Hydrobiologia 297 : 229-239, 1995 .

229© 1995 Kluwer Academic Publishers . Printed in Belgium .

Heavy metal contamination of Lake George (Uganda) and its wetlands

Patrick Denny', Roland Bailey 2 , Eldad Tukahirwa3 & Paul Mafabi 41 Department of Environmental Engineering, IHE, Delft, P.O. Box 3015, 2601 DA Delft, The Netherlands2 Division of Life Sciences, King's College, London, Campden Hill Road, London, W8 7AH, UK3 Makerere University Institute of Environment and Natural Resources, P O . Box 7062, Kampala, Uganda4 Ministry of Energy, Minerals, and Environment Protection, Uganda National Wetlands Conservation and Man-agement Programme, PO . Box, 9629, Kampala, Uganda

Received 16 December 1993 ; accepted 21 December 1993

Key words: Lake George, heavy metals, wetlands, Ramsar, copper cobalt

Abstract

Lake George is a shallow equatorial lake in Uganda which, around its fringe, has a wetland designated as a Ramsarsite of high international importance . A nearby copper mine has caused serious metal pollution of the surroundingland. Results show that heavy metals from mine waste, particularly copper and cobalt, are entering KahenderoSwamp, part of the Ramsar wetland, and contaminating the lake. In the lake, metals pass along the food chain . Thedistribution of metals in the wetland and possible modes of transport into the lake are discussed . The consequencesof the metal contamination are considered and recommendations for its clean-up, especially in the light of futuredevelopments, are made .

Introduction

Lake George, Uganda, lies within Queen ElizabethNational Park (Rwenzori National Park) on the equa-tor and is connected by the Kazinga Channel to LakeEdward (Fig . 1). It supports a wetland with a highbiological diversity value of international importancewhich, through the Ministry of Environment Protec-tion, was declared a Ramsar site in 1988 . Heavy metalpollution from a copper mining complex at Kilembein the foothills of the Rwenzori Mountains has affect-ed soils and vegetation within the boundaries of theNational Park and is detrimental to the wetland andaquatic ecosystems of Lake George . This paper focus-es attention on the transfer of toxic metals from theland, through the wetland into the food web of LakeGeorge, the provider of a sustained commercial fishery .

The wetland occupies some 180 km 2 around thefringe of Lake George. It is dominated by Cyperuspapyrus L. swamp with C. latifolius Poir and some Cla-dium mariscus var jamaicense Crantz . (Lock, 1973 ;Pomeroy, 1990) . The swamp is rooted into the sedi-ment at the landward edge but a thick rhizomatous mat

forms a floating raft over water on the lakeward side .The dynamics and functioning of this type of swampvegetation is discussed in Denny (1993) . A special fea-ture of the wetland is the most unusual floating forest ofFicus sycomorus L . at the northern inflow. It is inhab-ited by the relatively scarce sitatunga and rare birds,including two Red Data Book species, the shoebill andpapyrus yellow warbler .

Lake George is a shallow (mean depth, 2 .5 to 3 m ;Dunn et al., 1969), polymictic, eutrophic lake witha very high algal biomass. In terms of the range ofsalinities of African lakes, it lies around the middlewith sodium and bicarbonate as the dominant ions(Denny, 1985 ; Talling, 1992) . The high alkalinity andphotosynthetic activity of the algal biomass accountsfor some very high pH values . In the 1960s The lakewas the focus of an extensive International BiologicalProgramme (IBP) of research which detailed physico-chemical and biological characteristics (Viner, 1969 ;Ganf, 1974a, 1974b) .

At the western arm of the lake a narrow channelexpands into a shallow bay, Hamukunga (Hamukun-gu) Bay (Fig . 1). Irangara Island at the mouth of

230

r5 S.

r.` r

L.Edward

N

0

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40

km

0

1

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km

KASESEy

z

smelter' Chimney

4

x

HamukungaBay

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samebest

KahenderoSwampX 4, 31L VC

KAHENDERO

t1

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IrangaraIsland

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R .Rukoki

V

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XL. George

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IFig . 1 . A map of the study area to show the location of places mentioned in the text . Dashed line = seasonal river ; 1 = Station1 ; t = North-south transect across Hamukunga Bay ; x = Open water sample point in Lake George . The inset shows the studyarea (boxed) in relation to the Lake George, Kazinga Channel and Lake Edward system .

the channel effectively separates limnologically, themain body of Lake George from Hamukunga Bay .A papyrus swamp, the Kahendero Swamp, caps thenorthern shores of the Bay. This swamp receives themain impact of any pollution . The current survey hasfocused on the Kahendero swamp and the immediatearea of Hamukunga Bay .

The copper mines were opened in 1959 and func-tioned for about 20 years . In 1979 mining and mineprocessing ceased due to civil unrest and the break-down of infrastructure in Uganda . A legacy of minetailings and pollution remains . Full details of the min-ing processes can be found in an Environmental ImpactAssessment (EIA) carried out by Driver and Tukahirwain 1990 .

Pollution evolves from two main areas (i) frommines in the hills draining into the Kamulik-wezi/Nyamwamba River catchment and (ii) at the pro-cessing plant near Kasese where the ore was crushedand copper extracted. The tailings, which are richin cobalt sulphide, are stacked near to the process-ing plant. From these an erosion of fine sedimentsand leachates contaminates the National Park . Efflu-ent from the tailings passes down an open channel,through a culvert under the Fort Portal road, and thenspreads out in a delta-like formation into the Acaciascrub of the Park . Imperata grass grows along the edgeof the channel and around the delta deposition area . Themain area of deposition is bare. From here particulatesand leachates are washed in the rainy season towardsLake George, some eight kilometres away . Subsequentevaporation in the dry season produces metal-rich pans .In 1974, a baseline survey by Edroma drew attentionto the serious environmental consequences of copperpollution within the National Park . Subsequent surveys(Bugenyi, 1979; Denny, 1989 ; Driver & Tukahirwa,1990;) have reinforced the environmental concerns butuntil now no remedial action has been taken .

Methodology

Survey procedure

Between 1987 and 1992 visits were made to KahenderoSwamp, Lake George, the Kasese mine processingregion and the surrounding area (Fig . 1) .

The pan areas and wetland were approached onfoot from the Kamulikwezi Ranger's Post. Plant andsurface soil samples were collected and transported

in polythene bags . Hippo trails through the swampallowed access to waist wading depths .

Access to the lakeward edge of the KahenderoSwamp and to sample sites in Hamukunga Bay, and theopen water of Lake George, was usually from Kahen-dero fishing village by dug-out canoe . Samples fromthe Kazinga Channel and Lake Edward were collectedfrom a research boat provided by the Uganda Instituteof Ecology in Queen Elizabeth National Park . In 1990 atransect with 4 sampling points was established acrossHamukunga Bay. This was accomplished by paddlinga canoe uniformly for 5, 15 and 25 minute intervalsfrom station 1 southwards, on a compass point duenorth, fixed on the chimney of the copper processingplant.

Where possible, water characteristics were mea-sured on the spot with a HACH portable meter . Nor-mally, measurements included surface water tempera-ture, pH, conductivity and dissolved oxygen concen-trations . Water samples for metal analyses were filteredon the spot through a 25 mm diameter micro-filter unitwith 0.45µm millipore filter discs attached to a 50 mlGillette Scimiar disposable hypodermic syringe . Fil-trates were collected and stored in Sterilin tubes with-out acidification or preservatives .

Vegetation, soil and sediment samples

Plant samples, soils and lake sediments were sun-driedand packaged in polythene bags . On return to Londonsamples were placed in a vacuum oven at 90 °C untildry, ground with a pestle and mortar and stored inSterilin tubes .

Plankton samples

Plankton samples were collected by a combination ofhorizontal and vertical hauls with a 55µm mesh net forphytoplankton and a 250µm mesh net for zooplankton .Samples for metal analyses were filtered onto 47 mmGF/C filter discs using a Millipore 'Sterifil' polysul-fone portable vacuum filter system of 250 ml capacity,with a Mityvac hand vacuum pump to provide suction .Filtration took place in the field or in the laboratorywithin 2 h of sample collection. Filter discs were driedin the sun . When dry, the zooplankton film curledand detached from the disc and could be transferredby forceps into a Sterilin tube for storage . Dried discsholding the phytoplankton were placed intact into Ster-ilin tubes . When required for metal analysis the phy-toplankton discs were re-dried in a vacuum oven at

23 1

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90 °C. Normally, the phytoplankton film could thenbe prised off the filter disc with a scalpel for analyses .Phytoplankton samples from Lake George, open water,contained particularly adhesive algae which tended tostick to the disc . In these circumstances the discs withthe adhering algae were acid digested together .

Animal samples

A pond-net and small mosquito-net seine were usedto collect macroinvertebrates, amphibian tadpoles andsmall fish. Larger fish were caught in monofilamentnylon gill nets with stretched mesh sizes ranging from20 to 80 mm . Molluscs, without shells, entire small-bodied organisms, and livers from larger fish, wereprepared for drying within 5 h of capture . Drying wascarried out in an improvised solar-oven . It compriseda glass-covered, shallow box lined with aluminiumfoil, placed in direct sunlight with side vents to allowair circulation . The oven temperature rose quickly andwithin a few hours dried material could be clicked offthe foil with a scalpel for storage in a Sterilin tube .

Metal analyses

Water samples were acidified with a few drops of con-centrated ARISTAR-grade nitric acid and analysed byflame atomic absorption spectrophotometry. If neces-sary, samples were diluted with glass double-distilledwater to the required range for AA analyses .

All other samples were dried to constant weight inan oven at 90 °C . Aliquots of sample were taken induplicate for acid digestion . Each digest was analysedin triplicate by flame atomization with background cor-rection as appropriate using an International Laborato-ries (IL) 157 atomic absorption spectrophotometer orVarian AA 1475 (for samples collected in December1992) .

Detection limits for metals in prepared sampleswas as follows: Zn = 0.01 mg 1-1 ; Cu, Co, Mn andCd = 0 .05 mg 1- ' ; Cr, Fe and Ni = 0 .1 mg 1 -1 .

Acid digestion procedures Acid digestions tookplace either on energy-controlled hotplates in 100 mlacid-washed conical flasks with reflux glass stoppers,or in acid-washed glass digestion tubes placed in athermostatically-controlled aluminium heating block .5 mls of concentrated ARISTAR nitric acid was addedto weighed aliquots of tissues (0 .03 to 0.20 g dryweight) and cold digestion was allowed to proceedfor 24 h. The digests were then heated to 100 °C andallowed to reflux gently until the solution was clear

(normally about 3 days) . Cooled digests were washedinto 10 or 25 ml volumetric flasks with glass, double-distilled water and made up to volume . The digest wasanalysed by flame atomic absorption spectrophotom-etry. If necessary, the digest was diluted lOx 50x or100x with double-distilled water to bring the solutioninto the appropriate range for analysis .

Replication and error

The normal procedure was to collect samples in dupli-cate and treat them independently. Duplicate sub-samples from each were taken for wet digestion . Eachwet digestion was analysed three times . There wasrarely any significant difference between replicate AAanalyses from acid digests . Nor were there majordifferences between duplicate sub-sample digestionsfrom the same sample . If there were significant dif-ferences the digestions were repeated when samplematerial was sufficient.

Tabulated data are normally presented as the aver-age of the results from the two wet digestions fromeach sample .

Animal materials other than zooplankton weretreated as separate samples and digested individual-ly.

In some cases, considerable variation in analyticalresults was found between field samples from the samearea. This reflects the variation in metal concentrationswhich could occur within a site . It was particularlynoticeable with soil samples from the pan areas wheremetal concentrations from samples within a 0 .5 metrerange could vary by an order of magnitude .

Results

Results may be compared with `normal' concentrationsof metals in waters, soils and biological tissues byreference to Table 1, bearing in mind the wide naturalranges that can occur.

Field measurements of water characteristics withinthe study area are given in Table 2 . The Kamulik-wezi/Nyamwamba River had a low conductivity ofminimal buffering capacity on the acid side of neu-tral . The processing plant effluent was very acid . Ithad a high conductivity as did the swamp waters to thelandward of Hamukunga Bay . The characteristicallyhigh pH value of Lake George is maintained from theBay into the Kazinga Channel, while a trend of slight-

Table 1 . Background concentrations of metals in the freshwater environmentNote: These data are only presented as a guide : there are very wide ranges of metalconcentrations in uncontaminated soils, water and biological tissues .

Sediments; plant and animal tissues : jig g- ' (ppm)Water samples : µg 1 - ' (ppb)

' Salomons and Forstner, 19842 Driver and Tukahirwa, 1990Forstner and Wittmann, 19814 From a variety of sources

ly decreasing conductivity values is apparent from theBay to the Channel at Mweya. The higher conductivityof Lake Edward is associated with its greater sodiumbicarbonate salinity . Detailed water characteristics ofLake George and Lake Edward are documented else-where (Tailing & Tailing, 1965 ; Viner, 1969) .

Table 3 presents the results of heavy metal analysesin sequence from the mine and processing area throughto Lake Edward . The Kamulikwezi/Nyamwamba Riv-er had inflated metal concentrations compared withthose in the waters of Hamukunga Bay which weremostly close to normal . At the same site on the riv-er, Driver & Tukahirwa (1990) recorded values for arange of heavy metals including: 2 .6 ppm Fe; 1 .0 ppmCo; 0.8 ppm Cu and Zn .

The effluent carried very high concentrations ofmetals predominantly as copper, cobalt and iron sul-phides and sulphates (Driver & Tukahirwa, 1990). At1 .8 km from the stockpile, Driver & Tukahirwa (1990)reported values of 30 ppm Zn ; 4 ppm Co; 2-3 ppm Crin the run-off water, as well as inflated values for Pband Cd .

2 3 3

At the landward edge of Kahendero Swamp atthe interface between the Imperata grassland and thepapyrus, there is a succession of vegetation fromImperata cylindrica (L .) Beauv. -i Cyperus latifoliusPoir --) Vossia cuspidata (Roxb.) Griff. - + Cyper-us papyrus L . interspersed with occasional stands ofTypha domingensis Pers. A floating raft of papyrusdominates the outer, lakeward edge .

Bare pan areas are scattered throughout the land-ward swamp with stunted tussocks of Imperata andoutcrops of papyrus . The surface soils of some panscontained over 100000 ppm iron, 49000 ppm cop-per and 9000 ppm cobalt . Fine, reddish and green-ish deposits (from iron and copper salts) were notice-able. Papyrus heads collected from here had cobaltand copper concentrations some 200-fold and 80-foldgreater respectively than those in similar material fromthe floating raft of vegetation at the water's edge inHamukunga Bay (Station 1, Table 3) .

Although rich in iron, organic sediments from ahippo trail in the swamp showed no sign of copper orcobalt accumulation . However, concentrations in small

Type of material Cu Co Zn Fe Mn Ni Cr

Fresh watersFresh waters in general 3 1 .8 0 .05 0 .5 <30 <5 0.3 0.5River waters'

_ 1 10 55 6 0.3 0.5

Various rocks, soils and sedimentsRocks and sediments 3 4-250 0.1-74 16-165 390-6700 11-90Lake sediments' 13 68 29 72Soils 2 20 8 50 38000 100

Plant tissues; algae and mossesWide range of plant material 3 2-196 0.7-8 .3 8-7023 100-17300 36-24220 <0.5-16Angiosperm tissues 2 14 0.48 160 140 14

Most aquatic invertebrates 4 20-200 0.7-1 .2 40-500 270 130-500 1-10

Most freshwater fishes 4 0.1-50 0.02-0.25 2-100 20-100 0.5-15 0.01-1 .0

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Table 2. Water characteristics from field site measurements .

animals from the pools along the trail were generallysubstantially higher than those in similar animals fromthe lakeward edge of the swamp. A noticeable excep-tion was the concentrations of zinc which, perhaps,reflects the high mobility of this metal .

Cu and Co concentrations in emergent plant sam-ples and small animals collected from the floating raftof vegetation at Station 1 were at background levels,but those for these, and other metals, in sedimentsfrom the same area analysed by the French Bureau deRecherches G6ologiques et Minidres, were elevated.

A transect taken from the swamp edge acrossHamukunga Bay (Fig . 1) showed that concentrationsof copper and cobalt in the phytoplankton and zoo-plankton were high, particularly in cobalt . Concentra-tions were highest near the swamp/lake interface anddecreased with distance from the swamp (Table 4) .This trend continued through Lake George and theKazinga Channel to Lake Edward . Concentrations insamples from Lake Edward were at background levels,some 20-fold lower than those in Hamukunga Bay . A

similar trend was reflected by the Nile cabbage, Pistiastratiotes (Table 5). All concentrations were substan-tially higher than those found in Pistia from KisubiBay, Lake Victoria .

Fish caught at the swamp edge in Hamukunga Bayhad distinctly higher concentrations of copper in theirlivers compared with fish from the Kazinga Chan-nel and Lake Edward (Table 3) . A similar trend wasdiscernable, but less obvious, with concentrations ofcobalt, zinc and manganese .

Discussion

That heavy metal pollution from the Kilembe min-ing complex is affecting wildlife grazing areas insideQueen Elizabeth National Park is indisputable (Edro-ma, 1974) . Seasonal rains and hot, dry periods, haveencouraged flushing of particulates and salting out ofmetals in pans substantial distances from the initialpoints of pollution . Some of these metal-enriched pans

Site Date Depth(m)

Temp(°C)

Conduct .µS cm- I

pH 02mg 1- '

Nyambwamba river 1987 47 6 .5

Processing plant, Effluent 1987 >1000 2.2

Kahendero swampPapyrus swamp (landward) 1991 31 .8 2600 6.9Hippo trail in swamp 1991 32 .3 1790 7.9

Hamukunga bayTransect from Station 1 :

transect time: 0 min 1990 0.8 31 .3 313 11 .2 13transect time: 5 min 1990 1 .5 32 .3 300 9.8 18 .9transect time: 15 min 1990 1 .1 30 .4 308 9.8 18 .9transect time: 25 min 1990 1 .7 29 .2 253 9.8 17 .5

Lake GeorgeOpen water 1990 1 .9 32 .7 215 9.9 14 .8

Lake EdwardKazinga channel, Mweya 1991 3.4 27 .4 210 9.6Kazinga channel mouth 1991 4.0 29 .0 896 9.0Kazinga channel mouth 1990 8.5 28 .1 848 8.9 8 .41000 m from Kazinga channel 1990 10 .5 27 .7 846 8.5 8 .7Katwe Bay, centre 1990 2.3 27 .6 464 8.5 12 .6Katwe Bay, towards shore 1990 2 .1 29 .2 439 8.9 11 .7

Table 3 . Heavy metal concentrations in the mining and processing area of Kilembe Mine,into Queen Elizabeth National Park to the floodplain and shore of Hamukunga Bay, LakeGeorge, and to Lake Edward .

Sediments ; plant and animal tissues : pg g - I (ppm) ND = below detection limit

Water samples: mine effluent, mg I - I (ppm)

river and lake waters, µg 1 - I (ppb)

Unpublished data from Bureau de Recherches G6ologiques et Minidres (BRGM), France .

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Sample/Location Date Cu Co Zn Fe Mn Ni Cr

MINE AND PROCESSING AREA

WaterNyamwamba River Water (it g I - I ) August 1987 80 III 160 NDLiquid effluent 50 m downstream from tailings of processing

plant (mg 1 -1 ) August 1987 158.) 2 .6 32 (XX) 18 .0

SedimentSediment 50 m downstream from tailings of processing plant August 1987 1270 19 .7 9135 197

Plant tissuesimperata leaves from plants 100 m from tailing area August 1987 35.4 6.8 658 42 .1

LAKE GEORGE FLOODPLAINPan area, 1 km from lake shore

Sediment, surface scraping

August 1989

453 996 55 .5Plant tissues, !mperata leaves 9.2 9.7 14.3

LAKE GEORGE FLOODPLAINPan area, landward edge of Kahendero Swamp

Sediment August 1989 7383 669 107Plant tissues, Cyperus papyrus heads August 1989 349 199 37.6

Inorganic Sediment, Surface scraping April 1991 49 2(8) 9250 1107(81KAHENDERO SWAMP

Organic Sediment, hippo trail 50 m into papyrus swamp April 1991 147 <((.5 907)X)

FaunaAnuran tadpoles

December 199251 .8 10.2 44.9 9375 2187 15.6 13 .7

Cichlid fry 33.11 1 .1 42.6 2025 640 8.5

4 .3Pilo sp . (snail) 79.8 20.3 49.3 1596 1451 17.4 2 .9Lethocerus sp . (water bug) 36.8 ND 107.0 836 629 ND ND

LAKE WARD SIDE OF SWAMP, HAMUKUNGA BAY

Water * (gag ) -1 ) (BRGM station 1) 3 .0 6(1 3(XX) 3(X) 20 20September 1992 30.0

Sediment Mixed * (BRGM station 1) September 1992 749 89 103 69

I(9)Organic (station I) December 1992 739 230

Plant tissues (station l)Pvcreu.s muntdii shoots 1 .75.6Vossia cuspidata shoots 2.3 0.9Cvperus papyrus heads 4.4 1 .0

Fauna (station 1)Cichlid fry

December 199213.9 ND 63.0 218 17 .4 ND ND

Haplochromi•s sp. (whole specimen) 13.7 ND 201 .4 970 31 .8 7.5

3 .7Biomphalaria sp. (snail) 32.5 ND 39.0 748 260.11 ND NDLethocerus sp . (water bug) 411.0 ND 205.6 308 25 .1 ND ND

Fish liver tissuesOreochromi.s leucostictus (station 1) April 1991

571 2.2 6354 16 .7O,-eochmmi.s leucostictus (station 1) December 1992 549.6 25.9 49.4 8028 14 .8 ND NDOreochromis leucostictus (station 1) December 1992 415.1 15.3 7(1.3 12774 12 .8 ND NDOrenchmmis leucostictus (station 2) April 1991

288 8.3 83.4 11887 16 .2ND 14 .4P, topterus sp . (station 1) December 1992 96.4 ND 137.4 565 9 .3

KAZINGA CHANNEL AND LAKE EDWARD

Fish liver tissues(a) Kazinga channel, Mweya

Orrochmmi* leuco•stietu,s April 1991

117 4.6 16.6 3742 11 .4Oreochmmis niloticus December 1992 16.3 6.5 44 .5 2252 7 .6 ND ND(b) Kazinga channel, mouth of Lake Edward

Orrachmmis leucostictus April 1991

24.8 13.9 55 .7 4644 13 .9

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Table 4 . Copper and cobalt concentrations in plankton .

Sample/Site

Date Cu

Coµg g-1 µg g-'

rest at the edge of Kahendero Swamp abutting LakeGeorge . Of particular interest in this paper is the mobil-isation and movement of metals in the swamp andthrough the aquatic food webs . Does the swamp pro-vide protection, and act as a buffer to heavy metalcontamination of the Lake George ecosystem?

Edroma (1974) showed that grasses and shrubsaway from the mining area in the Park contained from0.4 to 5.4 ppm Cu. A value of 349 ppm Cu in papyrusfrom near the pans (Table 3), therefore, indicates con-siderable contamination at the landward edge of theswamp. Inflated values of metals in the surface-floatingPistia stratiotes from hippo trail pools and the smallanimals therein, suggest contamination of the waterin the swamp for at least 50 metres . However, emer-

gent plants at the lakeward edge did not have elevatedvalues .

One possible explanation for the reduction in metalconcentrations through the swamp is that the emer-gent vegetation is acting as an effective shield . Naturalwetlands can perform this function (Guntenspergen,Stearns & Kadlec, 1990) and the construction of arti-ficial wetlands is now a technique exploited widely byman in industrial (Silver, 1990 ; Lan, Chen, Li & Wong,1990) and domestic wastewater treatment (Cooper &Findlater, 1990; Hammer, 1990) . Waterplants willtake up a proportion of the metals (Welsh & Denny,1980 ; Zhang, Ellis, Revitt & Shutes, 1990), and inso doing may affect adversely their own metabolism .This is supported by Mbeiza (1993) who has shownthat the total plant biomass of Kahendero Swamp inthe contaminated area of Hamukunga Bay is reducedsubstantially. Metals in emergent wetland ecosystemstend to become immobilized in the sediments but canre-mobilize under changed environmental conditionssuch as increased acidity and reduced oxygen tensions(Cooper & Findlater, 1990; Hammer, 1990) .

From available data it remains clear, however, thatmetals are getting into sediments and organisms inLake George . Whilst the concentrations of metals inthe waters of the Bay are higher than elsewhere in thelake system (Bugenyi, 1979 and Table 3) they are with-in the normal range for freshwaters (Table 1) . Yet accu-mulation in phytoplankton (Table 4) and the uptake ofmetals into Pistia plants (Table 5) can only be from thewater.

A proportion of metal-enriched water and sedi-ments therefore, must pass through the swamp, inwhich case the discovery that there was no significantmetal accumulation in the vegetation at the swamp/lakeinterface (Station 1) requires explanation . The hydrol-ogy of the swamps and associated floodplains of LakeGeorge is poorly understood but it could be that in thewet season, when the lake level rises and surface run-off is maximal, particulates and mobilized metals fromthe enriched pans are transported across the swampinto the lake. Alternatively, or at other times addition-ally, contaminated water from Kasese which is cooler(Bugenyi, 1979) and thus more dense than the lakewater could enter the lake beneath the floating edgesof the swamp . For wetland vegetation to act efficientlyin wastewater treatment the effluent must pass throughthe rooting zone (hence the term `root zone biotechnol-ogy'). Thus, even though the landward, rooted swampmay act as a mechanism for heavy metal binding, thelakeward, floating areas may not. Either mechanism

PHYTOPLANKTONHamukunga BayTransect from Station 1 due south

transect time: 0 min 1990 428 90transcct time: 5 min 1990 428transect time: 15 min 1990 286transect time: 25 min 1990 56.4

Random samples :Station 1 1991 277 20.8Station 1 (east) 1991 293 41 .7Open water 1991 189 20 .3

LAKE GEORGE,Open Water 1990 11 .2 -

LAKE EDWARD

Kazinga Channel, Mweya, 1991 22

4.2Kazinga Channel, mouth 1991 11 .6 4 .7

ZOOPLANKTONHamukunga BayRandom samples :Station 1 1991 1465

9.7Station I (east) 1991 67

8.4Open water 1991 35 .5 8 .6

Lake EdwardKazinga Channel, Mweya 1991 33 .7 12 .3Kazinga Channel, mouth 1991 9 .1

6.8

would explain the presence of contaminated sedimentsin Hamukunga Bay. The highest concentrations of met-als in the phytoplankton and Pistia near the swampfringe, and the decline with distance from the swamp,suggest that this area is the main source of contamina-tion to the lake .

The physico-chemical and microbiological pro-cesses of metal mobilisation, complexing and precip-itation under different conditions of temperature, pH,redox and organic ligands such as fulvic acid, willdetermine the availability of metal species for uptakeinto the food chain. Organic detritus, algae and sub-merged macrophytes and their associated micro floraand fauna are at the bottom of the chain . Depend-ing upon the metal, adsorption and absorption can berapid and occur from very low concentrations of metalin solution (Denny & Welsh, 1979 ; Welsh & Denny,1980). Metal ions can also be transferred from metal-enriched particulates directly into plant tissues (Ever-ard & Denny, 1985). Lake George has a very high algalbiomass (Ganf, 1974a; Talling, 1992) . With the warmconditions and fast turnover of phytoplankton (Ganf,1974b) a rapid uptake and cycling of metals withinthe phytoplankton biomass is possible . The high algalbiomass, therefore, probably provides a major reser-voir of metals for transfer up the food chain .

Data from zooplankton analyses (Table 4) providesome support for this conjecture, the amount of copperin the zooplankton reflecting that of the phytoplankton.The tilapias would consume zooplankton as juvenilesbut they are essentially herbivorous as adults . Ore-ochromis leucostictus feeds on phytoplankton, vegeta-tion and detritus whilst O. niloticus grazes phytoplank-ton. The liver accumulates a variety of heavy metals(Forstner & Wittmann, 1981 ; Dallinger et al., 1987)and especially copper (Radhakrishnaiah, 1988), a met-al known to have stressful (El- Domiaty, 1987) andlong-term effects on survival, growth and reproductionof fish (McKim & Benoit, 1971) . The elevated levelsof metals in the livers of fish caught in HamukungaBay (Table 3) suggest that they have been contaminat-ed by grazing on the metal-enriched phytoplankton inthe Bay . Protopterus sp., on the other hand, is carnivo-rous, with molluscs as a common feature of its diet . It isunwise to draw firm conclusions from one sample, butit is noticeable that copper and cobalt concentrationsin its liver were substantially lower than in tilapias . Itspreferred food likewise, had some ten-fold lower con-centrations of those metals (Table 3) compared withphytoplankton .

Conclusions and recommendations

Some twenty years ago Edroma (1974) alerted us tothe serious metal contamination problems occurringin Queen Elizabeth National Park (Rwenzori NationalPark) from the copper mining activities at Kasese andKilembe. It has been shown that the internationallyimportant Ramsar wetlands of Lake George are affect-ed by the pollution . Although the Kahendero wetlandsact as a partial buffer to the movement of metals intothe lake, contamination of the lake occurs . The conse-quence is a transfer of metals into the food web .

Of immediate concern is the effect of this on thefish populations of the lake . Tilapias form the basisof a fishery and are a main protein source for localvillagers. We observed a tendency towards a reductionin the size of fish livers per unit body weight from fishcaught in Hamukunga Bay compared with elsewhere .It is possible that metal toxicity may affect the growthand behaviour of the fish . The relative effects of thecobalt and copper on the biodiversity and food websin the wetlands and in Lake George are unknown andneed to be investigated further .

There are advanced plans to re-process the tailingsfor cobalt abstraction and, later, to re-commence min-ing. Investment from western countries, will under-pin the development. Therefore, there is a window ofopportunity to clean up the waste (present and future)and by so doing fulfil also the stipulations of the `EarthSummit' proclaimed at the United Nations Conferenceon Environment and Development, UNCED, (1992) .In Article 14 on Biological Diversity, Contracting Par-ties agreed that : `Each Contraction Party shall intro-duce appropriate procedures requiring environmentalimpact assessment of its proposed projects . . . . with aview to avoiding or minimizing significant adverseeffects . .' Articles 18 and 20 of the Convention addressthe need for transfer of technologies from developedto developing countries .

An Environmental Impact Study by the Bureau deRecherches Gdologiques et Minieres, France, has beenundertaken to assess the extent of the current pollutionfrom the Kilembe mining complex and to advise thedevelopers on the protection of the environment inthe new plans . It is imperative that the highest stan-dards of environment protection are obtained . Thereare plans to remove or stabilize the contaminated soilsfrom the effluent, but the metal-enriched pans nearthe lake edge should be removed also to prevent fur-ther transfer into Lake George . The developers mustensure that no further particulates are washed down

237

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Table 5 . Copper and cobalt distribution in tissues of Pistia stratiotes.

into the National Park and that liquid effluents con-form to international standards . The construction ofan artificial, self-contained wastewater-treatment wet-land at the outflow of the processing complex shouldensure `polishing' of the treated effluent before finaldischarge .

Most importantly, a programme of survey and mon-itoring of the immediate area of development, andwithin Queen Elizabeth National Park, the Ramsarwetland and the Lake George ecosystem, must beestablished as an environmental audit . Any poten-tial deterioration in environmental quality can thenbe identified and rectified quickly, and managementprocedures modified accordingly . Infrastructural andcapacity building support for in house surveying, mon-itoring and environmental auditing of the schemethrough appropriate financial, technical and trainingback-up needs to be ensured .

Acknowledgments

This project has been supported by the British Councilthrough an inter-University Link scheme since 1989 .Dr D. Taylor and Dr G. Howard of the World Conserva-tion Union (IUCN) through the East African RegionalOffice and the Uganda Wetlands programme kindlyprovided transport for field visits and support to localstaff. Access to Hamukunga Bay and Lake George

was achieved by assistance from the village Chief,Fisheries Officer and fishermen of Kahendero village .We are especially grateful to Dr S . Sumba (Director),Mr Franco Busenene (Ag . Chief Research Officer) andstaff of the Uganda Institute of Ecology at Queen Eliza-beth National Park who provided laboratory space andboating facilities for sample collection in the KazingaChannel and Lake Edward. Mr A . Latif, Chief War-den of Queen Elizabeth National Park allowed us freemovement in the park and provided armed rangers asguides .

We are grateful to Dr Jason Weeks and Dr PhilRainbow (Queen Mary and Westfield College) and toMr Tony Cullen and Dr Steven Smith (King's College)for help with metal analyses. M. Francois Blanchardof the Bureau de Recherches Gdologique et Minit res,France, kindly allowed publication of some of the datafrom their Environmental Impact Study of the Kilembemining area .

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