Layout 1 INTRODUCTION Heavy metals/trace elements are one of the major oc- curring constituents in sediments. Duffus (2002) suggested the densities of heavy metals range from 3.5 to 7.0 gcm–3 (see also Gautam et al., 2015). However, since the threshold varies according to the author, it is impossible to come up with a precise consensus value. Therefore, any idea of defining “heavy metals” on the basis of density must be abandoned as it yields nothing but confusion (Duffus, 2002). An alternative (and theoretically more acceptable) name for this group of elements is “trace elements” but this is less widely used. Since heavy metals/trace elements occur naturally in rock-forming and ore minerals, there is a range of normal background concentrations in soil sedi- ments, water and living organisms (Alloway and Ayres, 1997). Pollution causes a high abnormal concentration of metals/elements relative to the normal background levels. Therefore, the only presence of the metals/elements is in- sufficient evidence of pollution. Heavy metals/trace elements are considered serious pollutants not only because of their persistence and non- degradability in the environment, but also because most of them have toxic effects on living organisms when they exceed certain concentrations. Additionally, aquatic or- ganisms can bio-accumulate, bio-magnificate or bio-trans- fer metals to concentrations high enough to bring harmful effects (Raulinaitis et al. 2012). Bottom sediment is the loose sand, clay, silt and other soil particles and organic matter that settle or deposit at the bottom of a water body. Sediments are originated from soil erosion (surface erosion in the watershed) or decom- position of plants, animals and microorganisms within the waterbody. Wind, water, and ice are erosion agents that help to carry these particles to streams, rivers and lakes (Davies and Abowei, 2009; Ezekiel et al., 2011). Sedi- ments are a sink for many pollutants and trace substances of low solubility and low degree of degradability. There- fore, pollutants are conserved in sediments over a long period of time according to their chemical persistence, physical, chemical and biochemical characteristics of the sub-strata (Adeyemo et al., 2008). A large proportion of heavy metals/trace elements are originated and dispersed into the air and/or directly into rivers, lakes, sea and reser- voirs (Todorovic et al., 2014) from anthropogenic activi- ties such as industrial processes, mining, automobile emissions, agricultural activities, wastewater discharge, and urban runoff. Natural sources contributing to the pres- ence of heavy metals in sediments include weathering and dissolution of minerals, parent rocks, and soils (Decena et al., 2018). Sediments can therefore act as natural geo- sorbent and primary sink for pollutants, including heavy Advances in Oceanography and Limnology, 2018; 9(2): 68-78 ARTICLE DOI: 10.4081/aiol.2018.7576 This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). The heavy metals/trace elements contents of sediments from Owalla Reservoir, Osun State, Southwest Nigeria Adedeji Idowu Aduwo,* Israel Funso Adeniyi Limnology and Hydrobiology Laboratory, Zoology Department, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria *Corresponding author: anjiaduwo@gmail.com ABSTRACT The heavy metals/trace elements contents of sediment samples from Owalla Reservoir were analyzed every three months in two annual cycles (March 2011 – February 2013). The main aim was to measure concentrations of selected elements in sediment samples, their variations in space and seasons and the level of pollution and/or contamination. The bottom sediment samples were collected with a Van Veen Grab and the elemental analysis in the laboratory was based on air-dried samples following standard methods. The overall hierarchy of heavy metals/trace elements in the sediments of the reservoir was in the decreasing order of concentrations: Fe >Mn >As >Zn >Ni >Co >Cr >Cu >Pb >Cd. The concentrations of the heavy metals did not follow any definite pattern from the upstream-down- stream basin, although most of them (Cu, Fe, Mn, Zn, and Co) showed significant differences (P<0.05) in their horizontal variations. Mn, Pb, Co, Fe, Ni, and Zn were significantly (P<0.05) higher at the open water region than in the littoral region. All the elements except Ni did not show significant seasonal variations (P>0.05). Most of the elements in the reservoir sediment have concentrations within the background levels and concentrations defined in environmental regulations and guidelines, except for As and Cd. The con- tamination factors (Cf) for most metals (Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) suggested low contamination in the sediments (Cf <1.0). Conversely, the sediments were moderately contaminated with Cd (Cf=2.41) and very highly contaminated with As (Cf=19.33). Key words: Anthropogenic; enrichments; pollution; grab; contamination factor; bioaccumulation; bottom sediment.. Received: May 2018. Accepted: December 2018. No n- co mm er cia l u se on ly The heavy metals/trace elements contents of sediments from Owalla Reservoir, Osun State, Southwest, Nigeria 69 metals/trace elements (Podlasinska and Szydlowski, 2017; Decena et al., 2018). High concentrations of these contaminants in sediments could be remobilized into the aquatic ecosystems through water and sediment interac- tions resulting in their bioaccumulation in the tissues of various biota, affecting the distribution and density of benthic organisms (Decena et al., 2018). Polluted aquatic sediments can also cause environmental damage by re- leasing metals/trace elements into the surrounding waters, directly contaminating aquatic plants and animals. One of the most common methods for assessing pol- lution levels and quantifying metals and elemental enrich- ment in bottom sediments is to compare measured concentrations with a background level or with a concen- tration posing serious environmental risks (e.g. maximum concentration defined in environmental regulations and guidelines) usually referred to as Environmental Quality Standards (Raulinaitis et al. 2012). The geochemical background concentration is defined as the concentration of an element reflecting the natural processes not in- fluenced by human activities. The discrimination between the natural and anthropogenic origin of heavy metals/trace elements is a relative measure that can be determined by the composition in the parent material. The geochemical background concentration allows the distinction of pol- luted areas from unpolluted ones and is useful for assess- ing the extent of human activities and the fate (i.e. mobilization, migration, and deposition/uptake of sub- stances in the environment) of these elements. Further- more, it also allows recognizing areas with the higher local background because of the occurrence of mineral- ization (Dung et al., 2013). Sediment Environmental Quality Standards are the “measuring stick” against which the acceptability of the presence and concentration of sub- stances in sediment is determined. They help us better as- sess the quality of the sediment and how clean/healthy it could be. The environmental quality standards are based and designed to protect human and environmental health from the toxicological effects of contaminants (BCMECCS, 2017). The aim of this study is to i) determine the concentra- tions of selected heavy metals/trace elements in sediment samples from Owalla Reservoir, ii) their variations in space and seasons, iii) the level of anthropogenic impact in the catchment basin, and iv) heavy metal/trace elements contamination, if any, in the reservoir. METHODS Study area Owalla Reservoir is located within latitudes 07° 53.5’ and 07° 59.0’ N and longitudes 004° 31.5’ and 004° 35.0’ E with an average elevation of 336±8 m above the mean sea level (Fig. 1). The reservoir is situated in the North of Osogbo, Osun State capital, about 35 km North of Ile-Ife, approximately 200 km North-East of Lagos, 360 km South-West of Abuja the capital city of Nigeria and bor- dered by Odo Otin, Ifelodun, Irepodun and Orolu Local Government Areas of Osun State at the northern, eastern, western and southern parts respectively. The dam wall is about 677 m long and 27.5 m high. The total length of the reservoir is about 12 km from the dam wall to the up- stream (i.e. the point at which the major river, Erinle River, enters into the reservoir) with a maximum width of 3.5 km. It has an area of about 14.5 km2 and capacity of about 94×106 m3. The climate of the study area is typically tropical (Ojo, 1977). The area is underlain by the Basement Complex of Southwestern Nigeria (Kogbe, 1976; Rahaman, 1976; Rahaman and Ocan, 1978). Vegetation is the lowland tropical rainforest vegetation (Keay, 1959; Agboola, 1979; Tijani and Onodera, 2009). The soils belong to the Fer- ruginous tropical red soil (laterites) with minimal soil degradation and erosion (Smyth and Montgomery, 1962; Tijani and Onodera, 2009). The major source of heavy metals/trace elements around the study area could be orig- inated from natural sources mostly by mineralization of parent rock materials. Anthropogenic sources are mainly from domestic and agricultural activities within the catch- ment basin. According to Oladejo and Ofoezie (2006) and Ugbomoiko and Ofoezie (2007), the municipalities sur- rounding the reservoir (i.e. Ilie, Oba, Bara, Onipakiti, Kuti, Igbokiti, Idiroko, Eko-Ende, and Ore, etc.) have a population of less than 5000 persons each, with an annual growth rate of about 3%. The inhabitants are mostly Yoruba, many of whom are peasant farmers, fishermen or petty traders in agricultural goods. There is no evidence of heavy industrialization. Riparian vegetation of the area includes cultivated crops such as vegetables, cashew, mango, cocoa, kola nut, oranges, oil palm, cassava, yam, maize, banana and plantain. Sampling stations and programme A total of twenty (20) sampling stations were estab- lished across the entire reservoir, representing three dif- ferent sections (upstream, mid-basin and downstream) and two zones (littoral and open water) (Fig. 1; Supple- mentary Tab. 1). Sampling was carried out every three months aboard a plank boat powered by an outboard-en- gine in two annual cycles (March 2011 – February 2013). Sample collections/field determination and laboratory analysis A Van-Veen Grab of 0.04 m2 area (0.2 m × 0.2 m) was used to collect the bottom sediments for the elemental analysis. Samples were taken and kept in a labeled poly- No n- co mm er cia l u se on ly A.I. Aduwo and I.F. Adeniyi70 thene bag for laboratory analysis of the heavy metal/trace elements contents of the sediments and analysis of the el- ements in the sediments was based on air-dried samples, dried in clean trays at room temperature. The laboratory analyses of the heavy metals/trace elements were done using the Atomic Absorption Spectrophotometry (AAS) (APHA, 1995; International Atomic Energy Agency, 2003; Supplementary Tab. 2). Adequate and proper qual- ity control and quality assurance (QA/QC) measures were observed both at the field and during the laboratory analy- sis to ensure accurate results and reliability of the data ob- tained following the appropriate Standard Methods and Instructions. The data obtained were analysed computing descriptive statistics, analysis of variance-ANOVA, and Cluster Analysis using Microsoft Excel and Paleontolog- ical Statistics (PAST, ver. 3.17) software. Cluster analysis was computed using the correlation index and the paired group method. Calculation of the contamination factor of the heavy metals/trace elements The contamination factor (Cf) is an indicator of sedi- ment contamination used in evaluating pollution in an aquatic environment by a given toxic substance (Decena et al., 2018). The contamination factor and the degree of contamination (Cd) of the heavy metals/trace elements an- alyzed for the study were used to determine the contami- nation status of sediments. The Cf values for each metal/element based on their overall average values from all the stations in the reservoir were calculated according to the equation given below: Contamination factor (Cf) = Metal/element content of sediment Background value of the metal/element Where the background value of the metal/element =world surface rock average (Geochemical background in the earth crust). Cf values for describing the contaminations level of the heavy metals/trace elements in the sediment according to Saha and Hossain (2011) are as given below: Contamination factor Level of contamination Cf <1 Low contamination 1 ≤ Cf <3 Moderate contamination 3 ≤ Cf <6 Considerable contamination Cf >6 Very high contamination. Fig. 1. A) Lake Map and (numbers from 1 to 20) sampling stations. Upper reach: stations 1-4; mid-basin: 5-11, 18; lower reach: 12-17, 19-20. littoral, 3-4, 7-9, 11, 13-15, 17-20; open water, 1-2, 5-6, 10, 12, 16. B) Map of the study area. No n- co mm er cia l u se on ly The heavy metals/trace elements contents of sediments from Owalla Reservoir, Osun State, Southwest, Nigeria 71 The degree of contamination (Cd) is defined as the sum of all contamination factors (Cf) for all the metals studied in the given waterbody. RESULTS Overall variations in Owalla Reservoir The concentrations of the individual heavy metals/trace elements in the reservoir occurred over the wide range of 0.00 – 377.41 µg g–1 (Tab. 1). Based on the mean values, they could be grouped into two categories, i.e.: <100 µg g–1=As >Zn >Ni >Co>Cr >Cu >Pb >Cd >100 µg g–1=Fe >Mn Comparison with other reservoirs and lakes Heavy metals/trace elements in sediments recorded in the present study showed marked similarities with the mass concentrations and hierarchy obtained in some reser- voirs in Nigeria and other parts of the world (Tab. 2). Contamination factor and comparison with toxicological reference values In Tab. 3, contamination factors (Cf) were computed as the ratio between the concentrations of trace elements in the Owalla Reservoir and the corresponding average con- centrations in the Earth crust. The degree of contamination, Cd, was 22.72. The maximum Cf, 19.33, was estimated for cadmium and the least, 0.003, for iron. Excluding As, the remaining elements, i.e. Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn had Cf <1 (Table 3). Most of the heavy metals/trace el- ements in the sediments showed concentrations within the background levels and guidelines, with the exception of As and Cd, which concentrations were greater than the geo- chemical background level. Moreover, As concentration was greater than the Canadian Guidelines. Cd value was greater than the values for all the guidelines (i.e. UK-Cefas, Dutch Standard, Canadian Guidelines, and US Environ- mental Protection Agency). Spatial variations The Co, Cr and Fe contents of the reservoir sediment had highest mean values upstream and decreased through the middle portion of the reservoir and towards the dam site (Table 4). The lowest values were recorded at the down- stream basin. Mn showed lowest mean values upstream and increased through the middle portion of the reservoir and downstream. As, Cd, Cu, Ni, Pb, and Zn showed highest mean values at the middle area of the reservoir; Cd and Ni had lowest mean values upstream, whereas As, Cu, Pb, and Zn had lowest values downstream. Co, Cu, Fe, Mn, and Zn showed significant differences (P≤0.05) in their mean values along the (upstream-down- stream) reaches of the reservoir, whereas the other metals/elements were not significantly different (P>0.05) (Tab. 4). All the heavy/trace metals analysed from the sed- iment samples collected throughout the study period showed higher mean values at the open water region than in the littoral region of the reservoir (Tab. 5). Differences were significant (P<0.05) for Co, Fe, Mn, Ni, Pb, and Zn (Tab. 5). The relationships among the investigated sampling sta- tions based on the concentrations of the metals/elements in the sediments have been analysed using a cluster analysis (Fig. 2). Heavy metal concentrations grouped the stations into five clusters. Nevertheless, the grouping of stations did not follow any definite and congruent spatial pattern, con- firming the absence of any spatial patterns also in the con- centrations of the heavy metals/trace elements. Seasonal variations As, Cd, Cu, Ni, Pb, and Zn had higher mean values dur- ing the rainy season than the dry season. On the contrary, Tab. 1. Descriptive statistics of the concentrations of heavy metals/trace elements from the Owalla Reservoir sediment samples. Descriptive statistics S/N Parameter Unit N Min Max Mean Median SEM SD Skewness Kurtosis 1 Arsenic (As) µg g–1 152 0.07 37.16 12.06 11.40 0.48 5.91 1.31 5.14 2 Cadmium (Cd) µg g–1 152 1.85 3.44 2.90 2.91 0.02 0.23 -2.17 8.67 3 Cobalt (Co) µg g–1 152 2.77 20.78 6.81 6.30 0.24 2.90 1.50 3.38 4 Chromium (Cr) µg g–1 152 1.47 8.63 3.88 3.39 0.15 1.85 0.46 -0.98 5 Copper (Cu) µg g–1 152 0.03 32.20 3.19 2.33 0.27 3.39 4.37 33.02 6 Iron (Fe) µg g–1 152 41.99 377.41 176.22 181.51 4.40 54.27 0.26 1.02 7 Manganese (Mn) µg g–1 152 23.05 189.88 120.46 128.55 2.96 36.51 -0.59 -0.40 8 Nickel (Ni) µg g–1 152 0.50 18.58 7.26 6.77 0.27 3.27 0.96 1.22 9 Lead (Pb) µg g–1 152 0.00 10.70 3.05 2.18 0.22 2.73 0.89 -0.16 10 Zinc (Zn) µg g–1 152 0.00 45.51 8.99 4.69 0.72 8.90 1.11 1.00 No n- co mm er cia l u se on ly A.I. Aduwo and I.F. Adeniyi72 Co, Cr, Fe, and Mn had higher mean values during the dry season than the rainy season. Nevertheless, only Ni showed significant seasonal differences (P<0.05) (Tab. 6). DISCUSSION The similarities in the heavy metals/trace elements content recorded from the present study with the levels obtained for some reservoirs in Nigeria and other parts of the world with respect to mass concentrations and hierar- chy is an evidence of their natural occurrence in rock- forming and ore minerals soil and sediments globally. However, the variations observed in a few elements from the selected reservoirs could be ascribed to the nature of the catchment basin and the underlying bedrock geology of the area and the type of anthropogenic activities around the basin. For example, land use pattern is recognized as a crucial factor influencing the concentrations of these el- ements in terrestrial and aquatic environments in various regions of the world (Szarek-Gwiazda et al., 2018). Un- like most organic pollutants, such as organo-halides, heavy metals occur naturally in a range of normal back- ground concentrations in soil sediments, water and living organisms (Alloway and Ayres, 1997). Pollution gives rise to an anomalously high concentration of metals relative to the normal background levels. Therefore, the presence of metals may not be a sufficient evidence of pollution. However, several countries have set Environmental Qual- ity Standard (EQS) or background levels of metals in sed- iment. Such interim guidelines are basically designed for soil and sediment remediation with a wide range of pol- lutants including, but not limited to, metals (Asibor, 2008). The concentrations of most of the heavy metals/trace elements from the present study were gener- ally lower than the geochemical background level (earth crust) representing the world average concentrations of the metals in sediments. Exceptions were Arsenic (As) and Cadmium (Cd) that were present with higher average values in the reservoir. Asibor (2008) suggested that one Tab. 2. Comparison of the heavy metals/trace elements average concentrations (µg g–1) in sediments from reservoirs and lakes in Nigeria and other countries around the world, and the present study. Reservoir/Lake Metals S/N Name Location As Cd Co Cr Cu Fe Mn Ni Pb Zn Reference 1 Opa Nigeria - - - - 12.28 2523.87 1137.03 - 0.32 55.68 Nathaniel, 2002 2 Opa Nigeria - - - 183.00 - 67468.22 2525.89 - 140.00 155.67 Ogunkoya, 2013 3 Aiba Nigeria - - - - 5.82 42.03 28.00 - 0.44 11.91 Olutona et al., 2012 4 Asejire Nigeria - - - 0.03 43.50 39873.00 1.52 0.05 71.9 20.80 Asibor, 2008 5 Awassa Ethiopia - - 5.49 8.27 8.69 - - 20.20 15.70 93.80 Yohannes et al., 2013* 6 Owabi Ghana - - - - 0.03 4.03 - - 0.10 0.40 Akot and Abankwa, 2014 7 Nasser Egypt - - 113.60 158.30 59.90 - 784.00 - 86.60 119.10 Lasheen, 1987 8 Nasser Egypt - 0.18 - 30.79 21.78 12418.00 279.60 27.56 10.91 35.38 Goher et al., 2014 9 Mariut Egypt - - - - 38.0 25600.00 958.00 - 7.30 94.00 Biney et al., 1994**** 10 Nubia Sudan - - 131.9 332.6 54.7 - 1196.00 - 79.40 138.80 Lasheen, 1987 11 Victoria Tanzania 4.74 1.34 - - 24.13 - - - 22.98 105.85 Machiwa, 2003 12 Masinga Kenya - - - 33.06 17.31 - 525.25 - 12.71 68.51 Nzeve et al., 2014 13 Yilong China - - - 86.73 31.40 - - 35.99 53.19 86.82 Bai et al., 2011* 14 Dongting China 29.71 4.65 - 88.29 47.48 - - - 60.99 185.25 Li et al., 2013 15 Texoma USA - - 9.00 30.00 38.00 - - 17.00 10.00 89.00 An et al., 2003* 16 Erie USA - 3.60 - 64.90 34.30 - - - 98.70 162.60 Opfer et al., 2011** 17 Manchar Pakistan - - 4.70 20.00 21.00 - - 20.10 18.90 96.60 Arain et al., 2008* 18 Veeranam India - - - 88.20 94.12 - - 63.61 30.06 180.08 Suresh et al., 2012* 19 Laguna Philippines - - - 16.90 103.00 - - 13.00 20.00 13.50 Hallare et al., 2005* 20 Karla Greece - - 27.40 298.40 38.30 - - 182.80 34.30 31.20 Skordas et al., 2015 21 Kariba Zimbabwe - 0.06 - 29.30 16.10 - - - 9.40 42.40 Kishe and Machiwa, 2003** 22 Itá Brazil - 6.33 - 90.27 179.86 105301.94 2265.12 - 20.57 232.04 Bonai et al., 2009 23 Wallace Australia - 2.61 28.00 28.00 162.13 2.58 327.25 51.25 29.88 446.25 Birch et al., 2001 24 Trasimeno Italy 9.00 - - 71.70 22.30 30004.00 1566.00 49.00 20.60 59.10 Baudo and Muntau, 1986*** 25 European Lakes (Average) - 2.41 - 142.00 96.00 33300.00 2336.00 66.00 135.00 1082.00 Morgantini and Peruzzi, 2014 26 World Average - - - 90.00 45.00 47000.00 850.00 - 20.00 95.00 Filho et al., 2015 27 Owalla Nigeria 12.06 2.90 6.81 3.88 3.19 176.22 120.46 7.26 3.05 8.99 This Study *Cited by Skordas et al., 2015; **cited by Jahangir et al., 2014; ***cited by Morgantini and Peruzzi, 2014; ****cited by Asibor, 2008. No n- co mm er cia l u se on ly The heavy metals/trace elements contents of sediments from Owalla Reservoir, Osun State, Southwest, Nigeria 73 Tab. 3. Concentrations of heavy metals/trace elements in Owalla Reservoir, geochemical background levels and the toxicological ref- erence values of the metals in river and lake sediments. Concentrations expressed as µg g–1. This study Geochemical background UK-CefasA Dutch Std.A Canadian Gdls.A S/N Metal Value Cf Earth crusta Shaleb AL–1 AL–2 TV RV TEL PEL US-EPAc TLESd 1 As 12.06 2.41 5.00** - 20.00 100.00 29.00 55.00 7.24 41.60 - 5.90 2 Cd 2.90 19.33 0.15 0.30 0.40 5.00 0.80 7.50 0.70 4.20 0.60 0.60 3 Co 6.81 0.27 25.00* - - - - - - - - - 4 Cr 3.88 0.04 100.00 90.00 40.00 400.00 100.00 380.00 52.30 160.00 26.00 37.30 5 Cu 3.19 0.06 55.00 40.00 40.00 400.00 35.00 90.00 18.70 108.00 16.00 35.70 6 Fe 176.22 0.003 56300.00 46700.00 - - - - - - - - 7 Mn 120.46 0.14 850.00 950.00 - - - - - - - 460.00 8 Ni 7.26 0.10 75.00 68.00 20.00 200.00 35.00 45.00 15.90 42.80 16.00 18.00 9 Pb 3.05 0.24 12.50 20.00 50.00 500.00 85.00 530.00 30.00 112.00 31.00 35.00 10 Zn 8.99 0.13 70.00 95.00 130.00 800.00 140.00 720.00 124.00 271.00 110.00 123.00 UK-Cefas, United Kingdom Centre for Environment Fisheries and Aquaculture Science; Dutch Std., Dutch standards; Canadian Gdls, Canadian guidelines; AL-1, action level 1 (contaminants below this level are of no concern/unlikely to influence licensing decision); AL-2, action level 2 (contaminants above this level are considered unsuitable for the environment); TV, target value (target below which the risk to the environment are considered to be negligible); RV, reference value (maximum allowable level, above which the risks to the environment are unacceptable); TEL, threshold effect level (exposure to this level is likely to affect some sensitive species); PEL, probable effect level (exposure to this level is likely to cause an adverse effect to a wider range of species); US-EPA, United State Environmental Protection Agency; TLES, threshold level effect in sediments; AESPS, 2015; aTurekian and Wedepohl, 1961; bTaylor, 1964; cUS-EPA, 1999; dBurton, 2002; *Faboya et al., 2012; **Martin and Whitfield, 1983) a,b,ccited by Goher et al., 2014; **cited by Smedley and Kinniburgh, 2002. Tab. 4. Variations of heavy metals/trace elements concentrations in sediments along the major axis of Owalla Reservoir. Reach ANOVA Upstream Mid-basin Downstream S/N Parameter Unit Mean ± SE Mean ± SE Mean ± SE F P Heavy/trace metal contents of sediment (N=32) (N=90) (N=30) 1 Arsenic (As) µg g–1 11.33±0.74 12.64±0.71 11.12±0.86 1.049 0.353 2 Cadmium (Cd) µg g–1 2.82±0.07 2.92±0.02 2.91±0.02 2.275 0.106 3 Cobalt (Co) µg g–1 8.02±0.67 6.82±0.28 5.53±0.31 6.070 0.003* 4 Chromium (Cr) µg g–1 4.34±0.38 3.79±0.18 3.69±0.35 1.275 0.282 5 Copper (Cu) µg g–1 3.07±0.41 3.66±0.42 1.90±0.30 3.161 0.045* 6 Iron (Fe) µg g–1 193.48±11.39 175.52±5.02 159.89±10.54 3.065 0.050* 7 Manganese (Mn) µg g–1 104.92±6.00 123.23±3.90 128.73±6.20 4.085 0.019* 8 Nickel (Ni) µg g–1 6.69±0.55 7.55±0.34 7.00±0.67 0.939 0.393 9 Lead (Pb) µg g–1 3.22±0.43 3.24±0.30 2.27±0.49 1.509 0.225 10 Zinc (Zn) µg g–1 8.79±1.45 10.33±1.02 5.21±1.04 3.863 0.023* Degree of freedoms, between groups: 2; within groups: 149; *P≤0.05. Tab. 5. Horizontal variations of the heavy metals/trace elements concentrations in the sediments across the major axis of Owalla Reservoir. Region ANOVA Open-Water Littoral S/N Parameter Unit Mean ± SE Mean ± SE F P Heavy/trace metal contents of sediment (N=56) (N=96) 1 Arsenic (As) µg g–1 12.10±0.72 12.04±0.63 0.004 0.953 2 Cadmium (Cd) µg g–1 2.91±0.04 2.89±0.02 0.506 0.478 3 Cobalt (Co) µg g–1 8.29±0.46 5.95±0.21 26.780 7.189×10–7* 4 Chromium (Cr) µg g–1 4.22±0.25 3.69±0.19 2.932 0.089 5 Copper (Cu) µg g–1 3.52±0.31 3.00±0.40 0.831 0.364 6 Iron (Fe) µg g–1 197.54±6.28 163.78±5.57 14.950 1.640×10–4* 7 Manganese (Mn) µg g–1 131.06±4.63 114.27±3.71 7.818 0.006* 8 Nickel (Ni) µg g–1 8.49±0.52 6.54±0.26 13.660 3.06×10–4* 9 Lead (Pb) µg g–1 3.81±0.38 2.60±0.26 7.338 0.008* 10 Zinc (Zn) µg g–1 12.60±1.34 6.89±0.76 15.96 1.01×10–4* Degree of freedoms, between groups: 1; within groups: 150; *P≤0.05. No n- co mm er cia l u se on ly A.I. Aduwo and I.F. Adeniyi74 of the main reasons of the low metals concentrations he observed in the sediments of Asejire reservoir was the low level of industrialization within the catchment basin. On the other hand, Kavitha and Kumar (2013) associated the accumulation of heavy metals in the sediments and biota of receiving waterbodies to rapid industrialization, urban- ization, modern civilization, economic development and increase in population. According to Smedley and Kin- niburgh (2015), Arsenic is a ubiquitous element found in the atmosphere, soils, rocks, natural waters, and organ- isms. It is mobilized in the environment through a com- bination of natural processes such as weathering reactions, biological activity, and volcanic emissions as well as through a range of anthropogenic activities. Most envi- ronmental arsenic problems are the result of mobilization under natural conditions, but man has an important impact through mining activity, combustion of fossil fuels, the use of arsenical pesticides, herbicides and crop desiccants and the use of arsenic as an additive to livestock feed, par- ticularly for poultry. From the study area, some of the likely sources of the elevated levels of arsenic may include natural enrichment. The typical richness of arsenic in the Earth’s crust is between approximately 2 and 5 mg kg–1. However, an enriched amount may be found in shale and coal deposits of sedimen- tary and igneous rocks. Arsenic adsorption to mineral sur- faces (including Fe, Mn, and Al-rich soil and sediments) act as an important sink. Arsenic exists mainly in the atmos- phere in various forms such as (As2O) adsorbed on particu- late matter, which circulates and is returned to the Earth by Tab. 6. Seasonal variations of heavy metals/trace elements concentrations in the sediments samples of Owalla Reservoir. Season ANOVA Dry season Rainy season S/N Parameter Unit N Mean ± SE Mean ± SE F P 1 Arsenic (As) µg g–1 76 11.66±0.89 12.46±0.36 0.687 0.409 2 Cadmium (Cd) µg g–1 76 2.86±0.03 2.93±0.02 3.138 0.079 3 Cobalt (Co) µg g–1 76 6.97±0.36 6.66±0.30 0.419 0.518 4 Chromium (Cr) µg g–1 76 3.96±0.24 3.81±0.18 0.276 0.600 5 Copper (Cu) µg g–1 76 2.93±0.28 3.46±0.47 0.931 0.336 6 Iron (Fe) µg g–1 76 179.93±6.51 172.50±5.94 0.711 0.400 7 Manganese (Mn) µg g–1 76 123.20±3.90 117.72±4.47 0.855 0.357 8 Nickel (Ni) µg g–1 76 6.55±0.36 7.98±0.38 7.646 0.006* 9 Lead (Pb) µg g–1 76 2.92±0.29 3.17±0.33 0.333 0.565 10 Zinc (Zn) µg g–1 76 8.55±1.05 9.44±1.00 0.372 0.543 Degree of freedoms, between groups: 1; within groups: 150; *P≤0.05. Fig. 2. Cluster analysis of the sampling stations based on the heavy metals/trace element contents of the sediments. No n- co mm er cia l u se on ly The heavy metals/trace elements contents of sediments from Owalla Reservoir, Osun State, Southwest, Nigeria 75 wet or dry deposition and simultaneous oxidation and re- conversion of arsenic to non-volatile forms (Chatterjee et al., 2017). As shown in this study, Fe and Mn in the sedi- ments had concentrations about ten times higher than the other elements and both have been found to be naturally as- sociated with arsenic occurrence in sediments. Natural sources of cadmium from the area could be originated from parent rock materials, soil, plants, and an- imal matter sunk into the sediment through one or more natural processes like weathering, erosion, and decompo- sition in the area. According to Mislin and Ravera (1986), cadmium is found in varying amounts as a natural com- ponent of the surface environment in rocks, overburden and soils, water, air, plant and animal tissues with an av- erage concentration in the earth’s crust reported being be- tween 0.15 µg g–1 and 0.11 µg g–1. Anthropogenic sources of arsenic and cadmium from the area could come mainly from domestic and agricultural activities within the catchment basin, considering that sub- sistence agriculture has been the mainstay of more than 90% of the riparian population. Since the majority of the farmers in the area practice subsistence agriculture with no access to mechanized agriculture, the use of agro-chemicals to boost crop productions and protections is usually a com- mon practice. As and Cd have been found in various con- centrations in agro-chemicals used to boost (fertilizers, manures, etc.) and protect crop production (herbicides, pes- ticides, fungicides, rodenticides, etc.). For example, since the industrial revolution, the use of arsenic as an insecticide, fungicide, and herbicide gradually peaked in 1950s when it was one of the most common pesticides in use, because it was an inexpensive by-product of the smelting of copper, iron, silver, cobalt, nickel, lead, gold, zinc, manganese, and tin. Also, agricultural workers preferred to use arsenic and not lead because of its accumulative poison characteristics over insects and pests (Gupta, et al., 2017). Likewise, phos- phate fertilizers and manures used to boost crop produc- tions can contain varying amounts of cadmium (De Boo, 1990; Roberts, 2014), representing examples of anthro- pogenic sources of cadmium in the ecosystems. Arsenic is listed by the US Environmental Protection Agency (USEPA) as one of the priority pollutants and is listed among the most hazardous substances having a significant potential threat to human health (Gupta, et al., 2017). Ar- senic, through ages, is being persistently regarded as a high- profile poison and was related to several conspicuous murder cases, for instance, in the infamous death of Napoleon Bonaparte in 1851, which was claimed to be a political murder by some conspiracy theorists (Gupta, et al., 2017). The U.S. Department of Health and the Euro- pean Commission’s Institute of Health and Consumer Pro- tection have recently summarized the adverse health effects of cadmium on the kidney, renal cortex, pulmonary, car- diovascular, and musculoskeletal systems; moreover, Cd was reported as a human carcinogen. For example, cad- mium poisoning known as Itai-itai disease, a disease caus- ing softening of the bones and kidney failure, was originally discovered in association with rice cultivation in Asia (Roberts, 2014). Concentrations of the heavy metals/trace elements did not follow any definite pattern from the upstream-down- stream basin, although most of them showed significant dif- ference in their horizontal variations. Further, the highest mean concentrations of the different metals were obtained in various basins of the reservoir. This could probably be explained by the different inputs of pollutants (domestic and agricultural) coming from local populations located within the different areas of the watershed (upstream, mid- basin and downstream). This could also confirm that most of the current anthropogenic inputs of these elements into the sediment of the reservoir were mainly of domestic and agricultural sources. Most of the elements with significant horizontal variations are generally regarded as nutritional essential elements (i.e. Co, Cu, Fe, Mn, and Zn). In this re- gard, heavy metals/trace elements have been grouped into toxic (As, Cd, Hg, and Pb) and nutritional essential (Ca, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, and Zn) elements by Alonso et al. (2004). Most heavy metals (Co, Fe, Mn, Ni, Pb, and Zn) had significant higher mean values in the open water region than the littoral region of the reservoir. The occurrence of background concentrations of trace elements in soils and sediments is, besides the lithology, also influenced by their clay and organic matter content. Therefore, clay and or- ganic matter content are often used to calculate ‘corrected’ background values for trace metal concentrations in soils and sediments (De-Saedeleer et al., 2010). Also, according to Parizanganeh (2008), the elemental concentration of sed- iments not only depend on anthropogenic and lithogenic sources, but also upon the textural characteristics, hydrogen ion concentration (pH), organic matter content, mineralog- ical composition and depositional environments of sedi- ments. It is generally believed that metals are associated with smaller grain-size particles. Therefore, in this study, the higher concentrations of the heavy metals in the open water region could be due to the higher clay and organic matter content that characterizes this area compared to the littoral region. Moreover, the peripheral shallow areas are subject to fluctuating temperatures and erosion of shore ma- terials through wave action. The result is a coastline region of relatively coarse sediment, especially evident near un- protected shores (Cole, 1975). Increased acidity tends to dissolve and mobilize heavy metals/trace elements in sediments, causing an increase of concentrations in lake water. The hydrogen ion concen- tration of sediments in the open water zone is usually higher because this stratum of water is generally charac- terized by decay rather than the production of organic No n- co mm er cia l u se on ly A.I. Aduwo and I.F. Adeniyi76 matter. The sediments in central zones are fine particles largely made up of materials produced within the lake (Cole, 1975). As suggested by Parizanganeh (2008), the open water region also represents the depositional envi- ronment where most of the suspended materials in the water column are usually deposited, whereas the littoral is the eroding region where the action of water move- ments drags most suspended particles towards the deep stable and less disturbed region. Most of the heavy metals/trace elements from the reservoir did not show significant seasonal difference. This could be explained considering that the concentra- tions of the heavy metals/trace elements in the sediments from this study were the result of the long-term accumu- lations of these elements over a long period of time rather than fresh anthropogenic enrichments from influx of al- lochthonous materials due to high incidences of flooding and erosions during the rainy season. CONCLUSIONS The overall hierarchy of concentrations of heavy met- als/trace elements in the reservoir sediment samples based on the mean values was in the decreasing order: Fe >Mn >As >Zn >Ni >Co >Cr >Cu >Pb >Cd. The concentrations of the heavy metals did not follow any definite common pat- tern from the upstream-downstream basin, although most of them (Cu, Fe, Mn, Zn and Co) showed significant differ- ences in their horizontal variations, but with various differ- ent spatial gradients and highest values in different areas of the reservoir. These unpredictable patterns are an indication that the populations living along the shores in different areas of the watershed might have produced different amounts and types of pollutants (domestic and agricultural sources) discharged to the reservoir. Many heavy metal/trace ele- ments analyzed from the sediment samples (Mn, Pb, Co, Fe, Ni, and Zn) showed higher mean values at the open water region than in the littoral region. Except Ni, all the heavy metal/trace elements did not show significant seasonal dif- ference in their mean values across the dry and rainy sea- sons. With the exception of As and Cd, most of the elements in the sediments showed concentrations within the back- ground levels defined in environmental regulations and guidelines. Cf values of the different metals from the sedi- ment samples indicated a low sediment contamination with Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn (Cf <1.0). Conversely, sediments were moderately contaminated with Cd (Cf=2.41) and very highly contaminated with As (Cf=19.33). ACKNOWLEDGMENTS The authors wish to acknowledge the contribution of Mr. Adebayo Amubieya and Godwin Zach for their as- sistance during the field collection. We are grateful to two anonymous reviewers and the Editor for valuable com- ments and suggestions on an earlier version of the man- uscript. REFERENCES Adeyemo OK, Adedokun OA, Yusuf RK, Adeleye EA, 2008. Seasonal changes in physicochemical parameters and nutri- ent load of river sediments in Ibadan City, Nigeria. Glob. NEST J. 10:326-336. Agboola SA, 1979. An agricultural atlas of Nigeria. Oxford Uni- versity Press, Oxford: 248 pp. Akot O, Abankwa E, 2014. Heavy metals contamination and speciation in sediments of the Owabi Reservoir. Environ. Res. J. 8:10-16. Alloway BJ, Ayres DC, 1997. Chemical principles of environ- mental pollution. 2nd ed. Chapman and Hall, London: 302 pp. Alonso ML, Montaña FP, Miranda M, Castillo C, Hernandez J, Benedito JL, 2004. Interactions between toxic (As, Cd, Hg, and Pb) and nutritional essential (Ca, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, Zn) elements in the tissues of cattle from NW Spain. BioMetals 17:389-397. APHA, 1995. Standard methods for the examination of water and wastewater. 19th ed. APHA, AWWA, WEF, Washington: 1220 pages. Asibor IG, 2008. The macroinvertebrate fauna and sediment characteristics of Asejire Reservoir, Southwest, Nigeria. Ph.D. Thesis, Obafemi Awolowo University, Ile-Ife. BCMECCS (British Columbia Ministry of Environment and Climate Change Strategy), 2017. Environmental Quality Standards. Available from: https://www2.gov.bc.ca/gov/con- tent/environment/air-land-water/site-remediation/ guidance- resources/technical guidance Birch G, Siaka M, Owens C, 2001. The source of anthropogenic heavy metals in fluvial sediments of a rural catchment: Cox River, Australia. Water Air Soil Pollut. 126:13-35. Bonai NC, Souza-Franco G M, Fogolari O, Mocelin DJC., Dal Magro J, 2009. Distribution of metals in the sediment of the Itá Reservoir, Brazil. Acta Limnol. Bras. 21245-250. Burton AG, 2002. Sediment quality criteria in use around the world. Limnology 3:65-75. Chatterjee S, Moogoui R, Gupta DK, 2017. Arsenic: Source, oc- currence, cycle, and detection, p. 14-29 In: D.K. Gupta and S. Chatterjee (eds.), Arsenic contamination in the environ- ment-The issues and solutions. Springer, Cham. Cole GA, 1975. Textbook of limnology. The C.V. Mosby Co., Saint Louis: 283 pp. Davies OA, Abowei JFN, 2009. Sediment quality of lower reaches of Okpoka Creek, Niger Delta, Nigeria. Eur. J. Sci. Res. 26:437-442. De Boo W, 1990. Cadmium in agriculture. Toxicol. Environ. Chem. 27:55-63. Decena SCP, Arguilles MS, Robel LL, 2018. Assessing heavy metal contamination in surface sediments in an urban river in the Philippines. Pol. J. Environ. Stud. 27:1983-1995. De-Saedeleer V, Cappuyns V, De-Cooman W, Swennen R, 2010. Influence of major elements on the heavy metal composition of river sediments. Geologica Belgica 13:257-268. No n- co mm er cia l u se on ly The heavy metals/trace elements contents of sediments from Owalla Reservoir, Osun State, Southwest, Nigeria 77 Duffus JH, 2002. “Heavy metal”– a meaningless term? Pure Appl. Chem. 74:793-807. Dung TTT, Cappuyns V, Swennen R, Phung NK, 2013. From geochemical background determination to pollution assess- ment of heavy metals in sediments and soils. Rev. Environ. Sci. Bio/Technol. 12:335-353. Environmental Statement for Port of Southampton (ESPS), 2015. Sediment Quality: Southampton approach Chanel Dredge, p. 107-121. In: Report R/3742/8/R.2015. Marine and Environment Research, ABPmer. Ezekiel EN, Hart AI, Abowei JFN, 2011. The sediment physical and chemical characteristics in Sombreiro River, Niger Delta, Nigeria. Res. J. Environ. Earth Sci. 3:341-349. Faboya OL, Sojinu OS, Sonibare OO, 2012. An assessment of heavy metals contamination in surface sediments of the Niger Delta, Nigeria. Can. J. Pure Appl. Sci. 6:2169-2174. Filho FJ, Marins RV, de Lacerda LD, Aguiar JE, Peres TF, 2015. Background values for evaluation of heavy metal contami- nation in sediments in the Parnaíba River Delta estuary, NE/Brazil. Mar. Pollut. Bull. 91:424-428. Gautam RK, Sharma SK, Mahiya S, Chattopadhyaya MC, 2015. Metals in aquatic media: Transport, toxicity and technolo- gies for remediation, p. 1-24. In: S.K. Sharma (ed.), Heavy metals in water, presence, removal and safety. Royal Society of Chemistry, Cambridge. Goher ME, Farhat HI, Abdo MH, Salem SG, 2014. Metal pol- lution assessment in the surface sediment of Lake Nasser, Egypt. Egyp. J. Aquat. Res. 40:213-224. Gupta DK, Tiwari S, Razafindrabe BHN, Chatterjee S, 2017. Arsenic contamination from historical aspects to the present, p.1-10. In: D.K. Gupta and S. Chatterjee (eds.), Arsenic con- tamination in the environment-The issues and solutions. Springer, Cham. International Atomic Energy Agency, 2003. Collection and preparation of bottom sediment samples for analysis of ra- dionuclides and trace elements. IAEA-TECDOC-1360. In- ternational Atomic Energy Agency, Vienna: 130 pp. Jahangir TM, Khuhawar MY, Leghari SM, Mahar MT, Mahar KP, 2014. Water quality and sediment assessment of Man- char Lake, Sindh, Pakistan: after effects of the super flood of 2010. Arab. J. Geosci. 8:3259-3283. Kavitha P, Kumar SP, 2013. Evaluation and sediment quality as- sessment of two perennial ponds in Kanyakumari District, Tamil Nadu, South India. Int. J. Res. Environ. Sci. Technol. 3:135-144. Keay RWJ, 1959. An outline of Nigerian vegetation. 3rd ed. Gov- ernment Printer, Nigeria: 46 pp. Kogbe CA, 1976. Geology of Nigeria. University of Life Center for Advanced Studies. Elizabethan Publishing Co., Lagos: 436 pp. Lasheen MR, 1987. The distribution of trace metals in Aswan High Dam Reservoir and River Nile ecosystems, p. 235-254 In: T.C. Hutchinson and K.M. Meema (eds.), Lead, mercury, cadmium, and arsenic in the environment. J. Wiley & Sons, Chichester. Li F, Huang J, Zeng G, Yuan X, Li X, Liang J, Wang X, Tang X, Bai B, 2013. Spatial risk assessment and sources identifica- tion of heavy metals in surface sediments from Dongting Lake, Middle China. J. Geochem. Explor. 132:75-83. Machiwa JF, 2003. Metal Concentrations in sediments and Fish of Lake Victoria, near and away from catchments with gold mining activities. Tanz. J. Sci. 29:43-54. Martin JM, Whitefield M, 1983. The significance of the river input of chemical elements to the ocean, p. 265-296. In: C.S. Wong, E. Boyle, K.W. Bruland and E.D. Goldberg (eds.), Trace metals in seawater. Plenum Press, New York. Mislin H, Ravera O, 1986. Cadmium in the environment. Birkhäuser Verlag, Basel: 144 pp. Morgantini N, Peruzzi L, 2014. Surface sediment quality in Trasimeno Lake. Proceedings of 15th World Lake Confer- ence, Perugia, Italy. Nathaniel IT, 2002. The Macro-invertebrate benthic fauna and bottom sediment studies of Opa Reservoir. M.Phil. Thesis, Zoology Department, Obafemi Awolowo University. Nzeve JK, Kitur EC, Njuguna SG, 2014. Determination of heavy metals in sediments of Masinga Reservoir, Kenya. J. Envi- ron. Earth Sci. 4:125-132. Ogunkoya OO, 2013. ‘All rivers run into the sea, yet the sea is not full…’ Obafemi Awolowo University Inaugural Lecture Series 256: 61 pp. Ojo O, 1977. The climates of West Africa. Heinemann Educa- tional Books, Ibadan: 219 pp. Oladejo SO, Ofoezie IE, 2006. Unabated schistosomiasis trans- mission in Erinle River Dam, Osun State, Nigeria: evidence of neglect of environmental effects of development projects. Trop. Medi. Int. Health 11:843-850. Olutona GO, Aribisala OG, Akintunde EA, 2012. A study of chemical speciation of metals in the aquatic bottom sedi- ment of Aiba reservoir, Iwo, Nigeria. Afr. J. Environ. Sci. Technol. 6:312-321. Parizanganeh A, 2008. Grain size effects on trace metals in con- taminated sediments along the Iranian coast of the Caspian Sea, p. 329-336. In: M. Sengupta and R. Dalwani (eds.), Pro- ceedings of the 12th World Lake Conference. Podlasińska J, Szydłowski K, 2017. Assessment of heavy metal pollution in bottom sediments of small water reservoirs with different catchment management. Pol. Acad. Sci. 3:987-997. Rahaman MA, 1976. A review of the basement geology of South Western Nigeria, p. 41-58. In: C.A. Kogbe (ed.), Geology of Nigeria. Elizabethan Publication Co., Lagos. Rahaman MA, Ocan O, 1978. On relationships in the Precambrian migmatite-Gneisses of Nigeria. J. Mining Geol. 15:23-30. Raulinaitis M, Ignatavičius G, Sinkevičius S, Oškinis V, 2012. Assessment of heavy metal contamination and spatial dis- tribution in surface and subsurface sediment layers in the northern part of Lake Babrukas. Ekologija 58:33-43. Roberts TL, 2014. Cadmium and phosphorous fertilizers: The issues and the science. Procedia Engin. 83:52-59. Saha PK, Hossain MD, 2011. Assessment of heavy metal contam- ination and sediment quality in the Buriganga River, Bangladesh, p. 384-388. Proceedings 2nd Int. Conf. on Envi- ronmental Science and Technology. IACSIT Press, Singapore. Skordas K, Kelepertzis E, Kosmidis D, Panagiotaki P, Vafidis D, 2015. Assessment of nutrients and heavy metals in the surface sediments of the artificial lake water reservoir Karla, Thessaly, Greece. Environ Earth Sc. 73:4483–4493. Smedley PL, Kinniburgh DG, 2002. A review of the source, be- haviour and distribution of arsenic in natural waters. Appl. Geochem. 17:517-568. Smedley PL, Kinniburgh DG, 2015. Source and behaviour of No n- co mm er cia l u se on ly A.I. Aduwo and I.F. Adeniyi78 arsenic in natural waters. British Geological Survey Report: 61 pp. Smyth AJ, Montgomery FR, 1962. Soil and land use in Central Western Nigeria. The Government Printer, Ibadan: 265 pp. Szarek-Gwiazda E, Mazurkiewicz-Boroń G, Gwiazda R, Urban J, 2018. Chemical variability of water and sediment over time and along a mountain river subjected to natural and human impact. Knowl. Manag. Aquat. Ec. 419:1-14. Taylor SR, 1964. Abundance of chemical elements in the conti- nental crust: a new table. Geochim. Cosmochim. Acta 28:1273-1285. Tijani MN, Onodera S, 2009. Hydrogeochemical assessment of metals contamination in an urban drainage system: A case study of Osogbo Township, South-West, Nigeria. J. Water Resour. Protect. 1:164-173. Todorovic ZB, Randelovic LM, Marjanovic JZ, Todorovic VM, Cakic MD, Cvetkovic OG, 2014. The assessment and dis- tribution of heavy metals in surface sediments from the reservoir “Barje” (Serbia). Adv. Technol. 3:85-95. Turekian, KK, Wedepohl KH, 1961. Distribution of the elements in some major units of the earth’ crust. Geol. Soc. Am. Bull. 72:175-192. Ugbomoiko US, Ofoezie IE, 2007. Multiple infection diagnoses of intestinal helminthiasis in the assessment of health and environmental effect of development projects in Nigeria. J. Helminthol. 81:227-231. US Environmental Protection Agency, 1999. Screening level ecological risk assessment protocol for hazardous waste combustion facilities. Appendix E: Toxicity reference val- ues. Solid Waste and Emergency Response. Report EPA530- D99-001C. Available from: www.epa.gov/osw No n- co mm er cia l u se on ly