in the NHS core are also above base line values (seeTable V).

(iv) Silver. Due to the analytical detection limit (0.3 µg g−1), this element wasnot detected in sediment samples considered as base line, nor was it detec-ted in the 2–4 cm layer of the NHS core and subsequent layers. In NHNcore layers, silver was not detected below 2–4 cm, due to a detection limitof 0.1 µg g−1. Moreover, silver was detected in the two upper layers of theNHS core reaching maximum concentration at 0–1 cm, and in the upper layerof NHN core (see Table V). Concentrations were higher at the NHS than atthe HNN sampling site. Silver contents in the suspended load were higher thanmaximum sediment concentration, with values higher at the NHS than NHNsampling site ([Ag]NHS1 = 4.58±0.36 µg g−1 and [Ag]NHS2 = 3.46±0.40 µgg−1 in the southern margin, [Ag]NHN1 = 1.68±0.18 µg g−1 and [Ag]NHN2 =1.88±0.23 µg g−1 in the northern margin). Concentration ratio between themean suspended load content and upper layer concentration are 4.76±0.78 atNHS, and 8.8±3.7 at NHN.

40 S. RIBEIRO GUEVARA ET AL.

It should be noted also that grain size distribution in core sediments differed fromthat in suspended load. The sand fraction is the principal component in core sed-iments while the suspended load is build-up by smaller grain sizes, mainly silt.Adsorption of pollutants to sediments strongly depends on grain size, increasingwith smaller grain sizes. Therefore, pollutant elements may be more concentratedin the suspended load than in core sediments, if adsorption may be the domin-ant binding mechanism. Another aspect is that elemental concentrations in coresediments represent a mean value integrated over an accumulation period for eachlayer, e.g. 24 yr for the NHS sediment. In contrast, the data from the suspended loadcorrespond to a very short collection time with regards to sediment accumulationperiods. From this, we may conclude that pollutant inputs increased with time,resulting in the higher suspended load contents when compared with the contents ofthe upper sediment layer contents. This will also explain concentrations decreasing.The combined effect of both phenomena explains the higher concentration obtainedfrom silver and bromine measurements in NHS and NHN suspended load thanthose in upper core layers (see Table V).

San Carlos de Bariloche is the most important city in the catchment area, witha population of about 100 000 inhabitants. The city was founded in 1902, and hadabout 500 inhabitants in 1910. The population grew most rapid in 1970–1980 (seeTable VI). Since 1970 Bariloche City population increased by a factor of 4, andfrom 1947 on by a factor of 15 (see Table VI). Sediment dating of core layer 0–1 cmof the NHS sediment core gave a time period corresponding to 1975–1998, andcore layer 1–2 cm corresponded to the 1952–1975 period. Consequently, the upperlayer received integrated inputs from a population that ranged between 45 000and 100 000 inhabitants, while the layer below received integrated inputs from apopulation between 9000 and 45 000 inhabitants. In the reference core NHN, layers0–2 cm and 2–4 cm correspond to 1982–1998 and 1966–1982, respectively, and ifsediment dating were extrapolated, layer 4–6 cm would correspond to 1950–1966.

5. Conclusions

In the present work we studied the vertical distribution of the immobile elements incore sediments of the southern part of the lake, in the 0–4 cm and 4–40 cm range,and compared it with the composition of suspended load. We conclude that nosignificant alteration has occurred in these sediments regarding the clastic fraction.Organic matter contents in core sediments do not vary significantly with depth,and confirm a rather constant sediment deposition. We conclude that the meanconcentration values for the 4–10 cm core layers can be safely considered baseline values.

Caesium, chromium and cobalt of the NHS core and of the suspended loadbehave like immobile elements (REE and Sc), with concentrations close to baseline values. No relevant variations were observed either in upper layers of the

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 41

TAB

LE

V

Hea

vym

etal

and

othe

rpo

llut

ant

elem

ent

conc

entr

atio

nva

lues

ofth

esu

spen

ded

load

and

core

sedi

men

tsof

Lak

eN

ahue

lH

uapi

mea

sure

dby

INA

A(µ

gg−

1)

Sam

ple

Ant

imon

yA

rsen

icB

rom

ine

Cae

sium

Cob

alt

Chr

omiu

mM

ercu

ryS

ilve

r

Sus

pend

edlo

adN

HS

10.

533±

0.07

918

.50±

7.3

41.5

0±4.

33.

42±0

.27

14.4

4±0.

6358

.8±4

.10.

850±

0.14

4.58

0±0.

36

Sus

pend

edlo

adN

HS

20.

490±

0.07

3–

24.4

0±3.

93.

47±0

.26

16.2

0±0.

6473

.6±4

.41.

180±

0.19

3.46

0±0.

40

Sus

pend

edlo

adN

HN

10.

603±

0.09

211

.20±

3.9

53.6

0±5.

02.

99±0

.25

19.2

7±0.

8368

.7±5

.41.

710±

0.23

1.68

0±0.

18

Sus

pend

edlo

adN

HN

20.

571±

0.08

1–

35.6

0±3.

62.

05±0

.17

12.7

5±0.

6640

.2±3

.21.

470±

0.21

1.88

0±0.

23

Cor

eN

HS

0–1

cm0.

321±

0.09

36.

27±0

.80

6.43

±0.6

92.

92±0

.22

15.7

9±0.

7255

.4±4

.0<

0.6

0.84

4±0.

080

Cor

eN

HS

1–2

cm0.

450±

0.19

6.50

±1.1

3.10

±1.1

2.69

±0.2

117

.24±

0.79

48.8

±3.7

0.59

0±0.

270.

281±

0.08

2

Cor

eN

HS

2–4

cm0.

317±

0.06

05.

06±0

.58

3.04

±0.4

72.

93±0

.22

18.8

8±0.

8657

.3±3

.90.

495±

0.09

8<

0.3

Cor

eN

HN

0–2

cm0.

448±

0.77

13.9

0±0.

9923

.70±

1.9

2.97

±0.3

121

.05±

0.74

44.1

±2.7

–0.

202±

0.08

4

Cor

eN

HN

2–4

cm0.

510±

0.98

11.5

1±0.

8712

.00±

1.0

3.32

±0.3

218

.09±

0.78

40.2

±2.7

–<

0.1

Cor

eN

HN

4–6

cm0.

450±

0.69

7.43

±0.7

56.

84±0

.66

2.79

±0.3

218

.66±

0.64

43.3

±2.7

–<

0.1

Bas

eli

ne0.

287±

0.04

84.

89±0

.51

3.08

±0.4

13.

17±0

.24

18.5

3±0.

7662

.0±4

.10.

261±

0.07

4<

0.3

42 S. RIBEIRO GUEVARA ET AL.

TABLE VI

Evolution of San Carlos de Bari-loche City populationa

Year Population

1902 Foundation

1910 500b

1947 6562

1960 15995

1970 26276

1980 60158

1991 81130

1996 100000b

a Source: Instituto Nacional de Es-tadı́stica y Censo.b Estimation.

NHN core. Therefore we conclude that no relevant inputs of these elements tothe lake have occurred. This is in contrast to chromium enrichment found in riverbed sediments at the mouth of Ñireco River (Kestelman et al., 1997). Ñireco Riverflows through San Carlos de Bariloche City and discharges in Lake Nahuel Huapiclose to the NHS sampling site (see Figure 1).

Antimony and arsenic are enriched in the suspended load above base line values,but can not be ascribed to external inputs to the sediment.

Also mercury is enriched in the suspended load compared to base line values, re-flecting anthropogenic inputs of this element to the Lake Nahuel Huapi ecosystem,although core sediment concentrations do not confirm this.

Bromine concentration is highly enriched in suspended load samples and inupper sediment layers. These variations can only be ascribed by external inputs tothe sediment. Silver concentration behave similar as bromine, with high concen-tration values in the suspended load and upper core layers, but higher values nearBariloche City than in northern part of the lake.

From the analytical data we conclude that silver inputs to the Lake NahuelHuapi ecosystem are related to activities around San Carlos de Bariloche Cityinhabitants. Silver was detected only in upper layers of both sediment cores. Inthe suspended load of both sampling points, it occurred in higher concentrationsthan in the core sediments, due to grain size dilution and time integration effects.Silver concentrations decreased in the sediment with increasing distance from Bar-iloche City, but the concentration ratio between suspended load and core sedimentsremains almost constant at both sampling points, indicating lateral transport withinthe water column. Sediment dating also suggests that silver concentrations correl-

HEAVY METAL CONTAMINATION IN LAKE NAHUEL HUAPI 43

ate to the population increase of Bariloche City. Like for silver, we find that alsobromine is strongly related to human activities.

Acknowledgements

We thank reactor RA-6 operation staff for their assistance in sample analysis, andRicardo Sánchez for the construction and repair of the suspended load traps. Wealso want to acknowledge G. Román Ross and A. J. Kestelman for their valuablecontributions to this work. This research was partially supported by Project PEI No.0214/97, CONICET, and PICT No. 13-03999, ANPCYT. The authors are gratefulfor the reviewer’s suggestions.

References

Appleby, P. G. and Oldfield, F.: 1978, ‘The calculation of lead-210 dates assuming a constant rate ofsupply of unsupported 210Pb to the sediment’, CATENA 5, 1–8.

Appleby, P. G., Nolan, P. J., Gifford, D. W., Godfrey, M. J., Oldfield, F., Anderson, N. J. and Bat-tarbee, R. W.: 1986, ‘210Pb dating by low background gamma counting’, Hydrobiologia 143,21–27.

Bachtiar, T., Coakley, J. P. and Risk, M. J.: 1996, ‘Tracing sewage-contaminated sediments inHamilton Harbour using selected geochemical indicators’, Sci. Tot. Environ. 179, 3–16

Bamba, B. M., Leermakers, M. and Baeyens, W.: 1997, ‘Influence of sediment preservation on totalmercury and methylmercury analyses’, Water, Air, and Soil Pollut. 107, 277–288.

Binford, M. W.: 1990, ‘Calculation and uncertainty analysis of 210Pb date for PIRLA project lakesediment cores’, J. Paleolimnology 3, 253–267.

Condie, K. C.: 1993, ‘Chemical compositions and evolution of the upper continental crust:Contrasting results from surface samples and shales’, Chem. Geol. 104, 1–37.

Cullers, R. L., Basu, A. and Suttner, L. J.: 1988, ‘Geochemical signature of provenance in sand-sizematerial in soils and stream sediments near the Tovacco Root Batholith, Montana, U.S.A.’, Chem.Geol. 70, 335–348.

Cullers, R. L.: 1994, ‘The control on the major and trace element variation of shales, siltstones,and sandstones of Pennsylvanian-Permian age from uplifted continental blocks in Colorado toplatform sediment in Kansas, U.S.A.’, Geochim. et Cosmochim. Acta 22, 4955–4972.

Cullers, R. L.: 1995, ‘The controls on the major – and trace – element variation of shales, siltstoneand sanstones of Ordovician to Tertiary age in the Wet Mountains region, Colorado, U.S.A.’,Chem. Geol. 123, 107–131.

Deely, J. M., Tunnicliff, J. C., Orange, C. J. and Edgerley, W. H.: 1992, ‘Heavy metals in surfacesediments of Waiwhetu Stream, Lower Hutt, New Zealand’, New Zealand Journal of Marine andFreshwater Research 26, 417–427.

Firestone, R. B. and Shirley, V.: 1996, Table of Isotopes, John Wiley & Sons, New York, U.S.A.Hardaway, C., Wei-Jenq, Sheu and Meriwether, J. R.: 1998, ‘The effect of diagenetic processes on

the radiochronology of soft sediments using 210Pb and 137Cs’, Microchem. J. 58, 127–134.Haskin, L. A., Haskin, M. A., Frey, F. A. and Wildeman, T. R.: 1968, ‘Relative and Absolute Ter-

restrial Abundances of the Rare Earths’, in L. H. Ahrens (ed.), Origin and Distribution of theElements, Pergamon, Oxford, pp. 889–911.

44 S. RIBEIRO GUEVARA ET AL.

Hatje, V., Bidone, E. D. and Maddock, J. L.: 1997, ‘Estimation of the natural and anthropogeniccomponents of heavy metal fluxes in fresh water Sinos river, Río Grande do Sul state, SouthBrazil’, Environ. Tech. 19, 483–487.

Joshi, S. R.: 1987, ‘Nondestructive determination of lead-210 and radium-226 in sediments by directphoton analysis’, J. Radioanal. Nucl. Chem. 116(1), 169–182.

Joshi, S. R.: 1989, ‘Determination of 241Am in sediments by direct counting of low-energy photons’,Applied Radiative Isotopes 40(8), 691–699.

Joshi, S. R. and Shukla, B. S.: 1991, ‘Ab initio derivation of formulations for 210Pb dating ofsediments’, J. Radioanal. Nucl. Chem. 148(1), 73–79.

Kestelman, A. J., Román Ross, G., Arribére, M. A. and Ribeiro Guevara, S.: 1997, ‘Heavy metalsand rare earth geochemistry in unpolluted and contaminated areas of the Nahuel Huapi NationalPark, Río Negro, Argentina’, Project 8409/RB final report, International Atomic Energy Agency,(in Spanish).

Longmore McCallan, M. E., O’Leary, B. M. and Rose, C. W.: 1983, ‘Caesium-137 profiles inthe sediments of a partial-meromictic lake on Great Sandy Island (Fraser Island), Queensland,Australia’, Hydrobiol. 103, 21–27.

Markert, B., Pedrozo, F., Geller, W., Friese, K., Korhammer, S., Baffico, G., Díaz, M. and Wölfl,S.: 1997, ‘A contribution to the study of the heavy-metal and nutritional element status of somelakes in the southern Andes of Patagonia (Argentina)’, Sci. Tot. Environ. 206, 1–15.

Mughabghab, S. F., Divadeenam, M. and Holden, N. E.: 1981, Neutron Cross Sections, Vols. 1 and2, Academic Press, New York, U.S.A.

Pedrozo, F., Baffico, G. and Temporetti, P.: 1999, ‘Aporte Atmosférico de Nutrientes’, in R. Alcalde,H. Labollita and F. Pedrozo (eds), Lago Nahuel Huapi. Estudio de Calidad de Aguas, AIC, DPAand CRUB, UNC, Bariloche, Argentina, pp. 1–9.

Ribeiro Guevara, S., Arribére, M., Masaferro, J., Villarosa, G. and Kestelman, A. J.: 1999, ‘Lead-210sedimentation rates of lake sediment cores by using high resolution gamma-ray spectrometry’,Proceedings of the II South American Symposium on Isotope Geology, Córdoba, Argentina,September 1999, pp. 441–446.

Ritchie, J. C., McHenry, J. C. and Gill, A. C.: 1973, ‘Dating recent reservoir sediments’, J. Limnol.Oceanogr. 18(2), 254–263.

Robbins, J. A. and Herche, L. R.: 1993, Models and uncertainty in 210Pb dating of sediments’,Radiochem. Limnol. 25, 217–222.

Schelske, C. L., Peplow, A., Brenner, M. and Spencer, C. N.: 1994, ‘Low-background gammacounting: applications for 210Pb dating of sediments’, J. Paleolimnol. 10, 115–128.

Tuli, K. J.: 1995, Nuclear Wallet Cards, National Brookhaven Laboratory, U.S.A.Von Gunten, H. R. and Moser, R. N.: 1993, ‘How reliable is the 210Pb dating method? Old and new

results from Switzerland’, J. Paleolimnol. 9, 161–178.US Environmental Protection Agency, U.S.A.: 1997, ‘Mercury study report Congress’, EPA-452/R-

97.