INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 7, No 2, 2016

Transcription

1 INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 7, No 2, 2016 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN Impact of Olushosun dumpsite on groundwater quality in Ojota area of Lagos state Akinade-Solomon, Olorunfemi 1, Adegbie, A. T 2, Ibitola M.P 3, Appia, J.Y 4, Abe, Oluwaseun 5 1- Nigerian Institute for Oceanography and Marine research [NIOMR] 3, Wilmot Point Road, Victoria Island, P.M.B , Lagos, Nigeria. fakinadesolomon@yahoo.com ABSTRACT The impact of buried refuse at Olushosun landfill in Ikeja local Government area of Lagos State on the groundwater quality has been carried out using geophysical methods. The geophysical method involved Vertical Electrical soundings using schlumberger array. Eight geoelectric sections were delineated from the interpretation of data acquired and were used to show the extent of pollution. It was concluded that the bulk of the pollution is concentrated within the central portion of the landfill and the groundwater in the area has been polluted to a depth of about 98 meters. Keywords: Geophysical, landfill, pollution, groundwater, geoelectric, schlumberger array. 1. Introduction The disposal of refuse is a major environmental problem in most Nigerian cities, and in particular, the metropolitan, Lagos State region. Lagos seems to have acquired the unenviable status of being one of the dirtiest cities in the world. An important element in this regard is the inability of the city management authorities to cope effectively with waste disposal. Human activities create vast amount of various wastes and pollutants. The release of these materials into the environment sometimes causes serious health problems. Among health hazards that have resulted from the lack of an effective disposal system are periodic epidemics and communicable diseases (Adesina, 1986). Refuse for dumping comes primarily from household and commercial solid wastes. When refuse is placed in a dump, it undergoes a number of biological, physical and chemical changes. The biological degradation of organic compounds results in the generation of gases, aliphatic acids and other liquids (Qasin et al, 1970; Riley et al, 1976). These gases, aliphatic acids and other liquids can permeate through the soil and result in the pollution of groundwater. Groundwater pollution occurs due to the contamination potential of leachate from waste (Makey, 1982). Leachate is the liquid produced when rainwater percolates through solid waste and reacts with the products of decomposition, chemicals and other materials in the waste. Leachate consists of decomposed organic and inorganic matter, which is subsequently mixed with water to produce effluents. The composition of leachate is highly variable in nature depending on numerous environmental factors and the characteristics of refuse (Chem et al, 1974; Yen, 1974; Pavoni et al, 1975). Purely domestic refuse origin commonly contain high concentration of soluble organic matter and inorganic ions while those from dumps of industrial wastes may have particular variations reflecting the type of waste from which they come such as high phenols or high heavy metal concentrations. If a landfill contaminates groundwater, a plume of contamination will occur and wells in that plume will be Submitted on April 2016 published on November

2 contaminated. In the Lagos area, the cheap mode of disposal of huge refuse matters being generated domestically and industrially has over the years, been reducing the potential sources of utilizable water for the growing population (Asiwaju-Bello and Akande, 2001). Information on sub-surface geology and aquifer characteristics below the metropolis indicate a complex lithology of alternating sand and clay deposits with three aquifer horizons (Longe et al, 1987). The water table aquifer (average thickness of 8m) which is mostly exploited is exposed to the danger of pollution by leachate effluent from indiscriminately sited refuse dump grounds (Asiwaju-Bello and Akande, 2001). This research work aims at integrating geophysical method to determine the depth of pollution of the subsurface within the study area, generate the geoelectric section of the subsurface and to delineate possible leachate polluted zones. The geophysical method employed is the Vertical Electrical Sounding. 2. Study area Lagos was the capital of Nigeria until the early 1990 s when the capital territory was moved to Abuja. It however remains the commercial and economic center of the nation, the most populous and the most urban center in Africa and one of the most populated cities in the world (Myers, 1991). Such geographic and demographic factors have resulted in its growing population and consequently, high rate of waste generation. There are many landfills serving the metropolis and most are surrounded by commercial and residential areas. The study area, Olushosun landfill (figure 1) is located north of Lagos State along Lagos- Ibadan expressway. It is the largest in Africa and one of the largest in the world with an area coverage of sq. meters. The site receives up to 10,000 tons of rubbish every day and accommodates about 85% of refuse in Lagos State. Olushosun landfill was opened up by Lagos Waste Management Authorities in The site was initially a burrow pit where laterite was obtained for construction of roads around Lagos. It was excavated to produce artificial gully and later abandoned. 2.1 Geology and Hydrogeology of Lagos State The area of study is within Lagos State. Lagos State, southwest of Nigeria is located within the sedimentary basin of Nigeria variously described as the eastern portion of Dahomey Basin. The basin is predominantly composed of sand, silt, sandstone, limestone and clay (Jones and Hockey, 1964). Various workers have described the stratigraphy to consist of Abeokuta group (Ise, Afowo and Araromi Formations), Ewekoro, Oshosun, and Ilaro Formations and Benin formation (coastal plain sands). Lagos State is basically a sedimentary area located within the western part of Nigeria, a zone of coastal creek and lagoon (Pugh, 1954). The area is also developed by barrier beaches associated with sand deposition (Hill and Webb, 1958). The subsurface geology reveals two basic lithologies: clay and sand deposits. These deposits may be interbedded in places with sandy clay or clayey sand and occasionally with vegetable remains and peat. The water bearing strata of Lagos State consists of sand, gravel or admixtures from fine through medium to coarse sand and gravel (Longe et al, 1987). Basically, there are four major aquiferous units that are being tapped for the purpose of water supply in the Lagos metropolis: 117

3 The first aquifer extends from the ground level to roughly 12m below the ground. It consists of alternation layers of clay and sand. This aquifer is prone to contamination from surficial activities because of its limited depth. Figure 1: Satellite image showing the study area The second aquifer is encountered between 20 and 100m below the sea level and it can be found around Ikeja and Ojota axis. This aquifer is of greater importance for water supply purposes throughout Lagos metropolis (Jones and Hockey, 1964). The third aquifer is encountered in the central part of Lagos at a depth ranging from m below the sea level. The fourth aquifer is located at an elevation of approximately 450m below the sea level. It is separated from the third aquifer by a rather thick layer of shale of the Ewekoro formation. Only few boreholes tap water from this aquifer (Jones and Hockey, 1964). 3. Materials and methods Electrical resistivity method commonly used for hydrogeological, mining, geotechnical investigations, and environmental surveys (Loke, 2001) was adopted for the study. Of the two main types of procedure employed in resistivity survey, only the vertical electrical sounding method was used. A total of fourteen vertical electrical sounding (VES) were carried out at Olusosun landfill site using Schlumberger array (figure 2). The current electrode separation (AB/2) was varied from a minimum of 2.0m to a maximum of 310m at the VES locations. ABEM 1000 resistivity meter was used in obtaining the apparent resistivity of each VES location. The apparent resistivity was then plotted against the electrode spacing (AB/2) on a log-log graph. Apparent resistivity sounding curves were interpreted using partial curve matching 118

4 techniques, utilizing two layer Schlumberger master curves and appropriate auxiliary curves to obtain an initial layer resistivity and thickness used as a starting model for a computer iterated interpretation technique known as WINRESIST. 4. Results, Interpretation and discussion Figure 2: Data acquisition map of the study area The result of the vertical electrical sounding is summarized in Table 1: VE Geoelectric Resistivity(Ω Thickness( Depth(m Elevatio S layers m) m) ) n (m) No Probable lithology Topsoil 6.6 Clay (polluted) Sand (polluted) Sand Topsoil Clay(pollute d) Clay(pollute d) Sand(pollute d) Sand Topsoil Decomposed refuse 119

5 material Decomposed refuse material Sand Topsoil Decomposed refuse material Decomposed refuse material Sand(pollute d) Sand Topsoil Decomposed refuse material + sand Decomposed refuse material + sand Sand Sand Topsoil Clay Clay Clay Sand(pollute d) Sand Topsoil Clayey sand Sand(pollute d) Sand Topsoil Laterite Clay Sandy clay Clay Sandy clay 120

6 Clay Sandy clay Topsoil Clayey sand Clayey sand Sand Clayey sand Sand Topsoil Laterite Sandy clay Sand Clay Topsoil Laterite Sandy clay Sand Clay Topsoil Laterite Sandy clay Sand Sandy clay Clay Topsoil Laterite Sandy clay Sand Clay Topsoil Laterite Sandy clay Clayey sand Sand Clay Sandy clay Eight geoelectric sections were delineated from the vertical electrical sounding (Figures ). 4.1 Geoelectric Section 1 This geoelectric section consists of VES 1 and 2 taken below the heap of the dumpsite. A maximum of five geoelectric layers were obtained: 121

7 The first geoelectric layer is the topsoil and has resistivity between 4.5 and 5.8Ωm with a thickness range of m. The second geoelectric layer is composed of clay with resistivity range Ωm and thickness m. This layer is polluted due to the flow of leachate. The third geoelectric layer of resistivity Ωm and thickness m represents a conductive region due to the leachate from decomposed refuse materials. Lithologically, this layer is composed of highly porous sand and serves as the first aquifer in the area. However, this layer has been polluted by the leachate from the decomposed refuse material thus its low resistivity values ( Ωm). The fourth geoelectric layer of VES 1 consists of sand of resistivity 369.6Ωm and a depth below 15.8m. The fourth geoelectric layer of VES 2 also consists of sand, however, this layer has been polluted thus its low resistivity value of 20.5Ωm and thickness 41.2m. The fifth geoelectric layer of VES 2 has resistivity Ωm and a depth below 48.4m. This layer is composed of sand and constitutes a clean water bearing aquifer, free of pollution. 4.2 Geoelectric Section 2 This geoelectric section consists of VES 3, 4 and 5. A maximum of five geoelectric layers were obtained: The first geoelectric layer is the topsoil with resistivity between Ωm and thickness range m. The second geoelectric layer has resistivity between Ωm and thickness m. This layer consists of the decomposed refuse material in the dumpsite. The third geoelectric layer has resistivity between Ωm and thickness m. This layer also consists of decomposed refuse material in the dumpsite. The fourth geoelectric layer composed of sand has resistivity range between Ωm and thickness of m. The low resistivity bed (32.4Ωm) indicates a region of pollution. The fifth geoelectric layer is composed of sand with resistivity range Ωm. the depth for this geoelectric layer cannot be determined as the electrode current terminated within this layer. Figure 3: Geoelectric section 1 Figure 4: Geoelectric section 2 122

8 4.3 Geoelectric Section 3 This geoelectric section consists of VES 6 and 7. The first geoelectric layer is the topsoil of resistivity between Ωm and thickness m. The second geoelectric layer has resistivity between Ωm and thickness m and is composed of clay. The third geoelectric layer has resistivity between Ωm and thickness m. it is composed of sand. This region reflects a high level of pollution with a vertical extent of about 90m indicating that the first aquifer beneath these VES points has been polluted and could constitute a danger to the health of the citizens. The fourth geoelectric layer is composed of sand. It has resistivity between Ωm the thickness of this layer is unknown as the electrode current terminated within this layer. However, the resistivity signature coupled with the chargeability information shows that the layer is composed of clean sand with very good quality fresh water. 4.4 Geoelectric Section 4 This geoelectric section consists of VES 8, 9 and 10. These vertical electrical soundings were obtained along a plumped virgin area. The first geoelectric layer is the topsoil of resistivity range Ωm and thickness m. The second geoelectric layer is composed of laterite and clayey sand and has resistivity between Ωm and thickness m. The third geoelectric layer is composed of sandy clay. It has resistivity range Ωm and thickness m. The fourth geoelectric layer has resistivity layer has resistivity range Ωm and thickness m. This layer is composed of clay however this has been displaced to the fourth geoelectric layer of VES 10 suggesting the possibility of a fault between VES 9 and 10. The third geoelectric layer beneath VES 10 is composed of sand with resistivity 617.6Ωm and thickness of 62.5m and constitutes the first aquifer beneath this point. Figure 5: Geoelectric section 3 Figure 6: Geoelectric section 4 123

9 4.5 Geoelectric section 5 VES 11, 12, 13 and 14 are contained in this geoelectric section. These vertical electrical soundings were obtained outside the dumpsite. The first geoelectric layer is the topsoil with resistivity between Ωm and thickness m. The second geoelectric layer of resistivity Ωm and thickness m is composed of laterite. The third geoelectric layer of resistivity Ωm and thickness m is composed of sandy clay. The fourth geoelectric layer is composed of sand and clayey sand with resistivity range Ωm and thickness m. This layer constitutes an aquifer unit along this traverse and does not show any evidence of pollution. The fifth geoelectric layer has resistivity between Ωm and thickness of 41.1m beneath VES 14. However, beneath VES 11 and 13, the thickness could not be ascertained as the current electrode terminated within this layer and beneath VES 12, it is composed of sandy clay of resistivity Ωm. 4.6 Geoelectric Section 6 This geoelectric section consists of VES 13, 1, 4, 7 and 9 in nearly East-West direction.the first geoelectric layer is topsoil with resistivity range Ωm and thickness m. The second geoelectric layer has resistivity between Ωm and thickness m and is composed of laterite, clayey sand, decomposed refuse material and clay. The third geoelectric layer has resistivity between Ωm and thickness m. It is composed of sandy clay, sand, decomposed refuse material and clay. Of interest, along this traverse is a thick sand bed that constitutes the first and second aquifer unit along this traverse. The sand bed is characterized by varying electrical resistivity reflective of the degree of pollution. The Eastern and Western VES point (VES 13 and 9 respectively) appear to be free from pollution while the bulk of the pollution appears to concentrate within the central portion of the dumpsite. A vertical depth of up to 98m at the central portion exhibits pollution signature. Figure 7: Geoelectric section 5 Figure 8: Geoelectric section 6 124

10 4.7 Geoelectric Section 7 This geoelectric section consists of VES 12, 2, 3 and 10 in the East-West direction towards the Northern part of the dumpsite. The first geoelectric layer is the topsoil. It has resistivity between Ωm and thickness m. The second geoelectric layer is composed of laterite, clay (polluted) and decomposed refuse material. It has resistivity between 3.2Ωm and 700.4Ωm with thickness m. The third geoelectric layer is composed of sandy clay, clay (polluted) and decomposed refuse material with resistivity range Ωm and thickness m. Along this traverse, the polluted zone appears to be concentrated at the central portion similar to that of the geoelectric section 6. However, this was observed at shallow depth, restricting the pollution into the first aquifer in the Northern region of the dumpsite. A vertical extent of about 40m has been delineated within the central region believed to have been polluted by the leachate from the decomposed refuse material. 4.8 Geoelectric section 8 This geoelectric section consists of VES 14, 5, 6 and 8 taken along the East-West direction in the Southern portion of the dumpsite. The first geoelectric layer is topsoil with resistivity range Ωm and thickness m. The second geoelectric layer with resistivity between Ωm and thickness m is composed of laterite, clay and decomposed refuse material. The third geoelectric layer comprises sandy clay, decomposed refuse material and clay. Resistivity range is Ωm and thickness m. The fourth geoelectric layer has resistivity between Ωm with a thickness of 19.8 to 63.3m and is composed of clayey sand, sand, clay and sandy clay. The fifth geoelectric layer has resistivity between Ωm with a thickness of m and is composed of sand, sandy clay and clay. However, the sand contained under VES 6 has been partly polluted due to the leachate from the decomposed refuse material present within the dumpsite. The sixth geoelectric layer of VES 6, 8 and 14 had resistivity between 16.1 and 9424Ωm with a thickness of 41.1 to 57.6m and is composed of clay and sandy clay. The resistivity of the seventh layer of VES 8 and 14 ranges from Ωm and is composed of sandy clay and clay. The eighth layer of VES 8 has a resistivity of Ωm and a depth below 174.7m. The pollution along this traverse ranges from a depth of m at the central portion in the Southern region. In general, the geoelectric sections have revealed that the bulk of the pollution is concentrated within the central portion of the dumpsite and has not extended significantly outside the dumpsite. However, the vertical extent has shown that the first and second aquifer within the central area of the dumpsite have been highly impacted by the leachate from the decomposed refuse material. 125

11 Figure 9: Geoelectric section 7 Figure 10: Geoeletric section 8 5. Conclusion Olushosun landfill was opened by Lagos State Waste Management Authorities in 1982 as a disposal site for refuse materials. The effect of the leachate generated by this refuse dumpsite on the groundwater has been considered using geophysical techniques. The results show that the leachate from the dumpsite has penetrated through the unsaturated zone to the saturated zones into the groundwater, thereby contaminating the aquifer zone within the environment. The depth of contamination is about 98m from within the central portion of the dumpsite and hence a source of potential hazard to the health of the citizens in the area. Thus, there is need for Lagos State Waste Management Board as well as the Ministry of Environment and Physical Planning to undertake urgent measures to protect the groundwater in the area. 6. References 1. Adesina, H.O. (1986), Urban Environmental and Epidemic Diseases. Proceedings of the National Conference on Development and the Environment (Nigerian Institute of Social and Economic Research), pp Asiwaju-bello, Y.A. and Akande, O.O (2001), Urban Groundwater Pollution: Case Study Of A Refuse Disposal Site In Lagos Metropolis. Journal Water Resources, 12, pp Chem, K.Y and Bowerman, F.R (1974), Mechanism of Leachate formation in: Recycling and disposal of solid wastes; Industrial, Agricultural, Domestic. Edited by T.F Yen, An Arbor Mich; An Arbor Science Publication. 4. Jones and Hockey (1964), Geology of Southwest Nigeria. Geological Survey of Nigeria Bulletin, no. 31, pp Loke, M.H (2001), An Handout on Electrical Imaging Surveys for Environmental and Engineering Studies. pp

12 6. Longe, E.O, Malomo, S and Olorunniwo, M.A (1987), Hydrogeology of Lagos Metropolis. Journal African Earth Science, 6(3), pp Makey, K.S (1982), National Buffers for Sludge Leachate Stabilization of Groundwater Geophysics, 20(4), pp Pavoni, J.L, Heer Jr., J.E and Hagerty, D.L (1975), Handbook of Solid Waste Disposal Materials and Energy Recovery Van Nostrand Reinhold Company, New York. 9. Qasin, S.R and Burchinal, J.C (1970), Leaching of pollutants from refuse beds. American Society of Civil Engineers. Journal of the Sanitary Engineering Division, 96 (541), pp Riley, R.D, Benedict, R.G, Carlson D.A and Seabloom R.W (1977), Chemical and Biological Studies on leachate from a lip. Journal of applied bacteriology 42, pp Yen, T.F (1974), Biodegradation and Biodeterioration In T.F Yen (Editor), Recycling and disposal of solid wastes; Industrial, Agricultural, Domestic. An Arbor Mich; An Arbor Science Publication. 127