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Abstract. Two industrial and highly populated cities namely Ibadan and Lagos, both located in. Southwestern Nigeria have urban migration problems and ...

EVALUATION OF GROUNDWATER QUALITY CHARACTERISTICS NEAR TWO WASTE SITES IN IBADAN AND LAGOS, NIGERIA A. IKEM1∗, O. OSIBANJO2 , M. K. C. SRIDHAR3 and A. SOBANDE4 1 Environmental Engineering Program, Department of Chemical Engineering, Tuskegee University, AL 36088, U.S.A.; 2 Department of Chemistry, University of Ibadan, Ibadan, Nigeria; 3 Department of Public Health and Preventive Medicine, University of Ibadan, Ibadan, Nigeria; 4 Environmental Consultants, 7273 Bennell Drive, Reynoldsburg, OH 43068, U.S.A. (∗ author for correspondence, e-mail: [email protected], fax: (334) 727 8942)

(Received 12 October 2000; accepted 9 February 2002)

Abstract. Two industrial and highly populated cities namely Ibadan and Lagos, both located in Southwestern Nigeria have urban migration problems and resource limitations. As a result, the development of residential areas near waste sites and the indiscriminate dumping of municipal waste are common in both cities. Orita-Aperin and Oworonsoki neighborhoods in Ibadan and Lagos, respectively, both located near a waste site were studied. The two areas relied on the wells as sources of drinking water hence poor drinking water quality may have health consequences. A total of 51 groundwater samples (30 wells in Ibadan and 21 wells in Lagos) were monitored seasonally for two years in Ibadan and a year in Lagos. Results from this study revealed that some of the groundwater quality constituents determined exceeded the World Health Organization (WHO) standards for drinking water irrespective of source of pollution. Some of the groundwater samples were poor in quality in terms of pH, conductivity, total dissolved solids, chloride, nitrate, ammonia, COD, Al, Cd, Cr, Fe, Pb, Ni and total coliforms recorded. Thus, groundwater from some of these private wells requires further purification to ensure its fitness for human consumption. F-test, one-way parametric analysis of variance (ANOVA), Mann-Whitney ‘T’ test and Kruskal-Wallis H-test applied to upgradient and downgradient concentrations suggest impact of the waste sites on groundwater quality. The Mann-Whitney test only suggests that the downgradient values of Fe (Ibadan: dry season (1), sulfate (Ibadan: rainy season (2) and pH (Lagos: dry season (2) were significant at 5% level of significance. For Ibadan wells, the Kruskal-Wallis test showed that variances of specific conductivity, dissolved solids, and chloride for Ibadan upgradient and downgradient values were unequal. Also variances of specific conductivity and dissolved solids for upgradient and downgradient values in Lagos were unequal at 5% level of significance. Three downgradient wells in Lagos and four downgradient wells in Ibadan had significant impact due to leachate migration into drinking wells. Keywords: downgradient, groundwater, leachate, sediment, soil, upgradient, waste sites, water quality constituents, wells

1. Introduction The impact of leachates on groundwater and other water resources has attracted a lot of attention worldwide because of its overwhelming environmental significance. Leachate migrations from waste sites or landfills and the release of pollutants from sediment (under certain conditions) pose a high risk to groundwater resource if not Water, Air, and Soil Pollution 140: 307–333, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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adequately managed. Their impact on groundwater continues to raise concern and have become the subject of recent and past investigations (Ahmed and Sulaiman, 2001; Fatta et al., 1999; Kjelsen et al., 1998; Bjerg et al., 1995; Robinson and Gronow, 1992; Cariera and Masciopinto, 1998; Loizidou and Kapetanios, 1993; Gallorini et al., 1993; Khan et al., 1990; Kunkle and Shade, 1976). Empirical investigations (Fatta et al., 1999; Bjerg et al., 1995; Loizidou and Kapetanios, 1993) as well as modeling techniques (McCreanor and Reinhart, 2000; Lee et al., 1997; Syriopolou and Koussis, 1987; Koussis et al., 1989; Ostendorf et al., 1984) have been used to assess the pollution of groundwater by leachate from a landfill. Mathematical models have the advantage of being able to predict different scenarios without involving tedious and time-consuming experimentation. However, these models and their predictions have to be tested and verified with field studies. In this study, field investigations, which involved the collection and analyses of samples, were used to generate groundwater quality data for two cities with severe potential pollution problems. 1.1. S TATISTICAL APPROACHES FOR EVALUATING GROUNDWATER DATA Several statistical methods have been used for analyzing and evaluating groundwater-monitoring data. The procedures include the students’ t-test (Montgomery and Loftis, 1987), alternatives to student’s t-test (US EPA, 1988; Doctor et al., 1986; Splitstone, 1989), graphical tools and non-parametric methods (Goodman, 1987; Fisher, 1989; Hensel, 1987; Hirsch and Slack, 1984). Water quality data are usually analyzed with parametric statistical procedures requiring the normality assumption for accuracy of their attained significance levels. However, the data are typically non-normally distributed. When parametric procedures are applied to non-normal data, the powers of parametric procedures are low and their results may be in error (Hensel, 1987; McBean and Rovers, 1998). Non-parametric methods have been found to be more accurate for analyses of environmental data especially groundwater data (Hensel, 1987). The fundamental characteristic of non-parametric tests is that the ranks are utilized instead of data values (McBean and Rovers, 1998). Ranking process for the analysis of environmental data has been adjudged to be over 95% more powerful than parametric tests (NCASI, 1985). A non-parametric approach is advantageous because the statistics are valid for all data distributions (NCASI, 1985; Goodman, 1987; Doctor et al., 1986), are easily computed and non-detect or less than values can be incorporated in the data analysis (Doctor et al., 1986; Hensel, 1987). In this study, parametric and non-parametric procedures were used to assess the homogeneity of variances of upgradient (background) and downgradient (downhill) concentrations of wells studied. Also further statistical comparisons between downgradient and upgradient concentrations that had unequal variances were made to identify wells that were contaminated.

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1.2. BACKGROUND AND AIM OF STUDY Nigeria is the largest country on the West coast of Africa with an estimated population of 123.9 million with about 43% of the populace currently living in cities or urban areas (World Bank, 2000a). The rate of urbanization in Nigeria is alarming and the major cities are growing at rates between 10–15% per annum and medium cities at about 7–10%. The siting and development of residential quarters near waste sites are common due to shortage of building land to cope with the increasing rate of migration and consequent population explosion. Environmental problems arising from the improper disposal of solid and liquid wastes near residential areas, poor wastes collection and handling, poor access roads, and the poor state of physical infrastructure especially in big cities like Ibadan and Lagos affect the quality of life. In Nigeria’s two major cities, Ibadan and Lagos, potable water from the water supply boards are inadequate for the teeming population and fears of drinking water contamination of municipal waters is high since water treatment and distribution is facing many problems. According to the World Bank (2000b), only 39% of people that live in cities have improved water source. Thus, a large percentage of the population of the neighborhoods studied (over 70%) depends on groundwater from their private wells as their main source of quality drinking water. Therefore, it became necessary to ascertain the water quality of these areas since poor water quality may have an adverse health impact. Groundwater quality characteristics near waste sites or landfills have been reported in the literature but similar studies in Nigeria are scarce or lacking. Against the background information described above, the aims of this investigation were: (1) to assess the groundwater qualities near the two sites located in Ibadan and Lagos by comparing water quality data with World Health Organization (WHO) drinking water standards, (2) to characterize the refuse, waste soils, sediments, surface waters and most importantly the speciation of metals on the two sites because of their environmental implications, and (3) to apply two non-parametric procedures namely; the Mann-Whitney and Kruskal-Wallis tests to selected sampled wells that served as upgradient and downgradient wells to assess if the leachates emanating from the waste sites had impact on groundwater quality. 2. Materials and Methods 2.1. D ESCRIPTION OF STUDY AREAS The Ibadan waste site (longitude 6◦ 01 56 E to 6◦ 01 57 E and latitude 8◦ 14 11 N to 8◦ 14 13 N) is in Orita-Aperin, a residential area located within Ibadan City, Nigeria. The Ibadan sampling locations is presented in Figure 1a. The Ibadan waste site has been in use since the 1950s and is now abandoned due to legal prohibition of waste discharge on the site. However, illegal local dumping of mostly market

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Figure 1a. Map of Ibadan sampling locations.

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wastes is still continuing on one edge of the site irrespective of the ban. The stream, which runs beside the waste site on the Northeastern boundary, carries pollutants along its path and empties into other water resources. Geologically, Ibadan area is located within the basement complex and the rock is largely crystalline in structure. It consists of banded gneiss and biotite gneiss and groundwater flow within this system is through fractures. The Lagos waste site (longitude 3◦ 22 E to 3◦ 23 E and latitude 6◦ 33 N to 6◦ 34 N) is in Oworonsoki area on the northeastern part of Lagos. The sampling locations near the Lagos waste site are presented in Figure 1b. On the eastern side of the Oworonsoki area is the Lagos lagoon. The eastern and northern areas of the site were swamps prior to the land filling and subsequent siting of residential properties beside the waste site. A 10 m wide channel that runs beside the waste site helps in the transport and redistribution of pollutants. The waste site is surrounded by industrial lands to the west, residential to the southeast and northeast and a swamp to the north. The composition of the waste received at this site is estimated to be 80% domestic including market wastes and 20% industrial waste. Since 1977, over 344 000 tonnes of waste have been deposited on this site (Lagos Waste Disposal Board, 1991) and dumping of toxic, hazardous and liquid wastes on the waste site have not been reported. Geologically, the Lagos area is made of sedimentary deposits that are largely alluvial and flow of materials and pollutants is through pores. 2.2. M ETHODOLOGY 2.2.1. Quality Assurance All chemicals used were of reagent grade and ultra pure deionized water was used throughout the experimentation. Washing procedures, sampling container types, chain-of-custody procedures, sampling for general parameters and heavymetals determinations, sample holding times and preservation techniques conform to standard methods for water and wastewater analysis (APHA, 1985). Procedural blanks, reagent blanks, preparation of standard solutions under clean laboratory environment, calibration of the Perkin-Elmer 2382 atomic absorption spectrophotometer (AAS) using certified standards and the analyses of calibrated standards after 15 samples to ensure that the instrument remained calibrated were some of the measures taken during the experimentation. Recovery studies for the metals analyzed using the AAS ranged between 87 and 98% and percent error for the cation-anion balances of all the water quality results were within ±20%. 2.2.2. Refuse Characterization, Soil and Sediment Analyses Refuse disposed on the Ibadan and Lagos waste sites were collected randomly into long polythene bags and transported to the laboratory. To ascertain the percentage compositions of the waste types, known weights of the refuse samples were sorted and weighed. Waste soils and sediment samples near the sites were

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Figure 1b. Map of Lagos sampling locations.

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collected randomly into polythene bags with a plastic scoop at a depth of 0–15 cm. The samples were transported to the laboratory and then air-dried in the open at normal room temperature. The pH of the soil and sediment were measured in water with a 1:2.5 soil/solution ratio after equilibration for 30 min (McLean, 1982). Particle size distribution was by pipette method (Day, 1965), organic carbon (%) was by Walkley-Black method (Jackson, 1957), sulfate was by turbidimetry with BaCl2 (IITA, 1979; APHA, 1985), phosphate by ascorbic acid method (IITA, 1979; APHA, 1985), nitrate determination was by phenoldisulphonic acid method (Taras, 1950) and ammonia was by nesslerization (HMSO, 1972). 2.2.3. Metal Speciation The main objective of the speciation analysis was to determine the concentrations of metals bound to the different soil and sediment fractions through chemical extraction schemes. Measurement of the metal fractions and their distribution in the soils and sediments collected will provide information on the pollution risks especially the mobility of toxic metals into groundwater and surface waters. Heavy metals are not biodegradable and they can accumulate in the food chain. Exposures to heavy metals can cause significant health problems in man and animals (Underwood, 1971). A wide variety of single and sequential techniques are available in the literature for the analysis of metals in soils and sediments using different extracting solutions (Pérez Cid et al., 2001; Maiz et al., 2000; Schramel et al., 2000; Singh et al., 1998; Kersten and Förstner, 1995; Lake et al., 1984; Hall, 1991) but the most applied procedure in the literature is that of Tessier et al. (1979). Sequential extraction procedures provide differentiation between several association forms of metals with the soil and sediment components (Maiz et al., 2000; Kersten and Förstner, 1995; Lake et al., 1984). Most schemes generally seek to use extractants in decreasing order of pH (Kersten and Förstner, 1995). The method by Hall (1991) was employed to determine the partitioning of metals of environmental significance in soil and sediment fractions. In brief, extraction with 0.1 M hydroxylamine hydrochloride (NH2 OH·HCl; pH: 2) is supposed to remove manganese compounds, exchangeable cations, carbonate-bound metals, and in general easily reducible species. Extraction with acidified 30% hydrogen peroxide (H2 O2 ; pH: 2.5) is supposed to remove oxidizable and organically bound species. Organically bound metals represent those forms that may be complexed, chelated or adsorbed to organic matter in addition to components of living cells, their exudates and a spectrum of degradation by-products. Extraction with 0.5 M HCl is supposed to distinguish between residual (i.e. metals associated with rock fragments) and non-residual (i.e. indicates contribution of human activities to total metals) metals. 0.5 M HCl is able to remove all metals that include those extracted by the 0.1 M NH2 OH·HCl and part of the residual species. Concentrated nitric acid was used to estimate total metals. It is known to attack all solid phases of soil/sediment (Valin and Morse, 1982). Residual metals are largely stable because

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they are embedded in the crystalline structure of the soil/sediment. They contain primary and secondary minerals, which may hold trace metals within their crystal structure. Residual metals are usually not released in solution over a reasonable time span under the conditions normally encountered in nature (Tessier et al., 1979). The air-dried soil and sediment samples were sieved into 45 – 250 mg L−1 1.15 mg L−1 7.5 mg L−1 (poor ) 0–30 mg L−1 31–60 mg L−1 61–120 mg L−1 121–180 mg L−1 >180 mg L−1 >0.2 mg L−1 >3 µg L−1 >50 µg L−1 >1.5 mg L−1 >2.5 mg L−1 >10 µg L−1 >0.5 mg l L−1 >15 µg L−1 >200 mg L−1 >250 mg L−1 >50 mg L−1 >500 mg L−1 1000–1700 µS cm−1 >3.0 mg L−1 8.5 >Zero

50 40 10 40 53.3 6.7 0 10 46.7 43.3 56.6 26.7 16.7 3.3 20 46.7 26.7 3.3 20 97.7 0 0 10 36.7 3.3 26.7 0 0 0 13.3 36.7 60 3.3 40 16.7 0 100

Number of Samples analyzed each season: 30.

50 36.7 13.3 60 26.7 13.3 0 20 23.3 56.7 100 0 0 3.3 20 46.7 20 10 60 70 66.7 0 10 56.7 0 96.7 0 3.3 3.3 13.3 50 40 10 13.3 30 0 100

WHO guidelines

250 mg L−

0.2 mg L−1 3 µg L−1 50 µg L−1 1.5 mg L−1 2.5 mg L−1 10 µg L−1 0.5 mg L−1 15 µg L−1 200 mg L−1 250 mg L−1 50 mg L−1 500 mg L−1

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TABLE IV Classification of Lagos groundwater quality data and comparison with guidelines Parameter

1. Total alkalinity

2. Sulfate

3. Ammonium (NH+ 4)

4. COD

5. Total hardness

6. Al 7. Cd 8. Cr 9. Na 10. Fe 11. Pb 12. Ni 13. pH pH 14. Mn 15. Cu 16. Nitrate 17. Chloride 18. Total dissolved solids 19. Specific conductivity 20. TOC 21. Total coliforms

Classification

Comparison with classification (% Samples)

Rainy season

Dry season

200 mg L−1 0–20 mg L−1 >20–40 mg L−1 >40 mg L−1 >250 mg L−1 1.15 mg L−1 7.5 mg L−1 (poor) 0–30 mg L−1 31–60 mg L−1 61–120 mg L−1 121–180 mg L−1 0.2 mg L−1 >3 µg L−1 >50 µg L−1 >200 mg L−1 >2.5 mg L−1 >10 µg L−1 >15 µg L−1 8.5 >0.5 mg L−1 >1.5 mg L−1 >50 mg L−1 >250 mg L−1 >500 mg L−1 >500 µS cm−1 >3.0 mg L−1 >Zero

95.2 4.8 0 85.8 13.2 1 0 85.7 9.5 4.8 90.5 4.8 4.8 61.9 33.3 4.8 0 0 71.4 14.3 9.5 0 28.6 38.1 9.5 85.7 0 0 0 0 0 0 0 0 100

Number of samples analyzed each season: 21.

100 0 0 100 0 0 0 57.1 33.3 9.5 100 0 0 71.4 28.6 0 0 0 4.8 33.3 28.6 0 19.1 8.1 52.4 81 0 0 0 0 0 0 0 9.5 100

WHO guidelines

250 mg L−1

0.2 mg L−1 3 µg L−1 50 µg L−1 200 mg L−1 2.5 mg L−1 10 µg L−1 15 µg L−1 6.5–8.5

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3.4. S OIL AND SEDIMENT FRACTIONATION RESULTS

Sediments are chemical sinks and under favorable condition toxic metals can be remobilized into the aquatic environment. Samples of soil and sediment fractions treated with each of the four extractants separately to study the effect of grain size or particle size on the distribution of metals revealed increased concentrations of metals with decreasing grain size. Zhang et al. (1990) also reported similar pattern for the –2 µm sediment fraction. Variation in behavior of elements with grain size have been investigated and they are largely attributed to differences in their relative potential for sorption on to clay minerals, hydrous oxides and organic matter surfaces, all of which tend to be concentrated in the smaller grain sizes (Sager, 1992; Schoer, 1985). Extractions with each of the four extractants on separate samples of the fractions also showed that concentrated nitric acid extracted more metals irrespective of particle size followed by 0.5 M HCl but both results were comparable in some cases. With respect to nitric acid extraction and 45 – Ca > Mg > Mn > Fe > Al. Concerning the mobility of metals in soils, the water solubility of Fe, Mn, Zn, Co, and Cd increases with deceasing pH (Sager, 1992). Mobility of metals in soils and sediments are affected by a variety of factors (Sager, 1992; Petruzzelli, 1990) such as the type of interactive processes in soil (sorption, desorption, precipitation, etc.) and the characteristics of the soil involved (pH, amount of organic matter, type and content of clay, available surfaces, etc.). The study of metals distribution in soils and sediments may suffer inadequacies due to the errors (e.g. effect of air

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Figure 2a. Upgradient and downgradient concentrations of dissolved solids in Ibadan.

and oven drying, freezing, evaporation, time delays before analysis, etc.) that may occur during sample handling and analyses (Kersten and Förstner, 1995). Ibadan sediments were polluted with respect to Zn and Pb metals when compared to values for African unpolluted inland water sediments (CIFA, 1994). Similarly, the Lagos sediment samples were polluted with respect to Zn, Cd, Cu and Pb. The reported mean metal concentrations in African unpolluted inland water sediments in µg g−1 dry weight were as follows: Hg (0.05–0.3), Cd (0.11), Pb (19), Cu (33), Zn (95), Mn (770), and Fe (41000). 3.5. U PGRADIENT AND DOWNGRADIENT CONCENTRATIONS OF SELECTED WELLS COMPARED

Pictorial comparison of upgradient and downgradient concentrations of dissolved solids, conductivity, sulfate, chloride, Na, K, Fe and COD for Ibadan and Lagos suggests possible impact of the sites on groundwater. Figures 2a–b and 3a–b presents the upgradient and downgradient dissolved solids and chloride values for Ibadan and Lagos. In Ibadan, well number 28 (a downgradient well and less than 5 m from the waste site) had dissolved solids (Figure 2a) and chloride (Figure 2b) maximum values of 861 mg L−1 and 314.9 mg L−1 , respectively. Chloride is known to be a good indicator of pollution and well number 28 had significant impact from the waste site judging from the chloride and dissolved solids values. The chloride and dissolved solids values for well 28 were above their respective WHO drinking water standards. In Ibadan, the difference in chloride values between some upgradient and downgradient wells were up to 100 mg L−1 . Also the differences in dissolved solids concentrations were as much as 400 mg L−1 . Judging by the

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Figure 2b. Upgradient and downgradient concentrations of chloride in Ibadan.

Figure 3a. Upgradient and downgradient concentrations of dissolved solids in Lagos.

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Figure 3b. Upgradient and downgradient concentrations of chloride in Lagos.

concentrations of dissolved solids and chloride near the Lagos site, the impact of the waste site on the wells was less significant. The dissolved solids concentration decreased by 0.81 mg L−1 while that of chloride decreased by 0.67 mg L−1 from the waste site to the wells. The highest chloride value reported in Lagos was about 100 mg L−1 and this was below the WHO stipulated guideline for drinking water. Also, the dissolved solids concentrations recorded for Lagos upgradient and downgradient wells were all below the WHO standards for drinking water. Comparison of some water quality constituents of wells within 10 m and >10 m from the Ibadan waste site and wells within 20 m and >20 m from the Lagos waste site through parametric analysis of variance (F-test) suggests significant variations of conductivity, dissolved solids, chloride, ammonia and COD at 95% confidence level. Also one-way ANOVA applied on the data obtained at different sampling periods at Ibadan suggests statistically significant difference in variances between upgradient and downgradient data at 95% confidence level. Significant variations were observed for pH, specific conductivity, dissolved solids, chloride, and sulfate for Ibadan wells. One-way parametric ANOVA on the Lagos data suggests that variances of pH, specific conductivity, dissolved solids, chloride, sulfate, nitrate, ammonia, and COD were statistically different at 95% confidence level. However, the one-way parametric ANOVA is deficient because it mixes both upgradient and downgradient wells and as such could not provide information on the wells that were contaminated. Confirmation of where the differences existed was not conducted using Bonferroni t-statistic since parametric procedures have been adjudged to be less accurate than non-parametric procedures.

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Non-parametric procedures (Mann-Whitney ‘T’ test and Kruskal-Wallis H-test) were then applied to upgradient and downgradient wells to further evaluate leachate impact on groundwater quality. Applying the Mann-Whitney ‘T’ test to ranked analytical data, it was found that upgradient concentrations of almost all the water quality constituents analyzed except iron, sulfate, and pH did not differ significantly with downgradient concentrations at 95% confidence level. Thus, the null hypothesis that the downgradient values of Fe, sulfate and pH were the same as the upgradient values were rejected at 95% confidence level. The average Fe levels for upgradient and downgradient wells were 0.12 and 1.07 mg L−1 , respectively. The average sulfate concentrations for upgradient and downgradient wells were 4.2 and 27.3 mg L−1 , respectively. The average pH values for upgradient wells and downgradient wells were 6.13 and 5.59, respectively. The Kruskal-Wallis H-statistic conducted revealed that variances of pH, specific conductivity, dissolved solids, and chloride for Ibadan upgradient and downgradient wells were significantly different at 5% level of significance. Similarly, the Lagos upgradient and downgradient wells showed significant variation in variances of only specific conductivity and dissolved solids at 5% level of significance. To ascertain specific wells that had significant contamination, further comparison of average rank of background wells (average rank of selected upgradient wells was used as background value) and each downgradient wells were conducted on parameters that had unequal variance in the Kruskal-Wallis H-test. From the test, downgradient wells DG6, DG8 and DG9 in Lagos had significant contamination from dissolved solids. Similarly, downgradient wells DG1, DG2, DG 23 and DG 28 in Ibadan also had significant contamination from dissolved solids and chloride intrusion. Sulfate was also significant for DG1, DG2 and DG28 in Ibadan. In a similar study in Malaysia (Ahmed and Sulaiman, 2001), downstream borehole values of electrical conductivity, sodium, potassium, chloride, hardness, phosphate and nitrite were higher than the corresponding upstream borehole values. Also the degree of contamination of the underground water quality in the Athens area, Greece increased with decreasing distance from the landfill (Loizidou and Kapetanios, 1993). More detailed work especially on the geology of the two sites is imperative but the hydrochemical maps obtained for the Lagos dissolved solids (Figure 4a) and chloride (Figure 4b) concentrations over the sampling periods showed values below the WHO standards for drinking water. The effect of the Lagos waste site on the wells may not manifest immediately but may take years to cause significant damage to the aquifer. Deeper aquifers of the two sites may be at risk from possible contamination since most of the drinking wells studied were shallow and vertical migration of pollutants may be more significant than horizontal flow. The Lagos and Ibadan sampled areas are both located in elevated zones relative to the waste sites. Thus, the low migration of materials over time towards the wells probably may be due to the low hydraulic gradient from the waste site to the wells. The leachate emanating from the Lagos waste site is largely westward and in opposite

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Figure 4a. Dissolved solid distribution for Lagos sampled area.

direction to the wells. Elevations and water level measurements of sampling points (Figure 5) near the Ibadan waste site suggested a groundwater dominant flow pattern moving largely towards the west and also in the northwest direction. This also suggested a low migration of pollutants towards the wells especially on the eastern and northeastern side of the Ibadan waste site. 4. Conclusions The analytical results of the 51 wells monitored in this study irrespective of source of pollution, revealed that groundwater from these private wells requires further purification to ensure their suitability for human consumption because the levels of some of the water quality parameters exceeded the WHO guidelines for drinking water. This study also revealed that nutrients and trace metals in the soils and sediments from the two sites were mostly in form of exchangeable metals, carbonate bound metals, and as oxides and hydroxides. These metals are largely of anthropogenic sources from the extractions conducted. The leaching of trace metals

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Figure 4b. Chloride distribution for Lagos sampled area.

into groundwater will be enhanced under favorable conditions such as low pH and high hydraulic gradient from the waste site towards the wells studied. However, the high pH (pH 8.5–9.1) condition of the soils and sediments (Tables I and II) and the low hydraulic gradient from the waste sites to the wells monitored may have posed limitation on the leaching of toxic metals into groundwater. Based on the statistical analysis of selected wells, three downgradient wells in Lagos and four downgradient wells in Ibadan were significantly impacted by leachate migration during the sampling seasons. These areas need to be independently studied over time to fully evaluate the environmental impact of the waste sites on the water resources and the health of citizens in the two neighborhoods. The results of this study indicate the need for environmental education, adequate regulations and proper management of the waste sites by the Nigerian government. Well owners were educated on the implications of inadequate well protection from storm water/runoffs and siting wells near waste sites or septic tanks. Waste dumping on both sites by companies and individuals

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Figure 5. Elevation and dominant flow pattern near Ibadan waste site.

is now prohibited and the sites are gradually being reclaimed. The Federal government of Nigeria has set up the States and Federal Environmental Protection Agencies (FEPA) to help protect the Nigerian environment from pollution in general. Some of the measures already in place to help curtail the disastrous effects of leachate migration into groundwater include laws on discharge and disposal of wastes especially industrial wastes; new and better designed waste sites in Lagos and Ibadan cities; and drinking water guidelines for Nigeria. A huge water project to help provide drinking water to a larger population of Lagos metropolis was recently undertaken.

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