Arsenic pollution in aquifers located within limestone areas of Ogun State, Nigeria A.M. Gbadebo Department of Environmental Management and Toxicology, College of Environmental Management Resources, University of Agriculture, Abeokuta, Ogun State, Nigeria ABSTRACT: This paper presents the results of physico-chemical analysis of groundwater sam- ples obtained from twenty (20) hand-dug wells located in ten communities within the limestone belt of Ogun State with a view to determine the quality (or portability) in terms of arsenic con- centration of the water resources of the aquifers in these communities. The result indicates a min- imum of 0.04 mg/L obtained in the wells of Akinbo and a maximum of 0.16mg/L obtained in the wells of Lapeleke. The mean values of arsenic in all the sampled aquifers are generally higher than the maximum permissible value of 0.01mg/L recommended for drinking water by WHO. Also, the main arsenic concentration of soil, shale and limestone in the area are 22.3 g/g, 78.0 g/g and 514.0 g/g respectively while the pollution index (PI) for the water in the aquifers generally for the limestone area is 8.1. Thus both the measured values of this metal and the calculated PI inferred groundwater arsenic pollution in this sedimentary terrain. 1 INTRODUCTION Aquifers are the major source of clean water all over the world. However, the advent of industrial revolution and mechanized farming has drastically changed the quality of groundwater resources by introducing a wide range of chemical contaminants (Nickolas 1996). Besides, widespread problem of aquifer contamination by chlorinated solvent emanating from dry-cleaning industry together with hydrocarbon leakages from buried tanks are also sources of aquifer pollution. All these have deteriorated and degraded the quality of groundwater. Aquifer pollution may arise from the chemistry of the host rock thereby changing the chemical constituents of the interstitial water. A sand dominated aquifer is also more vulnerable to the influence of human activities than the aquifer located between two layers of impermeable materials. Thus geology plays a prominent role in determining the quality of underground water resources. Among the widely reported inorganic pollutants in groundwater, arsenic (As) is of major con- cern all over the world. Chronic arsenic poisoning has been reported in many parts of the world such as Bangladesh, India, Taiwan, Argentina and Chile due to the prolonged ingestion of polluted groundwater (Bhattacharya et al. 2002, Armienta 2003). According to Bhattacharjee et al. (2003) Bangladesh is the worst grip of mass poisoning the world has ever witnessed. Groundwater arsenic enrichment has been ascribed to either mineralization or geochemical mobilization. (Armienta et al. 1997, 2001) while Gonzalez-Hita et al. (1991) and Cebrian et al. (1994) have traced the sources of arsenic groundwater pollution in some rural areas to hydrothermal processes. Further, arsenic pollution in aquifer has also been ascribed to anthropogenic sources (Gutierrez et al. 1996). This study is carried out in open dug wells from selected communities located within the limestone belt where quarrying activities are currently in progress. The purpose of this study is to determine the arsenic concentration level in the well water widely consumed by the residents of these communities and at the same time highlights its health implications. This work is a follow up of research to a previous study in which high levels of arsenic were obtained in the soils collected 85 Natural Arsenic in Groundwater: Occurrence, Remediation and Management – Bundschuh, Bhattacharya and Chandrasekharam (eds) © 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X Copyright © 2005 Taylor & Francis Group plc, London, UK from these areas. So far, there is no reported case or observable symptom of endemic arsenic poi- soning in the study area. However, the findings are meant to draw the attention of the concern gov- ernment on this issue. 2 LOCATION AND GEOLOGICAL SETTING The study area (i.e. Ewekoro Limestone belt of the Dahomey Basin) falls within latitude 6°50ЈN and 7°15ЈN and longitude 3°05ЈE and 3°20ЈE. The major communities in which the well water were collected in the area include: Akinbo, Elebute, Itori, Otun, Lapeleke, Egbado, Papalanto, Ilaro, Aiyetoro, Igbogila (Fig. 1). The topography is divided into two by the southeast trending depression in which Ewekoro Formation is deposited. The outcrop area is slightly above 420m above the mean sea level (Tijani & Ayodeji 2001). The drainage is dense and the major rivers include Ogun, Igbin, Ewekoro, Yewa, Atuwara, Akinbo etc. The area falls within the tropical rain- forest zone of Nigeria with annual rainfall ranging between 1500 to 2000mm and mean annual temperature ranging between 27 and 35°C with two distinct dry and wet seasons (Iloeje 1991, Akanni 1992, Oguntoyinbo et al. 1983). Geologically, the area of the present study forms a part of the Dahomey Basin which is one of the sedimentary basins on the continental margin of the Gulf of Guinea extending from the southeastern Ghana in the west to the western flank of the Niger Delta (Reyment 1965, Jones & Hockey 1964). The Dahomey Basin was formed consequent to the opening of the South-Atlantic during the Neocomian. The stratigraphy and the hydrogeology of this basin (Table 1) has been widely studied by various authors (Jones & Hockey 1964, Fayose 1970, 86 Figure 1. Map of Ogun State showing the geological formations and well location. Copyright © 2005 Taylor & Francis Group plc, London, UK Omatsola & Adegoke 1981, Idowu et al. 1999). However, the sedimentary rock of the study areas in which the aquifers are located are essentially basal conglomerate and arkosic sandstone of Abeokuta Formation; the limestone and shale members of Ewekoro Formation; the massive and poorly consolidated sandstone of Ilaro Formation followed by poorly sorted coastal plain sands and alluvium. Also, the underlining crystalline basement complex rock types in the study area comprises of gneises, older granites, pegmatites etc. 3 METHODS A total of 20 dug well water samples from ten different communities in the study area were col- lected and analyzed. Total depth of the wells, static water levels, temperature, pH and conductivity were determined in the field while the arsenic concentration was determined in the laboratory. The pH of the water samples were measured using field pH meter and the conductivity was measured using WTWLF 95 conductivity meter. The temperature was measure using celcius thermometer. Further, twelve soil and rock samples were randomly collected from the study area and also analyzed for arsenic concentration using Atomic Absorption Spectrophotometer (Buck 200A model). Statistical analysis using regression method and various graphs were plotted to observe the correl- ation pattern of the arsenic concentration with different parameters. The use of Pollution Index (PI) was adapted to predict the level of hazard associated with the aquifers. 4 RESULTS AND DISCUSSION The chemical data on the water, rock and soils samples are shown in Tables 2 and 3. It is observed that Lapeleke has the highest arsenic concentration with mean value of 0.16 mg/L followed by Elebute and Aiyetoro with mean value of 0.10mg/L. The least value of 0.04 mg/L was obtained in the water from aquifer located in Akinbo. These values are higher than that recommended) for drinking water by World Health Organization (0.01 mg/L; WHO 1999 and 2001). 87 Table 1. Stratigraphic units of the Dahomey Basin (Tijani & Ayodeji 2001). Jones & Hockey (1964) Fayose (1970) Omatshola & Adegoke (1981) Age Formation Age Formation Age Formation Recent Alluvium Early Miocene Benin Pleistocene Coastal to Plain Pleistocene Coastal to Ogwashi Oligocene sands to Plain sands Late Oligocene Asaba Oligocene Eocene Ilaro Ososun Late Ilaro Early Ameki Middle & Oligocene Akinbo Early To Middle Paleocene Eocene Eocene Ewekoro Paleocene Ewekoro Early Eocene Araromi and Paleocene Imo Formation Late Maastrichtian Senonian Abeokuta Late Afowo Cretaceous Abeokuta Formation Upper to Maastrichtian Neocomian Ise Precambrian Crystalline Crystalline Basement Formation Copyright © 2005 Taylor & Francis Group plc, London, UK There seems to be a relationship between arsenic concentration and temperature of the aquifer (Fig. 2). In general, at temperatures Ͼ24°C, arsenic mobility into the water is greater, while the very low value of arsenic concentration in the aquifers of Akinbo community at temperature of 24.1°C may be attributed to either the low mineralization in the shale and limestone lithologic units or it’s low mobility in the aquifers in this area. According to Smith et al. (1998), industrial effluents and geological formations are the major sources contributing arsenic to groundwater. The maximum pH value of the analyzed groundwater ranged between 6.9–7.6. However, the values fall within the WHO (1999) recommended pH value range of 6.5–8.5. These values con- form well with the pH values of the arsenic polluted groundwater also from the shallow aquifers 88 Table 2. Range and mean values of physico-chemical characteristics and arsenic concentrations in groundwaters. Electrical Mean As Sample Temperature conductivity As conc. conc. PI location** (°C) pH Depth (m) (S/cm) (mg/L) (mg/L) (Aquifers) Akinbo 24.0–24.2 7.1–7.3 5.4–10.3 487–894 0.00–0.08 0.04 Elebute 23.0–23.5 7.0–7.5 3.6–4.2 316–334 0.06–0.14 0.10 Egbado 23.0–24.0 7.4–7.8 3.6–4.2 146–193 0.06–0.08 0.07 Lapeleke 24.0–24.1 7.1–8.0 1.9–2.3 151–532 0.09–0.22 0.16 Itori 23.8–24.2 6.9–7.2 6.8–8.8 399–874 0.03–0.09 0.06 Otun 23.5–23.8 7.0–7.1 11.6–12.9 41–332 0.03–0.06 0.05 8.1 Papalanto 23.9–24.1 7.2–7.6 3.5–3.7 388–392 0.00–0.12 0.06 Ilaro 24.1–24.2 6.8–7.0 8.2–8.5 350–850 0.05–0.20 0.08 Igbogila 23.7–25.0 6.9–7.0 9.1–9.4 402–905 0.08–0.19 0.09 Ayetoro 24.1–24.8 7.0–7.3 8.8–9.2 405–875 0.07–0.13 0.10 *WHO 25 6.0–8.5 0.01 *FEPA 1000 0.01 * Drinking water standards. ** Duplicate samples were collected at each site. Table 3. Arsenic concentration in soil and the lithological units in the study area. Sample type Sample no. Range of As concentration (mg/kg) Mean of As concentration (mg/kg) Soil 4 5.2–39.5 22.3 Shale 4 77.7–78.5 78.0 Limestone 4 478.0–514.2 514.0 0,00 0,05 0,10 0,15 0,20 23,1 23,4 23,7 24 24,3 24,6 Groundwater temperature (°C) As (mg/L) Figure 2. Relationship between As concentration levels with the groundwater temperatures in the wells. Copyright © 2005 Taylor & Francis Group plc, London, UK in the La Banda County, Argentina which Bejarano et al. (2003) described as generally near neu- tral to alkaline (pH 6.67–8.95). Bethany & Jankowaki (2003) opined that arsenic may be attached to mineral surfaces but are subsequently released due to changes in redox conditions with other oxyanions e.g. oxides of Fe, Mn and sulphide. Comparison of range values of pH and range val- ues of arsenic in Table 2 indicates that there is tendency that at higher pH, more arsenic may be released. There is a little deviation from this pattern in the mean – mean plot of pH and arsenic respectively (Fig. 3). This deviation is probably due to the abundance of bicarbonate materials in the limestone components of the aquifers which Bethany & Jankowski (2003) has noted to be another member which can scavenge arsenic. Probably, being a sedimentary terrain and a limestone belt, the maximum mean depth of aquifer in the study area is 12.3 m while the least mean depth is 3.62m. There is seemingly an inverse cor- relation between aquifer depth and arsenic concentration in the study area. A downward but not perfect progressive decrease in arsenic concentration was observed as the depth of aquifer increases in the study area (Fig. 4). At an aquifer mean depth of 2.12 m arsenic concentration of 0.16 mg/L was obtained while at a mean depth range of 7.79–7.85m the mean arsenic concentra- tion of 0.06–0.04 mg/L was obtained. Also at an aquifer depth of 12.27 m the arsenic mean con- centration recorded is 0.03 mg/L. This trending pattern in aquifer arsenic concentration has been observed by Welch et al. (2003) in Bangladesh where it was discovered that arsenic concentration exceed 100 mg/L at depth of between 23 and 30 m below land surface while the arsenic concentration were Ͻ5 mg/L in the deep aquifer. Also, Wagner et al. (2003) noticed that arsenic content of sedi- mentary aquifer in West Bengal, India decrease with the increasing depth. This was attributed to the vertical transport of arsenic within the aquifer. The electrical conductivity in the aquifer ranges between 137 and 686S/cm, which is equivalent of the Calculated Total Dissolved Solids (CTDS) range of between 82.2 and 401.6mg/L respectively when multiplied by a factor of 0.6. Thus the EC values are indications of the presence of dissolved solutes, hence also the pollutants in groundwaters. It is evident that arsenic is one of the major contaminants present in groundwater of all the aquifers. There appears to be an undulating kind of relationship between the electrical conductivity and the arsenic concentration in groundwaters of the different aquifers (Fig. 5). 89 0,00 0,05 0,10 0,15 0,20 6,8 7 7,2 7,4 7,6 7,8 Groundwater pH As (mg/L) Figure 3. Relationship between arsenic concentration and groundwater pH. 0,00 0,05 0,10 0,15 0,20 02468101214 Aquifer depth As (mg/L) Figure 4. Bivariate plot showing the variation of the groundwater As concentration with the depth of the aquifers. Copyright © 2005 Taylor & Francis Group plc, London, UK The calcium content of the underground water from the aquifers ranged from 4.87 to 30.45 mg/L while the magnesium is from 0.83 to 10.24mg/L. Both sodium and potassium con- stituents ranged from 14.35 to 56.66 mg/L and 1.02 to 44.59 mg/L respectively. Similarly, chloride mean concentration range of 11.57 to 85.08 mg/L and a bicarbonate range of 77.79 to 264.00 mg/L were obtained. Both the sulphate and nitrate ions mean concentrations ranged from 2.92 to 50.79 mg/L and 10.16 to 44.36 mg/L respectively. The chemical data revealed a similarity in the concentration of both the cation and anion constituents of the aquifers with the magnitude of concentration decreasing in the order of Na Ͼ K Ͼ Ca Ͼ Mg for the cations and HCO 3 Ͼ Cl Ͼ SO 4 Ͼ NO 3 for the anions. This sequence according to Olatunji et al. (2001) is an indication of the hydrolytic processes that has taken place between the water in the aquifer and the mineral components of the hosting bedrock under confined pressure and temperature. Also, the high concentrations of the ions in the underground water according to Tijani & Ayodeji (2001) may be related to the ionic affinity among the cations and the anions since sodium occurs with potassium and calcium occurs with magnesium in the aquifers while calcium has affinity for car- bonate or bicarbonate. On the basis of their chemical composition the water characterization in the aquifer of the study area fit into the three main types, viz. Na-(K)-Cl-SO 4 , Ca-(Mg)-Na-(K)-SO 4 and Ca-(Mg)-Na-(K)-HCO 3 das reported by the earlier works carried out by Tijani & Ayodeji (2001) on the surface and groundwater of the Dahomey basin. Apart from arsenic the value of nitrate (10.16 to 34.36 mg/L) was found to be above the WHO (1980) maximum recommended value of 10mg/L while other chemical compositions of the aquifers are within the recommended limits. This implies that both arsenic and nitrates are the major pollution sources of the aquifers in the study area. The pollution index (i.e. the ratio of arsenic concentration to the recommended standard values) calculated for the groundwater geologic resource in all the aquifers from the entire limestone belt in the study area far exceeds 1 (i.e. PI Ͼ 1). This according to Kim et al. (1978) further confirms the anthropogenic source in addition to the natural source of arsenic in the study area. The result of the regression analysis carried out on the measured parameters in all the wells from the study area (Table 4) indicate that there is generally a negative statistical relationship between the arsenic concentration (As conc ), transformed log of arsenic concentration (As conclogTf ) and other parameters. This negative relationship is statistically significant for the arsenic concentration, transformed log of arsenic concentration, depth and pH (i.e. t Ͼ 2.0) but statistically insignificant with the 90 0,00 0,05 0,10 0,15 0,20 0 100 200 300 400 500 600 700 800 Electrical conductivity (S/cm) As (mg/L) Figure 5. Relationship between As concentration and electrical conductivity of the groundwater. Table 4. Summary of values of regression analysis obtained for As conc and As conclogTf for the study areas. Constant Temp. (°C) pH Depth (m) EC (S/cm) R 2 As conc Ϫ0.653 0.105 Ϫ0.218 Ϫ1.882 Ϫ1.895 57.5 (Ϫ0.543) (1.932) (Ϫ2.001) (Ϫ2.308) (Ϫ1.544) As conclogTf Ϫ8.729 1.173 Ϫ2.696 Ϫ0.207 Ϫ2.214 58.2 (Ϫ0.620) (1.841) (Ϫ2.006) (Ϫ2.429) (Ϫ1.543) Values in the parenthesis are the t-values. Copyright © 2005 Taylor & Francis Group plc, London, UK electrical conductivity (t Ͻ 2.0). However, there exist a positive and statistically insignificant rela- tionship between the arsenic concentration, transform log of arsenic concentration and tempera- ture in the study area. In the aquifers of the study areas, a total of 57.5% and 58.2% of the observed variation in the arsenic concentration and transformed log of the arsenic concentration respect- ively were accounted for by both the temperature, pH, depth, electrical conductivity while the remaining 41.8–42.5% are likely to be accounted for by geology of the aquifer, infilteration and percolation effect, atmospheric deposition etc. The statistically significant relationship between the arsenic concentration, depth and pH and also very high percentage of the observed variation in the transformed log of the arsenic concen- tration could be attributed to the high content of arsenic in the limestone unit in which the aquifers are located. Thus, the deeper the aquifers penetrate into the limestone units, the higher the arsenic concentration. 5 CONCLUSIONS Occurrence of arsenic has been found in groundwater from all aquifers located within the lime- stone area of Ogun state under a neutral pH of 6.8–7.5. The near neutral pH favours the occurrence of arsenic in the groundwater environment (see viz. Bhumbla & Keefer 1994, Bhattacharya et al. 2002) in two main inorganic forms: As(III) and As(V). These two forms of inorganic arsenic are mostly found in natural and drinking water (Marisol Vega et al. 2003). The reduced form As(III) is more mobile and more toxic than the oxidized form As (v) . In this study, the total arsenic [As(III) ϩ As(V)] were determined. It is therefore necessary to reduce the carcinogenic effects of arsenic to the recommended standard by chemical oxidation methods using chloride, chloride dioxide and potassium permanganate (Welte 2002) or bacteria (CAsO 1 – genus Thiomonas and genus Ralstonia) oxidation under authotrophic conditions in a pH range of 4 to 8 with a pH opti- mum of 6 (Battaglia-Brunet et al. 2002). REFERENCES Akanni, C.O. 1992. Climate. In S.O. Onakomaiya, O.O. Oyesiku & F.J. Jegede, (eds), Ogun State in Maps. 207p. Ibadan: Rex Charles Publication. 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Fact sheet 210: URL: http://www.who.int/mediacentre/factsheets/fs210/en/print.html (Accessed on March 9, 2004) 92 Copyright © 2005 Taylor & Francis Group plc, London, UK . composition the water characterization in the aquifer of the study area fit into the three main types, viz. Na-(K)-Cl-SO 4 , Ca-(Mg)-Na-(K)-SO 4 and Ca-(Mg)-Na-(K)-HCO 3 das reported by the earlier. which high levels of arsenic were obtained in the soils collected 85 Natural Arsenic in Groundwater: Occurrence, Remediation and Management – Bundschuh, Bhattacharya and Chandrasekharam (eds) ©. portability) in terms of arsenic con- centration of the water resources of the aquifers in these communities. The result indicates a min- imum of 0.04 mg/L obtained in the wells of Akinbo and a maximum