sv-lncs RUHUNA JOURNAL OF SCIENCE Vol. 7: 43-57, December 2016 eISSN: 2536-8400  Faculty of Science University of Ruhuna  Faculty of Science, University of Ruhuna 43 Sri Lanka Assessing the geo-electric characteristics of Basement Complex rocks and its implication for groundwater prospecting in Ilorin Metropolis, Nigeria I. P. Ifabiyi 1, E. D. Ashaolu2 and O. Omotosho3 Department of Geography and Environmental Management, University of Ilorin, Ilorin, Nigeria Correpondence: 1tokunifabiyi@yahoo.com, 2damash007@yahoo.com 3olumideomotosho47@gmail.com Received: 6th October 2016, Revised: 21st December 2016, Accepted: 24th December 2016 Abstract. In Basement Complex rocks where rainfall is seasonal, water provision in dry season depends on regolith aquifer. For effective exploitation of groundwater resources, it is reasonable that geophysical investigation be conducted before development of well. In many instances, geophysical surveys may be expensive or nonexistent. Hence, there is a need for spatial analysis which might advise water engineers within such environments. Vertical Electrical Soundings (VES) data of 53 locations conducted with ABEM SAS-1000 terrameter using Schlumberger electrode configuration were obtained from the hydrogeology Department of Kwara state Ministry of Water Resources and Lower Niger River Basin and Rural Development Authority, Ilorin. VES locational coordinates were recorded using handheld GPS device. Sound curves were evaluated by partial curve matching approach and computer iteration using WinResist. The results depict six geo-electric regional successions, namely: top soil, lateritic clay, weathered basement, fairly-hard basement, thin fractured and hard basement. The geo-electric succession identified was plotted in Surfer 12 environment, using kriging interpolation method to show spatial distribution pattern of this zone. The spatial pattern is expected to give an insight to the nature of spatial variability of geo-electric layers and assist drillers as well as water resources policy makers in their operations. Keywords. Geo-electric, groundwater, basement complex, geophysical, GIS 1 Introduction Groundwater occurrence is often localized and confined to weathered or fractured zones in the Basement Complex region, and groundwater exploration in such terrain is always complex. The crystalline basement rocks mailto:2damash007@yahoo.com I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 44 Vol. 7: 43-57, December 2016 have low porosity and permeability, hence, have no water storage capacity in their unaltered form; which makes their groundwater prospects to be limited and often restricted to features produced by weathering and tectonic processes (Olayinka and Olorunfemi, 1992; Olorunfemi and Fasuyi, 1993; Oyedele et al., 2013). In tropical basement rock, weathering process creates superficial layers, with varying degrees of porosity and permeability. This unconsolidated superficial layers, if significantly thick, porous and permeable, makes good aquifer units. It is important to note that the concealed basement rock may contain faulted areas, incipient joints and fracture systems derived from earlier tectonic processes in such region. The detection and delineation of these hydro-geologic structures may facilitate the location of groundwater potential zones in a typical basement rock environment (Omosuyi, et al., 2003; Oyedele, et. al., 2013). Olorunfemi and Fasuyi (1993) submitted that the highest groundwater yield in basement terrains is found in areas where thick overburden overlies fractured zones. These fractured zones are often characterized by relatively low resistivity values. Olayinka and Olorunfemi (1992) argued that before a borehole is sited, a surface geophysical survey such as Vertical Electrical Resistivity Sounding (VES) should be conducted to identify the localized aquifer for a productive well. The Vertical Electrical Resistivity Sounding survey provides information about the subsurface that aid in aquifer delineation and identification of lithologic boundaries and geological structures (Bose, et al., 1973; Abiola, et al., 2013). Vertical Electrical Resistivity Sounding method has been used widely by scholars in groundwater prospecting especially in the Basement Complex terrains to get detailed information about hydrogeological settings for groundwater potentials (Olorunfemi, 1990; Olorunfemi and Olayinka, 1992; Olorunfemi and Fasuyi, 1993; Oladapo, et al., 2009; Anohanmoran. 2013; Ogundana and Talabi, 2014). VES is used to determine the vertical variation of electrical resistivity below the earth surface and the potential field generated by the current, and this is because electrical resistivity of most rock depends on the amount of water in their pores. This method proved useful in groundwater studies because neither the structure nor the dynamics of the soil was disturbed (Otobo and Ifedili, 2005; Adiat et al., 2009; Ariyo and Adeyemi, 2009; Anomoharan, 2011; Anomoharan, 2013). Despite the importance of VES in groundwater prospecting in a Basement Complex terrain, a better interpretation of hydrogeological data generated from this method often requires that their spatial location be incorporated into the analysis (Shahid and Nath, 2002). This will reveal the spatial variation of different geo-electric section of Basement Complex rock which can give a I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 45 Vol. 7: 43-57, December 2016 better understanding of the hydrogeological prospect, especially of a large area. Consequently, the incorporation of Geographical Information System (GIS) into studies of groundwater prospecting becomes imperative. According to Shahid and Nath (2002), in recent time, GIS is widely used for spatial modeling of hydrogeological prospect of a large area with more reliability on groundwater exploration. Further, GIS has proved to be an efficient tool in groundwater researches and the inclusion of subsurface information deduced from geo-electric survey can give more realistic picture of groundwater potentiality of an area (Saraf, et.al., 1998; Krishnamurthy, et.al., 1996; Murthy, 2000; Shahid and Nath, 2002; Amaresh and Ravi Prakash, 2003). Ilorin city is underlain by Precambrian Basement Complex; comprising mostly gneiss, granite, schist, undifferentiated meta-sediments rocks and overburden that are composed mainly of clay, sand and silt soils. The residents of this area often augment the public water supply by the Kwara State Water Corporation with groundwater (shallow and deep) because the supply is erratic and unreliable (Ifabiyi and Ahmed, 2011) and the coverage is limited to some areas (Ifabiyi and Ashaolu, 2013). The population of this city is rapidly increasing as new residential areas have sprung up and continue springing up in the last decade. All these new residential areas depend solely on groundwater for their domestic needs. On this basis, this study assessed and mapped the geo-electric characteristics of Basement Complex Rock of Ilorin, Nigeria in order to identify their spatial variation and implication on groundwater prospects of the city. 2 Material and Methods 2.1 The study area Ilorin the Kwara state capital is located between latitude 08°24'N and 08°38'N of the equator, and longitude 04°26' E and 04°37'E of the Greenwich meridian, and covers about 12km. Ilorin is one of the fastest growing urban centers in Nigeria. There has been a huge increase in the population of Ilorin since it became the state capital in 1976. The population growth rate is much higher than other cities at 2.9 percent of the national growth rate. The 2006 census put the population of Ilorin city to about 847,582 (NPC, 2006 provisional results). Ilorin has a tropical wet and dry climate. Wet season is experienced from April to October and dry season from November to March. Rainfall condition in Ilorin exhibits greater variability both temporarily and spatial. The annual mean rainfall is about 1,200mm, exhibiting the double maximal pattern between April and October of every year. Relative humidity varies seasonally with an average of 79.7%. I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 46 Vol. 7: 43-57, December 2016 The city is underlain by Precambrian Basement Complex, comprising mostly gneiss, granite, schist, undifferentiated meta-sediments rocks and overburden that are composed mainly of clay, sand and silt soils. The underlying pre-Cambrian igneous-metamorphic rock of Basement Complex is neither porous nor permeable except in places where they are deeply weathered or have zones of weakness. Some part of the town is also laid by Sedimentary rocks, which contains both primary and secondary laterites and alluvial deposits. Groundwater on the alluvium is recharged directly by rainfall or the adjoining overflowing river system. In the dry season, the alluvium sustains considerable subsurface groundwater flow. The alluvial deposits have been exploited, with successful wells and boreholes in Ilorin metropolis and its surrounding. The drainage system of Ilorin is dendritic in nature, and is dominated by Asa River, which flows from south to north and divides the city into two parts, the western and eastern parts. The map of the study area is presented in Figure 1. Fig 1. Geological map of the study area showing the sampled points I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 47 Vol. 7: 43-57, December 2016 2.2 Methods This study adopted the combination of secondary and primary (field work) data. The secondary data collected are the Vertical Electrical Soundings (VES) data conducted by the Kwara State Ministry of Water Resources and Lower Niger River Basin Development Authority, Ilorin. All the Vertical Electrical Soundings (VES) data collected were conducted with ABEM SAS- 1000 Terrameter using the Schlumberger electrode configuration, and the electrode spacing (AB/2) varied from 0.1 m to 200m. The results of geophysical survey carried out in 53 different locations in Ilorin were collected. The minimum number of VES carried out in each of these 53 locations was 9 and the maximum were 12. The sounding curves were evaluated by partial curve matching method and computer iterations using WinResist. The field work was carried out to get the coordinates (locations) of the sampled points using handheld Global Positioning System device. This was carried out to get the coordinates of all the locations used in the study, which facilitated the plotting of the data in Surfer 12 GIS software. The resistivity values from the interpretation of the field data using curve matching were used to generate geo-electrical succession. The data on geo- electric layers and the point location collected were used to plot the geo- electric map of the study area. This was carried out in Surfer 12 GIS software using kriging interpolation method to determine the spatial distribution of the identified layers in the study area. 3 Results and Discussion 3.1 General Pattern of Geophysical Characteristics of Groundwater in Ilorin The identified layers resistivity and thickness ranges across the sampled points in the study area are presented in Table 1. From the table, six regional geo-electric patterns are discernable, namely: top lateritic sand, lateritic clay, weathered basement, fairly hard basement, thin fractured and hard basement. However, the study looks at the vertical variations of the electrical resistivity recorded from one point to another across the study area, hence, overlaps observed in the reported VES range. The first layer consists of the top soil with resistivity values ranging from 30-3000 ohm-m and thickness ranging from 0.2-1.0 m. The lateritic clay layer is where resistivity ranges between 23- 1400 Ohm-m and thickness from 2-20 m. The third lithologic layer is characterized by highly weathered basement with resistivity values between 22-1000 Ohm-m and thickness of 2-45 m. Electrical resistivity values here are controlled by the degree of water saturation (Odunsanya and Amadi, 1990; Oladipo et al., 2009). I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 48 Vol. 7: 43-57, December 2016 Table 1: Ranges of geo-electric succession in Ilorin, Nigeria SN Layers Resistivity (ohms) Thickness (m) 1 Top Lateritic Soil 30-3000 0.2-1.0 2 Lateritic Clay 23-1400 2-20 3 Weathered Basement 25-1000 2-45 4 Fairly Hard Basement 22-600 6-42 5 Thin Fractured 55-145 20-40 6 Hard Basement 35-780 15-42 above The fourth layer is a fairly hard basement weathered and resistivity ranging from 22-600 ohm-m and thickness between 6-42m. The fifth layer represents a thin fractured zone and resistivity ranging from 55- 145 Ohm-m and thickness between 20-40m. The sixth layer represents the hard basement with resistivity values ranging from 35- 780 Ohm-m and thickness between 15- 42m and above across the study area. Figure 2 and Table 2 show the resistivity value of each geo-electric succession in the 53 sampled locations across the study area. Fig 2. Geo-electric section of Vertical Electrical Sounding in Ilorin Metropolis I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 49 Vol. 7: 43-57, December 2016 Table 2: Geo-electric succession in the study area (Res: Resistivity Ω-m; T: Thickness, m) SN Description Top Lateritic Soil Lateritic Clay Weathered Basement Fairly Hard Basement Thin fractured Hard Basement Res. (Ω-m) T (m) Res. ( Ω-m) T (m) Res. (Ω-m) T (m) Res. ( Ω-m) T (m) Res. ( Ω-m) T (m) Res. ( Ω-m) T (m) 1 Apata Yakuba 1500 0-1 300 2-3 80 3-20 160 20-40 0 0 270 40-above 2 Ganiki Sango 85 0-1 130 2-6 90 6-20 190 20-35 0 0 250 35-above 3 Royal Valley 400 0-1 320 2-3 145 3-15 200 15-30 0 0 400 30-above 4 Elekoyangan 120 0-1 NA NA 180 2-15 250 15-20 0 0 600 20-above 5 Sango Area 100 0-1 50 2-10 65 10-30 52 30-40 0 0 68 40-above 6 Oyun Area 100 0-1 120 2-3 65 3-10 200 10-30 0 0 350 30-above 7 Alagbado 120 0-1 250 2-6 70 6-20 150 20-35 145 35-40 180 40-above 8 Okelele 30 0-1 68 2-8 52 8-25 60 25-35 0 0 66 35-above 9 Olorunsogo 170 0-1 160 2-3 35 3-20 40 20-40 0 0 55 40-above 10 Agbabiaka area 400 0-1 700 2-15 170 15-30 180 30-40 0 0 190 42-above 11 Kilanko Area 270 0-1 NA NA 35 2-20 110 20-40 0 0 150 40-above 12 Tanke Area 110 0-1 80 2-20 200 20-40 365 40-40 0 0 380 42-above 13 Danialu 280 0-1 300 2-3 225 3-20 260 20-30 0 0 450 30-above 14 Fate Tanke 80 0-1 100 2-5 90 5-10 105 10-20 55 20-30 600 15-30 15 Wonderland Chapel 200 0-1 1400 2-15 0 0 600 15-30 0 0 780 30-above 16 Oloje Area 270 0-1 160 2-5 32 5-6 58 20-30 60 30-35 85 35-above 17 Asileke 290 0-1 220 2-15 0 0 250 15-25 0 0 450 25-above 18 Jooro 25 0-1 85 2-10 72 10-20 100 20-35 0 0 130 30-above 19 Okolowo Area 220 0-1 70 2-3 30 3-20 40 20-40 0 0 60 42-above 20 Yebumot Area 1200 0-1 900 2-10 150 10-35 70 35-42 0 0 180 42-above 21 Bal. Eng. Area 300 0-1 80 2-6 52 6-30 65 30-40 0 0 80 40-above 22 Hajj Camp 340 0-1 450 2-6 110 6-20 200 20-40 0 0 0 0 23 Alhikmah UNIV. Area) 710 0-1 350 2-6 60 6-25 125 25-40 0 0 0 0 24 Adewole Area 120 0-1 200 2-10 50 10-30 80 30-40 0 0 0 0 25 Egbejila 550 0-1 700 2-8 260 8-45 300 15-25 0 0 520 25-above 26 Asa Dam 650 0-1 1200 2-8 100 8-20 150 20-40 0 0 180 42-above 27 Pakata 80 0-1 30 2-6 25 6-15 35 15-30 0 0 40 30-above 28 Iqra college 3000 0-1 280 25 0 0 120 8-40 0 0 160 40-above 29 Baboko Market 38 0-1 130 2-5 13 5-30 22 30-40 0 0 40 40-above 30 Banni Area 80 0-1 130 2-6 0 0 200 6-23 0 0 400 25-above 31 Fufu Str. Sabooke 32 0-1 74 2-10 50 10-30 92 30-35 0 0 130 35-above 32 Post office area 200 0-1 92 2-10 70 10-30 85 30-35 0 0 100 35-above 33 Min. of Agric area 260 0-1 150 2-10 45 10-30 50 30-35 above 0 0 60 35-above 34 Ododosowapo Amilegbe 340 0-1 100 2-10 56 10-30 43 30-35 0 0 68 35-above 35 Onikanga GRA 1200 0-1 1000 2-10 1000 10-30 55 30-35 0 0 60 35-above 36 Sakama Niger Rd 20 0-1 35 2-10 330 10-30 220 30-35 0 0 270 35-above 37 Okaka Niger Rd 720 0-1 1102 2-10 180 10-30 50 30-30 0 0 99 30-above 38 Agbadam Lake Rd 50 0-1 65 2-10 90 10-30 80 30-35 above 0 0 110 30-40 39 Abdulrasaq Rd GRA 200 0-1 180 2-8 140 8-30 170 30-35 above 0 0 175 35-above 40 Unilorin Garden 650 0-1 300 2-8 50 8-30 145 30-35 above 0 0 160 40-above I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 50 Vol. 7: 43-57, December 2016 41 Tipper Garage area 200 0-1 310 2-8 170 8-30 300 30-35 above 0 0 350 30-above 42 Behind Fed Sec. 1000 0-1 350 2-10 10-30 128 30-35 0 0 165 35-above 43 Odofin Lane Basin 100 0-11 260 30-35 above 0 0 450 40-above 44 Water View area 100 0-1 380 2-8 240 8-30 300 30-35 0 0 370 35-above 45 Adelodun Rd Fate 140 0-1 90 2-8 55 8-30 70 30-35 0 0 92 35-above 46 Edun area 265 0-1 130 2-8 55 8-30 78 30-35 0 0 82 35-above 47 Behind Amusement Park 35 0-1 23 2-10 33 10-30 40 30-35 0 0 45 35-above 48 Mustapha Idiagbede Basin 140 0-1 100 2-10 70 10-30 95 30-35 0 0 120 35-above 49 Adeleye Str. GRA 800 0-1 300 2-10 55 10-30 120 30-35 0 0 130 35-above 50 GSS Area 80 0-1 2-10 28 10-30 40 30-35 0 0 60 35-above 51 Kwara ADP area 300 0-1 360 2-10 55 10-30 60 30-35 0 0 100 35-above 52 Ile eleru Ojagbooro 40 0-1 2-10 35 10-30 30 30-35 115 35 35-above 53 Office Rd GRA 300 0-1 97 2-10 32 10-30 51 30-35 above 0 0 55 40-above 3.2 Geo-electric Succession and their characteristics in Ilorin Metropolis 3.2.1 Top Lateritic Soil and Lateritic Clay The average resistivity and thickness values of top lateritic soil in Apata Yakuba, Ganiki Sango and Royal Valley are 662 ohm-m and 1m respectively which indicate lower clay proportion. The average resistivity and thickness values of top lateritic soil in Elekoyangan, Sango area and Oyun area are 106 ohm-m and 2m respectively which indicated that the predominant composition of the top soil is lateritic clay. In Apata Yakuba, Ganiki Sango and Royal Valley, the average resistivity values and thickness of lateritic layer are 250ohm-m and 4m which implies that the layers composed of clayey sand. Also, the average resistivity and thickness values of top lateritic soil in Alagbado, Okelele and Olorunshogo areas are 107 ohm-m and 2m respectively which indicates that the predominant composition of the top soil is lateritic clay. The average resistivity and thickness values of top lateritic soil in Agbabiaka area, Kilanko area and Tanke area are 107 ohm-m and 2m respectively which is an indication that the predominant composition of the top soil is lateritic clay. Figure 3 shows map of the spatial distribution pattern of lateritic clay in the study area as it varies from one location to another. I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 51 Vol. 7: 43-57, December 2016 Fig 3. Distribution pattern of Lateritic clay depth (m) in Ilorin 3.2.2 Weathered Basement In Apata Yakuba, Ganiki Sango and Royal Valley, the weathered basement average resistivity and thickness are 105ohm-m and 18m which indicates a saturation characterized by a moderately low resistivity layer. The weathered basement average resistivity and thickness are 103ohm-m and 18m in Elekoyangan, Sango area and Oyun area, suggesting some level of saturation characterized a moderately lower resistivity layer. In Alagbado, Okelele and Olorunshogo, the weathered basement average resistivity and thickness are 51ohm-m and 22m, respectively. This agrees with the findings of Ogunlana and Talabi (2014), indicating that the material composition is largely clay, sandy clay and clayey sand. This is evidenced in the high degree of water logging of this area, particularly in dry season. In Agbabiaka, Kilanko and Tanke area of the city. The weathered basement average resistivity and thickness are 135ohm-m and 30m respectively, which indicates existence of some degree of fractures and water saturation weathered basement. The weathered basement average resistivity and thickness for Danialu and Fate Tanke are 158ohm-m and 15m respectively, indicating a saturation characterized by a moderately low resistivity layer. Figure 4 shows map of the spatial distribution pattern of weathered basement in the study area as it varies from one location to another. I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 52 Vol. 7: 43-57, December 2016 Fig 4. Distribution pattern of Weathered Basement Depth (m) in Ilorin 3.2.3 Fairly Hard Basement The fairly hard basement in Elekoyangan, Sango area and Oyun area, average and thickness resistivity are 167ohm-m and 30m. This falls within the weathered and fresh basement rock which is also characterized by potential aquiferous units and hard rock in the areas. Also, its resistivity and thickness values are 339ohm-m and 30m, the result agree with Oyedele and Olayinka (2012) that the groundwater potential of the aquifer may be significantly enhanced if the geo-electric basement has a fairly low resistivity. Relatively low values of geo-electric basement resistivity (200 – 640 Ωm) are indicative of good groundwater potential. The fairly low bedrock resistivity confirms the presence of fractures and hence water contained within the fissures (Becson and Jones, 1988; Olayinka and Olorunfemi, 1992; Ayodele and Olayinka, 2012). Figure 5 shows map of the spatial distribution pattern of fairly hard basement in the study area as it varies from one location to another. I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 53 Vol. 7: 43-57, December 2016 Fig 5. Distribution pattern of Fairly Weathered Basement Depth (m) in Ilorin 3.2.4 Thin Fractured In Alagbado, there is a presence of thin fracture zone which indicates a good potential aquiferous units in the area. Fate Tanke area also show thin fractured zone in the study area which served as reservoir in the area. Oloje area have thin fractured zone. The resistivity and the thickness values are 60ohm-m and 35m which indicates that the area are water bearing zone due to its low resistivity values. The coloured part of Figure 6, shows map of the spatial distribution pattern of thin fractured in Ilorin, while the plain area are the locations without thin fracture zone in the study area. I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 54 Vol. 7: 43-57, December 2016 Fig 6. Distribution pattern of Thin Fractured Depth (m) in Ilorin 3.2.5 Hard Basement The hard basement in Alagbado, Okelele and Olorunshogo, also varies and the average resistivity and thickness values are 100ohm-m and 38m which indicate fine grained with intercalation of sandy clay. Fresh hard rock is the last layer in Apata Yakuba, Ganiki Sango and Royal Valley, the section is relatively deep in the area and average resistivity and thickness values are 307ohm-m and 35m, the resistivity values are somehow high because of its crystalline nature. The hard basement in Agbabiaka area, Kilanko area and Tanke area also varies and the average resistivity and thickness values are 240ohm-m and 41m which indicate fine grained with intercalation of sandy clay. The hard rock average resistivity and thickness values for Danialu, Fate Tanke and Wonderland Chapel area, are 439ohm-m and 30m. The result agrees with Oyedele and Olayinka (2012), that the groundwater potential of the aquifer may be significantly enhanced if the geo-electric basement has a fairly low resistivity. Relatively low values of geo-electric basement resistivity are indicative of good groundwater potential and the fairly low I. P. Ifabiyi et al. Geo-electric characteristics of Basement Complex rocks Ruhuna Journal of Science 55 Vol. 7: 43-57, December 2016 bedrock resistivity confirms the presence of fractures, which shows that water is contained within the fissures (Becson and Jones, 1988, and Olayinka and Olorunfemi, 1992; Ayodele and Olayinka, 2012). Figure 7 shows the spatial distribution pattern of hard basement in the study area as it varies from one location to another. Fig 7. Spatial Distribution Pattern of Hard Basement Depth (m) in Ilorin 4 Conclusion In conclusion, understanding the spatial distribution of geo-electric succession, especially in a large area is a key to maximizing groundwater prospecting. This study has revealed four to six geo-electric successions in Ilorin metropolis and took a step further to present a spatial distribution pattern of the identified layers in the study area. As revealed by the study, from the third layer to the sixth layer shows good groundwater potential. In addition, the resistivity values obtained shows that there are economically viable groundwater resources in the study area, especially in the weathered basement, fairly hard basement, thin fractured and hard basement lithology. Although, the level of groundwater availability varies from one location to the I. P. Ifabiyi et al. 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