Lakes, Wetlands and People of East Africa's Rift …kenya-rift-lakes.org/downloads/research...
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Hydrobiologia 488: 27–41, 2002.D.M. Harper, R. Boar, M. Everard & P. Hickley (eds), Lake Naivasha, Kenya.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
27
Geochemical and physical characteristics of river and lakesediments at Naivasha, Kenya
Håkan Tarras-Wahlberg1,∗, Mark Everard2 & David M. Harper3
1Swedish Geological AB, Box 19090, 104 32 Stockholm, Sweden2The Natural Step UK, 9 Imperial Square, Cheltenham, Gloucestershire GL50 1QB, U.K.3Department of Biology, Leicester University, Leicester LE1 7RH, U.K.∗Author for correspondence. E-mail: [email protected]
Key words: Lake Naivasha, sediment stratigraphy, sediment geochemistry, metals
Abstract
Prior studies on Lake Naivasha relevant to understanding sediment dynamics include a bathymetric map, a pa-leolimnological study of fossil invertebrate assemblages in lake sediment, an overview of lake level fluctuationsthroughout the 20th century, and identification of a dynamic assemblage of macrophyte zones that has respondedboth to these changes in lake level and to more recent, alien species. Sediment samples collected from the riverssystems and the lake were examined physically and chemically. River sediment characteristics reflect geologyand geomorphological processes in the catchment, whereas lake sediment stratigraphy has responded to pastlake level changes. Such changes have caused significant changes in aquatic vegetation assemblages. Present daysediment dynamics in the lake are governed by the presence of river point sources in the north and wave-inducedre-suspension, such that sediments introduced by rivers are transported in easterly and southerly directions, andare eventually deposited in the eastern, central and southern parts of the lake. Sedimentary deposition is alsooccurring in northern areas that once were protected by papyrus swamp vegetation but now only have a narrowfringe, highlighting the important role of swamp vegetation in filtering out suspended particulates and therebycontrolling water quality in the lake. Geochemical analyses of river and lake sediments indicate that they representfairly undisturbed background conditions. Higher-than-expected concentrations of cadmium, iron, nickel and zincfound in both river and lake sediment are likely to derive from volcanic rocks and/or lateritic soils found in the lakecatchment.
Introduction
Lakes are strongly influenced by the natural and hu-man characteristics of their catchments. River sedi-ment in low order channels derive from a plethora ofhillslope and channel sources in the upstream catch-ment (Bowie & Mutchler, 1986). During downstreamtransport, sediment from different sources will mixand, therefore, investigations of the geochemical andphysical characteristics of river sediment may provideevidence of processes occurring in the entire catch-ment. As a consequence, stream sediment samplinghas long been used as a tool to prospect for metalli-ferous mineral deposits, and it is generally accepted
that such surveys provide more representative and costeffective results than surveys of soils or vegetation(Levinson, 1980).
Geochemical surveys of river sediment are increas-ingly being used also in environmental investigations,and applications include:
(i) assessments of environmental impacts, as sedi-ments’ contaminant concentrations downstreamof industrial, agricultural or other human activ-ities often are elevated (Darnley et al., 1995);
(ii) human and animal health studies as geologicallycontrolled deficiencies/excesses of certain nutri-ents/toxins in water and/or food products may
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cause health problems (Fordyce et al., 1996); and(iii) to provide a geochemical baseline against
which any future pertubations may be appraised(Williams et al., 2000).
Similarly, investigations of lake sediments can alsobe used in environmental surveys. When rivers enter alake, the decrease of velocity results in sediment de-position. Investigations of geochemical changes withsediment depth may, thus, provide indications ofchanges in the characteristics of sediment input withtime. Such investigations may therefore be used todetermine both the character of contamination and, ifsedimentation rates are known, changes in contamin-ant loading with time (Nicholson, 1996).
Environmental applications of geochemistry of-ten investigate metal contamination, and it is gen-erally acknowledged that metals in their dissolvedionic state control direct toxic effects (Förstner, 1983;1988; Stumm & Morgan, 1996). However, metals inaquatic systems are usually predominantly associatedwith sediment (Förstner, 1983; Windom et al., 1991).Therefore, the concentration of metals in sediment isgenerally a more sensitive and accurate indicator ofcontamination of an aquatic system, compared to theconcentration of dissolved metals (Carelli et al., 1995).It is also recognised that sediment-bound metals havethe potential of changing to the ionic state (Willfordet al., 1987). Investigations of sediment geochemistryis, thus, directly relevant to assessing the possibilityof toxic effects of metal contamination. Furthermore,as metals are present in much higher concentrationsin sediment than in water, the analysis of sedimentis comparatively easy to perform and, therefore, lesscostly, enabling more samples to be analysed, and agreater geographical coverage.
The interpretation of sediment geochemistry datais, however, not without its problems. In rivers, dif-ferent morphological units often have quite differentgeochemical characteristics, resulting from physicalsorting processes within the stream bed (Ladd et al.,1998). Similarily, metals are often found preferentiallyconcentrated in the finer fraction of sediment (Först-ner, 1983). Consequently, care needs to be taken toensure that the same or at least similar morphologicalunits and size fractions are sampled in order for resultsfrom different rivers or even reaches of the same riverto be comparable (Ladd et al., 1998). Further, whereasenvironmental criteria for water have been establishedfor a varity of purposes, including the protection ofaquatic life, attempts to elaborate similar criteria for
sediment have been less succesful. It is difficult to de-termine to what extent potentially harmful elementsin sediment are, or may become, available for bio-logical uptake. Consequently, environmental criteriafor sediment are generally seen to be of limited valuedue to variations through time and space in the con-ditions that determine bioavailability (Darnley et al.,1995; Maher et al., 1999). Non-statutory systems forsediment quality guidelines have, nevertheless, beendeveloped in a number of countries, including Canadaand Sweden.
Studies of the input of nutrients and other materialfrom the Naivasha catchment to lake have been un-dertaken by Gaudet & Melack (1981), Harper et al.(1993), Harper et al. (1995), Everard et al. (2002), andKitaka et al. (2002). None have yet investigated lakeand river sediments. The aims of the study were: (i) toinvestigate the geochemical and physical characterist-ics of river and lake sediments, (ii) to investigate thesediment stratigraphy of the lake, and (iii) to assess theimplications of (i) and (ii) in informing of how to bestmanage Lake Naivasha and its catchment.
Study site description
Water level and vegetation changes
The water level and aquatic vegetation of Lake Na-ivasha changes continuously (Becht & Harper, 2002),and such fluctuations will have significant effects uponsediment characteristics. The lake was first describedin the writing of European explorers in the 1880s butits earliest record is that according to local tradition,there was a period immediately before the arrival ofthe first Europeans when the lake was almost com-pletely dry (Ase et al., 1986). Towards the end of the19th century, writings by European travellers indicatethat the lake’s level was continuously rising until 1895,when a maximum lake level of 1896 m above sea levelwas reached. This historic high water level is some 4–5m higher than any level reached since that time. Levelsthen declined around the turn of the century, so thatwhen continuous recordings commenced in 1908 thelevel was 1890 m (Ase, 1987). Thereafter, the recordsshow a progressive decline in lake levels during the1920s and 1930s, culminating in a historic (recorded)low of 1882 m in 1946. The lake level remained lowuntil about 1956, whereafter it rose to an intermediatelevel of 1887 m in 1980 (Ase et al., 1986). Lake leveldeclined progressively during the following decade,
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rising rapidly by 1 m in May 1988, declining for a dec-ade then rising by 3 m in 1997. Both these latter eventswere following heavy rains associated with the ENSOclimatic phenomenon (see Becht & Harper (2002) fordetails of lake level changes).
The vegetation of Lake Naivasha is, in turn, influ-enced by changing lake levels. Beadle (1932) providedthe first scientific description of the lake’s vegetation.At that time, during a period of declining lake levels,papyrus (Cyperus papyrus) formed an extensive float-ing swamp in the whole northern part of the lake,north of an east–west horizontal line along the north-ern edge of Crescent Island (known colloquially as theNorth Swamp). Elsewhere varying widths of it fringedthe lake and formed a multitude of small, mobile is-lands. Various submerged macrophytes and the bluewater lily (Nymphaea nouchalii var. caerulea) grew inshallow lagoons inside these papyrus islands. To thelakeward side, as water deepened, the floating-leavedfollowed by dense beds of submerged macrophytesformed clear zonation. This pattern was consistentthroughout the water level changes up to the mid-1970s. Thereafter, it underwent dramatic and continu-ous changes. The driving forces behind these changeshave been various anthropogenic influences, includingthe introduction of alien species of animals and plants(Harper, 1984; Tarras-Wahlberg, 1986; Gouder et al.,1998), and the clearance of lakeside lands for intensiveagriculture, (Harper et al., 1990).
Lake bathymetry
Ase et al. (1986) provide a bathymetric map of thelake referring to a lake-surface of 1889 masl (repro-duced as Fig. 1). The map shows that the lake maybe divided into three semi-separate basins: the mainlake basin, the Crescent island crater in the east andLake Oloidien in the south. The main lake is charac-terized by a smooth flat relief with a maximum depthjust off Hippo Point in the southwest. The flat bottomtopography contrasts sharply with the hilly topographyof the surrounding land, indicating that the lake basinis filled with large amounts of sediments (Ase et al.,1986). A shallower area in the northern end of thelake is interpreted as a deltaic sediment accumulationderiving from the inflows of the Gilgil, Karati andMalewa rivers. At the time of the survey by Ase et al.(1986), the deltaic feature coincided closely with thepapyrus-dominated North Swamp. A feedback pro-cess is, thus, indicated in which the shallow delta areaprovides a preferred habitat for papyrus whilst at the
same time the papyrus swamp functions as a sedimenttrap and protects the delta from wave erosion and, inturn, promotes delta development.
Lake sedimentology
Verschuren (1996) conducted stratigraphic and geo-chronological analyses of three sediment cores takenfrom the main lake. He noted that sedimentationpatterns in the lake vary due to the effects of wind-induced re-suspension and re-deposition of sediments,and the presence of river point sources of sediment inthe north. As a result, two of the cores sampled did notrepresent longer periods of undisturbed sedimentation.In the central part of the lake, however, he sampledwhat was interpreted as a complete and uninterrup-ted sequence of sediments deposited during the 20thcentury, non-conformably overlying sediments datingback as far as the 16th–17th centuries. In this core, heidentified four main units: Unit 0, Unit I, Unit II andUnit III.
• Unit 0, found at the base of the core at 114–105 cm depths, was a peat horizon with abundantfragments of Cyperus papyrus and Ceratophyllum.Verschuren interpreted it as representing a periodwhen a low lake level transformed the lake intoa shallow, fragmented wetland overgrown by apapyrus swamp. A radiocarbon age of 290 ± 50years of sediments directly overlying this horizonindicates that Unit 0 was laid down, not during themid 19th century lowstand but during an earlierevent in the 16th–17th century.
• The clayey muds of Unit I, between 105 and82 cm, was interpreted as being composed of sed-iments laid down during low to intermediate waterlevels between the 16th–17th century and the endof the 19th century.
• Unit II sediments, between 82 and 44 cm, wereinterpreted as being laid down during high lakelevels in the late 19th and early 20th century. At48–58 cm there was a black layer with a significantadmixture of coarse partially decomposed papyrusmaterial. The unit’s black color was imparted bythe decomposed papyrus fragments, and the unitwas interpreted as representing sedimentation inshallow areas near papyrus stands, which grew onexposed mudflats, created as lake levels decreasedin the 1920s and 1930s.
• Lake levels declined in the 1920s, 1930s and 1940sand reached a lowstand of 1881.5 masl in 1946.
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Figure 1. Bathymetric map of Lake Naivasha. Water depth contours are drawn at 2-m intervals. The lake contour represents a water level of
approximately 1887 m above sea level, (adapted from Ase et al., 1986).
The transition to Unit III sediments at 44 cm wereinterpreted as marking this return to lower lakelevels. Near the transition of Unit II and III, Ver-schuren noted the presence of a thin, light felthorizon, dated at 1941 ±7 years, composed ofnumerous needle shaped diatoms. The mechanismof formation is unclear, though Verschuren spec-ulates that its origin could be either a result ofmass sedimentation of diatoms triggered by unfa-vorable conditions during low lake levels, or thesedimentary redistribution and associated aggreg-ation of previously deposited diatoms. Above thislevel, Unit III consisted of a fairly homogenous or-ganic, clayey mud indicating that the depositionalenvironment has remained similar over the past40 years. The sedimentation rate in the main lakeduring this time was estimated at about 1 cm peryear.
Methods
Sampling was undertaken during the dry season inJuly/August 1997 and during October 2000.
Sediment samples from rivers were collected fromsixteen river sites during the dry season, between 19thJuly and 7th August 1997. Sample sites were selectedin low (first to third) order reaches to represent theoverall characteristics of river sediments in the entirelake catchment (Fig. 2). During this time, the Kar-ati river contained water in isolated pools only. Thesamples were taken with an Ekman grab from act-ive, oxygenated, mid-river sediments at sites wherethe channel is straight and the flow fairly rapid (withthe exception of the Karati samples, which were takenfrom standing pools of water and sample S10, takenfrom the Turasha dam in the Malewa catchment). Ateach site, about 1 kg of sediment was collected intwo to three grabs and these were subsequently mixed.A subsample of approximately 150 g (wet weight)
31
was retrieved from the master sample. This subsamplewas subsequently wet sieved through a 2-mm mesh(discarding the +2 mm fraction), and air dried atapproximately 40 ◦C.
The sample were then divided in two, and one sub-sample was investigated under the microscope (×10magnification) in order to determine grain size, angu-larity and sorting in accordance with tables providedin Pettijohn (1975). The other sub-sample was sentfor analysis of bulk and trace geochemistry by X–rayFluorescence spectrometry (XRF) at the Anglo Amer-ican research Laboratories in Johannesburg, SouthAfrica.
Care was taken to avoid cross sample contamina-tion and all equipment used were thoroughly cleaned,first in river water and then in de-ionised water. Theaccuracy and repeatability of the sampling procedureand the XRF analysis was checked by sampling andanalysing the sediment from one site in duplicate.
Sampling of lake sediment was performed by usinga sediment corer in 1997, and by using an Ekman grabin 2000.
Eighteen lake sites were sampled during 5–18 Au-gust 1997. Sediment cores were collected in 110-cmlong Perspex tubes, attached to a single-drive pistoncorer, which was deployed from a small boat. Thelocation of sample sites was fixed by using a hand-held Garmin GPS 12, Global Position System. Theexact sampling locations are shown in Figure 3. Thecores were extruded and described with regards totheir physical and biological character and appear-ance. Each stratigraphic unit identified was describedin detail.
Two cores were selected for geochemical analysis,one from the central lake (no. 12) and one in the North-ern swamp (no. 9), close to the outflow of the Malewariver. One sample of 50–100 g (wet weight) was col-lected from each of the stratigraphic units identifiedin the two cores, making a total of 10 samples. Carewas taken to take these samples from the centre of thecore, so as to avoid cross-sample contamination. Thesamples were kept cool in a refrigerator before beingsent for geochemical analysis at the Anglo-Americanresearch laboratories in Johannesburg, South Africa.The analysis was performed by plasma emission spec-troscopy (ICP-AES) following digestion in aqua regia.The accuracy and repeatability of the sampling andanalysis was checked by sampling and analysing onestratigraphic unit twice. Seven lake sediment sampleswere retrieved during the period 20–24 October 2000.At each site, about one kilo of sediment was collec-
ted in three grabs, which were subsequently mixed. Asubsample of approximately 150 grams (wet weight)was retrieved from the master sample. This subsamplewas air dried at approximately 40 ◦C, and thereafterkept cool in a refrigerator before being sent for ana-lysis of Loss on Ignition (LOI) and metal content atthe Leeds University School of Geography’s laborat-ories in the UK. The metal analysis was performedby plasma emission spectroscopy (ICP-AES) follow-ing digestion in aqua regia. Again, the accuracy andrepeatability of the sampling and analytical procedurewas checked by sampling and analysing one sampletwice.
The evaluation of sediment analysis was carriedout by comparison of the observed major and minorelement sediment concentrations with global aver-ages for shale (Turekian & Wedepohl, 1961; Mason& More, 1982), and Environment Canada’s provi-sional sediment quality guidelines for the protectionof aquatic life (Environment Canada, 1995). The lat-ter assume 100% bioavailablity for sediment-boundmetals which, in turn, make them possibly over pro-tective (Williams et al., 2000). Further, caution mustattend comparisons between river and lake sedimentsas the analytical procedures differ.
Results
River sediment description
Site descriptions are provided in Table 1, together witha sediment description and a linkage to RHS sitesexamined by Everard et al. (2002). Sediment char-acteristics differ markedly between the three riverssampled, reflecting the hydrological, topographic, andgeological differences in their catchments.
Sediment in the Karati is generally immature (i.e.,poorly sorted and angular in nature), reflecting the factthat the river’s gradient beneath the Kinangop Plateauis steep and that it flows only during the rainy season,providing little time for sorting and abrasion. As aconsequence of the episodic nature of the river’s flow,Karati sediment contain both fine particles (clay andsilt) and coarse material (gravel and coarse sand).
Sediment in the Gilgil system is different from thatof the Karati. Everard et al. (2002) demonstrate thatthe topography of the Gilgil system is more gentlethan the Karati, with few rapids. The flow of waterin the Gilgil system are also perennial. Consequently,the sediments found at Gilgil sampling sites are mod-
32
Tabl
e1.
Des
crip
tions
ofth
esa
mpl
ing
loca
tions
,the
phys
ical
char
acte
rist
ics
ofth
esa
mpl
edse
dim
enta
ndth
ean
alys
ispe
rfor
med
Sam
ple
Sam
plin
gL
ocat
ion
desc
ript
ion
and
land
use
Riv
erm
orph
olog
ySa
mpl
ede
scri
ptio
nX
RF
(RH
Sda
tean
alys
is
num
ber∗
)
s119
Jul9
7R
iver
pool
near
brid
geov
erth
eK
arat
i.N
ear
smal
lN
oco
ntin
uous
wat
erflo
w.R
elat
ivel
yst
eep
grad
ient
Sand
y,cl
ayey
silt
(957
6)vi
llage
(10
huts
).M
ixof
natu
ralv
eget
atio
nan
dsm
all
with
grav
elly
rive
rbed
.Cha
nnel
wid
thof
2–4
man
dsu
ba–
subr
scal
eag
ricu
lture
(mai
ze).
Pool
used
for
was
hing
dept
hof
0.3
m(i
npo
ol)
m-p
sort
ed
s219
Jul9
7Po
olat
uppe
rpa
rtof
ade
epgo
rge
near
brid
geov
erth
eN
oco
ntin
uous
wat
erflo
w,S
teep
grad
ient
with
Sand
y,cl
ayey
silt
(no
RH
S)K
arat
i.N
ear
smal
lvill
age
(30
huts
).Pa
stur
esan
dsm
all
grav
elly
rive
rbed
.Cha
nnel
wid
thof
2–3
m,a
ndde
pth
subs
–su
br
scal
efa
rmin
g(b
eans
).of
0.8
m(i
npo
ol)
m-p
sort
ed
s319
Jul9
7A
tbri
dge
over
the
uppe
rK
arat
ion
the
Nja
beni
–so
uth
No
cont
inuo
usw
ater
flow
.Ste
epgr
adie
ntw
ithG
rave
lly,c
laye
ysa
ndY
(957
5)K
inan
gop
road
.Nea
rtw
ofa
rmho
uses
.Pas
ture
s,sm
all-
grav
elly
rive
rbed
.Cha
nnel
wid
thof
2m
and
dept
hof
a
scal
efa
rmin
gof
0.4
m(i
npo
ol)
m-p
sort
ed
s419
Jul9
7U
pstr
eam
ofbr
idge
over
Kar
atio
nth
eN
aiva
sha–
Nak
uru
No
cont
inuo
usw
ater
flow
silty
sand
Y
(No
RH
S†)
high
way
.Pas
ture
san
dex
tens
ive
wat
erin
gof
cattl
eG
entle
grad
ient
and
smoo
thri
verb
ed.C
hann
elw
idth
subr
–su
ba
of3
m,a
ndde
pth
of0.
15m
(in
pool
)m
sort
ed
s520
Jul9
7D
owns
trea
mof
brid
geov
erth
eG
ilgil
onth
eno
rthe
rnSm
allr
apid
atbr
idge
,oth
erw
ise
gent
legr
adie
ntan
dsi
ltyfin
esa
nd
(957
9)L
ake
road
.Den
seac
acia
vege
tatio
n,20
–100
mw
ide,
smoo
thri
verb
ed.C
hann
elw
idth
of5
m,a
ndde
pth
ofsu
ba–
subr
past
ures
.0.
5m
w-m
sort
ed
s620
Jul9
7D
owns
trea
mof
brid
geov
erth
eM
alew
aon
the
Gen
tlegr
adie
nt.S
low
flow
ing
wat
er.C
hann
elw
idth
slig
htly
silty
sand
(No
RH
S†)
Nai
vash
a-N
akur
uhi
ghw
ay.N
ear
Nai
vash
ato
wn,
of10
–12
m,a
ndde
pth
of0.
5m
subr
–r
exte
nsiv
ese
ttlem
ents
.Was
hing
and
bath
ing
wso
rted
dow
nstr
eam
s720
Jul9
7A
tbri
dge
over
Litt
leG
ilgil
atth
ePe
mbr
oke
scho
ol.
Smoo
than
dge
ntle
grad
ient
.Gra
ssed
bank
s.C
hann
elfin
esa
ndY
(957
1)E
upho
rbia
and
euca
lypt
usw
oodl
and.
Aca
cia
woo
dlan
dw
idth
of1
m,a
ndde
pth
of0.
1m
subr
dow
nstr
eam
.Sm
allv
illag
ene
arby
(50
hous
es).
mso
rted
s821
Jul9
7U
pstr
eam
ofbr
idge
over
Mal
ewa.
Den
seac
acia
Mai
nly
smoo
thw
ithge
ntle
grad
ient
.Sm
allr
apid
fine
sand
Y
(957
4)w
oodl
and.
Wat
erin
gpl
ace
for
cattl
e.be
low
sam
plin
gar
ea.C
hann
elw
idth
of20
m,a
ndsu
ba–
subr
dept
hof
0.5
mm
sort
ed
33
Tabl
e1.
Con
tinue
d
Sam
ple
Sam
plin
gL
ocat
ion
desc
ript
ion
and
land
use
Riv
erm
orph
olog
ySa
mpl
ede
scri
ptio
nX
RF
(RH
Sda
tean
alys
isnu
mbe
r∗)
s921
Jul9
7A
tpum
psat
ion
just
upst
ream
ofM
alew
a’s
confl
uenc
eFa
irly
smoo
thgr
adie
ntbu
twith
inte
rmitt
ent
smal
lFi
nesa
nd(9
573)
with
the
Tur
asha
.Sm
alls
cale
farm
ing
(mai
ze)
and
rapi
ds.C
hann
elw
idth
of16
m,a
ndde
pth
of0.
4m
suba
–su
brac
acia
woo
dlan
dm
sort
ed
s10
24Ju
l97
Ats
mal
lbri
dge
over
the
Tur
asha
dam
rese
rvoi
r.Sm
all
Res
ervo
irda
mw
ithsu
bmer
ged
mac
roph
ytes
and
Sand
ysi
ltY
(957
2)sc
ale
farm
ing
(mai
ze)
and
acac
iaw
oodl
and
float
ing
vege
tatio
nsu
br–
suba
wso
rted
s11
24Ju
l97
Atb
ridg
eov
erG
ilgil,
3km
east
ofG
ilgil
tow
nshi
p,Sm
ooth
and
gent
legr
adie
nt.G
rass
edba
nks.
Cha
nnel
Sand
ysi
ltY
(957
0)Pa
stur
esw
idth
of2–
4m
,and
dept
hof
0.4
msu
br–
suba
w-m
sort
ed
s12
24Ju
l97
Ats
mal
lbri
dge
dow
nstr
eam
ofbi
gger
brid
geov
erth
eSm
ooth
and
gent
legr
adie
nt.G
rass
edba
nks.
Cha
nnel
fine
sand
(956
8)M
alew
aon
the
OlK
alou
road
,eas
tof
OlK
alou
,w
idth
of5–
6m
,and
dept
hof
0.7
msu
ba–
subr
Past
ures
mso
rted
s14
25Ju
l97
On
the
Kar
atia
tthe
Del
amer
efa
rm,n
extt
ofe
nce
byN
oco
ntin
uous
wat
erflo
w.G
entle
grad
ient
with
silty
clay
Y(9
582)
the
nort
hern
swam
p.Pa
stur
esan
dne
arB
uffa
lo/c
attle
gras
sed
bank
s.C
hann
elw
idth
of1–
2m
,and
dept
hof
rw
ater
ing
plac
eof
0.1
m(i
npo
ol).
wso
rted
s15
25Ju
l97
Gilg
ilri
ver
onth
efa
rmof
Mr
A.S
imps
on.F
arm
land
Smoo
than
dge
ntle
grad
ient
.Cha
nnel
wid
thof
1–2
m,
clay
ey,s
andy
silt
Y(N
oR
HS)
<an
dpa
stur
es,n
ear
cattl
ew
ater
ing
loca
tion
and
dept
hof
0.1
m(i
npo
ol).
s–
mr
mso
rted
m1
7A
ug97
Eas
tern
frin
geof
Gilg
ilpa
pyru
ssw
amp,
onM
erul
aR
iver
frin
ged
bysh
allo
wpo
ols
ofw
ater
with
silty
clay
Y(9
577)
farm
,sw
amp/
wet
land
vege
tatio
nfo
r>
50m
each
side
abun
dant
papy
rus
and
salv
inia
.Cha
nnel
wid
thof
5m,
r–
sran
dde
pth
of1
mw
sort
ed
m3
7A
ug97
Sout
hern
part
ofM
erul
afa
rm,a
tcat
tlew
ater
ing
plac
eG
entle
grad
ient
,ste
epgr
asse
dba
nds.
silty
sand
Y(9
569)
near
smal
lfar
mvi
llage
(∼10
hous
es)
Cha
nnel
wid
thof
15m
,and
dept
hof
1.5
msu
brm
-wso
rted
Not
es:∗
RH
Ssi
tenu
mbe
rsan
dba
ckgr
ound
data
are
refe
rred
toin
Eve
rard
etal
.(20
02).
†N
eith
erof
thes
esi
tes
are
anR
HS
site
,but
both
are
clos
eto
Mal
ewa
RH
Ssi
te95
81(K
AR
Ifa
rm).
<T
his
isno
tan
RH
Ssi
te,b
utis
upst
ream
ofR
HS
site
9579
and
form
edth
eup
per
limit
ofbi
rdsu
rvey
stre
tch
inE
vera
rdet
al.(
2002
).Sa
mpl
ede
scri
ptio
n:a
–an
gula
r;r–
roun
ded;
w–
wel
l;m
–m
oder
atel
y;p
–po
orly
Not
e:X
RF–
X-r
ayflu
ores
cenc
e–
maj
oran
dtr
ace
geoc
hem
istr
y.
34
Figure 2. Sample site locations in the river sediment survey.
erately to well sorted, and the individual grains aresub-rounded to rounded. Since the flow of the Gilgilis relatively slower and more gentle, the sediment isgenerally fine (fine silt or clayey silt).
Sediments in the Malewa system represent an in-termediary stage between the poorly sorted and an-gular sediment of the Karati and the fine, well-sortedsediment of the Gilgil. This follows from the upperreaches of the Malewa being steep and similar to thoseof the Karati (although these upland reaches were notsampled in this study), whereas the lower reaches aresimilar to the Gilgil. Compared to the Karati and theGilgil, the Malewa is, furthermore, a more forcefulriver. Consequently, the sediment in the lower reachesis, though generally well-sorted, coarser-grained thanthose of the Gilgil.
Lake sediment description
The lake stratigraphy is conveniently divided into sixsub-areas: central lake, delta front, northeast, northw-est, southeast, and southwest. These divisions are notabsolute, as differences between areas are gradational.The division reflects differences in water depth andlong term sediment dynamics, and serves to make bothanalysis and presentation of results more effective.
The Central Lake sub-area occurs in the centre ofLake Naivasha with depths of 5–6 m. The generalstratigraphy of the four cores corresponded to that sug-gested by Verschuren (1996), and is summarised inTable 2 . A thickness of 10–20 cm of sediments in thecentral lake sub-area, interpreted as being laid downduring the past 10–15 years, gives a current sediment-
35
Table 2. Idealised stratigraphic section of the central LakeNaivasha basin
Depth of Type of sediment and interpretationinterface
0 cm Brown, organic mudTrace macrophytes and papyrus, and trace diat-oms.Interpreted as sediment laid down during the last10–15 years during which time papyrus raftshave been more or less completely absent fromthis part of the lake.Distinct contact
10–20 cm Light brown, organic mudAbundant macrophytes and papyrus and tracediatoms.Interpreted as sediments laid down during pro-gressive increasing water levels from about 1950to 1980. A few cm thick layer of yellowish browndiatomaceous felt may in places be present at thebase of this unit. Together with the unit abovethey seem to correspond to Unit III of Verschuren(1996).Gradational contact
40–50 cm Black rooty and diatomaceous mudAbundant macrophytes, papyrus and abundantdiatoms.Interpreted as sediments laid down during a pro-gressive lowering of water levels in the early 20th
century when papyrus stands were continuouslygerminating on exposed mudflats. Probably cor-responding to Unit II of Verschuren (1996).Distinct non-conformity
70–75 cm Dark brown peaty mudAbundant macrophytes, papyrus and diatoms.Interpreted as sediment laid down during lowlake level stand of the mid 19th century. Probablytruncated by a period when the lake may havebeen completely dry. Probably corresponding toUnit I of Verschuren (1996).Distinct non-conformity
80–90 cm Black PeatAbundant papyrus and trace macrophytes.Hard and massive peat laid down when the wholelake must have been a shallow, fragmented wet-land overgrown with papyrus. Probably corres-ponding to Unit 0 of Verschuren (1996) and thuslaid down during 16th to 17th century.
ation rate of about 1 cm per year. The upper 40–50 cmappears to have been laid down conformably. Belowthis level, two distinct non-conformities, caused byerosion during times when lake levels were low, were
identified. A distinct horizon of yellowish brown, di-atomaceous felt was identified at the base of the lightbrown, organic mud in cores 10 and 15 only. The factthat this unit was not noted in all cores indicates thatthe unit is not continuous over the whole central basin.An explanation for the unit’s formation must there-fore be sought in a process that was not basin-wide.A system of smaller sub-basins, each with its specificsalinity and water quality conditions, situated withina larger wetland, may have created conditions for themass mortality and subsequent sedimentation of diat-oms in some basins but not others. A similar situation,albeit at a larger scale, occurs in the present day wherewater quality, and hence diatom assemblages, varysomewhat between the three sub-basins of the mainlake, that is the Main Lake, Oloidien Bay and CrescentIsland crater lake (Verschuren, 1996).
The Delta front sub-area occurs along the lakewardfringe of the North Swamp, at depths up to about 1 m.The deltafront sub-area is dynamic where sediment-ation patterns are sensitive to changes in water level,changes in location of river inflows and the extentof swamp vegetation. Consequently, sediments in thearea are an heterogeneous mix of massive brown clays,sandy mud, organic mud and peat horizons.
The Northeast sub-area, at depths of about 1–3m, contains sandy muds derived from the river inlets.These sediments are continuously being reworked andre-suspended by wave action. Below these active sedi-ments are stiff grey clays, interpreted as having formedfrom the previous aerial exposure and mineralizationof lacustrine sediments, and brown clays which areinterpreted as representing deltaic mudflat deposits.
The Northwest sub-area, depths of 2–3 m, containssediments which appears to correspond to the upperthree units of the central lake stratigraphy with twobrown organic mud units overlying a black rooted mudunit.
Core 11 was situated only about 3 km west of theMalewa outflow. Nevertheless, there was no evidenceof material directly derived from the river in this core.It is therefore indicated that river sediment input is,subsequent to being introduced to the lake, transportedin an easterly direction only.
The Southeast sub-area, together with one coretaken about 4 km further north, 2.5 km west of Cres-cent Island (core 5), but included in this group becauseof its stratigraphic similarity, appear to correspond tothe upper four units of the central lake stratigraphywith brown organic mud units above a black or darkbrown rooted mud unit over a dark brown peaty mud.
36
Figure 3. Sample site locations in the two lake sediment surveys. Sites in the 2000 survey are identified by the prescript L.
In the Southwest Bay sub-area, at depths of 3–4 m,a far greater rate of deposition was evident. The strati-graphies was somewhat different from the central lakestratigraphy. The two cores consisted of organic mudunits, interpreted as corresponding to the upper twounits of the central lake stratigraphy. However, thoughboth cores were nearly 100 cm in length, neither in-tersected the organic rich, black mud units laid downin other parts of the lake in early to middle part ofthis century. Consequently, the sedimentation rate inthis relatively sheltered part of the lake appears to bemore rapid than in the central part of the lake. A 10-cmthick brown organic mud unit with abundant Salviniaand papyrus fragments at a depth of about 40 cm isinterpreted as a unit laid from the 10–15 years beforethe 1987 low level. At this time, both papyrus raftsand Salvinia mats were common in this part of thelake (Tarras-Wahlberg, 1986). Such an interpretationleads to a sedimentation rate of about 3 cm per year inthis part of the lake, which is considerably more thanestimated sedimentation rates in other parts of the lake.
Present-day sediment dynamics
Comparisons of the sediment composition at thesediment-water interface of the 18 cores enables ananalysis of present day sedimentation patterns.
Near the outflow of the Malewa the surface sed-iments are either massive brown clay or sandy mudunits. The clay is interpreted as a deltaic deposit (amud flat) which is now actively being eroded by waveaction, whereas the sandy mud is material directlyderived from the Malewa and Gilgil Rivers. Con-sequently, the sedimentary regime of the pro-delta areais dominated by erosion and sedimentary transport,rather than sedimentation. Presumably, sedimentationand associated deltaic growth occurs in the reducedareas now protected by swamp vegetation.
The surface sediments in the bay in the north-eastern part of the lake and areas directly west andnorth west of Crescent Island are, like the pro-deltaarea, dominated by sandy muds and clay units. Con-sequently, the sedimentary regime here is also dom-inated by transport and erosion. However, the stiffclays here are not only brown clays (interpreted asdeltaic deposits) but also stiff clays that are grey ordark grey in colour. These stiff clays are interpreted
37
as having formed from lacustrine sediments that wereexposed to air, such that they became mineralised andoxidised. This may be a consequence of historic lowlake levels, and is consistent with the inferences aboutdelta-forming processes based on RHS and other geo-morphological observations by Everard et al. (2002).Organic muds dominate the surface sediments col-lected elsewhere across the lake in this study and,hence, these areas represent depositional sedimentaryenvironments.
Geochemistry
Tables 3 and 4 show the results of the geochemicalanalyses. Lake and river sediments are geochemicallysimilar, and the contents of major and minor elementsare mostly comparable to global averages (Table 3).The only noticeable differences between river and lakesediments are that river sediment contain somewhatmore P2O5 and Pb, somewhat less Cu and Ni, andsignificantly less SO3, than lake sediment. These dif-ferences are, with the exception of SO3, less than oneorder of magnitude and may arise from the differentanalytical methods used. The higher content of SO3in lake sediment compared to river sediment is, how-ever, to be expected due to the tendency of animalsand plants, more common in the lake compared to therivers, to concentrate sulphur. This hypothesis is cor-roborated by the fact that the only two lake sedimentsamples to have low SO3 concentrations are 12 A and12 D, both of which represent inorganic clay units.
River sample S10, taken from the Turasha dam inthe Malewa catchment, differs from the other samplesin that it contains more organic and volatile materi-als (as indicated by the low total percentage). Thisreflects, the low energy environment of the dam wherefine organic material can settle out to a greater extentthan in-stream.
All sediment samples contain elevated levels ofFe2O3, and the concentrations of 6–13% are consider-ably higher than the 4% expected in an average shale.The high iron content could either be caused by thebed rock (i.e., the volcanic rocks of the area) beingnaturally enriched in iron, or caused by the erosion ofiron rich lateritic soils, which are common throughoutthe area.
The content of minor and potentially toxic metalsand metalloids are, in comparison to global averages,mostly unremarkable. Exceptions are elevated con-centrations of Zn in all samples, Ni in lake sediment
and in the Turasha dam sediment, and Cd in two lakesediment samples.
Zinc concentrations are similar in all samples,including samples taken from remote parts of the cath-ment (e.g., S12 and S3), which strongly indicatesthat relatively high concentrations are natural in ori-gin. Zinc concentrations vary from 88 to 190 mgkg−1, which can be compared to the ‘threshold effectlevel’ of the sediment quality criteria, set at 123 mgkg−1. The latter assumes however, 100% bioavailabil-ity, which is unlikely to be the case, so the Zn levelsare not likely to be of concern.
Nickel levels in sediment are elevated in com-parison to the sediment quality guidelines, althoughconsiderably lower than the global average shale. Fur-ther, the Ni concentrations are considered as only‘somewhat elevated levels’, as defined by the SwedishEnvironmental Protetion Agency (SEPA, 1998). Thus,it may be the case that the Canadian criteria are overlyconservative. Nickel is, nevertheless, enriched in ba-sic rocks such as the basalts of the Nyandarua range,which may explain the levels encountered. Further,Ni concentrations are somewhat higher in lake sedi-ment and in the Turasha dam sediment compared toriver sediments, indicating that this metal, for reas-ons unknown, is preferentially enriched in still watersediment compared to active river sediment. Pos-sible anthropogenic sources of Ni include batteries,electroplating and paints. None of these sources ap-pear overly likely in the Lake Naivasha area, whichharbours little or no such industry.
The lake sediment samples 9C and 12B contain7 and 6 mg kg−1 Cd, respectively, which is signi-ficantly above global average levels and also abovethe ‘probable effect level’ of the Canadian sedimentquality criteria. There are a number of explanations forthese seemingly high concentrations. Firstly, the con-centrations are fairly close to the low limit of detectionof the analytical method used (2 mg kg−1) and there-fore analytical error cannot be ruled out. Secondly,cadmium is a common contaminant of rock phos-phorus, the major ingredient of chemical fertilisers(Driver et al., 1999). Excessive inputs of Cd into thelake may therefore result from the use and release ofchemical fertilisers. Both units represent sediments atdepth below the sediment–water interface (39–49 and8–44 cm, respectively). The high Cd concentrationsmay thus be a result of Cd being mobilised from theupper oxidising sediments and re-precipitating out atdepths where conditions are reducing. Thus a horizonenriched in Cd could be generated from the concentra-
38
Tabl
e3.
Con
cent
ratio
nof
sele
cted
maj
oran
dm
inor
elem
ents
inri
ver
sedi
men
tsam
ples
from
Lak
eN
aiva
sha’
sca
tchm
ent.
Ana
lyse
sw
ere
perf
orm
edby
X-R
ayFl
uore
scen
ce(X
RF)
.Low
erto
tals
are
caus
edby
orga
nic
mat
ter
and
othe
rvo
latil
em
ater
ial
Sam
ple
Tota
lSi
O2
Al 2
O3
Fe2O
3K
2O
Na 2
OM
gOP 2
O5
SO3
CaO
MnO
As
Co
Cr
Cu
Ni
PbZ
n
%%
%%
%%
%%
%%
%m
g/kg
mg/
kgm
g/kg
mg/
kgm
g/kg
mg/
kgm
g/kg
S392
5712
133.
22.
20.
530.
310.
040.
910.
22<
10<
6<
100
22
1415
0
S491
5713
114.
22.
70.
730.
290.
041.
20.
41<
10<
6<
100
37
1715
0
S791
5713
123.
62.
00.
810.
200.
031.
70.
38<
10<
6<
100
410
1915
0
S890
5514
9.9
3.6
2.3
0.85
0.26
0.05
1.8
0.23
<10
8<
100
811
1614
0
S10
8349
1411
2.0
1.1
0.84
0.54
0.08
1.6
0.31
<10
14<
100
2220
1217
0
S11
9058
1310
3.1
1.7
0.72
0.41
0.03
1.0
0.38
<10
<6
<10
05
1116
150
S14
9361
149.
32.
61.
41.
00.
310.
031.
60.
23<
10<
6<
100
2016
1217
0
S15
9061
138.
03.
72.
20.
660.
240.
041.
00.
20<
10<
6<
100
66
1414
0
M1
8958
159.
42.
00.
900.
880.
490.
040.
990.
12<
10<
6<
100
1915
1119
0
M1-
repe
at87
5615
9.3
2.0
0.90
0.86
0.28
0.04
1.0
0.14
<10
8<
100
1714
1119
0
M3
9155
1410
3.6
2.4
0.89
0.37
0.03
2.1
0.17
<10
11<
100
710
1313
0
Ave
rage
shal
e158
.115
.44.
023.
241.
302.
440.
170.
643.
10.
10–
1990
4568
2095
Thr
esho
ldef
fect
leve
l:5.
9–
3736
1835
123
Sedi
men
tqua
lity
crite
ria
for
prot
ectio
nof
aqua
ticlif
e2Pr
obab
leef
fect
leve
l:17
–90
197
3691
315
1M
ajor
elem
ents
take
nfr
omM
ason
&M
ore
(198
2)an
dm
inor
elem
ents
from
Tur
ekia
n&
Wed
epoh
l(19
61).
2E
nvir
onm
entC
anad
a(1
996)
39
Tabl
e4.
Con
cent
ratio
nof
sele
cted
maj
oran
dm
inor
elem
ents
inla
kese
dim
ent
sam
ples
from
Lak
eN
aiva
sha
and
itsca
tchm
ent.
Ana
lyse
sof
lake
sam
ples
wer
epe
rfor
med
byIC
P-A
ES
afte
rdi
gest
ion
inaq
uare
gia
LO
IA
l 2O
3Fe
2O
3M
gOP 2
O5
SO3
CaO
MnO
As
Cd
Co
Cr
Cu
Ni
PbZ
n
%%
%%
%%
%%
mg/
kgm
g/kg
mg/
kgm
g/kg
mg/
kgm
g/kg
mg/
kgm
g/kg
Cor
esa
mpl
es(1
997)
9A(0
–11c
m)
–17
110.
940.
180.
630.
940.
212
<2
99
2328
<5
160
9B(1
1–39
cm)
–14
9.0
0.65
0.16
0.70
1.1
0.22
3<
28
1829
41<
513
0
9C(3
9–49
cm)
–13
7.3
0.56
0.16
0.58
2.0
0.15
57
76
918
<5
110
9D(4
9–73
cm)
–9.
99.
50.
480.
160.
681.
30.
28<
2<
211
3617
38<
588
9E(7
3–85
cm)
–12
7.9
0.55
0.16
0.60
0.95
0.15
2<
29
3013
35<
514
0
9F(8
5–10
1cm
)–
137.
90.
550.
090.
500.
970.
122
<2
622
1322
<5
150
12A
(0–8
cm)
–11
7.3
0.48
0.11
0.05
0.67
0.15
<2
<2
1015
1019
517
0
12A
-rep
eat
–11
7.5
0.50
0.09
0.05
0.67
0.15
<2
<2
1111
1018
519
0
12B
(8–4
4cm
)–
106.
00.
510.
090.
301.
10.
224
610
316
216
160
12C
(44–
60cm
)–
167.
20.
630.
070.
250.
830.
084
<2
107
1824
718
0
12D
(60–
73cm
)–
198.
60.
700.
070.
080.
600.
065
<2
1114
1722
719
0
Gra
bsa
mpl
es(2
000)
L-1
14.
8–
––
––
––
––
––
––
––
L-1
224
––
––
––
–0.
98<
0.17
––
1517
<0.
316
0
L-1
331
––
––
––
–0.
84<
0.17
––
1113
<0.
314
0
L-1
420
––
––
––
–0.
55<
0.17
––
7.4
6.4
3.7
100
L-1
4-re
peat
20–
––
––
––
0.52
<0.
17–
–7.
65.
73.
710
0
L-1
626
––
––
––
–0.
99<
0.17
––
1715
<0.
318
0
L-1
733
––
––
––
––
––
––
––
–
L-1
839
––
––
––
–0.
87<
0.17
––
1319
3.6
112
Ave
rage
shal
e115
.44.
023.
240.
170.
643.
110.
10–
0.3
1990
4568
2095
Thr
esho
ldef
fect
leve
l:5.
90.
6–
3736
1835
123
Sedi
men
tcri
teri
afo
rpr
otec
tion
ofaq
uatic
life2
Prob
able
effe
ctle
vel:
173.
5–
9019
736
9131
5
1M
ajor
elem
ents
take
nfr
omM
ason
&M
ore
(198
2)an
dm
inor
elem
ents
from
Tur
ekia
n&
Wed
epoh
l(19
61).
2E
nvir
onm
entC
anad
a(1
996)
.
40
tion of existing natural Cd or, alternatively, through thesimilar concentration of contaminant Cd. The analysisof surface sediments performed in 2000 does not in-dicate any significant supply of contaminant Cd fromfertilisers. This follows since Cd levels are equally lowat sites situated near the big flower farms on the east-ern shore of the lake (L-13 and L-14), as at sites nearthe much less developed western shore (L-16 and L-18). Hence, the relatively high Cd levels encounteredare unlikely to be caused by contamination, but ratherdue to geochemical redistribution of naturally existingCd. The content of other minor and potentially toxicmetals and metalloids is low.
Conclusions
The description and analyses of Lake Naivasha sedi-ment have provided extensive data on both present andpast sedimentation patterns. Importantly, the findingsshow that natural lake level changes have caused dra-matic changes to the lake ecosystem in the past, andthat the instability of the lake’s ecosystem is in partnatural.
At present, sediment dynamics in the lake are gov-erned by the presence of river point sources in thenorth, the deposition of silt and sand sized materialnear the river outlets; and wave-induced re-suspensionof fine and organic rich sediment in the north, whichare transported in easterly and southerly directions anddeposited in the lake’s central, southern and easternparts. Deposition of fine, organic rich sediment isalso occurring in northern areas that are protected bypapyrus swamp vegetation.
The sedimentation rate in the central lake is foundto be about 1 cm per year, which is in agreements witha previous estimate (Verschuren, 1996). However, thesedimentation rate in Elsamere Bay in the southwestappears to be higher, at up to 3 cm per year.
In the north-western part of the lake, historic lowsin lake levels has lead to the formation of a possiblycontinuous layer of stiff grey, mineralised clay. Thislayer will act as an effective water barrier and its exist-ence indicates that ground water recharge in this partof the lake, suggested by among others Ase (1987)may be impossible.
The geochemical analysis of a selected number ofsediment samples from the lake and its catchment in-dicate that the concentrations of Fe, Zn, Cd and Nisediment are elevated compared to global averagesand/or sediment quality criteria. The reasons for the
relatively high concentrations are likely to be found inthe sediment source materials (volcanic rocks or later-ite soils), which are naturally rich in these elements.The concentrations of other potentially toxic elementsare low.
Lake Naivasha area is experiencing a period ofrapid development with ever-increasing use of landand irrigation water for flower farming and the ex-pansion of the Olkaria hydrothermal plants (Becht &Harper, 2002). In addition to these intensive uses,Everard et al. (2002) note an increasing intensity ofsubsistence agriculture in the catchment, includingwidespread poor land management practices. Con-sequently, it is not unlikely that the lake’s catchmentmay soon be severely affected by impacts related tothese developments as well as the pressures of a risingpopulation. In addition to these immediate pressures,the planned construction of a water supply reservoirfor the nearby town of Nakuru, that will export a sub-stantial proportion of the river Malewa’s water, willhave an immediate and large scale affect both on theMalewa River and the lake itself (Harper et al., 1995).
The present study indicates that the river and lakesediment of the Lake Naivasha catchment are stillrather pristine, and that anthropogenic impacts are notyet significant. The data obtained may therefore beused as reference values for assessing the possible en-vironmental impacts of future developments aroundthe lake.
Acknowledgements
This study formed past of the work of the Universityof Leicester research project at Lake Naivasha, whichsince 1987 has been authorised by the Office of thePresident, Government of Kenya under research per-mit to Dr D. M. Harper no. OP 13/001/12C 46. Theproject was funded by the Earthwatch Institute, Bo-ston, USA, and Oxford, England. The data collectionwould not have been possible without the assistance ofnumerous Earthwatch volunteers. Our sincere thanksgo to the numerous colleagues in Kenya in particularJill & Angus Simpson and Velia Carn.
References
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