Major and Trace Element Geochemistry of Lake Bogoria and Lake Nakuru, Kenya, During Extreme Draught

Home , Lake Bogoria

Chemie der Erde 73 (2013) 275–282

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Chemie der Erde

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Major and trace element geochemistry of Lake Bogoria and Lake Nakuru, Kenya, during extreme draught

a,∗ b a c d

Franz Jirsa , Martin Gruber , Anja Stojanovic , Steve Odour Omondi , Dieter Mader ,

e b

Wilfried Körner , Michael Schagerl

University of Vienna, Institute of Inorganic Chemistry, Währingerstrasse 42, A-1090 Vienna, Austria

University of Vienna, Department of Limnology, Althanstrasse 14, 1090 Vienna, Austria

Egerton University, Department of Biological Sciences, P.O. Box 536, Njoro, Kenya

University of Vienna, Department of Lithospheric Research, Althanstrasse 14, 1090 Vienna, Austria

University of Vienna, Department of Environmental Geosciences, Althanstrasse 14, 1090 Vienna, Austria

a r t i c l e i n f o a b s t r a c t

Article history: The physico-chemical properties of water samples from the two athalassic endorheic lakes Bogoria and

Received 30 May 2012

Nakuru in Kenya were analysed. Surface water samples were taken between July 2008 and October 2009

Accepted 9 September 2012

in weekly intervals from each lake. The following parameters were determined: pH, salinity, electric con-

ductivity, dissolved organic carbon (DOC), the major cations (FAAS and ICP-OES) and the major anions

Keywords:

(IC), as well as certain trace elements (ICP-OES). Samples of superﬁcial sediments were taken in October

Lake Nakuru

2009 and examined using Instrumental Neutron Activation Analysis (INAA) for their major and trace

Lake Bogoria

element content including rare earth elements (REE). Both lakes are highly alkaline with a dominance of

Saline lakes

REE Na > K > Si > Ca in cations and HCO3 > CO3 > Cl > F > SO4 in anions. Both lakes also exhibited high concentra-

DOC tions of Mo, As and ﬂuoride. Due to an extreme draught from March to October 2009, the water level of

Molybdenum Lake Nakuru dropped signiﬁcantly. This created drastic evapoconcentration, with the total salinity rising

Arsenic from about 20‰ up to 63‰. Most parameters (DOC, Na, K, Ca, F, Mo and As) increased with falling water

Iron levels. A clear change in the quality of DOC was observed, followed by an almost complete depletion of

dissolved Fe from the water phase. In Lake Bogoria the evapoconcentration effects were less pronounced

(total salinity changed from about 40‰ to 48‰). The distributions of REE in the superﬁcial sediments of

Lake Nakuru and Lake Bogoria are presented here for the ﬁrst time. The results show a high abundance

of the REE and a very distinct Eu depletion of Eu/Eu* = 0.33–0.45.

1. Introduction Turkana, features a string of smaller and shallower lakes, most of

them endorheic and athallasic. The most famous is Lake Nakuru,

The East African Rift Valley is the most extensive, presently known worldwide for its huge ﬂamingo populations and an enor-

active continental extension zone on earth (Dawson, 2008) and mous variety of other birds, which come to feed there. Intense

shows seismicity and, in some places, magmatism. Its geological water level ﬂuctuations, along with highly variable ion concentra-

exploration started following the geographical discoveries of the tions, have been reported from this shallow lake (Vareschi, 1982;

late 19th century and is still ongoing (Schlueter, 1997). In addi- Burgis and Morris, 1987). Due to its speciﬁc geological, chemi-

tion to these special geological features, the valley is known as cal and biological features, the lake has been the focus of intense

one of the major lakeland areas of the world, where various lake studies in the past. This yielded information on geology (McCall,

types are present (Burgis and Morris, 1987). The nature and extent 1967), abiotic factors (Vareschi, 1982), nutrient content (Odour

of volcanism played a key role in forming the morphological and and Schagerl, 2007) and biotic interactions (Schagerl and Oduor,

hydrological properties of the lake basins (Schlueter, 1997). Major 2008). The most recent efforts have concentrated on the impact

differences in the lakes occur especially regarding their content of human land use on the lake’s ecosystem (Raini, 2009) and the

in dissolved salts, ranging from freshwater to hypersaline. Espe- ﬂuctuation of algae populations in connection with mass mortality

cially the Eastern Rift Valley, south of its largest freshwater Lake of ﬂamingos (Krienitz and Kotut, 2010). The second saline, alka-

line lake known for its ﬂamingo populations is Lake Bogoria, only

about 100 km north of Lake Nakuru. In contrast to Lake Nakuru,

∗ it is deeper and partially fed by hot springs (Cioni et al., 1992;

Corresponding author. Tel.: +43 1 4277 526 27; fax: +43 1 4277 526 20.

E-mail address: [email protected] (F. Jirsa). McCall, 2010). Detailed publications are available on its geology and

276 F. Jirsa et al. / Chemie der Erde 73 (2013) 275–282

mineralogy, amongst others by Owen et al. (2008) and McCall

(2010), as well as on selected trace elements (Kerrich et al., 2002)

and biology (Odour and Schagerl, 2007; Krienitz and Kotut, 2010).

The ﬁrst chemical data on the lakes date back to the ﬁrst half of

the 20th century (Lake Bogoria: Beadle, 1932; Lake Nakuru: Jenkin,

1936). Although research into these aspects has continued, a gap in

our knowledge still exists for some chemical aspects. This is partic-

ularly true for the trace element concentrations in both the water

column and the lake sediments.

This survey covers a 16-month period including a phase of

extreme drought. We studied dissolved chemical species including

trace elements and organic carbon in both lakes in weekly intervals.

We also analysed the element composition of superﬁcial sediments

including rare earth elements (REE). This is the ﬁrst study including

hydrological variations and short-term sampling intervals over an

extended period of time. We compare two lakes of differing mixing

regimes, hydrology and morphology. Abiotic conditions, including

hydrochemistry, strongly inﬂuence the lake’s biocoenosis. Excep-

tionally high ion concentrations and huge ﬂuctuations altered the

chemical equilibria and inﬂuenced the bioavailability of elements

for the aquatic biota. This makes this survey also of interest for

aquatic ecologists.

2. Materials and methods

2.1. Study areas

The basins of the athallasic endorheic lakes Bogoria and Nakuru

were formed through tectonic and volcanic activities and lie on the

Eastern Kenyan branch of the Great Rift Valley (Schlueter, 1997).

The runoff from the alkaline volcanic rocks characteristic for the

Eastern Rift Valley (King, 1970) is rich in sodium and hydrogen car-

bonate, constituting the major source of these ions in the lake water

(Yuretich, 1982). Both lakes are located in semi-arid regions (Fig. 1)

with annual rainfall in their catchment areas of app. 700–900 mm

(McCall, 1967) showing high variability. The climate is charac-

terised by two rainy seasons, one from April to August (the so-called

long rains) and one during October and November (“short rains”).

Neither lake has a surface outﬂow; the water budget is there-

fore dependent on direct precipitation, evaporation, inﬂows and

anthropogenic use. Both catchment areas are completely occupied

by alkaline volcanic rocks (trachyphonolite, phonolite and basalts)

(McCall, 1967).

2.2. Lake Nakuru

Fig. 1. Geographical position of Lake Nakuru and Lake Bogoria, Kenya. Open circles:

sampling points. “S” south, “C” central, “N” north.

◦ ◦

Lake Nakuru (00 22 S, 36 05 E) is part of the

Naivasha–Elmentaita–Nakuru basin, a region where the East-

ern Rift reaches its highest elevation (Schlueter, 1997). It is

situated at approximately 1756 m above sea level, and is one of 2.3. Lake Bogoria

the three residual water bodies that resulted from the drying of

◦ ◦

a former large freshwater body that had ﬁlled the basin about Lake Bogoria (00 15 N, 36 07 E) is situated at an altitude of

10,000 y BP (Richardson and Richardson, 1972). The lake lies in a app. 990 m above sea level and covers an asymmetric half graben

graben between the Lion Hill Volcano and the Mau Escarpment, within the axial depression of the Gregory Rift Valley (Schlueter,

the west wall of the Rift Valley (McCall, 1967). Depending on 1997). It is app. 17 km long and 3.5 km wide, with a maximum

2 6 3

the water level, its surface area covers between 36 and 49 km , depth of about 10 m. The total volume is 231 10 m . The lake

and its mean depth ranges from less than 0.5 m to approximately is divided into three parts by projections of sediments around hot

6 3

3.5 m (Burgis and Morris, 1987) with a volume of 150 × 10 m springs, so-called sills. The lake waters display a salinity of >40‰

at a surface of 42 km (Melack and Kilham, 1974). The lake’s due to high sodium carbonate–bicarbonate concentrations; the pH

water is saline, alkaline with highly variable concentrations of the is >10.3. Besides direct precipitation, the lake is fed by some small

−1

salts (electric conductivity 9.5–165.0 mS cm ). The lake is fed by ephemeral tributaries and more than 200 hydrothermal springs,

precipitation, three seasonal surface streams, small freshwater two geysers and the direct runoff from the highlands to the south

springs in the north and the efﬂuent of the sewage treatment and east; its mixing regime has been described as meromictic.

system from Nakuru City, a rapidly growing industrial town (Raini, Moreover, a gradient of decreasing salinity has been described from

2009). south to north (Vincens et al., 1986; McCall, 2010).

F. Jirsa et al. / Chemie der Erde 73 (2013) 275–282 277

2.4. Measurements and analyses

Weather data (precipitation, wind, temperature) were recorded

with two HOBO Weather Stations positioned near the lakes

throughout the observation period from August 2008 to October

2009.

Temperature, pH and electric conductivity (EC) were deter-

mined in situ using portable instruments (WTW, Germany). Three

offshore sampling points were chosen at each lake, designated as

“south”, “central” and “north” (locations see Fig. 1) and marked

with moored buoys. Water samples were taken on a weekly basis

from an inﬂatable boat about 10 cm below the surface by dip-

ping acid-prewashed polyethylene (PE) bottles into the water.

They were immediately ﬁltered through sterilised glass–microﬁbre

ﬁlters grade MGC by Munktell (2 ␮m pore size). Samples for

dissolved organic carbon (DOC), dissolved nitrogen (DN) and ele-

ment determination were preserved with 2.7 mM sodium azide

(1:200, vol/vol) at a ﬁnal concentration of 13.5 ␮M (following

Kaplan, 1994), then stored cooled in the dark until analysis in Fig. 2. Monthly rainfall in Lake Nakuru and Lake Bogoria; columns: mean ± SE

1987–2006 at Lake Nakuru; open circles: L. Nakuru, closed circles: L. Bogoria.

Vienna. Carbonate and bicarbonate concentrations were deter-

mined immediately after sampling from additionally ﬁltered

samples by titration with 0.1 M HCl (Merck, Titrisol) using a Titrino

They were analysed using Instrumental Neutron Activation

702 (Metrohm) in the laboratory at Egerton University. Prior to

Analysis (INAA) at the Department of Lithospheric Research at

further analyses, samples were ﬁltered through 0.45 ␮m nylon ﬁl-

® the University of Vienna following methods described by Koeberl

ters (Acrodisc ). Metal ion concentrations were determined in the

® (1993) and Mader and Koeberl (2009). Approximately 100–115 mg

acidiﬁed samples (HNO3 TraceSELECT by Fluka) using inductively

of the individual samples were weighed and sealed in small

coupled plasma-optical emission spectrometry (ICP-OES) (Optima

polyethylene vials, as were about 70–100 mg of three international

5300 DL by Perkin-Elmer). Rh was used as internal reference

rock standards: the carbonaceous chondrite Allende (Smithsonian

standard, showing a recovery range between 95% and 105%. Sodium

Institution, Washington DC, USA; Jarosewich et al., 1987), the gran-

and potassium concentrations were determined in the acidiﬁed

ite AC-E (Centre de Recherche Pétrographique et Géochimique,

samples using ﬂame atomic absorption spectrometry (F-AAS) (Aan-

Nancy, France; Govindaraju, 1989), and the Devonian Ohio shale

alyst 300 by Perkin-Elmer) after diluting them accordingly prior to

SDO-1 (United States Geological Survey; Govindaraju, 1989). For

measurement. For ICP-OES and ﬂame-AAS, element concentrations

the irradiation the 250 kW TRIGA Mark II type reactor at the Insti-

of each sample were calculated from the corresponding regression

tute of Atomic and Subatomic Physics of the Vienna University

lines (correlation factor >0.9995) using ﬁve different dilutions of

of Technology was used. The samples were irradiated for about

a standard solution (Fluka). Dissolved organic matter (DOM) was 12 −2 −1

8 h at a ﬂux of about 2 × 10 n cm s . After a cooling period of

characterised by measuring the following parameters in the ﬁltered

5 days, the decontaminated samples were measured in three count-

sodium-azide-preserved samples: the amount of dissolved organic

ing cycles according to the half-lives of the nuclides, using high

carbon (DOC) and the ratio of absorbance at 250 nm and 365 nm

resolution HpGe (high-purity germanium) detectors with relative

(E2/E3). DOC was measured with a TOC-VCPH total organic car-

efﬁciencies of 39.8–45.3% and energy resolutions of 1.76–1.82 keV

bon analyser (Shimadzu) as non purgeable dissolved organic carbon 60

at 1332 keV of the Co peak. The rare earth element (REE) values

(NPDOC), diluting the samples accordingly, bringing them to pH ≈ 2

were normalised to chondrite composition, and the Eu-anomalies

with HCl (Fluka) and sparging them with carrier gas for 10 min prior 0.5

were calculated as follows: Eu/Eu* = EuN/(SmN · GdN) (Taylor and

to combustion. Element concentrations were calculated from the

McLennan, 1985).

corresponding regression lines (correlation factor >0.9995) using

Standard statistical analyses were performed with the SPSS 16.0

ﬁve different dilutions of a potassium hydrogen phthalate solution

software package. Signiﬁcant differences between the sites in Lake

as standard for the carbon measurements. The E2/E3 ratio was cal-

Bogoria and Lake Nakuru were checked with the Kruskal–Wallis-

culated after measuring the absorbance at 250 nm (E2) and 365 nm

Test. Signiﬁcant correlations of the sums of cations and anions were

(E3) with a Lambda 35 UV VIS Spectrometer (Perkin Elmer) to esti-

tested using the bivariate Spearman test.

mate the relative molecular weight distribution of fulvic acids in

the DOM, following the method described by De Haan (1983).

Dissolved nitrogen (DN) was measured as total-bound-nitrogen 3. Results and discussion

with the TNM-1 unit of the TOC-analyser described above, using

ﬁve different dilutions of potassium nitrate for calibration (factor 3.1. Rainfall

>0.9995).

For samples from May 2009 onwards, ion chromatography was Precipitation data for the two lakes are presented in Fig. 2 and

used to determine the concentrations of selected anions with an compared with the mean monthly rainfall from 1987 to 2006, as

ICS1000 with a RFC-30 reagent-free controller (Dionex Corp., CA, measured for Lake Nakuru by the Nakuru meteorological station.

USA), 38 mM KOH as eluent and a conductivity detector, Ion Pac For both lakes, rainfall declined signiﬁcantly compared to the long-

◦

AS15 4 mm analytical column, 1.2 mL/min ﬂow rate, 30 C; detec- time mean, especially during the rainy season from April to July.

− − 2− −

tion limits: Cl , F : 20 ppb, SO4 , NO3 : 50 ppb, accuracy 2%. The These deﬁciencies in precipitation led to a major drop in the depth

charge balance was calculated including all major ions. of Lake Nakuru and a drastic retraction of the waterline (Fig. 3). For

In October 2009, two sediment samples were taken from Lake Lake Bogoria, no such clear retraction took place, probably due to

Nakuru, as well as three samples from Lake Bogoria with a plastic its much smaller surface/volume ratio and the continuous inﬂow

core then transferred into poly ethylene (PE) tubes and dried at of thermal wells. Nonetheless, a reduction of volume was observed

◦

105 C to weight constancy immediately after sampling. here as well, indicated by rising concentrations of salts (see below).

278 F. Jirsa et al. / Chemie der Erde 73 (2013) 275–282

Table 1

−1

Physico-chemical parameters and soluble analytes from Lakes Nakuru and Bogoria; all values, where not noted differently, in mg L , b.d.: below detection limit (LOD),

− 2− − 3−

samples size for each lake: n = 204, except for Cl , SO4 , F , NO , where n = 78 for each lake.

Nakuru Bogoria

Min Median Max Min Median Max

◦

Temp. ( C) 18.9 24.8 32.0 24.9 28.3 33.4

pH 9.5 10.1 10.9 9.5 10.0 10.9

−1

Electr. conductivity (mS cm ) 23.1 38.3 96.2 50.7 67.0 81.5

Salinity (‰) 15.2 24.6 63.5 30.1 43.1 48.9

Dissolved organic carbon (DOC) 160 270 980 40 45 58

Dissolved nitrogen (DN) 10.9 21.1 60.1 2.1 3.5 4.6

Ratio E2/E3 3.6 11.9 14.6 3.4 7 8.8

Na 8490 13,560 46,560 19,900 25,860 30,040

K 232 447 1231 289 414 497

Mg <0.005 0.060 0.246 0.013 0.420 0.903

Ca 1.2 3.6 5.5 1.4 4.2 8.1

Si 45.4 105.1 143.8 16.2 32.0 101.7

Mn 0.006 0.021 0.079 0.022 0.240 0.328

Fe <0.010 0.052 0.191 0.020 0.110 0.241

Cu b.d. – b.d. b.d. – b.d.

Zn <0.010 0.011 0.068 <0.010 0.010 0.100

As 0.009 0.043 0.103 0.018 0.080 0.132

Sr 0.010 0.033 0.063 0.060 0.240 0.286

Mo 0.159 0.723 1.712 0.107 0.300 0.472

Cd b.d. – b.d. b.d. – b.d.

Ba <0.010 0.055 0.643 0.036 0.110 0.293

Pb b.d – b.d. b.d. – b.d.

Carbonate 3350 5930 19,370 8830 12,700 18,060

Bicarbonate 9540 16,290 51,050 24,400 34,950 47,320

Chloride 3220 5080 9950 4360 5240 6710

Sulphate 292 444 934 60 148 210

Fluoride 500 740 1370 530 1100 1310

Nitrate 16.7 23.6 40.2 9.5 13.2 15.5

Although distinct ﬂuctuations in the water level and shoreline of et al. (1986) for Lake Bogoria, who described a gradient in rising

Lake Nakuru have been described before, amongst others by Burgis salinity from north to south. Our sample points were situated far

and Morris (1987), the drastic reduction of lake water during the offshore, near the centre of the lake. Accordingly, the hot springs

present survey is exceptionally high; this led to a near dry-out of the that drain into the lake, most of which contain less salt than the

lake, a situation last described in 1962 (Burgis and Morris, 1987). lake water (Owen et al., 2008; Cioni et al., 1992), appear to have no

inﬂuence on the salinity there. In addition the major river ﬂowing

3.2. Water samples, dissolved components into the lake from the north discharged very little or no water dur-

ing the observation period. This also might have contributed to the

The results for the physico-chemical parameters, dissolved equal distribution of the salt content from south to north.

major and trace elements as well as dissolved organic carbon (DOC) The DOC concentrations in the two lakes are remarkable. Both

and dissolved bound nitrogen (DN) are presented in Table 1. The lakes show high DOC values, ranking them in the 0.4% of the

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statistical evaluation of the charge balance showed a signiﬁcant world’s lakes containing more than 40 mg L (Sobek et al., 2007).

correlation (p < 0.05) between the summed up charges of cations In this regard, they are comparable to some saline lakes from North

and anions. This demonstrates that all major ions in the two lakes America (Osburn et al., 2011). Both publications report maximum

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were analysed. The concentrations and values varied signiﬁcantly DOC concentrations of around 330 mg L . In Lake Bogoria, DOC

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during the survey due to drought at both lakes. We therefore amounted to around 50 mg L during the observation period;

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present minimum, median and maximum values from all three in Lake Nakuru, the values rose from around 200 mg L at the

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sampling points in each lake. In order to show the behaviour of cer- beginning of the survey to 980 mg L (Fig. 4) at the end. The latter

tain interesting components, mean values for the measurements of value exceeds those in any other lake. A major factor shaping

the three sites were calculated and presented over time in Fig. 4. the DOC levels appears to be the total salt content, a coherence

The statistical evaluation of salinity and electric conductivity from outlined by Osburn et al. (2011) for saline lakes in general. Our data

the three different sample sites at each lake showed no signiﬁcant show a signiﬁcant correlation (R = 0.97, p < 0.01) between electric

difference (p > 0.05). This contradicts results reported by Vincens conductivity and DOC throughout the survey (Fig. 5), underlining

Fig. 3. Photo of Lake Nakuru from the Baboon Cliff (southwest of the lake) in October 2009, the dotted line showing the shoreline in June 2008.

F. Jirsa et al. / Chemie der Erde 73 (2013) 275–282 279

Fig. 4. Selected parameters from Lakes Nakuru and Bogoria between July 2008 and October 2009.

the importance of evapoconcentration for the DOC amount even signiﬁcantly higher from July 2008 to July 2009 (mean = 12.1 ± 1.3),

in very highly saline solutions. The amount of dissolved-bound dropping to the same level as in Lake Bogoria (mean = 6.9 ± 1.5)

nitrogen (DN) also appears to be signiﬁcantly correlated to EC within a month and staying stable until the end of this survey.

(R = 0.94, p < 0.01) in Lake Nakuru. This agrees with the values This change appears to also be correlated to the salinity change

Curtis and Adams (1995) provided for saline lakes in North due to evaporation and starts when salinity reaches app. 35‰

America, conﬁrming these correlations even at exceptionally high (Fig. 4); the inﬂuence of salinity on the molecular size of dissolved

concentrations. organic matter (DOM) has been described amongst others by De

The differences in DOM between Lakes Nakuru and Bogoria are Haan et al. (1987) and Peuravuori and Pihlaja (1997); these authors

expressed in both quantity and quality. Although the E2/E3 ratio propose a reduction of apparent molecular size with rising ionic

allows only a rough estimation of the distribution of low molec- strength. Rising E2/E3 ratios, however, are positively correlated

ular weight DOC, signiﬁcant differences occur. In Lake Bogoria, with the proportion of low weight DOM and rising hydropho-

the E2/E3 ratio remained relatively low (mean 6.8 ± 0.8) for the bicity of the DOM. This appears contradictory to the observed

whole observation period, whereas in Lake Nakuru the values were signiﬁcant drop in E2/E3 values during increasing salinity in Lake

Nakuru. This raises the question whether the E2/E3 ratio is appro-

priate to describe the properties of DOM for these very special

ecosystems. For example, the very high phytoplankton biomass

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(25–770 mg L ) and a ﬂamingo population digesting 50–90% of

the daily primary production in the lake (Krienitz and Kotut, 2010)

clearly have a major inﬂuence on DOM quality and composi-

tion. Future investigations will have to examine these complex

issues.

The concentrations of the major ions measured in both lakes

agree with earlier investigations, exhibiting a clear dominance of

Na > K > Si > Ca and HCO3 > CO3 > Cl > F > SO4. Fig. 4 shows the con-

centrations of bicarbonate and carbonate as examples for readily

soluble ions and the total salinity over the survey period. It clearly

depicts rising concentrations from June 2008 to October 2009 in

both lakes, but a distinctly higher value for Lake Nakuru than for

Lake Bogoria.

Fluoride levels, measured from May 2009 onwards, are very high

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in both lakes, ranging from app. 520 to 1370 mg L in Lake Nakuru

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and from 1000 to 1300 mg L in Lake Bogoria. Values for Lake

Nakuru are in the same order of magnitude as those reported by

Fig. 5. Correlation of DOC and electric conductivity in Lake Nakuru; n = 204,

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2 Wambu and Muthakia (2011), i.e. 168 mg L in Lake Elementeita, R = 0.97.

280 F. Jirsa et al. / Chemie der Erde 73 (2013) 275–282

Table 2

Mean concentration of elements in superﬁcial sediments of Lakes Nakuru (n = 2) and Bogoria (n = 3) compared to reference material; PAAS = average 23 post-Archean shales

from Australia (adapted from Taylor and McLennan (1985); NASC = composite 40 shales, mainly N. American (Condie, 1993); UCC = upper continental crust (Taylor and

McLennan, 1985), Olkaria (Marshall et al., 2009), St. Helena (Chaffey et al., 1989)).

Element Nakuru Bogoria PAAS NASC UCC Olkaria St. Helena

Na (wt%) 9.81 7.92 – – 2.89 – –

K (wt%) 3.28 2.87 – – 2.8 – –

Fe (wt%) 4.63 5.33 – – 3.5 – –

Sc 5.19 4.69 – – 11 – 14.1

Cr 23.8 21.3 – – 35 0.3 25

Co 3.16 1.96 – – 10 0.2 10

Ni 38.4 34.1 – – 20 – 8

Zn 236 222 – – 71 519 82

As 6.1 3.35 – – 1.5 – –

Se 2.68 0.52 – – 0.05 1.7 –

Br 25.9 14.7 – – – – –

Rb 120 108 – – 112 677 773

Sr 47.9 39.4 – – 350 1.2 34

Zr 981 705 – – 190 2486 175

Sb <0.6 0.35 – – 0.2 – –

Cs 2.42 1.6 – – 3.7 12 –

Ba 116 99.4 – – 550 2.4 540

Hf 21 15.4 – – 5.8 73 108

Ta 13.4 9.81 – – 2.2 51 13

W 9.9 8.0 – – 2 – –

Au (ppb) <6.0 <4.6 – – 1.8 – –

Th 27.0 20.5 – – 10.7 – 10.5

U 15.9 6.25 – – 2.8 – 21

Ir (ppb) <2.1 <1.6 – – 0.02 – –

La 159 161 38.2 32 30 162 190

Ce 295 243 79.6 73 64 342 64.2

Nd 105 101 33.9 33 26 140 11.1

Sm 21.8 18.3 5.55 5.7 4.5 37 3.06

Eu 2.26 2.5 1.08 1.24 0.88 0.52 2.6

Gd 20.2 15.7 4.66 5.2 3.8 42 –

Tb 3.34 2.55 0.774 0.85 0.64 7.7 0.47

Tm 1.87 1.34 0.405 0.5 0.33 5.7 –

Yb 10.8 9.07 2.82 3.1 2.2 38 3.4

Lu 1.65 1.38 0.433 0.48 0.32 5.3 –

Eu/Eu* 0.33 0.45 – – 0.65 – –

LaN/YbN 9.9 12 – – 9.21 – –

another edorheic saline lake only about 20 km southeast of Lake inﬂuence; sediment analysis (see Section 3.3) provides a deeper

Nakuru and sharing an identical geological background. For Lake insight into possible human impacts here.

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Bogoria, Schlueter (1993) reports 1060 mg L . The levels in both As and Mo, both elements associated with volcanism, are

lakes probably reﬂect concentration effects in the lakes, similar present in the two lakes. Arsenic has been the focus of research

to other readily soluble compounds. Borehole and surface waters due to its toxicity and speciﬁc redox behaviour, and because it is

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from the region contain 0.002–21 mg L ﬂuoride (Gikunju et al., a source of energy for bacteria (Wolfe-Simon et al., 2010) in saline

1995; Wambu and Muthakia, 2011). This causes major problems in lakes. Recently, Hacini and Oelkers (2011) described As during com-

the region for humans, expressed in hyperﬂuorosis (Kahama et al., plete evaporation from an saline ephemeral lake in Algeria; the

−1

1997). values ranged from 1 to 3 mg L , which is one order of magnitude

Nitrate concentrations are high in both lakes compared to higher than in this survey. This shows the good solubility even in

−1

most other saline lakes, where they are usually below 1 mg L . these concentrations.

These high values can be attributed to the high density of Furthermore, Mo concentrations in Lake Nakuru and Bogoria

−1 −1

cyanobacteria, which show high rates of decay and subsequent (0.2–1.7 mg L in the former, 0.1–0.5 mg L in the latter) were

nitriﬁcation (Hammer, 1986). Again, in Lake Nakuru, evapocon- elevated compared to saline lakes in Alberta, Canada, for exam-

−1 −1

centration caused levels to rise up to app. 40 mg L at the end of ple, where values were between 0.002 to 0.009 mg L (Evans and

this survey. Such nitrate levels are exceeded in only few lakes, e.g. Prepas, 1997). Mo is mostly present in anionic species and there-

Lake Mono, California (Mason, 1967) and Lake Pretoria, South Africa fore shows high solubility at high pH (Alloway, 1994), explaining

(Ashton and Schoeman, 1983). the high levels at the present, highly alkaline conditions.

−1

Regarding trace elements, all measured values for Lake Bogoria Iron concentrations in saline lakes are usually <1 mg L

agree with those reported by Kerrich et al. (2002) and Owen et al. (Hammer, 1986 and references therein). The values found dur-

(2008). In both lakes the Cu, Cd and Pb values were below the limit ing this survey are in good accordance with this ﬁgure, but the

−1

of detection (LOD) of 5 ␮g L , which also corresponds to the ﬁnd- behaviour of iron shows different curves for the two lakes (Fig. 4). In

ings of Kerrich et al. (2002) for surface and bottom waters of Lake Lake Nakuru, concentrations follow the rising trend of salinity and

−1

Bogoria near the inﬂow of hydrothermal springs. This indicates no readily soluble ions until late August 2009 up to 0.191 mg L and

−1

human impact of these metals in either lake. The low concentra- then drop signiﬁcantly to somewhat above the LOD of 0.010 mg L .

tions of these metals as well as the moderate concentrations of Zn in This drop follows the decline in E2/E3 ratios in the lake, with a

the aqueous phase can also be explained by enhanced precipitation time shift. In Lake Bogoria, Fe concentrations rise after the short

of metal salts under strongly alkaline conditions. These values are rainy period in November–December 2008 within 2 months to over

−1

therefore not necessarily a sign for the absence of anthropogenic 0.200 mg L , then drop again.

F. Jirsa et al. / Chemie der Erde 73 (2013) 275–282 281

The behaviour of iron in shallow lakes is partially determined

by precipitation processes from the oxic zone of lakes into the

sediment and reduction in the anoxic layers with the follow-

ing release into the aqueous phase (Hammer, 1986). This alone,

however, cannot explain the concentration drop observed in Lake

Nakuru because oxic conditions did not change during this time.

Humic substances, which form a major part of the DOM, are

described as important chelators for iron (Thurman, 1985). Fe(III)

displays the highest equilibrium constants after Al and less impor-

tant metals such as Am and Th with carboxylic groups of humic

and fulvic acids (Tipping, 2002). Although the DOM composition in

the lake remains unclear (see above), the change in the E2/E3 ratio

indicates some change of quality, probably a reduction in size due

to enhanced salinity and ionic strength (De Haan et al., 1987). If

humic particles decompose and release their chelated metal ions,

metals are subject to hydroxide formation and then precipitation.

Fig. 6. Chondrite normalised REE patterns of sediments; values for elements marked

The timely succession of these processes could also explain the

with * were extrapolated.

time shift between the observed alteration in DOM quality and iron

depletion from the water column in Lake Nakuru. For Lake Bogoria

the source for rising iron concentrations is unclear. As the meromic- relatively ﬂat HREE; they exhibit a progression similar to those of

tic water regime stands in contrast to a possible upwelling of iron the average upper continental crust (UCC) and the post-Archean

from the monimolimnion, one hypothesis is that aeolian import ﬁne-grained clastic sediments (Post-Archean Australian Shale

of dust from the surrounding iron-rich rocks during the very dry (PAAS) and North American Shale Composite (NASC)) reported by

months January to March 2009 (Fig. 2) contributed to the rise in Taylor and McLennan (1985) and Condie (1993). The calculated

iron content. Eu depletion (Eu/Eu* = 0.33–0.45) is also comparable with those

of UCC, PAAS, and NASC, as well as with values for hydrothermal

3.3. Major and trace elements in superﬁcial sediments cherts from Lake Bogoria described by Kerrich et al. (2002). The

high abundance of REE in the investigated sediments corresponds

The mean concentrations of all elements investigated via with their volcanic geological background and is in the previously

INAA are presented in Table 2. As expected, the sediments published range, e.g. for the Greater Olkaria Volcanic Complex (see

exhibit extremely high sodium and elevated potassium abundance Table 2).

(9.81 wt% Na and 3.28 wt% K for Lake Nakuru, 7.92 wt% Na and

2.87 wt% K for Lake Bogoria). This is in accordance with the fact

4. Conclusions and outlook

that the alkalinity of African basins arises from rapid hydrolysis

of volcanic glass and lavas, producing high initial Na , SiO , and

2 In conclusion we think to have shown that in general both

3−

HCO concentrations (Jones and Deocampo, 2003; Jones et al.,

lakes depict the geology and volcanic origin of the Rift Valley ﬂoor

1977). The superﬁcial sediments of Lake Nakuru and Bogoria are

regarding element composition in both, the water column and the

described as a mixture of carbonates (calcite, dolomite), iron oxide,

sediments, but further investigation is needed to understand the

zeolite (analcime, natrolite) and diatomites (Macdonald, 1987;

origin and behaviour of some trace elements such as Zn or Fe.

Washbourn-Kamau, 1971; Owen et al., 2008). The volcanic origin

REE show a high abundance range in the sediments of both lakes.

of lake sediments is clearly evident in high Fe concentrations. The

Both lake sediments are characterised by LREE-enriched patterns

extremely high Zn concentrations, however, cannot be explained

with relatively ﬂat HREE (LaN/YbN 9.9/12) and negative Eu deple-

solely by the geological background; here, anthropogenic impacts

tion (Eu/Eu* 0.33/0.45) and are comparable with those of the UCC,

may be at work. The elevated Zn concentration agrees with the

PAAS, and NASC.

results of Jumba et al. (2007) and Raini (2009) for Lake Nakuru,

The effects of evapoconcentration during a period of extreme

presenting Zn as one of the major pollutants originating from the

drought were far more pronounced in Lake Nakuru than in Lake

industry and anthropogenic land use in and around Nakuru City.

Bogoria and led to exceptionally high concentrations, amongst oth-

For Lake Bogoria this explanation is implausible because there is

ers, of DOC and DN in the lake; ongoing investigations will help

neither a city nor intense agriculture in the catchment area. Cong

to understand changes in DOC during rising salinity and inherent

et al. (2010) and references therein describe the atmospheric wet

chemical processes certainly inﬂuencing trace element bioavail-

deposition of Zn, amongst other heavy metals, as a major source of

ability in those two highly productive ecosystems.

this element in remote areas such as Tibet and the Himalayas. Fur-

ther analysis is needed to locate the origin of these trace elements

Acknowledgements

for the two lakes investigated here.

This study was partly funded by the Austrian Science Fund,

3.4. REE patterns

project P-19911-B03. We want to express our sincere thanks to the

Egerton limnology team for the major support in both ﬁeld and lab-

Table 2 presents the mean REE concentrations of the stud-

oratory work. M. Bichler, E. Klapfer, H. Schachner (all Atominstitut

ied sediments. The obtained REE distribution patterns for both

Vienna) are thanked for the irradiations of the samples.

lake sediments were normalised to chondrite composition and

show light rare earth element (LREE)/heavy rare earth element

(HREE) fractionations and a Eu anomaly (Fig. 6). The sedi- References

ment samples of both lakes show a high REE abundance range

Alloway, B.J.E., 1994. Heavy Metals in Soils. Springer.

(LaN = 432–438 and YbN = 36–43), characteristic for the pyroclas-

Ashton, P.J., Schoeman, F.R., 1983. Limnological studies on the Pretoria Salt Pan,

tic rocks and volcanic sediments of the Kenyan Rift Valley.

a hypersaline Maar Lake. 1. Morphometric, physical and chemical-features.

The sediments are characterised by LREE-enriched patterns with Hydrobiologia 99 (1), 61–73.

282 F. Jirsa et al. / Chemie der Erde 73 (2013) 275–282

Beadle, L.C., 1932. Scientiﬁc results of the Cambridge Expedition to the East African Krienitz, L., Kotut, K., 2010. Fluctuating algal food populations and the occurrence

Lakes, 1930-1. 3. Observations on the bionomics of some East African Swamps. of Lesser Flamingos (Phoeniconaias minor) in three Kenyan Rift Valley Lakes. J.

J. Linn. Soc. Lond. Zool. 38 (258), 135–155. Phycol. 46 (6), 1088–1096.

Burgis, M., Morris, P., 1987. The Natural History of Lakes. Cambridge University Press, Macdonald, R., 1987. Quaternary peralkaline silicic rocks and caldera volcanoes of

Cambridge [Cambridgeshire], NY, 218 pp., 24 p. of plates. Kenya. Geol. Soc. Lond. Spec. Publ. 30 (1), 313–333.

Chaffey, D.J., Cliff, R.A., Wilson, B.M., 1989. Characterization of the St Helena magma Mader, D., Koeberl, C., 2009. Using Instrumental Neutron Activation Analysis for

source. Geol. Soc. Lond. Spec. Publ. 42 (1), 257–276. geochemical analyses of terrestrial impact structures: current analytical proce-

Cioni, R., Fanelli, G., Guidi, M., Kinyariro, J.K., Marini, L., 1992. Lake Bogoria hot dures at the University of Vienna Geochemistry Activation Analysis Laboratory.

springs (Kenya): geochemical features and geothermal implications. J. Volcanol. Appl. Radiat. Isot. 67 (12), 2100–2103.

Geotherm. Res. 50 (3), 231–246. Marshall, A.S., et al., 2009. Fractionation of Peralkaline Silicic Magmas: the Greater

Condie, K.C., 1993. Chemical-composition and evolution of the upper continental- Olkaria Volcanic Complex, Kenya Rift Valley. J. Petrol. 50 (2), 323–359.

crust – contrasting results from surface samples and shales. Chem. Geol. 104 Mason, D.T., 1967. Limnology of Mono Lake, California, vol. 83. UC Publications in

(1–4), 1–37. Zoology, 102 pp.

Cong, Z., Kang, S., Zhang, Y., Li, X., 2010. Atmospheric wet deposition of trace ele- McCall, G.J.H., 1967. Geology of the Nakuru-Thomson’s Falls-Lake Hannington Area.

ments to central Tibetan Plateau. Appl. Geochem. 25 (9), 1415–1421. Geological Survey of Kenya. Ministry of Natural Resources, Nairobi, 122 pp.

Curtis, P.J., Adams, H.E., 1995. Dissolved organic matter quantity and quality from McCall, J., 2010. Lake Bogoria, Kenya: hot and warm springs, geysers and Holocene

freshwater and saltwater lakes in east-central Alberta. Biogeochemistry 30, stromatolites. Earth-Sci. Rev. 103 (1-2), 71–79.

Dawson, J.B., 2008. The Gregory Rift Valley and Neogene-Recent Volcanoes of alkaline, saline lakes. Limnol. Oceanogr. 19 (5), 743–755.

northern Tanzania. Geological Society Memoir. Geological Society, London, vi, Odour, S.O., Schagerl, M., 2007. Temporal trends of ion contents and nutrients in

102 p. three Kenyan Rift Valley saline-alkaline lakes and their inﬂuence on phytoplank-

De Haan, H., 1983. Use of ultraviolett spectroscopy, gel ﬁltration, pyrolysis/mass ton biomass. Hydrobiologia (584), 59–68.

spectrometry and numbers of benzoate-metabolizing bacteria in the study of Osburn, C.L., Wigdahl, C.R., Fritz, S.C., Saros, J.E., 2011. Dissolved organic matter com-

humiﬁcation and degradation of organic matter. In: Christman, R.F., Gjessing, position and photoreactivity in prairie lakes of the U.S. Great Plains. Limnol.

De Haan, H., Jones, R.I., Salonen, K., 1987. Does ionic strength affect the conﬁguration Owen, R.A., Owen, R.B., Renaut, R.W., Scott, J.J., Jones, B., Ashley, G.M., 2008. Min-

of aquatic humic substances, as indicated by gel ﬁltration? Freshwater Biol. 17 eralogy and origin of rhizoliths on the margins of saline, alkaline Lake Bogoria,

(3), 453–459. Kenya Rift Valley. Sediment. Geol. 203, 143–163.

Evans, J.C., Prepas, E.E., 1997. Relative importance of iron and molybdenum in Peuravuori, J., Pihlaja, K., 1997. Molecular size distribution and spectroscopic prop-

restricting phytoplankton biomass in high phosphorus saline lakes. Limnol. erties of aquatic humic substances. Anal. Chim. Acta 337 (2), 133–149.

Oceanogr. 42 (3), 461–472. Raini, J.A., 2009. Impact of land use changes on water resources and biodiversity of

Gikunju, J.K., et al., 1995. Water ﬂuoride in the molo division of Nakuru District, Lake Nakuru catchmant basin, Kenya. Afr. J. Ecol. 47 (Suppl. 1), 39–45.

Kenya. Fluoride 28 (1), 17–20. Richardson, J.L., Richardson, A.E., 1972. History of an African Rift Lake and its climatic

Govindaraju, K., 1989. Compilation of working values and sample description for implications. Ecol. Monogr. 42 (4), 499–534.

272. Geostand. Newslett. 13, 1–113. Schagerl, M., Oduor, S.O., 2008. Phytoplankton community relationship to envi-

Hacini, M., Oelkers, E.H., 2011. Geochemistry and behavior of trace elements during ronmental variables in three Kenyan Rift Valley saline-alkaline lakes. Mar.

the complete evaporation of the Merouane Chott Ephemeral Lake: Southeast Freshwater Res. 59 (2), 125–136.

Algeria. Aquat. Geochem. 17 (1), 51–70. Schlueter, T., 1993. Comparison of the mineral composition of the lakes of the

Hammer, U.T., 1986. Saline Lake Ecosystems of the World. Monographiae Biologicae. East African Rift Systems (Western Rift and Gregory Rift). In: Thornweihe, U.,

Dr. W. Junk Publishers, Dordrecht, Boston, x, 616 p. Schandelmaier, H. (Eds.), Geoscientiﬁc Research in Northeast Africa. Balkema,

Jarosewich, E., Clarke, R.S., J.N, B. (Eds.), 1987. The Allende Meteorite Reference Sam- Rotterdam, pp. 657–662.

ple. Smithsonian Contributions to the Earth Sciences. Smithsonian Institution Schlueter, T., 1997. Geology of East Africa. Beiträge zur regionalen Geologie der Erde,

Press, Washington. 27. Gebrüder Bornträger, Berlin, Stuttgart.

Jenkin XIV, P.M., 1936. Reports on the Percy Sladen expedition to some Rift Val- Sobek, S., Tranvik, L.J., Prairie, Y.T., Kortelainen, P., Cole, J.J., 2007. Patterns and regu-

ley lakes in Kenya in 1929. VII. Summary of the ecological results: with special lation of dissolved organic carbon: an analysis of 7,500 widely distributed lakes.

reference to the alkaline lakes. J. Nat. Hist. Ser. 18 (103), 133–160. Limnol. Oceanogr. 52 (3), 1208–1219.

Jones, B.F., Deocampo, D.M., 2003. 5.13. Geochemistry of Saline lakes. In: Heinrich, Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evo-

D.H., Karl, K.T. (Eds.), Treatise on Geochemistry. Pergamon, Oxford, pp. 393–424. lution: An Examination of the Geochemical Record Preserved in Sedimentary

Jones, B.F., Eugster, H.P., Rettig, S.L., 1977. Hydrochemistry of the Lake Magadi Basin, Rocks. Blackwell Scientiﬁc.

Kenya. Geochim. Cosmochim. Acta 41 (1), 53–72. Thurman, E.M., 1985. Organic Geochemistry of Natural Waters. Developments in

Jumba, I.O., Kisia, S.M., Kock, R., 2007. Animal health problems attributed to envi- Biogeochemistry 2. M. Nijhoff; Distributors for the U.S. and Canada/Kluwer Aca-

ronmental contamination in Lake Nakuru National Park, Kenya: a case study on demic, Dordrecht, Boston/Hingham, MA, USA, xii, 497 p.

heavy metal poisoning in the waterbuck Kobus ellipsiprymnus defassa (Ruppel Tipping, E., 2002. Cation Binding by Humic Substances. Cambridge University Press,

1835). Arch. Environ. Contam. Toxicol. 52 (2), 270–281. Cambridge.

Kahama, R.W., Kariuki, D.N., Kariuki, H.N., Njenga, L.W., 1997. Fluorosis in children Vareschi, E., 1982. The Ecology of Lake Nakuru (Kenya). III. Abiotic factors and pri-

and sources of ﬂuoride around Lake Elementaita region of Kenya. Fluoride 30 mary production. Oecologia 55, 81–101.

(1), 19–25. Vincens, A., Casanova, J., Tiercelin, J.J., 1986. Palaeolimnology of Lake Bogoria (Kenya)

Kaplan, L.A., 1994. A ﬁeld and laboratory procedure to collect, process, and preserve during the 4500 BP high lacustrine phase. In: Frostick, L.E., Renault, R.W., Reid,

freshwater samples for dissolved organic carbon analysis. Limnol. Oceanogr. 39 I., Tiercelin, J.J. (Eds.), Sedimentation in the African Rifts. Geological Society of

(6), 1470–1476. London Special Publication, London, pp. 323–330.

Kerrich, R., Renaut, R.W., Bonli, T., 2002. Trace-element composition of cherts from Wambu, E.W., Muthakia, G.K., 2011. High ﬂuoride water in the Gilgil area of Nakuru

alkaline lakes in the East African Rift: a probe for ancient counterparts, sedimen- county. Fluoride 44 (1), 37–41.

tation in continental rifts. Soc. Sediment. Geol., 275–294. Washbourn-Kamau, C.K., 1971. Late Quaternary lakes in the Nakuru-Elmenteita

King, B.C., 1970. Vulcanicity and rift tectonics in East Africa. In: Clifford, T.N., Gass, Basin, Kenya. Geogr. J. 137 (4), 522–535.

I.G. (Eds.), African Magmatism and Tectonics. Oliver & Boyd, Edinburgh. Wolfe-Simon, F., et al., 2010. A bacterium that can grow by using arsenic instead of

Koeberl, C., 1993. Instrumental Neutron Activation Analysis of geochemical and cos- phosphorus. Science 332, 1163–1166.

mochemical samples: a fast and reliable method for small sample analysis. J. Yuretich, R.F., 1982. Possible inﬂuences upon lake development in the East African

Radioanal. Nucl. Chem. 168 (1), 47–60. Rift Valleys. J. Geol. 90 (3), 329–337.