Jun 15, 1996 - of rural Wisconsin lakes: Sc. Total Environ., 87/88:223-232. Gaudet, J. J., 1979a, Natural drawdown on Lake Naivasha, Kenya, and formation of ...
GEOPHYSICALRESEARCHLETTERS,VOL. 23, NO. 13, PAGES 1581-1584,JUNE 15, 1996
Mercury concentrationsin waters of Lake Naivasha watershed, Kenya
J.C.Bonzongo •, B.S.Ojiambo 2,W.B.Lyons •, S.Wilder I and K.Welch I Abstract. We presentthe firstmercurymeasurements madein Sample Location and Watershed Environment Africanwatersutilizingultracleancollectionandstate-of-the-art Lake Naivasha(0ø45'S.and 36ø20'E.)is locatedin the Eastern analyticaltechniques. Concentrations of totalmercury(HgT) in Rift Valley of Kenyaat 1,890m. It is a rathershallowlakewith a streamsenteringLake Naivasha,Kenya are similarto values
meandepthof 5 meters andanareaof 190Km2. It is a closed
obtainedfrom runoff water in the NorthernHemisphere.
basinlake thatexhibitslargefluctuations in waterlevel (Gaudet, 1979a; Gaudet and Melack, 1981; Ojiambo, 1992). Lake Introduction Naivashais unique amongthe rift valley lakes in that it has The understandingof the biogeochemicaldynamics of fresh, not saline water. Extensive papyrusswamps(Cyperus mercury(Hg) is important,in part, becauseof the acutetoxicity papyrusL.) existaroundits margins.Theselittoralmacrophytes of thiselement,evenat extremelylow concentrations (Fitzgerald havebeenextensivelystudied(Gaudet,1977) There are two perennialrivers;Malewa and Gilgil, and one and Clarkson,1991). In a review article,Fitzgerald(1979) sumephemeral river, Karati,draininginto the lake (Figure1). These marized the available information on the distributionof Hg on rivers start in the escarpments(i.e. NyahururuMountainsand terrestrial waters until that time. He demonstrated, based on the published data,that50 to 55ngL-1wasa reasonable estimate for KinangopPlateau)abovethe valley floor but they alsodrainthe the natural Hg concentrationin rivers and lakes. However, he higherportionsof the valley floor. Volcanicrocksin the area also speculatedthat the true "background" concentrationmay be consistof tephrites,basalts,trachytes,phonolites,ashes,tuffs, closerto 10ngL-1.Certainly, research in thepast-yearsspurred agglomeratesand acid lavas such as rhyolites (Gaudet and on by analyticaldevelopmentsand the acknowledgment of the Melack, 1981; Thompson and Dodson, 1963; Noble and great care, preparationand thoughtfulness neededto collectand Ojiambo, 1975). Hot springsand fumarolesalso are observed analyze samplesin order to obtain accurateresults(Gill and within the lake watershed.The rivers providethe lake with the Fitzgerald, 1985; Gill and Bruland, 1990; Vandal et al., 1993; majority of its water with the highestflows occurringbetween Mason et al., 1994), has shownthat Fitzgerald'sspeculationwas April andOctober(GaudetandMelack,1981).The latterportion of thistime periodis whatGaudetcallsthe"throughflow period" indeed correct. The determinationof "natural"backgroundconcentrations of metals, such as Hg, in rivers/streamsfrom all regions of the world, is neededin order to: (1) comparethe water quality of variousfiver systemsin orderto establishwhat, if any, geologic, hydrologicand/orbiologicconditionscontrolthe aquaticdistribution of Hg, (2) evaluatehow the concentrationof Hg varies with land usechangeor otheranthropogenic activitieswithin the immediate drainage, as well as (3) determine accurateglobal
of the rivers (Gaudet, 1979b).
Agricultureis the primary anthropogenicactivity within the watershedwith small,family farmingplotsdominating.Closerto the lake's perimeter,larger,agribusiness-like activitiesprevail, suchas flower cultivationand dairy cattleproduction.
Sample Collection and Analysis
All sampleswere collectedin acid-precleaned1L Teflon bottles whichcontainedultrapurewateracidifiedwith highpurity Although recently accurateand precisemeasurements of Hg HC1 in July 1994. The sampleswere collectedby an individual have been made in aquatic systems,including rivers/streamsin North Americaand Europe,few, if any, "background" measure- wearing polyethylenegloves, using the ultraclean free-metal samplingprotocoldescribed by Gill andFitzgerald(1985). Once mentsexist from other continents.In this note, we present,to our knowledge,the first Hg measurements from Africa. We have the acidified water was emptied, bottles were rinsed with the measuredthe Hg concentrations from Lake Naivasha,Kenya and sample waters before being filled. The bottles were inverted, in the three major streamsenteringthe lake (Figure 1). This was submergedand filled completely,cappedand double-bagged precleaned polyethylene bags. The samples were carefully boxed done in order to establishbackgroundconcentrations of Hg in andshippedvia air backto USA. Immediatelyuponarrivalat the thesewatersaswell asto compareremoteareavaluesin Africa to laboratory, the samples were removed from the shipping those in more industrialized areas of North America. container,unbagged,placedundera class100 laminarflow hood, acidified using high purity HC1 and analyzed in a clean 1Department ofGeology, University ofAlabama, Tuscaloosa, Alabama laboratoryenvironment.The sampleswerekept in the dark from 2Hydrologic Sciences Program, University ofNevada, Reno,Nevada the time of collectionuntil acidified.Sampleswere collectedand analyzedwithin 10 days. Verta et al. (1994), citing N. Bloom, claim that freshwatersamplesmay be storedat least two to four Copyright1996by theAmericanGeophysical Union. weeksunpreservedin Teflon bottlesat low temperaturesin the dark withoutany changein speciationandtotal Hg concentration. Thesesampleswere unfiltered,and hence,representtotal Hg Papernumber96GL01434 values.Analyseswere accomplished usingthe Au trap technique 0094-8534/96/96 GL-01434505.00 fluxes to the marine environment.
1581
1582
BONZONGO ET AL.' MERCURY 36 30'E
36 45'E
CONCENTRATIONS
Table2. Majorelement(in mM) andisotope data(in %0)from theLake Naivashadrainage
998
998 --
Sam-
0ø15,S 997
055's
pie
997
No. R2
996
996
R9 RI1 RI
LK3
River sampling locations
oø3•s
R4
CI' NO3' SO4 2' 68 61 55 180 390
8.7 1.1 11 101
