Contamination of Water and Sediments by Obsolete Pesticides at Vikuge Farm, Kibaha District, Tanzania

CONTAMINATION OF WATER AND SEDIMENTS BY OBSLOLETEPESTICIDES AT VIKUGE FARM, KIBAHA DISTRICT, TANZANIA

MJ Mihale and MA KishimbaChemistry Department, University of Dar es Salaam

P.O Box 35061, Dar es Salaam, Tanzania.

Accepted 24 December 2004

ABSTRACTSediments and water from Vikuge State Farm, Coast Region, Tanzania, where, in 1986, a“donation” of 170 m3 of partially expired pesticides were stored in an open shed- whicheventually collapsed, were analysed for 80 different pesticide residues and metabolites. DDT andHCH, two of the most persistent organochlorines pesticides, were the most dominant pesticidesfound in both matrices. In sediments the levels were up to 99,620 mg/kg dry weight of ΣDDT andup to 7,400 mg/kg dry weight of ΣHCH. The mean total HCH concentrations were 0.95 µg/l intap water, 0.28 mg/l in surface water and 3.09 µg/l in ground water. Of the four common HCHisomers, α-HCH and β-HCH had the highest on average concentrations in the water. Mean totalDDT concentrations were 1.76 µg/l in tap water, 0.18 µg/l in surface water and 9 µg/l inground water. The only other pesticides detected were in sediments and water are azinphos-methyl, an organphosphorous insecticide and thiabendazol, a systemic fungicide, respectively.During the rainy season, the well-water, which is used for domestic purposes by the villagers,has levels of pesticides higher than those allowed by WHO for drinking water. It is thusrecommended that immediate decontamination measures be undertaken. In the meantime, villagersshould sediment the well-water and should not use not use it at all during the rainy seasons.

INTRODUCTIONDeveloping countries (including Tanzania)cannot afford to purchase environmentfriendly pesticides that are more expensivebecause of their low purchasing power, afactor which forces these countries to opt forcheaper pesticides many of which havepronounced adverse long-term environmentaland public health effects. They are alsotempted to receive aid, in form of pesticides,for use in agriculture (the mainstay of theireconomies) and in public health programmesto combat vectors of diseases such asmalaria, sleeping sickness, etc.

A “donation” of 170 m3 of partly expiredpesticides from Greece in 1986 was receivedby the Tanzania Government through theMinistry of Agriculture without any properprior arrangements (Kishimba and Kalemera

1994). They were thus “temporarily” storedin an open shed with an earthen floor, whicheventually collapsed with obvious direenvironmental and public healthconsequences. Analyses of surface soilsamples from the old storage at the farmrevealed that the total pesticide content wasup to almost 40% by dry mass (Kishimbaand Mihale 2002). This paper reports on theanalyses of surface, ground and drinkingwater, together with sediments, collectedfrom and around the old storage site and thevillage, which is situated on lower ground,with respect to the former.

MATERIALS AND METHODSMaterialsAll the reagents and solvents used were ofanalytical grade and included eighty (80)different pesticide standards ordered fromDr. Ehrenstorfer GmbH (Ausburg,

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Germany). Working standard solutions weremade by dilution of the stock standards andmixtures of standards of differentconcentrations were used in most cases forthe screening of the pesticide residues andmetabolites. All volumetric glassware usedwas teflon stoppered. Varian Star 3400 andHewlett Packard 5890A gas chromatographsequipped with 63Ni Electron Capture, (EC)and Nitrogen-Phophorous, (NP) detectorswere used for analysis.

SamplingSediment samples were collected along thedrainage ditch in May 2001 using spatula.Samples were taken from different pointsalong the ditch uphill and downhill sides,and only up to 5 cm depth. All sampleswere wrapped in aluminium foil and putinto plastic bags.Water samples werecollected in August 2000, March and May2001 in one litre teflon stoppered glasssampling bottles. Sodium chloride (100 g)was added as a preservative. Tap-watersamples were taken from a tap downhill afterthe pipe has passed through the old storagesite. (Figure 1).

VIKUGE FARMWater pipe

PondDrinking water Recreation

Ground water 1

Drinking water

New storage site

Groundwater 2

Old storage site

Figure 1: Overview of Water Sampling Points

Groundwater samples were taken from twodifferent wells from which the villagers drawtheir water for domestic purposes. Surfacewater samples were taken from a pond in thenearby surroundings. Water samples were

transported to the laboratory and kept in arefrigerator at < 4 oC until extraction.ExtractionSediment samples were extracted by usingthe Solid Dispersion Extraction method(Akerblom 1995). The sediment sample was

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mixed thoroughly and two sub-samples (10geach) were taken simultaneously. Onesubsumable was put in a petri dish and driedat 105oC to constant weight fordetermination of the water content. Theother sub sample is for analysis. The subsample for analysis was grounded withsodium sulphate (30g) until a free-flowingpowder was formed. The powder was thenextracted by shaking successively withAcetone/Cyclohexane 1:1 v/v (50, 3 x 20ml) The extract was then filtered through aglasswool into a separating funnel and thenshaken with sodium chloride solution(200ml) and the phases were separated. Theorganic phase was collected in an E-flaskthrough a funnel containing oven driedanhydrous sodium sulphate (15g), while theaqueous phase was further extracted withethyl acetate/cyclohexane (15:85 v/v)(50ml). The extracts were drained throughthe same funnel containing anhydroussodium sulphate for drying. The funnel wasthen rinsed ethylacetate/ cyclohexane (15:85v/v) ( 3 x 10 ml). Finally the combineddried extract was concentrated in vacuo at 35oC to less than 2 ml.

Water samples were extracted using theLiquid-Liquid Extraction (LLE) methodwith ethyl acetate (Åkerblom 1995). Theunfiltered sample (1000 ml) wasquantitatively transferred into a separatingfunnel, and the sampling bottle was rinsedwith ethyl acetate (120 ml) which was thenmixed with a sample in a separating funnel.The combined contents were then shakenvigorously for about 2 minutes and laterallowed to stand to let the phases separate.The organic phase was drawn into an E-flaskthat contained oven dried anhydrous sodiumsulphate (18 g) for drying. The aqueousphase was repeatedly extracted as above,with ethyl acetate (2 x 60 ml) and theextracts were combined in one flask. The

extract was treated in the same way as thesediment extracts.

Clean upDilutions of up to 100,000 times was usedas a clean up procedure for sedimentextracts. Because of low levels of thepesticide residues and metabolites in water,the extracts which needed dilution werediluted only up to 100 times. Acid/alkalitreatment techniques were also used in theclean up of sediment and water samples asdescribed by Akerblom (1995).

Analysis and quantificationAnalysis for the residues and metaboliteswas done as described by Åkerblom (1995).Varian Star 3400 and Hewlett Packard5890A gas chromatographs equipped with63Ni Electron Capture (EC) and Nitrogen-Phophorous (NP) detectors were used for theanalysis. SE-30 and OV-1701 megaborecolumns (30 m x 0.32 mm x 0.5 µm) wereused in each detector. Nitrogen was used asboth a carrier and make up gas in the ECDat a flow rate of 30 ± 1ml/min. In the NPD,helium was used as a carrier gas at a flowrate of 0.5 - 1 ml/min and nitrogen, at aflow rate of 29 ± 1 ml/min, was used as themake up gas. The temperature programmewas 90 oC held for 1 min, 30 oC/min to 180oC, 4 oC/min to 260 oC held for 12 minutes.The injector and detector temperatures were250 oC and 300 oC, respectively.Identification of residues was effected byrunning samples and external referencestandards in GC and then comparing thechromatograms. A peak was not consideredrelevant unless it appeared in both columnsin a given detector. The method averagedetection limits are given in Table 1.

Recoveries of residues were in the range 59– 80 % for HCH and 70 – 99 % for DDTwhich is within the acceptable range(Åkerblom 1995).

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Table 1: Average method detection limits

Analyte Sediments [(mg/kg dry weight) x10-2 ] Water (µg/l)

α-HCH 3 0.4β-HCH 2 0.4γ-HCH 0.1 0.1δ-HCH 0.1 0.2p,p'-DDE 0.06 0.1p,p'-DDD 0.7 0.3o,p'-DDT 2 0.6p,p'-DDT 10 0.6

RESULTS AND DISCUSSIONSediments in the drainage ditch at VikugeFarm were analysed for organic mattercontent. The organic matter content in thesesediments was variable ranging from 0.9%to 21.5%, with a mean of 8.58%.

Levels of pesticide residues in sedimentsAll the HCH and DDT residues found to beabundant in the soil from the old storagesite (Kishimba and Mihale 2002) were alsodetected in the sediments. These include α-HCH, β-HCH, γ-HCH δ-HCH, p,p’-DDE,p,p’-DDD, o,p’-DDT and p,p’-DDT.

Levels of HCH residues in sedimentsThe concentration of total HCH insediments were in the range 280-7,400mg/kg dry weight (Table 1), with higherconcentrations in downhill samples. The β-isomer had overall higher concentrations ascompared to a-HCH. Other isomers'concentrations were below the methoddetection limits. There was an overall

increase in concentrations when movingfrom uphill to downhill.

Pesticide residues in sediments can be usedto provide an indication of pesticidedynamics in the environment (Kammerbauerand Moncada 1998). The high concentrationof ΣHCH implies the presence of a constantsupply of HCH especially for downhillsamples (Table 2). Detecting only γ-HCHand δ-HCH would indicate the storage oflindane on the site (Bacci 1994, Menone etal. 2001). The detection of only a- and b-isomers suggests that technical HCH (whichusually contains a high proportion of a-isomer) was in the consignment stored atVikuge. The non-detection of γ - and δ-isomers can be accounted for by theirrelatively small proportions in the technicalHCH and the photochemical isomerizationof the isomers and the rarity of detecting δ-isomer (Fu et al. 2001, Falandsyz et al.2001).

Table 2: Concentrations of HCH Residues in Sediment Samples.

Sampling Point Concentration (in mg/kg dry weight) x 103

α-HCH β-HCH γ-HCH δ-HCH SHCHUphill points i < 0.04 0.28 < 0.05 < 0.09 0.28

ii < 0.04 0.4 < 0.05 < 0.09 0.4iii < 0.04 1.55 < 0.05 < 0.09 1.58iv 0.2 3.8 < 0.05 < 0.09 4

Downhill points v 1.4 1.1 < 0.05 < 0.09 2.5vi 0.28 1.8 < 0.05 < 0.09 2.08vii 0.85 6.55 < 0.05 < 0.09 7.4viii 0.2 0.92 < 0.05 < 0.09 1.12

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The spatial distribution of ΣHCH showed anirregular pattern in sediment samples. Thisfact indicates a significant contamination bytechnical HCH and an accumulative propertyof the HCH isomers in sediments due totheir dispersible nature. β-HCH was presentin all the sediment samples. Theconcentration of β-HCH in technical HCHranges from 5-12%. However, the β- isomerwas found in a large proportion in sediment,almost in all uphill samples (Table 2). Therelatively higher percentage of β-isomer insediments even though it has a lowerpercentage in technical HCH implies that b-HCH is the most persistent isomer amongthe HCH isomers (Lee et al. 2001). This issupported by the low Henry’s law constantand low water solubility of the isomer(Tomlin 2000). The behaviour of HCHisomers in the environment may beexplained by volatility and water solubilityrather than adsorption to organic matter.Other transport processes such as run-off andinput through ground water (the leachingprocess) might also affect the finalcomposition of HCH isomers in sediments(Lee et al. 2001).

Levels of DDT residues and metabolitesin sedimentsThe concentration of SDDT ranged from3,490 mg/kg to 99,620 mg/kg dry weightand relatively high levels were found in the

downhill samples (Table 3). The trend inSDDT concentrations showed that downhillsamples had higher concentrations of almostall the DDT residues and metabolites (o,p’-DDT, p,p’-DDT, p,p-DDE and p,p’-DDD),than the uphill samples.

A pattern of distribution almost similar tothat of HCH residues in sediments wasobserved for the DDT residues andmetabolites in sediments (Table 3). TheDDT residues in the sediments had higherconcentrations than their metabolites. Ingeneral, the concentrations of the residuesincreased along the ditch when moving fromuphill to downhill. Comparison of the p,p’-DDE and p,p’-DDD in all the samplesshows that p,p’-DDD concentrations arerelatively higher than those of p,p’-DDEsuggesting the presence of TDE (DDD) as apesticide on its own) in the donatedconsignment.Because DDT is expected todegrade to its fairly stable degradationproducts, DDD and DDE in soil, theDDT/SDDT ratio is used as an indicator ofthe duration of time that the DDT residueshave been in the environment (Nowell et al.1999). In a situation like this where one ofits metabolites, DDD, is used as a pesticideson its own, this ratio is inaccurate. The onlyratio that can accurately predict thedegradation rate of DDTs in sediment is theDDE/DDT.

Table 3: Concentrations of DDT residues and metabolites in sediment samples

Sampling Concentration (in mg/kg dry weight) x 103

Point p,p’-DDE p,p’-DDD o,p’-DDT p,p’-DDT SDDT DDE/DDTUphill i 0.1 0.42 0.69 2.18 3.49 0.03points ii 0.27 1.03 1.69 2.67 5.66 0.06

iii 0.27 2.05 4.61 3.57 10.91 0.03iv 1.63 27.8 7.95 37.9 75.28 0.04

Downhill v 1.2 5.74 6.35 36.42 49.71 0.03points vi 0.89 3.8 74.28 20.65 99.62 0.01

vii 2.8 11 16.68 28.4 58.88 0.2viii 0.5 2.07 3.35 15.4 21.32 0.03

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In all the sediment samples analysed, theDDE/DDT ratios were less than one (Table3), which shows that there is delayeddegradation of DDT isomers. This can beaccounted for by the absence of favourableconditions for DDT degradation such as Na-bethonite clay surfaces, neutral to alkalinepH env i ronment and adap tedmicroorganisms which may be either sparseor absent altogether as they would have beeneliminated by the high concentrations of thepesticides. It has been reported that aerobicdegradation of DDT in sediments isprohibited or significantly decreased inpresence of high concentration of DDT (Wuet al. 1999).

Sediments act as a sink or trap forhydrophobic compounds, (such as DDT,HCH, PCBs) since they have high organicmatter. Once an organic compound entersthe sediments, it may find itself in a highlyreducing environment. Then, depending onits molecular characteristics, it may either bepreserved or rapidly reduced. Thus thepossibility of DDT to undergo reductivedechlorination in this situation is highlyfavoured. Chemicals with high octanol-watercoefficients, Kow, are readily adsorbed bynatural sediments. Since DDT has a highKow (pKow = 6.4), (Larson et al 1996), itwould be adsorbed and thus accumulate.But, because of its chemical nature, thereduction process would be very slow.Hence the amount of DDT in sediments wasfound to be high, as expected.

Levels of other pesticide residues i nsedimentsBesides HCHs and DDTs, other residuedetected in sediment samples was azinphos-methyl an organophosphorourous insecticidewhose concentration was 360 mg/kg dryweight. Though azinphos-methyl is knownto have a half-life of 1-3 days (Vogue et al2001), it remains unchanged in the soil forvarying lengths of time depending on soil

organic matter and texture. The detection ofazinphos-methyl, though in only onedownhill sediment sample, is a clearindication that the original consignmentconsisted of a large quantity of thispesticide, since, even under sterileconditions, its half-life is reported to be atleast 355 days (Vogue et al. 2001).

Levels of pesticide residues andmetabolites in waterLower levels of the same residues andmetabolites predominant in the sedimentswere found in most water samples.

Levels of HCH residues in waterThe concentrations of ΣHCHs in water were0.95 mg/l in tap water, up to 0.44 µg/l insurface water and up to 6.15 µg/l in groundwater, with means of 0.28 µg/l in surfacewater and 3.09 mg/l in ground water (Table4).

The concentration trend of HCHs showedthat ground water samples had higher levelsthan either tap or surface water samples.Between tap and surface water samples, tapwater samples had high concentrations of theHCHs compared to surface water samples(Table 4). This raises serious concern on thequality of the water.

Table 4 shows that a high proportion of theΣHCH in samples analysed is contributed byα- and β- isomers. This further supports thehypothesis of the presence of technical HCHin the consignment. High α - H C Hcontamination of groundwater indicates thatthere is a continuous supply of technicalHCH from the point source especially duringthe rainy seasons. This also shows that theisomer does not undergo any degradationbefore entering the water source (Nowell etal. 1999).

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Table 4: Concentrations of different HCH residues in waterSampling Point Concentration (in µg/L of Sample) x 10-2

α-HCH β-HCH γ-HCH δ-HCH ΣHCHTapwater tap 37 54 2 2 95

S1 < 0.4 < 0.4 < 0.1 < 0.2 -Surface water S2 1 < 0.4 < 0.1 < 0.2 1

S3 17 24 1 2 44S4 12 26 1 1 40G1 3 < 0.4 < 0.1 < 0.2 3G2 4 < 0.4 < 0.1 < 0.2 4G3 1 < 0.4 < 0.1 < 0.2 1

Ground water G4 1 < 0.4 < 0.1 < 0.2 1G5 470 141 0.2 4 615*G6 420 144 0.2 4 568*G7 2 < 0.4 < 0.1 < 0.2 2

*Rainy season (March, 2001) samples

Levels of DDT residues in waterTotal concentrations of DDT was 1.76 mg/lin tap water, up to 0.35 mg/l in surface

water and up to 32.71 µg/l in ground water,with means of 0.18 µg/l in surface water and9 mg/l in ground water (Table 5).

Table 5: Concentrations of different DDT residues in water.

Sampling Point Concentration (in µg/L of Sample) x10- 2p,p’-DDE p,p’-DDD o,p’-DDT p,p’-DDT ΣDDT

Tap water Tap < 0.1 3 22 151 176S1 < 0.1 4 4 27 35

Surfacewater S2 < 0.1 4 1 28 33S3 1 1 < 0.6 < 0.6 2S4 2 1 < 0.6 < 0.6 3G1 < 0.1 2 < 0.6 64 66G2 < 0.1 6 < 0.6 64 70G3 < 0.2 3 1 18 22

Groundwater G4 < 0.1 1 1 25 27G5 13 112 790 1908 2822*G6 13 109 793 2366 3271*G7 < 0.1 4 2 22 28

* Rainy season (March, 2001) samples

The distribution of individual DDT isomersand metabolites' concentrations in waterwere in descending order: p,p’-DDT > p,p’-DDD > o,p’-DDT in tapwater, p,p’-DDT >o,p’-DDT > p,p’-DDD > p,p’-DDE insurface water and p,p’-DDT > o,p’-DDT >p,p’-DDD > p,p’-DDE in ground water(Table 5). In all the water samples analysed,

p,p’-DDT had higher concentrations than theother residues followed by o,p’-DDT.

Of the DDT metabolites detected in water,p,p'-DDD had higher concentrations ascompared to p,p'-DDE. This still is anindication of the presence of DDD (TDE,rothane) in the donated consignment.

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Amongst the DDT compounds, the p,p’-isomer was more abundant than the o,p’-isomer probably because o,p’-isomer has arelative small percentage in the technicalDDT. It is also known that o,p’- D D Tundergoes a relatively rapid isomerisation top , p ’ -DDT and thus i ts proportiondiminishes (Nhan et al. 2001). Nevertheless,it is worth noting that o,p’-DDT residueswere found in almost all water samplesexcept two ground water samples (Table 5).It should be noted that although all DDTsare of known public health concern, the o,p'-DDT isomer is known to have endocrinedisruptive effects by reducing sperm counts(Ulrich 2000).

From Table 5, one can calculate the non-metabolised fraction of DDT, (DDT/SDDT),which, in the water samples analysed rangefrom 0.6 to 0.98. This high ratio of theparent compound indicates a continuousinput from the point source and there is noappreciable degradation of the residues insoil before entering the aquatic environment.This argument can be supported by the factthat DDT appears to break down morequickly once it enters a river or any otherwaterway (Nowell et a l 1999). This canexplain the fact that there is an almostconstant supply of the residues from thecontaminated site (point source), especiallyin downhill tap water and ground water.

The WHO tolerable daily intake (TDI) forDDT is set at 0.02 mg/kg body weight(WHO 1996). For a person who takes 2litres of water per day, this implies that a 60kg person is allowed to consume up to 0.6mg of DDT in a litre of water. Comparingthis with the highest DDT level of 32.71µg/l (0.033 mg/l) detected in groundwatercould mean that villagers at Vikuge who usethe water for domestic purposes are notpresently at any high health risk. However,in cases of heavy rains and heavy floods, thequality of these waters is questionable. Thiswas conclusion arises from the levels of thepesticide residues in the water after heavy

rains when water from the old storage areafloods the lower areas, includes the wells.

Pesticide adsorption to soil or sediment isinversely related to its water solubility. Thegreater the water solubility the greater themobility (Singh 2001), and the less theadsorption. Since the HCHs are more watersoluble than the DDTs, a high proportion ofthem was expected to be in the water ascompared to DDTs. When the concentrationsof ΣHCH (Table 4) are compared to those ofΣDDT (Table 5), the concentrations ofHCHs are lower than those of DDTscontrary to what was expected. This may notonly be due to rapid HCHs’ mineralizationand formation of volatile organiccompounds (Singh 2001), but also due tothe lower content of HCHs as compared toDDTs in the soil at the point source(Kishimba and Mihale 2002).

Despite being widely invoked in literature,water solubility is rarely an important factorinfluencing deep leaching of pesticides.While the water solubility of mostpesticides fall within the range of 10-5-10-2mg/l (Barbash et al. 1996), the pesticideconcentrations measured in water rangesfrom 10-1 – 1 x 105 mg/l. The latter gives abetter indication of the compounds mostlyencountered in water are those that are lesslikely to be encountered, especially whenappreciable time has elapsed after the releaseof the pesticides into the environment. Thusthe majority of the pesticides detected arethe chlorinated organic pesticide residues.Water solubility of a may sometimes givelittle information regarding the tendency of apesticide to associate with the solid phase inthe subsurface, where adsorption interactionsare likely to be significant (Barbash et al.1996). This study has shown that thatdespite their low water solubilities, HCHand DDT residues and metabolites and otherorganochlorine pesticides appreciablycontaminate water at Vikuge.

The high concentrations of organochlorinepesticides in well water indicate that affinity

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for organic matter is an unreliable indicatorwhether an individual pesticide residue or ametabolite will reach groundwater, probablybecause of the effects of non-equilibriumtransport or colloid transport (Barbash et al.1996). Since most pesticides used today areorganics and their chemical structures aremore similar to those of natural organicmatter, NOM, their degree of affinity toorganic matter is highly pronounced. This'like prefers like' rule of solution chemistrycauses the pesticides to partition into soilorganic matter, (sediments for example),from the aqueous solution. This tells us thatdetection of the residues in water implies ahigh concentration of the residues in deepsediments. This makes the sediments to besecondary contamination source bycontinuously releasing the residues in theaqueous media through sediment-waterinterface equilibration.

There is increasing evidence that DissolvedOrganic Matter, DOM, in water can bindsome pesticides. There have been reportsthat the apparent solubility of hydrophobicpesticides increase by the presence of DOM(Cheng 1990). Association of an organicchemical with dissolved organic matter canincrease the solubility of organic moleculesin water and a concomitant increase insubsurface mobility, a phenomenon referredto as facilitated transport (Larson et al.1997). It has been found that significantfraction of the dissolved DDT and otherresidues found in natural waters may bebound to dissolved humic materials.Soluble humic substances can interact withslightly soluble pesticides, therebyincreasing the concentration of the pesticidein the aqueous layer (Cheng 1990). Thus thehigh levels of DDT and other residuesdetected may be attributed by the presence ofdissolved organic matter in water.

Surface-active substances (surfactants) mayalso influence the rates and pathways oftransformation of pesticides in subsurfaceaqueous systems. The significance of this topesticide behaviour and fate in ground water

arises from the fact that surfactants arecommonly applied as adjuvants in pesticideformulations. Furthermore, natural organicmatter exhibits surface-active propertiesbecause it typically possesses bothhydrophobic and hydrophilic regions withinits overall structure. As a result, thesequestration of pesticides within morehydrophobic regions of the natural organicmatter may influence the accessibility ofthese molecules to other attacking chemicalspecies. Because most of the functionalgroups within the natural organic matter arenegatively charged, the association of ahydrophobic pesticide with the morehydrophobic region of NOM may reduce therate of attack of the molecule by anionicreactants, but accelerates reactions withcationic species. The fact that the rate ofdechlorination of DDT and DDD areaccelerated by cationic surfactants suggeststhat these reactions would be slowed inpresence of natural organic matter (Barbashet al. 1996).

Levels of other pesticide residues in waterThiabendazole was detected in surface andgroundwater at an appreciable amount at aconcentration range of up to 0.17 µg/l insurface-water and up to 0.59 µ g/l ingroundwater samples. The pesticide residuewas not detected in tap-water.

Groundwater contamination by pesticides isusually attributed to the magnitude of thesources, opportunities for interactionthrough the soil, chemical properties andgeological conditions. A pesticide can leavethe contaminated site either in its molecularform and enter surface or ground water, in aparticle associated form in run-off or throughplant uptake. The interaction of the chemicalstructure and environmental conditionscontrols the chemical’s behaviour andultimately its effect on the environment(Barbash et al. 1996).

CONCLUSIONThe findings above illustrate once again therelationship between water contamination

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and improper storage, spills and disposal ofpesticides. The results show the presence ofpesticides in tap, surface and groundwaterwells from the residential area around astorage site. Although the concentrationsdetected appear to be below the levels ofconcern - over 15 years after the pesticideswere improperly stored – one is leftwondering what the situation was a the firstyears after the storage. Still the results showthat pesticides, including the hydrophobicones such as DDTs and HCHs, cancontaminate the water including thatintended for domestic uses.

It is thus highly recommended that water fordomestic use should be subjected tosedimentation and filtration to remove NOMand DOM as an immediate and temporarymeasure in reducing the health risks andeffects to come. Further governmentintervention is highly needed particularly onthe supply of clean and safe drinking water.Decontamination by biological methods(bioremediation and phytoremediation) ofthe point source will be the only solutionfor preventing further pesticide residuespread through global distillation, run offand wind. Further studies are needed todetermine the levels of these residues in air,organisms and in the studied media on awider perspective in the course of assessingthe extent of environmental pollution byobsolete pesticides.

ACKNOWLEDGEMENTThis work was sponsored by SIDA throughSida-SAREC Agrochemicals Project of theFaculty of Science, UDSM and a scholarshipto MJ Mihale through the Directorate ofPostgraduate Studies, UDSM. We arehumbly indebted to Associate Professors M.Akerblom and H. Kylin of the Universitiesof Uppsala and Swedish University ofAgricultural Sciences, Uppsala, Sweden,respectively for logistical and technicalsupport.

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