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Asian Journal of Water, Environment and Pollution, Vol. 1, No. 1 & 2, pp. 87-97.
Comparative Studies of Water Chemistry of
Four Tropical Lakes in Kenya and India
J.W. Njenga
Chemistry Department
Jomo Kenyatta University of Agriculture and Technology
P.O. Box 62000, Nairobi, Kenya
Received January 2, 2004; revised and accepted April 20, 2004
Abstract: Water samples collected from three Rift Valley Lakes (Nakuru, Elementaita and Naivasha) in Kenya in
June, 2002 and one lake in southern India (Kolleru) in non-monsoon and monsoon were studied in order to understand
the water chemistry of the four tropical lakes. Results indicate that Lakes Nakuru and Elementaita are highly alkaline
in nature compared to Lakes Naivasha and Kolleru. Sodium is the major cation while chloride and bicarbonate are
the major anions contributing almost in equal proportion (48% each). Both carbonate and silicate weathering contribute
to the bicarbonate content in Lake Kolleru; however silicate weathering seems to be the major contributing factor in
the bicarbonate content in the Rift Valley lakes. Fluoride content was very high in the rift valley lakes.
The water chemistry of lakes Nakuru and Elementaita strongly reflects the dominance of evaporation and
crystallization mechanism. However data points for lakes Naivasha and Kolleru plot to the right of the boomerang
envelope�an indication that rock weathering is not the only mechanism controlling the water chemistry of these
lakes.
Results obtained indicate that if the waters were in equilibrium with minerals, the waters of lakes Naivasha and
Kolleru would be in equilibrium with kaolinite while that of Nakuru and Elementaita would be in the range of albite,
quartz and chlorite. The carbonate system suggests that dolomite and aragonite would be the possible minerals in
equilibrium in all the lakes.
Key words: Mineral solubility, carbonate and silicate weathering, water chemistry.
Introduction
Lakes have been used as ideal natural laboratories to study
a number of processes that are important in understanding
hydrogeochemical processes including evaporation,
dissolution, mixing, precipitation of minerals and
chemical exchange between water, sediment and
atmosphere.
The Rift Valley lakes and the ecosystem within and
around them including their catchment areas are of great
social, cultural, aesthetic and economic values to Kenya.
The lakes, their ecosystems and habitats are niches to a
wide variety of unique flora and fauna (Moor, 1984; Bugis
and Mavuti, 1987; Jackson, 2000). Lake Naivasha also
supports an outstanding horticulture and floriculture
sector that generates much needed job opportunities as
well as foreign exchange for Kenya. It also supports a
thriving fishery, livestock and a growing tourism sector.
Lake Nakuru is internationally important because of the
number of flamingos in the lake as well as over 450
species of birds in the surrounding area. Lake Kolleru
supports a thriving aquaculture sector.
Although the living resources in these habitats are
renewable, they are also vulnerable. Their sustainability
is threatened by natural, environmental and anthropo-
genic factors. As a result of rapid urbanization in the
surrounding area, the lakes have been subjected to
increasing environmental pressure and the environmental
88 J.W. Njenga
conditions of the lakes have been deteriorating. This is
evidenced by deaths of tens of thousands of flamingos
in Lake Nakuru since 1990 and increased phytoplankton
biomass in Lake Naivasha (SAPS reports, 2001, 2002;
Harper and Mavuti, 1990).
Lake Kolleru (India), once a paradise of birds like
Pelican philipinis (Chatterjee, 1996), has now changed
as a result of water quality deterioration due to sewage,
agricultural and industrial input. The lake no longer
attracts birds and masses of macrophites now cover most
of the lake.
Even though it is well known that both the concentra-
tion of total dissolved solids (TDS) and the relative
amounts or ratios of different ions influence the species
of organisms that can best survive in a lake, in recent
years, no substantial work has been carried out to study
the water chemistry especially of the smaller Rift Valley
lakes. Existing studies on water chemistry are mainly
concentrated on the great lakes of Africa namely, Victoria,
Tanganyika and Malawi and Turkana (Sprigel & Coulter,
1996, 2002). Studies have also been carried out on some
of the smaller lakes in the Rift Valley (Talling & Talling,
1965; Staum & Morgan, 1970; Kilham, 1971; Richardson
& Richardson, 1972; Gaudet & Melack, 1981; Kilham,
1990, Mungoma, 1990; Ojiambo and Lyons, 1996;
Jackson, 2000).
In India studies on various aspects of the Himalayan
and other lakes have been carried out by several authors
which include Vijay Raghavan (1971), Kaul et al. (1980),
Zutshi et al. (1980), Zutshi and Khan (1988), Das (1996),
Panograhy (2000) and Gupta et al. (2001). However very
little work has been reported on Lake Kolleru, that too
not exclusively on water chemistry (Seshavatharanan,
1992; Sreenivasa, 1997; Chatterjee, 1996, Sreenivasa
et al. 1999, 2000; Vikram Reddy, 2002).
Since the ionic concentration in a lake influences the
lake�s ability to assimilate pollutants and maintain
nutrients in solution, knowledge of the water chemistry
is necessary especially if the lake resources are to be fully
exploited. In the current study, aspects related to variation
in physicochemical parameters, ionic composition
together with weathering processes and possible sources
and mechanisms controlling the water chemistry in four
tropical lakes have been studied and are discussed. The
baseline data are necessary as a starting point for further
work in relation to the utility and pollution status of these
lakes. An attempt has also been made to assess the water
quality of lakes Naivasha and Kolleru in as far as
irrigation is concerned for which these waters are highly
utilized.
A comparison between the water chemistry of three
lakes in Kenya and one lake in India has been made to
understand the major mechanisms controlling the water
chemistry of these tropical lakes. Similar factors such as
climate (semi-arid region), tropical location, and
similarity in water utilization (agriculture and aqua-
culture) prompted us to compare these lakes in two
different continents/countries although they have slightly
varying geological set up in terms of volcanic origin.
Study Area
The Republic of Kenya extends between latitude 4.5° N
and 4.5° S and between longitudes 34° E and 42° E. A
major topographic feature is the north-south trending Rift
Valley, in which Lakes Turkana, Baringo, Bogoria,
Nakuru, Elementaita, Naivasha and Magadi are located
(Figure 1).
Figure 1: Study area and sampling sites in Kenya.
The current study concentrates on three of the Rift
Valley lakes namely, Lakes Nakuru, Elementaita and
Naivasha (Figure 1) and one lake in India (Figure 2).
The three Rift Valley lakes are the remains of a once
larger (625 km2) lake which is believed to have dried up
Comparative Studies of Water Chemistry of Four Tropical Lakes in Kenya and India 89
10,000 years ago due to changes in climatic conditions
(LNROA, 1993). The lakes are located in Nakuru District,
Rift Valley Province in Kenya. Lake Naivasha (with
unique characteristic e.g. freshwater lake among saline
lakes) is located at the highest part of the Kenyan Rift
Valley, 1890 m above sea level (LNROA, 1993). Lake
Nakuru is at 1778 m above sea level and Elementaita
lying at 1776 m above sea level. Both lakes Nakuru and
Elementaita are shallow with a mean depth of one metre.
There are four major geological systems in the lake
region: metamorphic rocks of Precambrian age,
sedimentary rocks of Carboniferous to Cretaceous age,
Tertiary and Quaternary Volcanics and unconsolidated
Tertiary and Quaternary sediments (Thompson &
Dobson, 1963; Clerk et al., 1970). The volcanic rocks in
the region are a mixture of acid and basic lava such as
tephrites, rhyolites and sodic rhyolites.
Lakes Nakuru and Naivasha are important for
biodiversity and have been listed as wetlands of
international importance (Ramsar sites), under Ramsar
Convention (1971) (Koyo et al., 2000).
Lake Kolleru is the largest natural freshwater wetland
in Andhra Pradesh (India). It lies between two major
South Indian rivers, the Krishna and Godavari. The lake
lies at longitude 81° 05¢ and 81° 21¢ E and latitude
17° 25¢ and 16° 28¢ N (Figure 2) at an altitude of 2 to
13.3 metres above sea level. The lake is a shallow
freshwater body with a normal water spread of 674 km2
which goes up to 954 km2 during highest floods and
comes down to 66 km2 during dry seasons. The lake depth
ranges from less than 1 to 3 metres with the central part
being 9 metres. The lake has some saline intrusions
through Upputeru River (Mital, 1993).
Geologically, the lake is believed to be of recent origin
formed by excessive silting by the Krishna and Godavari
rivers of the earlier lagoon separating completely from
the sea (Sreenivasa, 1997). The lake is surrounded by
alluvium on all sides. About 10 km towards the west,
northwest and northeast of the lake, geological formations
of Khodolites, Gondwana, Deccan traps and tertiary
sediments are present (Das, 1982; Sreenivasa, 1997).
Methodology
Water samples were collected in one litre plastic bottles
from various sampling sites (Figures 2 and 3). Lake
Kolleru was almost dry during non-monsoon period,
hence only a few samples were collected during this
season. Field measurements of pH, electrical conductivity
and dissolved oxygen were determined at the site using
Raccho (model no. 123). The pH electrode was calibrated
with pH 4 and pH 7 buffer solution. Chloride content
was determined by �Reddelkis� chloride ion selective
electrode in combination with a double junction reference
electrode (with inner junction 4 M KCl and outer junction
1 M KNO3 (Corning, 1981) and consort P602 ion meter
(Consort, 1994). Fluoride concentration was determined
Figure 3: Percentage Contribution of major
anions and cations.
Figure 2: Study area and sampling sites in India.
90 J.W. Njenga
using �Omega� ion selective electrode (Omega, 1993).
Sulphate was determined by titrimeric method using
barium perchlorate after passing the sample through
cation exchange resin (Fritz & Yamamura, 1955; Hartz
et al., 1979). Phosphate was determined by the ascorbic
acid method (Eaton, 1995). Silica by the molybdo silicate
method (Eaton, 1995). Bicarbonate was determined by
acid titration method while nitrate was determined by
brucine method (Trivedy and Goel, 1984). Cations were
analysed using ion chromatography (Metrohm model,
using 732 IC Detector, 709 IC Pump and 753 Suppressor
Module).
Results and Discussion
The major ion chemistry data of lakes Nakuru,
Elementaita and Naivasha are given in Table 1. TZ+ (sum
of cation in meq/l) and TZ� (sum of anion in meq/l) have
also been included to verify the analytical precision of
the data. Table 2 gives the ion chemistry of Lake Kolleru.
The imbalance between cations and anions may be due
to the high salinity contribution from saline soil leaching
and rapid evaporation of the ancient precipitation deposits
of CaCO3 i.e. additional source controlling the water
chemistry other than rock weathering and evaporation.
Lakes Nakuru and Elementaita are highly alkaline in
nature (pH 9.9 Elementaita and 10.3 Nakuru) while
lakes Naivasha and Kolleru are moderately alkaline [pH
range of 8.0-8.9 (Naivasha) and 8.3-8.7 (Kolleru non-
monsoon)] and near neutral pH 7.5-7.8 (Kolleru
monsoon). The high pH values in the Rift Valley lakes
can be explained fundamentally by the natural process
of weathering in the study area (Yuretich, 1982; Nanyaro,
1984). Contribution of photosynthetic activities, which
utilizes CO2 thereby shifting the equilibrium towards the
alkaline side in the lakes, cannot be overlooked (Melack,
1981; Mungoma 1990). Low pH in Lake Kolleru is due
to the dilution effect during monsoon period.
Electrical conductivity (EC) range between 39,300-
54,800 mS/cm (lakes Nakuru and Elementaita) and 220-
Table 1: Physicochemical parameters in lakes Naivasha, Nakuru and Elementaita
Station pH EC DO NO3 PO4 F Cl HCO3 H4SiO4 SO4 Na K Ca Mg TZ- TZ+
NVI 8.4 1,480 7.9 15.9 0.8 0.1 162 470 8.4 3.2 115 129 5.4 6.0 12.3 9.0
NV2 8.6 1,180 7.7 20.5 1.8 10.9 224 409 8.6 5.5 183 109 7.3 6.1 13.1 11.6
NV3 8.6 1,120 8.5 18.4 2.3 4.1 797 305 6.7 3.2 149 125 3.0 2.0 27.5 10.0
NV4 8.7 1,090 9.8 16.7 2.1 5.6 618 366 5.3 4.7 178 80 6.0 6.1 23.5 10.6
NV5 8.7 1,500 8.7 16.4 1.6 25.1 514 409 3.0 4.7 178 125 7.1 4.9 21.3 11.7
NV6 8.2 1,910 7.4 16.4 2.5 13.4 342 396 9.8 1.6 199 47 6.3 5.7 16.2 10.6
NV7 8.0 1,670 7.6 16.9 1.7 23.9 548 409 7.1 4.7 165 51 5.6 2.6 22.3 9.0
NV8 8.6 1,160 10.4 25.8 1.9 4.9 638 409 10.5 5.5 157 34 3.6 3.5 24.8 8.2
NV9 8.6 1,430 6.7 17.6 2.1 1.3 672 396 11.0 4.7 101 64 7.1 10.1 25.5 7.2
NV10 8.9 850 10 28.0 2.0 2.2 672 366 5.3 7.9 256 15 9.4 3.6 25.1 12.3
NV11 8.4 1,220 7.4 17.9 1.2 24.3 204 390 10.9 4.0 171 67 4.3 4.7 12.2 9.7
NV12 8.9 2,480 6.8 17.4 2.3 7.6 144 366 2.1 7.9 180 41 7.9 6.6 10.2 9.8
NV13 8.3 1,460 5.8 23.7 2.1 8.6 511 378 5.2 9.5 108 146 5.1 5.2 20.8 9.1
NV14 8.7 1,410 7.7 17.1 2.5 3.4 69 305 3.3 4.0 244 88 2.8 3.2 7.0 13.2
NV15 8.8 270 6.9 20.3 2.3 23.7 285 427 7.3 5.5 149 81 5.0 3.7 15.2 9.1
NV16 8.9 220 10.5 21.3 2.6 0.7 305 427 14 9 182 40 7.7 5.3 15.8 9.7
NK2 10.3 46,300 6.3 16.2 0.15 18.6 2,234 25,437 105 16 13,989 940 827 20 480 675
NK3 10.3 47,400 9.5 40.1 0.08 31.6 15,697 25,437 97 142 26,732 897 15.8 40 863 1,190
NK4 10.3 40,600 17.4 70 0.05 25.3 17,583 29,890 113 190 32,567 1,051 18.7 49 990 1,448
NK5 10.3 51,400 15.1 43 0.2 28.3 32,549 6,527 89 166 14,063 1,026 1293 74 1,028 708
NK7 10.3 52,700 13.9 39.1 0.01 28.6 23,461 10,490 95 205 12,993 567 8.4 27 838 582
NK8 10.3 39,300 12.8 58.9 0.15 34.9 25,326 12,993 115 395 33,562 730 122 21 935 1,486
NK11 10.3 42,900 8.2 40.3 0.06 39.4 23,652 6,100 95 154 25,940 935 283 71 770 1,172
NK13 10.3 54,800 17.9 90.3 0.07 32.8 23,598 25,498 86 186 32,567 956 125 39 1,087 1,450
NK12 7.7 46,640 9.4 45.4 0.08 21.0 23,113 27,572 90 249 6,421 1,087 206 216 1,109 335
EL1 9.9 60,500 3.6 0.12 0.24 9.7 30,030 24,278 104 97 29,531 4,301 21 23 1,247 1,396
EL2 9.9 58,500 1.7 0.15 0.47 9.7 40,730 25,904 135 97 33,142 4,681 35 49 1,575 1,566
EL3 9.9 61,500 2.1 0.15 0.15 9.6 45,598 21,229 89 130 33,398 3,549 110 70 1,637 1,554
EL4 9.9 60,500 6.5 0.13 0.13 9.6 42,606 27,327 63 114 35,487 2,372 75 119 1,652 1,617
All parameters in mg/l except pH and EC (mS/cm). TZ+ and TZ- in meq have also been included. NV�Naivasha,
NK�Nakuru, and EL�Elementaita.
Comparative Studies of Water Chemistry of Four Tropical Lakes in Kenya and India 91
2480 mS/cm (Lake Naivasha), 1860-6350 mS/cm (Kolleru
non-monsoon) and 847-3010 mS/cm (Kolleru monsoon).
The high EC in lakes Nakuru and Elementaita indicates
high concentration of dissolved ions in the two lakes.
The lakes are closed basins without outlets; hence high
ion concentration results from evaporative concentrations
of ions leached from the surrounding drainage basin. Lake
Naivasha has the lowest electrical conductivity. Factors
controlling the ion concentration in Lake Naivasha
include freshwater supply through rivers flowing into the
lake. Also, unlike other lakes in the region, the lake does
not lie in a closed basin but looses water and solute via
underground seepage (Gaudet & Melack, 1981). The high
conductivity in Lake Kolleru in non-monsoon season is
as a result of evaporation.
Dissolved oxygen ranged between 6.25-17.9 mg/l
(Lake Nakuru), 6.7-10.5 mg/l (Lake Naivasha), 7.4-8.9
mg/l (Kolleru non-monsoon) and 3.3-7.3 mg/l (Kolleru
monsoon). Supersaturation of oxygen (O2) in most
sampling sites in Lake Nakuru was observed most
probably as a result of photosynthetic activity in the
surface waters of the lake. This is supported by high
concentration of Spirulina plantensis enrichment in the
lake (Melack, 1981; SAPS, 2002; Ngene, 2002).
Nitrates range between 16-90 mg/l (Nakuru), 0.12-
0.15 mg/l (Elementaita), 15-23.6 mg/l (Naivasha), 0.23-
1.47 mg/l (Kolleru non-monsoon) and 0.01-0.4 mg/l
(Kolleru monsoon). Phosphate content ranges between
0.01-0.2 mg/l (Nakuru), 0.82-2.53 mg/l (Naivasha), 0.13-
0.47 mg/l (Elementaita), 0.16-2.07 mg/l (Kolleru non-
monsoon) and 0.02-0.47 mg/l (Kolleru monsoon).
Phosphorus and/or nitrogen have been identified as the
growth limiting nutrients in most water bodies (Moss,
1969; Sproulle & Kaliff, 1978; Vollenweider, 1978;
Sugunan, 1995).
Computation of the NO3 : PO4 ratio suggests that
nitrogen is the growth limiting nutrient in lakes Naivasha,
Elementaita and Kolleru while phosphorus is the growth
limiting nutrient in Lake Nakuru. However, the high
nutrient content in Lake Naivasha would suggest that
none of the nutrient is a limiting factor. Lake Nakuru is
naturally eutrophic (SAPS Reports, 2001, 2002) whereas
contribution of nutrients through anthropogenic activities
like sewage, industrial effluents and runoff from
Table 2: Physicochemical parameters in lake Kolleru
Station pH EC DO NO3 PO4 F Cl HCO3 H4SiO4 SO4 Na K Ca Mg TZ- TZ+
KM1 8.3 2,090 7.5 0.38 0.21 1.62 990 514 3.9 15.2 533 19 23 44 36.6 28.4
KM2 8.3 1,860 7.4 0.36 0.23 1.41 938 426 3.6 25.3 729 42 24 47 34.0 37.8
KM3 8.2 2,255 7.6 0.23 0.16 0.88 968 498 1.5 16.1 762 49 24 45 35.8 39.3
KM4 8.5 6,300 7.4 1.52 0.62 0.63 1061 740 22.5 32.7 1749 175 47 141 42.7 94.4
KM5 8.5 7,200 7.6 1.47 0.63 0.99 1099 601 21.7 36.0 1574 246 55 146 41.6 89.4
KM6 8.2 6,350 8 1.18 2.07 0.45 929 736 16.9 32.7 1835 237 45 146 38.9 100.1
KM7 9.0 5,000 7.7 1.22 0.53 0.45 938 926 17.6 39.3 1556 149 41 115 42.4 83.0
KS2 7.9 1,040 7.2 0.12 0.37 0.69 436 445 6.5 8.5 342 26 19 24 19.8 18.4
KS3 7.5 1,090 5.7 0.14 0.18 0.47 436 486 6.6 9.9 396 36 24 29 20.5 21.7
KS4 7.5 1,110 5.1 0.11 0.12 0.45 452 518 6.7 25.1 345 25 20 24 21.7 18.6
KS5 7.5 1,780 6.1 0.13 0.23 0.3 533 591 7.3 9.9 429 37 24 34 24.9 23.6
KS6 7.5 2,400 6 0.13 0.5 0.3 584 786 11.5 11.2 543 78 35 51 29.6 31.5
KS7 7.7 1,640 3.3 0.4 0.44 0.02 576 713 7.5 9.9 446 41 11 34 28.1 23.8
KS8 7.7 1,590 6.3 0.06 0.43 0.37 462 567 7.4 8.5 437 65 31 35 22.5 25.1
KS9 7.6 1,650 7.1 0.05 0.22 0.46 540 509 5.5 11.9 398 48 23 33 23.8 22.4
KS11 7.6 1,410 5.4 0.11 0.39 0.34 520 591 8.3 8.5 417 55 24 38 24.5 23.8
KS12 8.2 3,010 7.2 0.01 0.4 0.03 659 742 9.7 25.8 510 64 23 43 31.3 28.5
KS13 7.5 1,860 6.2 0.11 0.36 0.32 615 591 9.7 16.5 497 111 39 53 27.4 30.7
KS14 7.5 1,670 5.5 0.06 0.24 0.33 573 526 8.5 13.9 520 37 44 53 25.1 30.1
KS17 7.5 827 7.3 0.04 0.02 0.22 461 285 4.3 5.9 186 25 20 17 17.8 11.1
KS19 7.6 1,450 7 0.06 0.2 0.37 553 364 4.0 23.2 434 59 27 34 22.0 24.5
KS20 7.3 1,200 5.6 0.05 0.17 0.22 597 461 7.2 7.2 639 109 33 63 24.6 37.4
KS21 7.5 1,490 6 0.04 0.42 0.76 633 453 5.2 6.54 416 53 23 31 25.4 23.1
KS22 7.5 1,500 7.3 0.06 0.47 0.75 882 583 5.6 12.7 449 81 70 59 34.7 29.9
KS23 7.4 1,370 6.5 0.016 0.42 0.43 882 636 6.6 23.15 446 80 24 32 35.8 25.3
All parameters in mg/l except pH and EC (mS/cm). TZ+ and TZ- in meq have also been included, KM (Kolleru non-monsoon)
and KS (Kolleru monsoon).
92 J.W. Njenga
agricultural farms (SAPS, 2001) cannot be ruled out. High
phosphate concentration in Lake Naivasha could be as a
result of runoff from horticultural farms around the lake
(Wamukoya et al., 1997; Tang Zu, 1999). Nutrient
enrichment in Lake Kolleru is due to the input from the
sewage and industrial effluents (Chatterjee, 1996;
Sreenivasa, 1999). Birds excrete could also be a signifi-
cant source of phosphorus content in the Rift Valley lakes.
Anion
Chloride and bicarbonate (Figure 3) contribute over 90%
of the anions. High chloride content in the Rift Valley
lakes is primarily contributed from the alkaline/saline
soils in the drainage area (Gachiri & Davies, 1993).
Significant contribution of chloride in the rift valley
region from dry fallout is also possible as the areas are
rainfall deficient. In Lake Kolleru, however, it reflects
intrusion of seawater (Mital, 1993; Sarojini et al., 1997).
Chloride concentration in Lake Kolleru may also reflect
contribution by sea spray due to its proximity to the ocean.
Bicarbonate content is very high in lakes Nakuru and
Elementaita relative to the other two lakes. The major
source of bicarbonate are the carbonate rocks containing
calcite (CaCO3) and dolomite [CaMg(CO3)2]. Calcium
(Ca) and magnesium (Mg) can also be supplied from
Ca-silicates and Mg-silicates. Holland in 1978, after
comprehensive review of water chemistry and composi-
tion of rocks, concluded that 74% + 10% of calcium and
40% + 20% of magnesium in the river water are derived
from solutions of carbonate minerals and the remainder
from silicate minerals. Thus the bicarbonate derived from
carbonate weathering (HCO3)C and bicarbonate derived
from silicate weathering (HCO3)Si are divided between
the two sources and can be calculated following the
equation:
(HCO3)C = 0.74(Ca)t +0.4 (Mg)t
(HCO3)Si = (HCO3)t � (HCO3)C
(concentrations in milimoles/l)
(Holland, 1978; Raymahashay, 1996).
On the above basis, an attempt has been made in the
current study to quantify the carbonate and silicate
contribution to the bicarbonate content in the studied
lakes. This would help identify and explain the source of
bicarbonate in Rift Valley lakes. The results indicate that
both carbonate and silicate weathering contribute to
bicarbonate content in Lake Kolleru ((HCO3)C/(HCO3)Si
> 0.5 in most sampling sites) while silicate weathering is
the major contributing factor in the rift valley lakes
((HCO3)C/(HCO3)Si < 0.1 in most sampling sites).
Sulphate (SO4) contribution to anions was 1% in lakes
Naivasha and Kolleru and negligible percent contribution
in lakes Nakuru and Elementaita. Fluoride content in the
Rift Valley lakes was high. It ranges between 18-39 mg/l
in Lake Nakuru, 2-25 mg/l in Lake Naivasha and 9.65
mg/l (mean) in Lake Elementaita. High fluoride content
in the Kenya waters has been reported by a number of
researchers (Barkish, 1974; Jones et al., 1970; Clarke,
1970; Harper et al., 1990). The major source of fluoride
entering into the hydrological system in Kenya can be
traced to volcanic activity associated with Rift Valley
formation and chemical weathering of volcanic rocks
(Kilham & Hecky, 1973; Yuretich, 1982; Nanyaro et al.,
1984). The volcanic rocks of the Rift Valley system are
predominantly alkaline rocks rich in sodium and fluoride
(Harper & Mavuti, 1990). The rocks are richer in fluoride
here than the analogous rocks in other regions of the world
(Gachiri & Davies, 1993). Alkali basalt, basanites and
tephrites are the main varieties followed by phonolites
and trachytes (Williams, 1982). Evaporative concentra-
tion has also been reported to be responsible for the
extremely high fluoride concentrations found in Kenyan
lakes (Eugster, 1970; Jones et al., 1977; Nanyaro et al.,
1984; Clarke et al., 1990; Kilham & Mavuti, 1990).
Evaporative concentration in the Rift Valley lakes is
reported to be so effective that the fluoride concentra-
tion is several orders of magnitude higher than the normal
groundwater and river water (Gaciri & Davies, 1993).
Geochemical model proposed by Aswathayanarana
(2001) to account for high fluoride contents of natural
waters of northern Tanzania indicates that fluoride is
mainly derived from two sources: steady influx of
fluoride in the surface and groundwater by the leaching
of the East African Rift and also from episodic, massive
influx of fluoride which arose due to the leaching of the
highly soluble villiaumite (NaF) present in the volcanic
ash, exhalations and sublimates related to miocene.
Considerable amounts of fluoride are also discharged
direct into hydrological system in the form of waste
waters and other wastes resulting from mining and ore-
processing operations at the Kenyan Fluospar Mine in
the Kerio Valley (Western Kenya) (Gaciri & Davies,
1993). This is indicated by the widespread occurrence of
fluorosis among inhabitants and cattle in the surrounding
region (Njenga, 1984; Nyaora et al., 2002). Fluoride
content in Lake Naivasha is of special interest and needs
to be investigated further because the water is used for
irrigation purposes.
Comparative Studies of Water Chemistry of Four Tropical Lakes in Kenya and India 93
resistance to weathering of potassium and its use in clay
formation as well as in biological utilization.
Mechanisms Controlling Water Chemistry
The source of major ions in water can be defined by
plotting the samples according to the variations in weight
ratios of Na/(Ca+Na) as function of total dissolved solids
(TDS) (Gibbs, 1970). Gibbs� idea has been challenged
by various authors especially in regard to the water
chemistry of African lakes (Kilham, 1990; Berner &
Berner, 1996; Faure, 1998; Baca & Threlkeld, 2000). In
the current study the Gibbs diagrams (1970) together with
its modification by Kilham (1990) have been utilized to
decipher the major mechanisms controlling the ion
chemistry in the study area (Figure 5). The data plot for
lakes Nakuru and Elementaita is in agreement with both
observations (Gibbs, 1970; Kilham, 1990). However the
data points for lakes Naivasha and Kolleru plot outside
and to the right of the boomerang region (Gibbs, 1970)
but fall in the region indicated by Kilham (1990) for
African lakes (Figure 5). Kilham (1990) concluded that
rock weathering, evaporative concentrations and
precipitation of calcium carbonate largely control the
chemical composition of such waters. The mechanism
controlling water chemistry of Lake Kolleru seems to be
a combination of rock weathering as well as evaporation-
crystallization.
Cations
Sodium (Na+) is the dominant cation with the exception
of lake Naivasha. Na+ is about 90% of the total cations
in lakes Nakuru and Elementaita and 70% in Lake Kolleru
(Figure 3). Both Na and K together contribute more than
90% of the total cations in all the lakes. The scatter
diagram indicate that unlike most lakes where the major
cations are the divalent cations (Ca2+ and Mg2+), the Rift
Valley lakes have the monovalent cations (Na+) as the
major cations. The dominance of the monovalent ions is
further confirmed by the scatter diagrams (Figure 4). The
relatively high contribution of (Na + K) to the total cations
indicate that silicate weathering and/or contribution of
alkaline saline soil are the important sources of ions in
these waters. Predominance of (Na+K) over (Ca+Mg)
and the low contribution of calcium and magnesium
(Figure 4) can be attributed to precipitation of calcite
and dolomite as the pH is very high especially in lakes
Nakuru and Elementaita. Hecky and Kilham (1973)
reported that calcium and magnesium are removed from
solution through precipitation (calcite and dolomite) at
pH values above 9. Observations made on the water
chemistry of Pulicat (India) also indicate the dominance
of sodium and chloride with complete lack of bicarbonate
(Magaraju et al., 1990). The lower potassium content than
sodium content in all the lakes can be attributed to the
Figure 4: Scatter diagrams for Na+K vs TZ+, Ca+Mg
vs TZ+ and for Na+K vs Ca+Mg.
94 J.W. Njenga
Figure 5: Gibbs/Kilham diagram.
[_____ Gibbs (1970)/- - - - - - - Kilham (1990)]
Mineral Stability
Mineral stability is an important way in which the
geochemical approach to equilibrium between clay
minerals and natural waters can be verified through
thermodynamic data (Garrels & Christ, 1965). On the
basis of water analysis, silicate stability diagrams for
sodium (Na) and calcium (Ca) (Figure 6) and carbonate
stability diagram (Figure 7) were constructed. The
stability diagrams have been used to understand what
the mineral equilibrium would be if the waters were in
equilibrium. In all the lakes the data points fall in the
dolomite and aragonite region (Figure 7) indicating that
these waters could be in equilibrium with dolomite and
aragonite. The silicate systems (Figure 6) demonstrate
that if the lake water were in equilibrium, lakes Naivasha
and Kolleru would be in equilibrium with kaolinite.
Similar observation on the water chemistry of lake
Naivasha was made by Gaudet and Melack (1981). Lakes
Nakuru and Elementaita are in equilibrium with albite,
quartz and chlorite, which implies that the chemistry of
the waters would favour chlorite and quartz with
aragonite and dolomite.
Figure 6: Stability diagram for silicate systems.
Log (H4SiO
4)
Figure 7: Stability diagram for carbonate system.
Water Quality Assessment
Lakes Naivasha and Kolleru have been used as sources
of irrigation water. Using the data obtained during the
Comparative Studies of Water Chemistry of Four Tropical Lakes in Kenya and India 95
current study, evaluation of the water in terms of its
suitability for irrigation purposes (Richards, 1954) was
carried out. EC and sodium correlation is very important
in classifying irrigation water. While a high salt
concentration (high EC) in water leads to formation of
saline soil, high sodium concentration leads to develop-
ment of alkali soil. There is a significant relationship
between sodium absorption ratio (SAR) values of
irrigation water and the extent to which sodium is
adsorbed by the soils. If water used for irrigation is high
in sodium and low in calcium, the cation-exchange
complex may become saturated with sodium. This can
destroy the soil structure owing to dispersion of the clay
particles. The calculated values of SAR in the two lakes
range from 4-8 (Naivasha), 4-15 (Kolleru monsoon) and
15-30 (Kolleru non-monsoon). The plot of data on the
US salinity diagram (Richards, 1954) in which the EC is
taken as salinity hazard and SAR as alkalinity hazard
(Figure 8) shows that most of the points in lakes Naivasha
and Kolleru fall in the category C3S2 with a few points
falling on the C3S3 region (Figure 8). With the exception
Conclusion
1. Lakes Nakuru and Elementaita are highly alkaline
in nature while the waters of lakes Naivasha and
Kolleru are moderately alkaline and near neutral for
Kolleru in monsoon period.
2. Sodium is dominant in cations in all the lakes.
Depletion of the divalent cations (Ca and Mg) in
the highly alkaline lakes Nakuru and Elementaita
was noted.
3. Chloride and bicarbonate are the dominant anions
in all the four lakes.
4. Both carbonate and silicate weathering contribute
to bicarbonate content in Lake Kolleru while silicate
weathering is the major contributing factor in the
Rift Valley lakes.
5. The high (Na+K)/(Ca+Mg) ratio and the relatively
high contribution of Na+K to the total cations in the
Rift Valley lakes also suggest that silicate weathering
is the main source of the major ions.
6. NO3 : PO4 ratio suggests that nitrogen is the growth
limiting nutrient in lakes Elementaita and Kolleru
while phosphorus is the growth limiting nutrient in
lake Nakuru. However, the high nutrient content in
lake Naivasha would suggest that none of the
nutrient is a limiting factor.
7. Rock weathering, evaporative concentrations and
precipitation of calcium carbonate largely control
the chemical composition of lakes Nakuru and
Elementaita in the Rift Valley. The mechanism
controlling water chemistry of Lake Kolleru seems
to be a combination of rock weathering as well as
evaporation-crystallization.
8. The chemistry of water of lakes Nakuru, Elementaita
and Kolleru seem to favour aragonite formation
while the chemistry of Lake Naivasha favours
dolomite formation.
9. In the silicate system, lakes Naivasha and Kolleru
water is in equilibrium with kaolinite and favour
kaolinite formation, while lakes Nakuru and
Elementaita water is in the range of albite, quartz
and chlorite.
10. The water in both lakes Naivasha and Kolleru
(monsoon) is suitable for irrigation purposes. How-
ever, the waters would not be suitable during dry
seasons.
11. The high fluoride content in Lake Naivasha is of
great concern as it will limit its use in agriculture.
Figure 8: Quality criteria for irrigation water.
of the points falling in C3S3 region the water can be
used for irrigation purposes. However if dry season
persists, the water in both the lakes would not be suitable
for irrigation purposes as both salinity and sodium hazard
increases substantially in both the lakes. This is clearly
indicated by Kolleru non-monsoon samples, which fall
in S4C4 region (very high alkali and salinity hazards)
restricting its suitability for irrigation.
96 J.W. Njenga
Acknowledgements
The author is grateful to the Kenya Wildlife Society,
Kenya and the Forestry Department, Eluru, State of
Andhra Pradesh, India for their assistance during
sampling sessions. I also acknowledge with gratitude the
financial assistance given by JKUAT (Kenya) during the
fieldwork.
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