Groundwater contamination in the basement-complex area of Ile-Ife, southwestern Nigeria: A case study using the electrical-resistivity geophysical method A.A. Adepelumi · B.D. Ako · T.R. Ajayi Abstract Hydrogeoenvironmental studies were carried out at the sewage-disposal site of Obafemi Awolowo University campus, Ile-Ife, Nigeria. The objective of the survey was to determine the reliability of the electricalresistivity method in mapping pollution plumes in a bedrock environment. Fifty stations were occupied with the ABEM SAS 300C Terrameter using the Wenner array. The electrical-resistivity data were interpreted by a computer-iteration technique. Water samples were collected at a depth of 5.0 m in 20 test pits and analyzed for quality. The concentrations of Cr, Cd, Pb, Zn, and Cu are moderately above the World Health Organization recommended guidelines. Plumes of contaminated water issuing from the sewage ponds were delineated. The geoelectric sections reveal four subsurface layers, with increasing depth, lateritic clay, clayey sand/sand, and weathered/fractured bedrock, and fresh bedrock. The deepest layers, 3 and 4, constitute the main aquifer, which has a thickness of 3.1–67.1 m. The distribution of the elements in the sewage effluent confirms a hydrological communication between the disposal ponds and groundwater. The groundwater is contaminated, as shown by sampling and the geophysical results. Thus, the results demonstrate the reliability of the direct-current electrical-resistivity geophysical method in sensing and mapping pollution plumes in a crystalline bedrock environment. Résumé Des études géo-environnementales ont été réalisées sur le site d’épandages du campus universitaire d’Obafemi Awolowo, à Ile-Ife (Nigeria). L’objectif de ce Received: 27 January 2000 / Accepted: 20 September 2001 Published online: 1 November 2001 © Springer-Verlag 2001 A.A. Adepelumi (✉) Geophysics Department, Observatorio Nacional-CNPq, Rua Gal. Jose Cristino 77, CEP 20921-400, São Cristovão, Rio de Janeiro, Brazil e-mail: [email protected] Fax: +55-21-25803782 B.D. Ako · T.R. Ajayi Geology Department, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria Hydrogeology Journal (2001) 9:611–622

travail était de déterminer la fiabilité de la méthode des résistivités électriques pour cartographier les panaches de pollution dans un environnement de socle. Cinquante stations ont été soumises à mesures au moyen d’un ABEM SAS 300C Terrameter en utilisant le dispositif de Wenner. Les données de résistivité électrique ont été interprétées au moyen d’une technique de calcul itérative. Des échantillons d’eau ont été prélevés à une profondeur de 5,0 m dans 20 puits tests et analysés pour la qualité. Les concentrations en Cr, Cd, Pb, Zn et Cu sont légèrement au-dessus des valeurs recommandées par l’OMS. Des panaches d’eau contaminée provenant de bassins d’eaux usées ont été délimités. Les profils géoélectriques mettent en évidence quatre couches, qui sont successivement en profondeurs croissantes une argile latéritique, un sable ou un sable argileux, le substratum altéré, puis fissuré, et enfin le substratum non altéré. Les niveaux 3 et 4 les plus profonds constituent l’aquifère principal, de 3,1–67,1 m d’épaisseur. La distribution des éléments dans les effluents d’égouts confirme l’existence d’une communication hydrologique entre les bassins d’épandage et la nappe. Les eaux souterraines sont contaminées, comme le prouvent les résultats des prélèvements et de la géophysique. Par conséquent, les résultats démontrent la fiabilité de la méthode géophysique de résistivité électrique pour la détection et la cartographie de panaches de pollution dans un environnement de socle cristallin. Resumen Se ha efectuado un estudio hidrogeológico ambiental en el punto de vertido de las aguas residuales del Campus Universitario de Obafemi Awolowo, en IleIfe (Nigeria). El objetivo era determinar la validez del método de la resistividad eléctrica para delimitar penachos de contaminación en un medio rocoso. Se utilizó 50 estaciones con un Terrameter ABEM SAS 300C, utilizando la matriz de Wenner. Se interpretó los resultados por medio de una técnica iterativa automática. Se recogieron muestras de agua a una profundidad de 5 m en 20 pozos de ensayo, las cuales fueron posteriormente analizadas en laboratorio. Las concentraciones de cromo, cadmio, plomo, cinc y cobre son ligeramente superiores a los valores guía de la Organización Mundial de la Salud. Se delineó el penacho de agua contaminada procedente de las balsas de aguas residuales. Las secciones geoeléctricas revelan la existencia de cuatro capas, que DOI 10.1007/s10040-001-0160-x

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están formadas, de menor a mayor profundidad, por arcilla laterítica, arena arcillosa/arena, matriz rocosa meteorizada/fracturada, y roca sin alterar. Las dos capas más profundas constituyen el acuífero principal, que tiene un espesor comprendido entre 3,1–67,1 m. La caracterización química de las aguas residuales confirma que hay una conexión hidrológica entre las balsas de estabilización y las aguas subterráneas. Éstas muestran síntomas de contaminación, de acuerdo con el muestreo y la interpretación geofísica. Los datos evidencian la fiabilidad del método geofísico de la resistividad eléctrica por corriente directa para detectar y delimitar penachos contaminantes en un medio rocoso cristalino.

Keywords Contamination · Geophysical methods · Hydrochemistry · Nigeria · Waste disposal

Introduction Groundwater contamination by toxic chemicals that result from sewage disposal is an environmental problem. Trace metals are common constituents of sewage effluents and are used for the evaluation of toxicity in the environment (Richard and Richard 1977). Chromium (Cr), cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) are potentially toxic and they can affect biota at a water-soluble concentration of less than 1 ppm. Groundwater be-

Fig. 1 Geological map of the Obafemi Awolowo University, Ile-Ife campus, Nigeria, showing location of study area

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613 Fig. 2 Geology of the study area, showing locations of VES points, pits, and sewage ponds

comes polluted when these undesirable metals become dissolved in water and percolate into the aquifer. Obafemi Awolowo University (OAU), on the northwestern outskirts of Ile-Ife, 220 km northeast of Lagos, Nigeria, is at 7°30′N latitude and 4°30′E longitude. The university campus, located in Fig. 1, has a population of about 40,000. The campus is within a tropical rain forest and has two distinct seasons (wet, April–October; and dry, November–March). The annual mean rainfall is about 1,600 mm. The diurnal range in temperature is not significant, but the daily temperature can reach 29 °C and is seldom lower than 25 °C. The campus is subdivided into two main physiographic units, inselbergs and dissected pediments. The inselbergs constitute the promiHydrogeology Journal (2001) 9:611–622

nent hills with an average altitude of 400 m above sea level. The relatively low-lying, gently undulating pediments (within the gray gneiss, Fig. 1) have altitudes that range from 250–300 m and are dissected by several river valleys. Statement of Problem and Objectives An oxidation sewage-disposal method is used at the university campus. As shown in Figs. 1 and 2, two disposal ponds (labeled A and B) are located in the southwestern corner of the campus; each is 180×270 m. Liquid wastes, including laboratory solvents, acids containing salts and heavy metals, organic residues, and human wastes have DOI 10.1007/s10040-001-0160-x

614 Table 1 VES results. D1/D2 ... Dn Depth to the bottom of each successive layer; ρ1/ρ2/ρ3 ... ρn Resistivity of each successive layer VES station number

Layer resistivity ρ1/ρ2/ρ3 ... ρn (Ωm)

Depth D1/D2 ... Dn (m)

VES station number

Layer resistivity ρ1/ρ2/ρ3 ... ρn (Ωm)

Depth D1/D2 ... Dn (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

180/720/78/560 794/199/46/558 1,063/188/111/292 470/873/60/232 243/1,377/85/432 410/46/450 473/53/666 530/59/385 393/44/446 370/41/450 400/100/900 240/960/287/1,640 360/1,440/102/854/325 870/433/83/568 640/427/73/580 149/99/725 88/29/855 660/1,226/89/954 400/172/132/612 180/720/78/652 167/800/56/608 230/564/70/720 228/560/78/800 350/40/410 669/200/48/704

1.6/4.8/30.1 3.0/10.8/60.2 1.6/4.3/35.7 1.6/4.1/73.2 1.5/7.4/45.3 1.4/13.5 1.4/17.5 1.2/12.8 1.0/9.6 1.0/13.2 1.5/13.4 3.3/19.7/41.8 1.7/8.5/19.7/48.6 3.2/12.0/43.6 4.0/11.2/38.0 1.5/6.1 1.0/4.6 1.6/5.5/26.0 2.6/3.9/9.9 2.3/8.4/40.7 2.2/9.0/36.1 2.2/8.7/35.2 2.5/8.5/35.0 1.0/10.6 1.4/6.0/16.4

26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

600/186/78/560 770/470/190/750 900/240/72/466 400/172/132/612 374/52/510 258/36/563 310/77/540 323/84/512 298/65/600 317/73/528 331/101/586 309/89/602 392/68/583 343/121/62/579 400/138/72/610 270/61/498 365/70/608 325/61/600 400/74/508 368/72/516 415/89/528 264/55/479 325/88/502 321/167/82/602 366/173/100/538

1.5/6.2/15.1 3.0/10.1/48.2 3.3/19.7/41.8 2.4/4.3/48.2 1.2/10.5 3.1/10.2 3.0/10.0 2.9/9.7 1.7/9.0 3.0/10.7 3.2/12.4 2.8/12.1 2.6/9.4 2.3/9.0/11.7 2.2/8.0/12.1 3.2/9.6 4.0/14.0 1.5/6.0 1.4/7.6 1.0/4.6 1.4/4.2 1.6/4.7 3.1/10.2 2.6/3.9/9.9 2.3/8.4/12.7

been released into the ponds daily since 1962. The campus depends solely on surface and well water for its water supply; these sources are 1–2 km southeast and southwest of the sewage-disposal site. Health concerns about possible impacts of the sewage-disposal site on the water supply of the University campus have led to this study. The objective of the study around the OAU sewage-disposal area was to determine the reliability of the electrical-resistivity geophysical method in mapping pollution plumes in a bedrock environment (Donaldson 1984; Subba et al. 1997; Nixon and Murphy 1998), to evaluate the extent of groundwater contamination, and to correlate subsurface geologic structures with geophysical properties. Geologic Setting and Hydrogeologic Framework The Obafemi Awolowo University campus and the entire Ile-Ife area are located within the Ife-Ilesha schist belt, which is predominantly a migmatite gneiss–quartzite complex. Rahaman and Ocan (1978) classified the rocks of the Ife-Ilesha schist belt into the migmatite gneiss–quartzite complex as slightly migmatized to nonmigmatized metasedimentary and metaigneous rocks, and members of the older granite suite. The geology at the campus is shown in Fig. 1. The gray gneiss occurs in the pediment area and is the oldest recognizable rock within the migmatite–gneiss–quartzite complex. The granite gneiss outcrops as inselbergs and forms the three prominent hills on the campus, labeled 1, 2, and 3 in Fig. 1. This unit displays augen structures in some places. Hydrogeology Journal (2001) 9:611–622

The slightly migmatized to non-migmatized metasedimentary and metaigneous rocks of the campus (marked as Ms and Am in Fig. 1) belong lithologically to mafic–ultramafic rocks. They occur in the southern and eastern parts of the campus. A dolerite dyke cuts across the granite gneiss on hill 2 in the north-central part of the campus. Minor veins and pegmatites of various lengths and thicknesses cut across the country rock in both a concordant and a discordant manner. Two prominent NE-trending thrust faults (F1 and F2) occur in the northern and south-central parts of the campus, respectively (Fig. 1). The study area is underlain by regional gray gneiss and mica schist, and a sequence of lateritic clay (aquitards), clayey sand/sand, and weathered/fractured bedrock. The clayey sand/sand and weathered/fractured bedrock constitute the main aquifer, located within a bedrock depression that is the catchment area for the region. The relatively low-lying, gently undulating pediments are dissected by several river valleys. The young rivers of the campus site have cut their valleys into the pediments. The valleys sides are steep with slopes as great as 12°. The valleys typically are V-shaped and have narrow valley bottoms. The contact between the gently sloping pediments and the steep valley sides is marked by a prominent break of slope. Subsurface ridges and depressions in the bedrock surface control the groundwater flow pattern, as described by Okhue and Olorunfemi (1992). The direction of groundwater flow follows the subsurface configuration of the bedrock. Groundwater flows from the highland arDOI 10.1007/s10040-001-0160-x

615 Fig. 3 Isopach map of thickness of the overburden in the study area

ea in the west towards the east and northeast, which is generally marshy and waterlogged. The water level is at a depth of 4–11 m. Groundwater occurs under water-table conditions in the clay sand/sand aquifer as well as under semi-confined to confined conditions in the weathered/fractured zone. Groundwater is pumped from these two units by water wells in the campus, which are 30–50 m deep. They were drilled in 1999, 1–2 km southeast and southwest of the sewage site. Detailed information on the well specifications (type, casing, and screens) were not made available to the authors. The university dam and reservoir are at the western side of the sewagedisposal site, about 1 km away. Sewage contamination was monitored from the water extracted from the pits that surround the sewage-disposal site.

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Methods of Study Electrical-Resistivity Method The vertical electrical-resistivity sounding (VES) method involves injecting an artificially generated direct current or low frequency alternating current into the ground through two current electrodes. The resulting potential difference is measured by another pair of potential electrodes in the vicinity of the current flow. Although resistivity generally increases as porosity decreases, the electrical properties are controlled more by water quality than by the resistivities of the rock matrix (Kearey and Brooks 1984). Vertical electrical-resistivity soundings were conducted using a Wenner array in the vicinity of the sewage ponds (Fig. 2). Fifty vertical electrical-sounding stations were occupied using the ABEM SAS 300C Terrameter DOI 10.1007/s10040-001-0160-x

616

Fig. 4a, b Geoelectric sections. a A–A’. b B–B’. Lines of the sections are shown in Fig. 1

Table 2 Comparison of depths to contacts between lithologic units, VES results and pit logs. D1/D2...Dn Depth to the bottom of each successive layer VES no.

VES log D1/D2/... Dn (m)

Pit no.

Pit log D1/D2/...Dn (m)

Logged depth (m)

Lithologic descriptions from pit logs

3 4 7 8 10 14

1.6/4.3/35.7 3.0/4.1 1.4/17.5 1.2/12.8 1.0/13.2 3.2/12.0/43.6

3 4 7 18 19 14

1.5/4.5 2.8/4.0 1.5/18.0 1.3/13.0 1.0/13.0 3.1/12.7

4.5 4.0 20.0 13.0 13.0 14.0

15

4.0/11.2/38.0

15

4.0/14.1

16.5

16 17 22 25 28 34 41 42 43 45 47 49 50

1.5/6.1 1.0/4.6 2.2/8.7/35.2 1.4/6.0/16.4 3.3/19.7/41.8 1.7/8.5/38.6 3.2/9.6 4.0/14.0 1.5/6.1 1.0/4.6 1.6/4.7 2.6/3.9/9.9 2.3/8.4/12.7

16 17 8 11 20 6 13 10 9 5 12 2 1

1.4/6.0 1.0/4.5 2.2/9.0 1.5/6.1/16.5 3.5/20.0 1.6/8.8 3.4/10.0 3.8/16.0 1.5/6.0 1.0/5.0 1.7/5.1 3.0/4.1/10.0 2.41/8.0/13.0

6.0 4.5 9.0 16.5 20.0 8.8 10.0 16.0 6.0 5.0 5.1 10.0 13.0

Lateritic clay, clayey sand Lateritic clay, clayey sand Clayey sand, sand, weathered bedrock of mica schist Lateritic clay, sand Lateritic clay, sand Lateritic clay, clayey sand, weathered/fractured bedrock of gray gneiss Lateritic clay, clayey sand, weathered/fractured bedrock of gray gneiss Lateritic clay, clayey sand Clayey sand, fractured bedrock, fresh mica schist bedrock Lateritic clay, clayey sand Lateritic clay, sand Lateritic clay, sand Clayey sand, fractured bedrock, fresh gray gneiss bedrock Clayey sand, fractured bedrock, fresh gray gneiss bedrock Clayey sand, weathered bedrock, fresh gray gneiss bedrock Clayey sand, fractured bedrock, fresh gray gneiss bedrock Clayey sand, fractured bedrock, fresh mica schist bedrock Clayey sand, fractured bedrock, fresh mica schist bedrock Clayey sand, sand, fractured bedrock, fresh mica schist bedrock Clayey sand, sand, fractured bedrock, fresh mica schist bedrock

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617 Fig. 5 Distribution of apparent resistivity of the main aquifer at the study area

with an electrode separation of 1–48 m. The resulting sounding curves were interpreted by partial curve-matching (Orellana and Mooney 1966), using two-layer model curves with the corresponding auxiliary curves and computer-iteration technique of Ghosh (1971). Chemical Analyses Twenty test pits that coincide with some of the VES stations (Fig. 2) were dug manually for the purpose of obtaining groundwater samples for chemical analyses. The pits were not cased. Depths of the pits ranged from 5.0–20.0 m b.g.l. (below ground level), with an average diameter of 1.5 m. All pits extend below the water table. Pit locations are shown in Fig. 2. Water samples from the Hydrogeology Journal (2001) 9:611–622

test pits were taken regularly beginning at week zero, with a sampling frequency of every 4 weeks during 1998 and 1999, and a total of two samples per pit. The samples were kept in 500-ml plastic containers for 24 h prior to chemical analysis. Samples of raw sewage effluent were obtained from sewage pond B. The sewage and water samples were analyzed for Cr, Cd, Pb, Zn, and Cu contents using atomic-absorption spectrophotometry. These elements were selected because of their high concentrations in the sewage wastes and their relatively low abundance in the granitic and gneissic rocks around the sewage site. Analysis of the raw sewage effluents and water samples was carried out following the methods described in APHA, AWWA, and WEF (1975) using a Perkin–Elmer model 306 Atomic Absorption Spectrophotometer. DOI 10.1007/s10040-001-0160-x

618

Fig. 6 Concentrations of toxic elements in pit samples of groundwater, weeks 1, 4, 8, and 12 (1998–1999)

Table 3 Concentration of selected elements in the rawsewage effluent flowing into sewage pond B

Element

Cd Cr Cu Pb Zn

WHO (1983) Standard (mg/L) 0.04 50.00 50.00 25.00 100.00

Results and Discussion Vertical Electrical Soundings Computer-interpreted VES results are presented in Table 1. On the basis of the VES results, four distinct geologic layers were identified: (1) top soil (laterite in most places), (2) clayey sand/sand, (3) weathered/fractured bedrock, and (4) fresh bedrock. Figure 3 is an isopach map of the overburden obtained from test pits and from VES thicknesses. The overburden includes the topsoil, clayey sand/sand or sandy clay/clay, and the weathered/fractured basement, whose thickness ranges from 3.1–94.2 m, as partially confirmed by the dug pits. The Hydrogeology Journal (2001) 9:611–622

Concentration (mg/L) Week 1

Week 4

Week 8

Week 12

32.00 100.00 2,000.00 290.00 660.00

21.00 48.30 1,625.00 320.00 815.20

25.00 61.80 1,820.00 350.00 910.20

28.00 67.00 1,800.00 320.00 890.00

isopach map reveals areas with relatively thick overburden; these are marked T1, T2,and T3 (≥30 m) and correspond to the depressions in the basement rocks. The areas with relatively thin overburden, marked S1, S2, and S3 (≤10 m) correspond to the basement highs (Fig. 3). Figure 4a, b shows the geoelectric sections A–A’ and B–B’, obtained from (Fig. 2), by using the results obtained from the VES (Table 1). The sections provide insight into the subsurface sequence and the structural conditions in and around the sewage ponds. The geoelectric sections show a maximum of four subsurface layers. The deepest layer (layer 4) is composed of fresh bedrock and has a mean and standard deviation resistivity of 646.8±274.2 Ωm. Layer 4 was probably too deep for DOI 10.1007/s10040-001-0160-x

619 Fig. 7 Distribution of zinc in groundwater at the study area

VES to detect at sounding points 14, 15, and 27 because of the maximum electrode spacing of 1–48 m used during the survey. The next deepest layer (layer 3) is composed of highly weathered/fractured bedrock, and has a resistivity of 77.5 Ωm. The generally low resistivity values of this layer are probably caused by the inflow of sewage effluents from the oxidation pond. Its thickness ranges from 2.5–16.4 m. The top two layers consist of dry sand and laterite (layer 1) and of clayey sand/sand (layer 2). These are more resistive layers (88–2,030 Ωm), and, depending on moisture content and the relative amounts of sand and clayey sand in layer 2, either one may be more resistive than the other. At many of the sounding points, these two layers could not be distinHydrogeology Journal (2001) 9:611–622

guished effectively using resistivity. The thrust fault (F2) was effectively delineated around VES 43 in section B–B’. This fault zone/lithological boundary likely serves as a medium through which the sewage effluent flows into the surrounding groundwater, which accounts for the relatively high concentration of Cu, Pb, and Zn observed in pits 6 and 9, which were sited directly on the fault (Fig. 2). Also, this zone is extremely wet and marshy compared with the surrounding area. Figure 5 shows the distribution of apparent resistivity of the main aquifer of layers 2 and 3 combined. The map was prepared from the VES resistivity data and represents resistivities at a depth of 8 m. In general, low resistivities occur in the central part and towards the eastDOI 10.1007/s10040-001-0160-x

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Pb

Zn

10.00 7.80 20.00 1.00 3.00 10.00 10.00 4.00 0.01 3.00 2.00 0.06 1.00 1.00 1.00 8.00 3.00 3.00 2.00 0.00

20.00 15.00 56.00 0.00 4.00 12.00 0.00 0.00 0.00 16.00 0.00 2.10 0.00 40.00 38.00 10.00 0.00 46.00 34.00 1.00

1.00 130.00 91.00 0.00 203.00 270.00 0.00 94.00 79.00 98.00 0.00 144.00 0.00 0.00 0.00 0.10 0.10 9.00 8.40 0.10

10.00 216.00 530.00 340.00 240.00 370.00 120.00 180.00 630.00 420.00 360.00 280.00 270.00 84.00 80.00 100.00 80.00 41.00 38.00 10.00

12.80 21.10 19.10 1.10 3.51 8.23 10.01 3.46 0.01 2.89 2.32 0.02 1.04 1.91 1.17 9.10 4.82 5.21 1.26 0.56

21.10 15.20 55.60 0.03 4.60 12.00 0.01 0.01 0.01 18.50 0.06 2.10 0.01 44.00 33.70 9.10 0.05 50.00 37.10 1.97

Cr 62.00 811.00 326.00 76.00 182.00 245.00 237.00 305.00 168.00 107.00 243.00 166.00 85.00 378.00 266.00 235.00 201.00 54.10 33.20 7.90

Cu

in Fig. 2 200 m upgradient (northwest) of sewage pond A

51.00 789.00 305.00 64.00 138.00 206.00 202.00 255.00 136.00 78.00 193.00 106.00 53.00 300.00 206.00 181.00 180.00 22.20 19.10 1.50

Cd

Cu

Cd

Cr

Week 4

Week 1

Concentration (mg/L)

a Locations of pits shown b The neutral point that is

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20b

Pit no.a

1.21 165.30 126.00 1.00 239.00 307.00 1.00 142.00 99.10 108.00 1.00 168.00 1.00 1.00 1.00 1.00 1.00 11.00 12.10 1.00

Pb 21.00 254.00 565.00 387.00 294.00 408.00 153.00 205.00 700.10 463.00 394.00 306.00 300.00 102.00 100.00 123.00 120.00 72.40 67.10 19.10

Zn 12.72 21.08 19.41 1.01 3.47 8.88 0.01 4.21 0.01 3.51 2.57 0.04 1.08 1.49 1.68 9.10 4.10 4.76 2.58 0.82

Cd

Week 8

Table 4 Results of chemical analyses of water samples obtained from pits around sewage ponds A and B

21.62 15.65 54.41 0.01 4.79 10.53 0.00 0.01 0.01 17.26 0.01 2.10 0.00 42.70 32.90 9.32 0.01 48.20 37.10 2.00

Cr 61.00 800.00 315.00 70.00 163.00 222.00 228.00 276.00 145.00 81.00 214.00 139.00 72.00 326.00 234.00 203.10 200.00 43.20 27.30 6.20

Cu 1.00 160.00 101.00 1.00 230.00 300.00 1.00 140.00 99.00 102.00 1.00 162.00 1.00 1.00 1.00 1.00 1.00 10.80 11.20 1.00

Pb 15.30 231.00 546.10 378.00 288.00 341.00 122.00 193.00 659.00 437.00 375.00 296.00 291.00 90.10 90.00 115.00 100.00 68.50 54.20 15.20

Zn

11.43 19.48 20.00 1.00 3.23 8.91 0.01 3.97 0.00 3.22 2.38 0.02 1.00 1.48 0.90 9.48 0.72 3.74 1.92 0.08

Cd

19.47 14.80 55.50 0.00 4.47 11.00 0.00 0.01 0.01 17.00 0.00 2.00 0.00 41.40 32.10 9.30 0.05 47.60 36.12 1.22

Cr

Week 12

55.20 765.00 292.00 61.00 147.00 231.00 197.00 250.10 132.0 73.20 198.20 121.00 48.00 290.00 221.00 194.00 182.50 27.80 17.40 3.70

Cu

1.00 152.00 96.00 1.00 200.00 292.00 1.00 100.10 90.10 100.00 1.00 110.00 1.00 1.00 1.00 1.00 1.00 10.60 11.00 1.00

Pb

12.00 196.00 503.00 333.20 256.00 325.00 118.00 187.00 623.00 425.00 354.00 283.00 267.00 88.10 87.10 102.00 96.00 51.00 44.10 11.62

Zn

620

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621

ern side; towards the north, south, and west, the values increase continuously. A comparison of Figs. 1 and 5 suggests that this occurrence of low resistivities is probably, in part, caused by a band of weathered schist (low resistivity) surrounded by gneisses (higher resistivity). It is assumed that the leachates and seepages contributed to these low values. Groundwater from within the zone of low resistivity (pits 1,2, 5, 6, 7, 9, 10, 12, 13, and 17) tends to have relatively high toxic metal contents(see next section). The lateral decrease in resistivity eastwards coincides with the presumed migration path of the contaminated groundwater plume. The movement of the groundwater and the contaminants follows the topography of the area. The low resistivities in the center (ρ <60Ωm) of the study area, which trends NE, indicate the influence of the contact between the granite gneiss–gray gneiss–mica schist and the polluted area. The sewage site is bounded by basement highs in the northern, southern, and western sectors, with the eastcentral sector forming a basement depression. The groundwater flow pattern essentially follows the basement lows toward the east and southeast. The high topography and the presence of thick laterite prevent the lateral and vertical flow of the sewage effluents towards the west. The lateritic hard pan is very thin or completely absent in the eastern flank of the sewage-disposal site. Lithologic logs of the pits delineate four major geologic layers. The top layer is composed of dry sand and laterite; the second layer is composed of wet clayey sand/sand; the third layer, which was encountered in all the pits, consists of highly weathered/fractured basement; and the fourth layer consists of fresh bedrock that consists of mica schist to gray gneiss, which was observed in the pits in the eastern sector. Table 2 compares the VES-derived thickness with the thicknesses observed in the pits. The two sets of thicknesses generally agree to within 10%. Pit depths could not exceed 20 m because of the waterlogged nature of the site. Chemical Analyses Table 3 gives the results of the chemical analyses for the raw sewage effluents. Concentrations of all analyzed toxic elements substantially exceed the World Health Organization (WHO 1983) standards. Samples from a neutral point (pit 20), 200 m northwest of sewage pond A (Fig. 2), confirm that the neutral area was not affected by sewage effluent. This site is at a higher elevation and, thus, up-gradient of the sewage disposal site, and a thick lateritic clay occurs there. Results of the chemical analyses of Cr, Cd, Zn, Cu, and Pb from the pit samples are shown in Table 4. The highest values are east and southeast of the sewage ponds, whereas the lowest values occur on the northwestern and southwestern flanks, where the thick lateritic clay occurs. Figure 6 shows the concentrations of toxic elements in pit samples of groundwater. Each element varied little in concentration during the 12-week monitoring period. However, the concentrations moderately Hydrogeology Journal (2001) 9:611–622

exceed the water-quality standards of the WHO (1983). A very low level of pollution was detected upgradient of the sewage pond in the western sector (pits 4, 14, 15, 16, and 25), which is relatively high topographically. Figure 7 shows two major contamination plumes (P1 and P2) north and east of the sewage sites. Based on the pattern of the contours, arrows depict the flow direction of the contaminants, probably along the existing fault (F2)/lithological boundaries (Figs. 1 and 2). Figures 4 and 6 both indicate that the contaminants around the sewage ponds flow in two principal directions, i.e., SW to NE, and W to E. Areas labeled U1 and U2 were largely unaffected by the sewage effluents.

Conclusions The results of the geoelectrical soundings delineate swells and swales in the surface of the crystalline bedrock, which is overlain by about ~50 m of surficial deposits (sandy clay/sand layer underlain by weathered/fractured basement) in a tropical rainforest setting. The soundings also identified plumes of contaminated water issuing from the sewage ponds. The result of the geophysical mapping was corroborated by the geologic logging of the pits and the chemical analyses of the water samples obtained from them. The toxic elements Cr, Cd, Zn, Cu, and Pb occur in groundwater in excess of the WHO (1983) recommended guidelines because of the impact of the sewage-pond effluents. The results of the study demonstrate the reliability of the direct current electrical-resistivity method in sensing and mapping pollution plumes in a crystalline bedrock environment. Acknowledgements The authors greatly appreciate the financial assistance of Shell Petroleum Development Company, Port Harcourt, Nigeria; and thank the Observatorio Nacional-CNPq, Brazil, for granting access to its computer facilities. Also acknowledged are David Campbell and Robert J. Bisdorf of the US Geological Survey, whose reviews improved the content of the paper. The Center for Energy Research at Obafemi Awolowo University, Ile-Ife, assisted with the analyses of the water samples. Stephen Olawore, Kole Ajayi, and Mosun Adebanjo-Adepelumi assisted with the fieldwork. The useful suggestions of M.O. Olorunfemi of the Department of Geology, Obafemi Awolowo University, Ile-Ife, are deeply appreciated.

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Rahaman MA, Ocan OO (1978) On relationship in the Precambrian migmatitic gneiss of Nigeria. J Mining Geol 15:23–30 Richard AG, Richard AM (1977) Trace metals in sediments of Ravitan Bay. Mar Pollut Bull 8:188–192 Subba Rao NS, Gurunadha Rao VVS, Gupta CP (1997) Groundwater pollution due to discharge of industrial effluents in Venkatapura area, Visakhapatnam, Andhra Pradesh, India. Environ Geol 33(4):289–294 WHO (1983) International standard for drinking water, 3rd edn. WHO, Geneva, Switzerla

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