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Water quality assessment of the Asata River catchment area in Enugu Metropolis, Southeast Nigeria
Journal of African Earth Sciences 121 (2016) 247e254
Contents lists available at ScienceDirect
Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci
Water quality assessment of the Asata River catchment area in Enugu Metropolis, Southeast Nigeria Olawale Olakunle Osinowo Department of Geology, University of Ibadan, Ibadan, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2016 Received in revised form 5 June 2016 Accepted 8 June 2016 Available online 14 June 2016
Hydrogeochemical mapping of the Asata River Catchment area in the Enugu metropolis, southeast Nigeria was carried out in order to assess the quality of the surface and groundwater and based on the analyses of the hydrogeochemical data, establish the level of chemical contaminations which inhibit the availability of potable water in the area. Forty (40) water samples comprising five (5) springs, nineteen (19) surface (streams/rivers) and sixteen (16) groundwater (well/borehole) samples were collected and analysed for the presence and degree of contamination of nine (9) major chemical contaminants. Hydrochemical analyses indicate that Electrical Conductivity (EC) which has a linear relationship with Total Dissolved Solid (TDS) ranges between 015 and 887 mS/cm, pH between 4.4 and 8.3, nitrate (NO 3) ranges between 40 and 130 mg/l and chloride (Cl) between 7 and 130 mg/l. The concentrations of the dissolved chemical constituents defined the pollution trend and the rate of dispersion of contaminants. The degree of contaminants followed a simple trend, where the level of contamination of the dissolved chemical constituents is least in sampled spring water, with measured chemical constituents of EC, pH, NO 3 and Cl range from 15 to 354 mS/cm; 6.4e6.5; 4.0e70 mg/l and 8e36 mg/l, respectively. However, the value of the measured chemical constituent of EC, pH, NO 3 and Cl gradually increases down the stream in both the surface (63e354 mS/cm; 4.5e7.7; 7.1e110 mg/l; 8e41 mg/l) and groundwater (56 e531 mS/cm; 4.5e7.5; 40e130 mg/l; 7e130 mg/l), respectively. Noticeable peaks in contamination levels characterised sections of the study area where human population or their activities is highest. The result of the hydrogeochemical mapping indicate that Enugu coal mine operation, the industrial activities, fertilizer applied to plants cultivated on river banks and domestic human wastes which are indiscriminately dumped along river channels are the major sources of chemical contamination in the Asata River catchment area. An adequate water resource management scheme is urgently needed to rescue the shallow regolith aquifer from being permanently damaged. Acts such as construction of uncased toilet pits and septic tanks into the thin shallow regolith aquifer, application of inorganic fertilizers along river bank farms and indiscriminate dumping of untreated industrial and human wastes should also be discouraged. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Hydrogeochemical mapping Contamination Asata River catchment Regolith aquifer Inorganic fertilizers
1. Introduction One of the major problems facing the inhabitants of Enugu metropolis is the source of potable water. The problem persisted despite concerted efforts by several government administrations to address it. The acute water shortage is related to the challenge of meeting up with the water demands of a metropolis which is experiencing geometrical population and industrial growth. The population of the metropolis is currently place at over seven
E-mail addresses: [email protected], [email protected]. http://dx.doi.org/10.1016/j.jafrearsci.2016.06.009 1464-343X/© 2016 Elsevier Ltd. All rights reserved.
hundred and fifty thousand (750,000) people and has been projected to exceed 1,000,000 by the year 2020 (Wikipedia, 2016). Enugu metropolis is underlain by the Enugu Shale which only supports thin shallow regolith aquifer of low yield (Egboka, 1985). The aquifer is not prolific enough to yield sufficient quantity of water into boreholes that penetrate it (Agbo and Onuoha 1989; Egboka, 1985). Surface water around the metropolis is heavily contaminated by acid mine drainage of Enugu coal mines, unchecked and indiscriminate dumping of domestic, and industrial pollutants into the streams as well as heavy fertilizer pollution coming from river bank farming culture of the inhabitants. This has in no small measure contributed to the degradation of surface and
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groundwater around the Enugu metropolis. Enugu metropolis is the capital city of Enugu-State. It is situated between longitude 7 250 E and 7 37.10 E and latitude 6 210 N and 6 300 N, covering an area of about 432 square kilometres. Climate in Enugu part of eastern Nigeria is characterised by wet humid climate, wet season from March to September while dry season spreads from November to February. Annual rainfall is about 953 mm and temperature ranges between 21 Ce35 C. The three main land use in the study area includes; residential, industrial and agricultural land use. Agricultural land use involves irrigation farming, especially along river banks. This practise also involves the use of both organic and inorganic fertilizer to boost farm production. River Asata is one of the major rivers that drain the Enugu metropolis. It flows through the main city of Enugu urban. It is a perennial river having many other streams that feed it (Fig. 1). The river is about 19.8 km long; it ranges in diameter from 1.3 m to 5.1 m with average depth of 0.6 m (could be as deep as 1.5 m in some deeper parts of the stream). It has an average discharge of 0.4 m3/sec. Upstream, the discharge increases down the gradient with discharge rate reaching 1.29 m3/sec during the dry season. The increase in discharge down the gradient is attributed to contribution of many adjoining tributaries. The river has numerous head streams with spring points at the upper part of the escarpment. These springs exist at the contact between the sandstone and shale unit boundary, located between the Upper and Lower Mamu Formations. 2. Geology and hydrogeology Enugu metropolis is located in the eastern border of the Anambra Basin which occupies the lower part of the Benue trough.
It is underlain by a thick sequence of sedimentary deposit of Cretaceous age. Seven sedimentary sequences in all, the oldest, which is the Albian Asu River Group, unconformably overlie the Basement Complex rocks of southeastern Nigeria. The Asu River Group is overlain by the Turonian Ezeaku Formation which is immediately succeeded by Campanian Awgu Shale, Nkporo Shale of Santonian age, Mamu Formation, Ajali and Nsukka Formations of Maestrichtian age (Hoque and Nwajide, 1984) (Fig. 2a). The study area is underlain by Enugu Shale, a lateral equivalent of Nkporo Formation (Reyment, 1965). The Enugu Shale is overlain by Mamu Formation and itself overlain by the Ajali Sandstone in quick succession (Fig. 2b). The rocks in the study area have not been affected by any major tectonic activities, rather gentle late Cretaceous tectonic activities which are indicated as gentle folds, faults and some joint sets (Agbo and Onuoha 1989). De Swardt and Casey (1961) reported the occurrence of fault with displacements ranging from a few centimetres to over 60 m which occur in the coal e bearing part of the Mamu Formation. De Swardt and Casey (1961) also reported some faults which were described as unexposed Nyaba fault. Other faults identified include Hades faults system, Ogbette and Obudu faults (Hoque and Nwajide, 1984; Ojoh, 1990; Uma, 1992). 2.1. Aquifer systems Three major types of aquifer systems were identified around Enugu and the environs, namely; unconfined, confined and shallow regolith aquifer, situated at the upper, middle and the basal part of the Enugu escarpment respectively. The unconfined aquifer is formed by the thick sandy sequences of the Ajali Sandstone and the sandy beds of the upper part of the Mamu Formation (Egboka and Uma, 1986). This aquifer is prolific with thickness ranging from less
Fig. 1. Dendritic drainage pattern of Asata River Catchment area of Enugu Metropolis.
O.O. Osinowo / Journal of African Earth Sciences 121 (2016) 247e254
249
Fig. 2. (a) The Geology of southeastern Nigeria (Ojoh, 1990) (b) The geology of the Enugu metropolis.
than 30 m to about 150 m (Uma, 1992) and rainfall is the major source of recharge. The depths to water table in the aquifer ranges from zero at the spring line by the surface of the escarpment to about 70 m around Ninth Mile corner (Uma, 1992). The confined aquifer system is made up of separate discontinuous, but hydrologically continuous beds of sands and silts, usually less than 15 m. It comprises of four sand units separated by shale and coal beds, which are lumped together as one confined aquifer systems due to the homogeneous natures of each cycle of Mamu
Formation and hydraulic continuity between the separate units resulting from the fractured nature of the coal and shale beds. The confined aquifer is recharge directly by rainfall and from overlying unconfined aquifer via the joints and connecting fractures. The third and the aquifer of interest in this study is the shallow regolith aquifer, it overlies the Enugu Shale and serves the Enugu metropolis. It is a mixture of weathered regolith materials such as laterite, light coloured clay and silt admixtures (Daniel, 1992). The thickness of the regolith aquifer ranges from less than 1 m to about
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Fig. 2. (continued).
20 m (Fig. 3) and increases down slope, especially toward the outskirt of the Enugu metropolis around Amechi, Obiagu, Ugwaji where most of the wells tap relatively thick layer of the aquifer. The porosity as well as the permeability of the regolith aquifer is low and thus the aquifer is low in yield. Depth to water in the aquifer as discovered during hydrogeological mapping (borehole depth sounding) ranges from 2.1 m around Uwani to about 9.3 m around Amechi Akwunanaw toward the foot of the escarpment. Recharge of the aquifer is mainly from rainfall over the widely distributed surface of the unconfined regolith aquifer. The groundwater flow pattern around the study area is presented in Fig. 4. 3. Materials and methods 3.1. Sample collection and measurement procedures Forty (40) water samples comprising five (5) spring samples, nineteen (19) surface (streams and rivers) water samples and sixteen (16) groundwater (well/borehole) samples were collected and analysed for nine (9) parameters; that is, Electrical conductivity (EC); measured in mS/cm, degree of acidity and alkalinity (pH), Nitrate (NO 3 ), Chloride (C1 ), Carbonate (CO3 ), Bicarbonate (HCO3 ), 2þ 2þ Total Carbonate Hardness, Calcium (Ca ) and Magnesium (Mg ). Measurements of discharge rates of springs and streams were also taking at the time of sampling. Borehole sampling also includes taking measurements of water table as well as the borehole/well installation depths. This enabled the construction of water table flow pattern within the Asata River catchment area. Each of the sample collection points was georeferenced and this allowed the ease of presenting hydrogeochemical distributions across the study area as well present a down-stream variation of dissolved chemical contaminants which also aided the determination of contaminants plumes.
Measurement of Electrical conductivity (EC) and pH were done in situ using hand held electronic meter (WQC-24 Portable Water Quality Meter) while the concentrations of Nitrate, Chloride, Carbonate, Bicarbonate, Total Carbonate Hardness, Calcium and Magnesium in sampled waters were carried out in the laboratory, using €tten titration method. The Wissenschaftlich-Technische-Werksta (WTW) GmbH testing kits were used to measure the concentration of dissolved ions (cations and anions). It contains various reagents which upon reaction with the water samples impose changes in sample’s colour. The colour change (upon addition of recommended drops of the reagents) indicates the presence of metallic or non-metallic ions of the tested salt(s), such as Ca2þ, Mg2þ NO 3 , Cl salts, and others, present in the sampled water. The concentrations of the ions of these metal or non-metal in milligram per litre (mg/l) were read-off by colour matching/comparison with sets of standard colours. 4. Results and discussions 4.1. Surface and groundwater hydrochemistry Hydrogeochemical analyses of five spring water samples collected during hydrogeochemical mapping indicate that the spring water represents background dissolved solids as Electrical Conductivity (EC) value range from 15 to 116 mS/cm, degree of alkalinity/acidity (pH) range from 6.4 to 6.5, also measured con centrations of bicarbonate (HCO 3 ), nitrate (NO3 ) and chloride ions (Cl ) range from 13.5 to 15.3 mg/l, 4.0e38 mg/l and 8e9 mg/l, respectively. Surface water collected from streams and rivers in the study area present higher TDS as indicated by the measured values of EC, pH, HCO 3 , NO3 and Cl which range from 063 to 354 mS/cm; 10.9e43.6 mg/l; 4.5e7.7; 7.1e110 mg/l; 8e41 mg/l, respectively, while the groundwater likewise indicate higher dissolved
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Fig. 3. Vertical stratigraphic profile of the Enugu shallow regolith aquifer.
251
The chemistry of spring water defines the background input into the Asata River catchment area. Generally, lesser amount of solids were observed to have dissolved into the spring water, evident with generally low EC value. Spring water samples also present generally weak acidity with average pH value of 6.5, however, the pH readings recorded from spring water sampled around Udi mine is as low as 4.5, suggesting the influence of acid mine drainage reported by Uma (1992). Dissolved nitrate concentration in sampled spring water is less than 40 mg/l and chloride ion concentration average value of 9 mg/l. The likely primary sources of chloride in spring water include dissolution of chlorine (Cl2) or hydrogen chloride (HCl) gas into the rainwater, dissolution of rock salt or salt water intrusion. Since the last two are unlikely possibilities because of the geology and nonproximity to sea water, chloride ions in ground and surface water of Asata River catchment is likely to have originated from dissolved chlorine and hydrogen chloride gases in the atmosphere (White, 2010). Bicarbonate concentration in water is a measure of the carbonate ions; there are two likely possible sources of carbonate ions into the recharging spring water, either from the dissolution of carbon dioxide in the atmosphere into rain water or from carbonaceous rock framework. The rock distribution around the study area as revealed by the geology of Enugu and environs indicates the occurrence of carbonaceous rock members (carbonaceous shale) in the Upper Mamu Formation (De Swardt and Casey, 1961), and thus explains why the carbonate concentration is relatively high in sampled spring water (15.3 mg/l). Generally, surface water chemistry indicates higher TDS with EC value, as high as 354 mS/cm. Most of the surface water samples tested displayed alkaline pH, stream samples collected upstream were slightly acidic to slightly neutral (6.8e7.2), except for samples collected very close to the coal mines, where pH reading dropped to 4.5. Surface water’s pH generally increases downstream becoming more alkaline as pH readings increases to 8.5 at the most urbanized part of the metropolis. TDS generally increases as the streams drain
Fig. 4. Groundwater flow pattern in the study area.
constituents as evident in EC value of 056e564 mS/cm; pH 4.5e7.5; HCO 3 13.1e61.0 mg/l; NO3 and 40e130 mg/l; and Cl 7e130 mg/l, respectively.
more urbanized and industrialised part of the metropolis, here nitrate readings of sampled surface water rose form an average value of 40 mg/l to 130 mg/l and carbonate concentration likewise
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picked from 28 mg/l to 205 mg/l. Also, chloride recorded similar surge pattern from an average value of 7 mg/l to 130 mg/l. Water samples collected from streams/rivers close to refuse dumps or at locations where refuse were dumped directly into the stream channels present characteristically high bicarbonate readings, as high as 61 mg/l. The high HCO 3 is attributed to the high volume of carbon dioxide (CO2) gas released from decomposing dead plants and animal remains. The generated carbon dioxide gas, which is readily soluble in water contributes significantly to HCO 3 ion concentration in water. Analyses of groundwater hydrogeochemical data indicate that conductivity readings range from 056 to 564 mS/cm while the degree of acidity ranges from acidic to slightly alkaline with pH value of 4.5e7.5. However, many of the sampled borehole/well water were slightly alkaline with pH value reading above 7.0. The concentration of nitrate ions in groundwater is observed to be relatively high having values in the range of 40e130 mg/l. The high nitrate values reflect likely contribution from human wastes through shallow uncased toilet pits and septic chambers (soak away) constructed into the thin shallow regolith aquifer. Nitrate value is highest in highly populated areas like Asata River layout, Uwani and Army Barracks in Akwunanaw. Here, the nitrate concentration ranges between 100 and 120 mg/l. A simple ground water hydrogeochemistry profile from sampled borehole water, situated upstream, before the densely populated sector of the metropolis and down the stream, through the metropolis and subsequently to the outskirt of the metropolis presented a simple trend where tested parameters increase in value towards the densely populated part of the metropolis and thereafter gently
decreases toward the outskirt of the Enugu Metropolis. The reduction in contaminants’ concentration, away from the densely populated regions, towards the outskirt of the metropolis is due to decrease in contaminants’ input and dilution effect from those streams that did not traversed the densely populated part of the metropolis. Fig. 5 presents the distribution and correlation pattern between nitrate (NO 3 ), Chloride (Cl ) and bicarbonate (HCO3 ) ions across the Asata River catchment area. The figure indicates the level of contamination as well as the proportions of contaminants of nitrate, Chloride and bicarbonate ions in the sampled water at different sampled locations along Asata River. 4.2. Down-stream contamination trend Nitrate profile across the Asata River catchment area of the Enugu metropolis indicates a general down-stream increase in nitrate contaminant with peaks coinciding with regions where human population or their activities is highest (Fig. 6). This again corroborates the possibility of human activities contributing significantly to nitrate contamination of both surface and ground water within the river catchment. Down-stream chloride concentration plot also presents a generally increasing trend. High chloride concentration in water sampled at Gariki and Emene, especially from stream sampled at locations where river bank/ irrigation farming was very active, together with abnormally high carbonate concentrations readings from stream water samples at Ogbette Layout and O’Conor layout further strengthens the argument that anthropogenic inputs remain the main source of surface and groundwater contamination in the study area.
Fig. 5. Trend and correlation pattern between Nitrate (NO 3 ), Chloride (Cl ) and Bicarbonate (HCO3 ) ions.
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Fig. 6. Downstream contaminants concentration trends across the Asata River catchment area of the Enugu Metropolis.
5. Conclusions
Acknowledgements
The water quality assessment of Asata River catchment area of Enugu metropolis as examined through the analyses of hydrogeochemical data of sampled spring, surface and ground water indicate geogenic as the primary source of dissolved chemical constituents. The contributions from interactions of water with rocks and soils account for the background chemical constituents, as indicated by the spring water chemistry. Anthropogenic sources such as the Enugu coal mine operations, the industrial activities, agricultural practises and the input from urbanisation remain the main contributors of contaminants to the surface and groundwater within the river catchment area. The thin regolith aquifer as the main source of groundwater to the inhabitants of Enugu metropolis must be rescued from total destruction through adequate water management scheme which must include discouraging the human activities which have been identified to contribute significantly to surface and groundwater contamination.
The invaluable contributions and suggestions offered by the reviewers and the editorial team members greatly improved the quality of this paper. Professor Kalu O. Uma (of blessed memory) is acknowledged for his mentorship and initiating this research a long time ago. References Agbo, J.U., Onuoha, K.M., 1989. Aquifer systems and groundwater flow patterns in the coal-mining area. J. Min. Geol. 25, 1002. Daniel, D.C., 1992. Environmental Science: Action for a Sustainable Future. The Benjamin Cummings Pub. Company, Inc, New York, p. 549. De Swardt, A.M.J., Casey, O.P., 1961. The coal resources of Niger. Geol. Surv. Niger. Bull. 28, 100. Egboka, B.C.E., 1985. Water Resources Problems in the Enugu Area of Anambra State, Nigeria Water Resources and Environmental Pollution Unit (WREPU). Department of Geological Anambra State University of Technology, pp. 95e97. Egboka, B.C.E., Uma, K.O., 1986. Acid Mine Drainage in Enugu Coal Mine of Anambra State, Nigeria. International Mine Water Association (I.M.W.A), Granada Spain.
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Hoque, M., Nwajide, C.S., 1984. Tectona-sedimentological evolution of an elongate intracratonic basin (Aulacogen): the case of the Benue Trough of Nigeria. Nig. Jour. Min. Geoli 21, 1e2. Ojoh, K.A., 1990. The southern part of the Benue trough, Nigeria creataceous stratigraphy, basin analysis, Paleo-oceanography and the aerodynamic evolution of the equatorial domain of the South Atlantic. Elf Bull. 1.7 (2), 67e74. Reyment, R.A., 1965. Aspect of the Geology of Nigeria. University of Ibadan, Ibadan,
Nigeria, p. 145. Uma, K.O., 1992. Origin of acid mine drainage in Enugu. Environ. Geol. Water Sci. 20, 181e194. White, W.B., 2010. Springwater geochemistry. In: Kresic, N., Stevanovic, Z. (Eds.), Groundwwater Hydrology of Springs. Butterworth-Heinemann, USA, pp. 231e268.
Contents lists available at ScienceDirect
Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci
Water quality assessment of the Asata River catchment area in Enugu Metropolis, Southeast Nigeria Olawale Olakunle Osinowo Department of Geology, University of Ibadan, Ibadan, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2016 Received in revised form 5 June 2016 Accepted 8 June 2016 Available online 14 June 2016
Hydrogeochemical mapping of the Asata River Catchment area in the Enugu metropolis, southeast Nigeria was carried out in order to assess the quality of the surface and groundwater and based on the analyses of the hydrogeochemical data, establish the level of chemical contaminations which inhibit the availability of potable water in the area. Forty (40) water samples comprising five (5) springs, nineteen (19) surface (streams/rivers) and sixteen (16) groundwater (well/borehole) samples were collected and analysed for the presence and degree of contamination of nine (9) major chemical contaminants. Hydrochemical analyses indicate that Electrical Conductivity (EC) which has a linear relationship with Total Dissolved Solid (TDS) ranges between 015 and 887 mS/cm, pH between 4.4 and 8.3, nitrate (NO 3) ranges between 40 and 130 mg/l and chloride (Cl) between 7 and 130 mg/l. The concentrations of the dissolved chemical constituents defined the pollution trend and the rate of dispersion of contaminants. The degree of contaminants followed a simple trend, where the level of contamination of the dissolved chemical constituents is least in sampled spring water, with measured chemical constituents of EC, pH, NO 3 and Cl range from 15 to 354 mS/cm; 6.4e6.5; 4.0e70 mg/l and 8e36 mg/l, respectively. However, the value of the measured chemical constituent of EC, pH, NO 3 and Cl gradually increases down the stream in both the surface (63e354 mS/cm; 4.5e7.7; 7.1e110 mg/l; 8e41 mg/l) and groundwater (56 e531 mS/cm; 4.5e7.5; 40e130 mg/l; 7e130 mg/l), respectively. Noticeable peaks in contamination levels characterised sections of the study area where human population or their activities is highest. The result of the hydrogeochemical mapping indicate that Enugu coal mine operation, the industrial activities, fertilizer applied to plants cultivated on river banks and domestic human wastes which are indiscriminately dumped along river channels are the major sources of chemical contamination in the Asata River catchment area. An adequate water resource management scheme is urgently needed to rescue the shallow regolith aquifer from being permanently damaged. Acts such as construction of uncased toilet pits and septic tanks into the thin shallow regolith aquifer, application of inorganic fertilizers along river bank farms and indiscriminate dumping of untreated industrial and human wastes should also be discouraged. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Hydrogeochemical mapping Contamination Asata River catchment Regolith aquifer Inorganic fertilizers
1. Introduction One of the major problems facing the inhabitants of Enugu metropolis is the source of potable water. The problem persisted despite concerted efforts by several government administrations to address it. The acute water shortage is related to the challenge of meeting up with the water demands of a metropolis which is experiencing geometrical population and industrial growth. The population of the metropolis is currently place at over seven
E-mail addresses: [email protected], [email protected]. http://dx.doi.org/10.1016/j.jafrearsci.2016.06.009 1464-343X/© 2016 Elsevier Ltd. All rights reserved.
hundred and fifty thousand (750,000) people and has been projected to exceed 1,000,000 by the year 2020 (Wikipedia, 2016). Enugu metropolis is underlain by the Enugu Shale which only supports thin shallow regolith aquifer of low yield (Egboka, 1985). The aquifer is not prolific enough to yield sufficient quantity of water into boreholes that penetrate it (Agbo and Onuoha 1989; Egboka, 1985). Surface water around the metropolis is heavily contaminated by acid mine drainage of Enugu coal mines, unchecked and indiscriminate dumping of domestic, and industrial pollutants into the streams as well as heavy fertilizer pollution coming from river bank farming culture of the inhabitants. This has in no small measure contributed to the degradation of surface and
248
O.O. Osinowo / Journal of African Earth Sciences 121 (2016) 247e254
groundwater around the Enugu metropolis. Enugu metropolis is the capital city of Enugu-State. It is situated between longitude 7 250 E and 7 37.10 E and latitude 6 210 N and 6 300 N, covering an area of about 432 square kilometres. Climate in Enugu part of eastern Nigeria is characterised by wet humid climate, wet season from March to September while dry season spreads from November to February. Annual rainfall is about 953 mm and temperature ranges between 21 Ce35 C. The three main land use in the study area includes; residential, industrial and agricultural land use. Agricultural land use involves irrigation farming, especially along river banks. This practise also involves the use of both organic and inorganic fertilizer to boost farm production. River Asata is one of the major rivers that drain the Enugu metropolis. It flows through the main city of Enugu urban. It is a perennial river having many other streams that feed it (Fig. 1). The river is about 19.8 km long; it ranges in diameter from 1.3 m to 5.1 m with average depth of 0.6 m (could be as deep as 1.5 m in some deeper parts of the stream). It has an average discharge of 0.4 m3/sec. Upstream, the discharge increases down the gradient with discharge rate reaching 1.29 m3/sec during the dry season. The increase in discharge down the gradient is attributed to contribution of many adjoining tributaries. The river has numerous head streams with spring points at the upper part of the escarpment. These springs exist at the contact between the sandstone and shale unit boundary, located between the Upper and Lower Mamu Formations. 2. Geology and hydrogeology Enugu metropolis is located in the eastern border of the Anambra Basin which occupies the lower part of the Benue trough.
It is underlain by a thick sequence of sedimentary deposit of Cretaceous age. Seven sedimentary sequences in all, the oldest, which is the Albian Asu River Group, unconformably overlie the Basement Complex rocks of southeastern Nigeria. The Asu River Group is overlain by the Turonian Ezeaku Formation which is immediately succeeded by Campanian Awgu Shale, Nkporo Shale of Santonian age, Mamu Formation, Ajali and Nsukka Formations of Maestrichtian age (Hoque and Nwajide, 1984) (Fig. 2a). The study area is underlain by Enugu Shale, a lateral equivalent of Nkporo Formation (Reyment, 1965). The Enugu Shale is overlain by Mamu Formation and itself overlain by the Ajali Sandstone in quick succession (Fig. 2b). The rocks in the study area have not been affected by any major tectonic activities, rather gentle late Cretaceous tectonic activities which are indicated as gentle folds, faults and some joint sets (Agbo and Onuoha 1989). De Swardt and Casey (1961) reported the occurrence of fault with displacements ranging from a few centimetres to over 60 m which occur in the coal e bearing part of the Mamu Formation. De Swardt and Casey (1961) also reported some faults which were described as unexposed Nyaba fault. Other faults identified include Hades faults system, Ogbette and Obudu faults (Hoque and Nwajide, 1984; Ojoh, 1990; Uma, 1992). 2.1. Aquifer systems Three major types of aquifer systems were identified around Enugu and the environs, namely; unconfined, confined and shallow regolith aquifer, situated at the upper, middle and the basal part of the Enugu escarpment respectively. The unconfined aquifer is formed by the thick sandy sequences of the Ajali Sandstone and the sandy beds of the upper part of the Mamu Formation (Egboka and Uma, 1986). This aquifer is prolific with thickness ranging from less
Fig. 1. Dendritic drainage pattern of Asata River Catchment area of Enugu Metropolis.
O.O. Osinowo / Journal of African Earth Sciences 121 (2016) 247e254
249
Fig. 2. (a) The Geology of southeastern Nigeria (Ojoh, 1990) (b) The geology of the Enugu metropolis.
than 30 m to about 150 m (Uma, 1992) and rainfall is the major source of recharge. The depths to water table in the aquifer ranges from zero at the spring line by the surface of the escarpment to about 70 m around Ninth Mile corner (Uma, 1992). The confined aquifer system is made up of separate discontinuous, but hydrologically continuous beds of sands and silts, usually less than 15 m. It comprises of four sand units separated by shale and coal beds, which are lumped together as one confined aquifer systems due to the homogeneous natures of each cycle of Mamu
Formation and hydraulic continuity between the separate units resulting from the fractured nature of the coal and shale beds. The confined aquifer is recharge directly by rainfall and from overlying unconfined aquifer via the joints and connecting fractures. The third and the aquifer of interest in this study is the shallow regolith aquifer, it overlies the Enugu Shale and serves the Enugu metropolis. It is a mixture of weathered regolith materials such as laterite, light coloured clay and silt admixtures (Daniel, 1992). The thickness of the regolith aquifer ranges from less than 1 m to about
250
O.O. Osinowo / Journal of African Earth Sciences 121 (2016) 247e254
Fig. 2. (continued).
20 m (Fig. 3) and increases down slope, especially toward the outskirt of the Enugu metropolis around Amechi, Obiagu, Ugwaji where most of the wells tap relatively thick layer of the aquifer. The porosity as well as the permeability of the regolith aquifer is low and thus the aquifer is low in yield. Depth to water in the aquifer as discovered during hydrogeological mapping (borehole depth sounding) ranges from 2.1 m around Uwani to about 9.3 m around Amechi Akwunanaw toward the foot of the escarpment. Recharge of the aquifer is mainly from rainfall over the widely distributed surface of the unconfined regolith aquifer. The groundwater flow pattern around the study area is presented in Fig. 4. 3. Materials and methods 3.1. Sample collection and measurement procedures Forty (40) water samples comprising five (5) spring samples, nineteen (19) surface (streams and rivers) water samples and sixteen (16) groundwater (well/borehole) samples were collected and analysed for nine (9) parameters; that is, Electrical conductivity (EC); measured in mS/cm, degree of acidity and alkalinity (pH), Nitrate (NO 3 ), Chloride (C1 ), Carbonate (CO3 ), Bicarbonate (HCO3 ), 2þ 2þ Total Carbonate Hardness, Calcium (Ca ) and Magnesium (Mg ). Measurements of discharge rates of springs and streams were also taking at the time of sampling. Borehole sampling also includes taking measurements of water table as well as the borehole/well installation depths. This enabled the construction of water table flow pattern within the Asata River catchment area. Each of the sample collection points was georeferenced and this allowed the ease of presenting hydrogeochemical distributions across the study area as well present a down-stream variation of dissolved chemical contaminants which also aided the determination of contaminants plumes.
Measurement of Electrical conductivity (EC) and pH were done in situ using hand held electronic meter (WQC-24 Portable Water Quality Meter) while the concentrations of Nitrate, Chloride, Carbonate, Bicarbonate, Total Carbonate Hardness, Calcium and Magnesium in sampled waters were carried out in the laboratory, using €tten titration method. The Wissenschaftlich-Technische-Werksta (WTW) GmbH testing kits were used to measure the concentration of dissolved ions (cations and anions). It contains various reagents which upon reaction with the water samples impose changes in sample’s colour. The colour change (upon addition of recommended drops of the reagents) indicates the presence of metallic or non-metallic ions of the tested salt(s), such as Ca2þ, Mg2þ NO 3 , Cl salts, and others, present in the sampled water. The concentrations of the ions of these metal or non-metal in milligram per litre (mg/l) were read-off by colour matching/comparison with sets of standard colours. 4. Results and discussions 4.1. Surface and groundwater hydrochemistry Hydrogeochemical analyses of five spring water samples collected during hydrogeochemical mapping indicate that the spring water represents background dissolved solids as Electrical Conductivity (EC) value range from 15 to 116 mS/cm, degree of alkalinity/acidity (pH) range from 6.4 to 6.5, also measured con centrations of bicarbonate (HCO 3 ), nitrate (NO3 ) and chloride ions (Cl ) range from 13.5 to 15.3 mg/l, 4.0e38 mg/l and 8e9 mg/l, respectively. Surface water collected from streams and rivers in the study area present higher TDS as indicated by the measured values of EC, pH, HCO 3 , NO3 and Cl which range from 063 to 354 mS/cm; 10.9e43.6 mg/l; 4.5e7.7; 7.1e110 mg/l; 8e41 mg/l, respectively, while the groundwater likewise indicate higher dissolved
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Fig. 3. Vertical stratigraphic profile of the Enugu shallow regolith aquifer.
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The chemistry of spring water defines the background input into the Asata River catchment area. Generally, lesser amount of solids were observed to have dissolved into the spring water, evident with generally low EC value. Spring water samples also present generally weak acidity with average pH value of 6.5, however, the pH readings recorded from spring water sampled around Udi mine is as low as 4.5, suggesting the influence of acid mine drainage reported by Uma (1992). Dissolved nitrate concentration in sampled spring water is less than 40 mg/l and chloride ion concentration average value of 9 mg/l. The likely primary sources of chloride in spring water include dissolution of chlorine (Cl2) or hydrogen chloride (HCl) gas into the rainwater, dissolution of rock salt or salt water intrusion. Since the last two are unlikely possibilities because of the geology and nonproximity to sea water, chloride ions in ground and surface water of Asata River catchment is likely to have originated from dissolved chlorine and hydrogen chloride gases in the atmosphere (White, 2010). Bicarbonate concentration in water is a measure of the carbonate ions; there are two likely possible sources of carbonate ions into the recharging spring water, either from the dissolution of carbon dioxide in the atmosphere into rain water or from carbonaceous rock framework. The rock distribution around the study area as revealed by the geology of Enugu and environs indicates the occurrence of carbonaceous rock members (carbonaceous shale) in the Upper Mamu Formation (De Swardt and Casey, 1961), and thus explains why the carbonate concentration is relatively high in sampled spring water (15.3 mg/l). Generally, surface water chemistry indicates higher TDS with EC value, as high as 354 mS/cm. Most of the surface water samples tested displayed alkaline pH, stream samples collected upstream were slightly acidic to slightly neutral (6.8e7.2), except for samples collected very close to the coal mines, where pH reading dropped to 4.5. Surface water’s pH generally increases downstream becoming more alkaline as pH readings increases to 8.5 at the most urbanized part of the metropolis. TDS generally increases as the streams drain
Fig. 4. Groundwater flow pattern in the study area.
constituents as evident in EC value of 056e564 mS/cm; pH 4.5e7.5; HCO 3 13.1e61.0 mg/l; NO3 and 40e130 mg/l; and Cl 7e130 mg/l, respectively.
more urbanized and industrialised part of the metropolis, here nitrate readings of sampled surface water rose form an average value of 40 mg/l to 130 mg/l and carbonate concentration likewise
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picked from 28 mg/l to 205 mg/l. Also, chloride recorded similar surge pattern from an average value of 7 mg/l to 130 mg/l. Water samples collected from streams/rivers close to refuse dumps or at locations where refuse were dumped directly into the stream channels present characteristically high bicarbonate readings, as high as 61 mg/l. The high HCO 3 is attributed to the high volume of carbon dioxide (CO2) gas released from decomposing dead plants and animal remains. The generated carbon dioxide gas, which is readily soluble in water contributes significantly to HCO 3 ion concentration in water. Analyses of groundwater hydrogeochemical data indicate that conductivity readings range from 056 to 564 mS/cm while the degree of acidity ranges from acidic to slightly alkaline with pH value of 4.5e7.5. However, many of the sampled borehole/well water were slightly alkaline with pH value reading above 7.0. The concentration of nitrate ions in groundwater is observed to be relatively high having values in the range of 40e130 mg/l. The high nitrate values reflect likely contribution from human wastes through shallow uncased toilet pits and septic chambers (soak away) constructed into the thin shallow regolith aquifer. Nitrate value is highest in highly populated areas like Asata River layout, Uwani and Army Barracks in Akwunanaw. Here, the nitrate concentration ranges between 100 and 120 mg/l. A simple ground water hydrogeochemistry profile from sampled borehole water, situated upstream, before the densely populated sector of the metropolis and down the stream, through the metropolis and subsequently to the outskirt of the metropolis presented a simple trend where tested parameters increase in value towards the densely populated part of the metropolis and thereafter gently
decreases toward the outskirt of the Enugu Metropolis. The reduction in contaminants’ concentration, away from the densely populated regions, towards the outskirt of the metropolis is due to decrease in contaminants’ input and dilution effect from those streams that did not traversed the densely populated part of the metropolis. Fig. 5 presents the distribution and correlation pattern between nitrate (NO 3 ), Chloride (Cl ) and bicarbonate (HCO3 ) ions across the Asata River catchment area. The figure indicates the level of contamination as well as the proportions of contaminants of nitrate, Chloride and bicarbonate ions in the sampled water at different sampled locations along Asata River. 4.2. Down-stream contamination trend Nitrate profile across the Asata River catchment area of the Enugu metropolis indicates a general down-stream increase in nitrate contaminant with peaks coinciding with regions where human population or their activities is highest (Fig. 6). This again corroborates the possibility of human activities contributing significantly to nitrate contamination of both surface and ground water within the river catchment. Down-stream chloride concentration plot also presents a generally increasing trend. High chloride concentration in water sampled at Gariki and Emene, especially from stream sampled at locations where river bank/ irrigation farming was very active, together with abnormally high carbonate concentrations readings from stream water samples at Ogbette Layout and O’Conor layout further strengthens the argument that anthropogenic inputs remain the main source of surface and groundwater contamination in the study area.
Fig. 5. Trend and correlation pattern between Nitrate (NO 3 ), Chloride (Cl ) and Bicarbonate (HCO3 ) ions.
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Fig. 6. Downstream contaminants concentration trends across the Asata River catchment area of the Enugu Metropolis.
5. Conclusions
Acknowledgements
The water quality assessment of Asata River catchment area of Enugu metropolis as examined through the analyses of hydrogeochemical data of sampled spring, surface and ground water indicate geogenic as the primary source of dissolved chemical constituents. The contributions from interactions of water with rocks and soils account for the background chemical constituents, as indicated by the spring water chemistry. Anthropogenic sources such as the Enugu coal mine operations, the industrial activities, agricultural practises and the input from urbanisation remain the main contributors of contaminants to the surface and groundwater within the river catchment area. The thin regolith aquifer as the main source of groundwater to the inhabitants of Enugu metropolis must be rescued from total destruction through adequate water management scheme which must include discouraging the human activities which have been identified to contribute significantly to surface and groundwater contamination.
The invaluable contributions and suggestions offered by the reviewers and the editorial team members greatly improved the quality of this paper. Professor Kalu O. Uma (of blessed memory) is acknowledged for his mentorship and initiating this research a long time ago. References Agbo, J.U., Onuoha, K.M., 1989. Aquifer systems and groundwater flow patterns in the coal-mining area. J. Min. Geol. 25, 1002. Daniel, D.C., 1992. Environmental Science: Action for a Sustainable Future. The Benjamin Cummings Pub. Company, Inc, New York, p. 549. De Swardt, A.M.J., Casey, O.P., 1961. The coal resources of Niger. Geol. Surv. Niger. Bull. 28, 100. Egboka, B.C.E., 1985. Water Resources Problems in the Enugu Area of Anambra State, Nigeria Water Resources and Environmental Pollution Unit (WREPU). Department of Geological Anambra State University of Technology, pp. 95e97. Egboka, B.C.E., Uma, K.O., 1986. Acid Mine Drainage in Enugu Coal Mine of Anambra State, Nigeria. International Mine Water Association (I.M.W.A), Granada Spain.
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Hoque, M., Nwajide, C.S., 1984. Tectona-sedimentological evolution of an elongate intracratonic basin (Aulacogen): the case of the Benue Trough of Nigeria. Nig. Jour. Min. Geoli 21, 1e2. Ojoh, K.A., 1990. The southern part of the Benue trough, Nigeria creataceous stratigraphy, basin analysis, Paleo-oceanography and the aerodynamic evolution of the equatorial domain of the South Atlantic. Elf Bull. 1.7 (2), 67e74. Reyment, R.A., 1965. Aspect of the Geology of Nigeria. University of Ibadan, Ibadan,
Nigeria, p. 145. Uma, K.O., 1992. Origin of acid mine drainage in Enugu. Environ. Geol. Water Sci. 20, 181e194. White, W.B., 2010. Springwater geochemistry. In: Kresic, N., Stevanovic, Z. (Eds.), Groundwwater Hydrology of Springs. Butterworth-Heinemann, USA, pp. 231e268.