Heavy metal contamination in stream water and sediments of gold mining areas of South Western Nigeria

This study assessed the seasonal variation in heavy metal contamination of stream water and sediments in the gold mining area of Atakunmosa West local Government, Osun State, Nigeria. Twelve villages of prominence in illegal gold mining were selected for the study covering dry and wet seasons of 2012. Stream water and sediment samples were randomly sampled for both seasons. Samples were analyzed with atomic absorption spectrometric method (AAS) and X-ray fluorescence spectroscopy (XRF) for Cd, Zn, Pb, Fe, Zn, Mn, Cu and As. The trend observed for the metals analyzed in the stream water for both seasons are Cu > Zn > Fe > Cd > Pb > As. In stream sediments, higher mean concentration values were generally recorded in the dry season than in wet season. Four metals (Cd, Pb, Cu and Fe) in stream waters and sediments were found to be higher than allowed limits both by the World Health Organisation (WHO, 2004) and the Nigerian standard for drinking water quality (NSDWQ, 2007) in some villages for both seasons, attesting to pollution of the environment resulting from the mining operations and this call for concern.


INTRODUCTION
Introduction of harmful substances into the environment has many adverse effects on human health, biological systems, agricultural productivity and natural ecosystem (Alloway and Ayres, 1993).Environmental pollution by heavy metals has become a question of public concern considering their hard consequences.It produces substances whose existence poses major threats to quality living and existence of man and animals when *Corresponding author.E-mail: dotmark4great@yahoo.co.uk.Tel: +2348033951110.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License above allowable limits (Fatoki et al., 2002).The ecosystem has been constantly disturbed by mining operations (Howard and Beck, 1986).Its adverse effects on both surface and groundwater in such mining areas where adequate measures are not in place to safeguard them are always immeasurable, rendering the water unsafe for human usage and inhabitable for aquatic and benthic organisms.Tailings from small scale and even abandoned gold mining operations have been shown to contaminate streams and sediment as well as agricultural products with high concentrations of toxic metals (Grosser et al., 1994).This could result in metal poisoning in areas where water from such stream is used for some domestic applications.
Official mining of gold began in Nigeria in 1913 and gradually rose to a peak between 1933 and 1943, with 1.4 tonnes of the metal produced.Production however became halted due to world war outbreak.The discovery of oil in economic value finally shifted attention from mining.Gold occurs in alluvial and eluvia placers and primary veins in the schist belts, in some North West and South West communities in the country.
The area selected for the study is known for unofficial, small-scale artisanal gold mining operations for decades with less detailed and updated information on the environmental impact of the mining operation in relation to heavy metal pollution.This is evident in the uncountable number of pits dug close to streams which serve as water supply for both agricultural and domestic purposes (Figure 1).Thus, this work aimed at carrying out a heavy metal analysis of stream water and sediment samples from the mining areas, with a view to assessing the impact of mining operations on the aquatic environment.

Study area
Twelve villages in the mining area of Osun state, with high rate of artisanal mining of gold were selected for the study.The villages are Sabo, Oke-ora, Okutu-Omo, Oko Ogboni, Alaba, Itagunmodi, Ariyelepe, Alaba-Oke, Iyere, Igun, Aba-Isobo and Mokuro.Figure 2a and b shows the map of the state and the sampling points obtained using a hand-held global positioning system (GPS).The control site is Tonkere village in Ife Central Local Government area of Osun State, there is no evidence of mining or 'digging' for gold or any precious metal.Streams were located inside farm lands similar to those obtained in the study area with cash crops like Cocoa or Cassava grown on the farms.

Sampling and sample preparation
One liter each of water samples were collected in washed plastic bottles and was digested using standard procedure and used for atomic absorption (AAS) analysis for metal concentration determination.Stream sediment samples, on the other hand, were collected randomly around each mining site with stainless steel trowel to the depth of 0 to 15 cm.In all, 52 stream water and 35 sediment samples were collected covering both dry and wet seasons.Sediment samples were air dried for 2 to 3 days and kept in labeled polythene bags.They were sieved using a 2 mm mesh to remove large grains, then properly grinded with an agate mortar and made into pellets of 13 mm diameter and 1 mm thickness with an hydraulic pelletizing machine for X-ray fluorescence (XRF) analysis.

Analysis of samples
Stream water samples were digested with aqua-regia (HNO3:HCl 1:3) and analyzed with atomic absorption spectrophotometer (AAS) using PG990 atomic absorption spectrophotometer.Analysis of sediment samples for metal concentration was carried out with an ECLIPSE III, X-ray fluorescence spectrometer (XRF).To ensure quality for the procedure, estuarine standard reference material (SRM 1646a) was irradiated for 1000 s, with the X-ray tube operating at 25 Kv and 50 µA, respectively.Table 1 shows the results obtained for the triplicate analysis of the standard reference materials for estuarine sediments (SRM 1646a purchased from NIST) for data validation.Seven elements (Cd, Pb, Cu, Fe, Mn, Cr and Zn) were detected.Comparison was made between experimental and certified value for the standard reference material used as a measure of quality for the analytical procedures used for stream sediment analysis.Reasonable agreement was observed for the elements with available certified values and this gives a measure of validation for the data generated.

RESULTS
Results obtained from the AAS analysis of the stream water samples collected from April to August and October to December, covering both seasons of 2012 are presented in Table 2.The table showed generally higher mean concentration of heavy metals in the dry season compared to wet season (Figure 3).The mean concentration obtained for Cd in stream water samples ranged from 0.007 ± 0.002 mg/L to 0.027 ± 0.005 mg/L recorded in the dry season and 0.004 ± 0.001 to 0.011 ± 0.004 mg/L obtained in the wet season (Figure 3A).Concentration of lead ranged from 0.002 ± 0.001 to 0.012 ± 0.004 mg/L in the dry season and 0.001 ± 0.001 to 0.007 ± 0.003 mg/L in the wet season.In control samples, mean values obtained for Pb was also higher in dry than wet season (Figure 3B) with (0.003 ± 0.002 to 0.002 ± 0.001) mg/L recorded in both dry and wet seasons, respectively.Arsenic was more pronounced in Itagunmodi and Alaba-Oke water samples with mean concentration in dry season well above those of wet season (Figure 3C), concentration ranges between (0.005 ± 0.001 to 0.011 ± 0.004) mg/L and (0.003 ± 0.001 to 0.008 ± 0.002) mg/L in dry and wet seasons, respectively.The control site recorded lower concentrations of As in both seasons (0.005 ± 0.002 mg/L in dry season and 0.003 ± 0.001mg/L in the wet season) compared to mining sites.Copper was high in all the sites compared with other heavy metals, with the (Figure 3E).Occurrence of iron in the stream water samples in this study did not follow a regular pattern for both seasons (Figure 3F).Mean concentration of the Fe was lower at Oke-Ora, Ariyelepe, Igun and Itagunmodi in dry season.Samples with the lowest and highest concentration were found in Alaba village.However, the mean concentrations values of Fe were generally higher than 0.100 ± 0.013 mg/L obtained in control samples for the dry season but lower in some villages than the 0.192 ± 0.024 mg/L recorded in the wet season for control sample and the mean concentration was much lower in control samples for both seasons Results of the stream sediment analysis for both seasons are presented in Table 3.Generally, the mean concentrations of metals in sediments samples in all the twelve villages were higher in dry season than wet season.The mean concentration values for Fe ranged from 24.886 ± 2.039 to 42.077 ± 3.043 mg/kg in dry season and 17.755 ± 2.911 to 36.800 ± 3.901 mg/kg in wet season, having the highest mean value of 42.077 ± 3.043 mg/kg in the dry and 36.800± 3.901 mg/kg in wet season.The highest values of 26.113 ± 2.711 mg/kg for Fe were found at Itagunmodu in both seasons, while the lowest value 24.886 ± 2.039 mg/kg for both seasons were recorded in sediment samples from Alaba-Oke.Concentrations of non-essential elements Pb and Cd in the stream sediment ranged from 2.114 ± 0.410 to 4.991 ± 0.518 mg/kg of Pb and 2.911 ± 0.560 to 4.341 ± 1.111 mg/kg of Cd in wet season and a higher concentration of 3.951 ± 1.001 to 5.447 ± 1.326 mg/kg and 4.007 ± 0.792 to 6.809 ± 1.315 mg/kg of the metals in the dry season, respectively.The highest values of Cd and Pb (6.809 ± 1.315 mg/kg and 5.447 ± 1.328) obtained in this study were from sediment samples of Itagunmodi village while sediment from Oko-Ogboni had lowest concentrations of Cd and Pb (4.007 ± 0.792 and 3.951 ± 1.001) mg/kg in dry season (Table 3).In the wet season however, the highest value of Cd and Pb were found in sediment samples from Oke-Ora and Itagunmodu villages and the lowest values for these metals were found at Alaba-Oke (2.911 ± 0.560 mg/kg) and Mokuro (2.114 ± 1.400 mg/kg).In dry season, except samples from Igun and Itagunmodi with concentrations of 6.418 ± 1.297 and 6.809 ± 1.315 mg/kg, all other sediment samples had concentrations of Cd below 6 mg/kg recommended limit for sediment.Zinc ranged from 1.305 ± 0.099 to 2.813 ± 0.300 mg/kg in the dry season and 0.506 ± 0.028 to 2.100 ± 0.104 mg/kg of the metal in the wet season (Table 3).The concentration of Mn also ranged from 2.200 ± 0.100 mg/kg to 4.531 ± 0.261 mg/kg in the dry season and1.088± 0.151 mg/kg to 3.820 ± 0.222 mg/kg in the wet season, while that of Cr was between 0.889 ± 0.034 mg/kg to 4.002 ± 0.301 mg/kg in the dry season and 0.321 ± 0.107 mg/kg to 2.711 ± 0.201 mg/kg in the wet season.Generally, the metals in the sediment followed these order in both dry and wet seasons: Cr<Zn<Mn<Pb<Cu<Cd<Fe (dry season) Cr<Zn<Mn<Pb<Cd<Cu<Fe (wet season) Sediment samples from the control site had lower mean concentration values for all elements except Cu and Cr in the dry season and Cu and Cr in wet season.

DISCUSSION
The higher concentration of metals observed in the dry season might be due to effect of evaporation which reduces the volume of the water in the stream thus concentrating the mineral component of the stream.Erosion and runoff into the streams are minimal which reduces the volume of water and therefore increased the concentration of the toxic heavy metals.Higher mining (digging) activity observed during the dry season which turns up more contaminants into the stream during washing (separation) to concentrate the precious metal will also be another reason.Cd is known to cause damage to all types of body cells by increasing their permeability and allowing other heavy metals enter into it.A safe limit of 0.003 mg/l has been recommended for Cd by the Nigerian standard for drinking water quality (NSDWQ, 2007).However, this limit was exceeded in all stream water samples in both the dry and wet season in the study area, which makes the water unsafe with respect to Cd contamination.Mean concentration values (0.005 ± 0.001 mg/L in dry season and 0.004 ± 0.001 mg/L in wet season) recorded for this metal in samples from control site were also higher than the recommended safe limit clearly indicating that Cd contamination of the stream water might be coming from other sources other than mining.The control site selected for the study was a farm land where chemicals like fertilizers are constantly used.
Lead is another non-essential heavy metal that is found in association with gold and a major pollutant in mining environments.It has been reported to be a probable human carcinogen (USEPA, 2007).It causes serious damage to brain and nerve cells.Lead is bioaccumulated in humans and can be transferred from mother to child during pregnancy.Apart from the use of herbicides and other farming chemicals which might have lead as impurity, one other major source of lead contaminant in the study area is the formation of acid mine drainage.The Nigerian regulating body has stipulated a safe limit of 0.01 mg/L for water meant for domestic applications to which most of the stream waters in the study area are been used.Other possible anthropogenic sources of the metal in stream water are burning of fossil fuel and contribution from road side dust (Oluyemi et al., 2009) etc., however, all these other sources were absent in the study area limiting the possible contamination to contribution from the active digging for gold metal as well as contamination from using of agriculture related chemicals.Arsenic is often found as a by-product of both acid mine drainage and of neutral pH leaching of mining wastes from many precious and base metal ore deposits.Arsenic minerals associated with gold mining include arsenopyrite (FeS 2 , FeAs, FeAsS) and tennatite [(Cu, Fe, Zn,) As 4 S] (Smedley, 1996).Oxidation of these minerals has been known to impact on the environment; they act as sources of sulphate acidity and heavy metals contamination of streams and groundwater (Appelo and Postma, 1993).Exposure to As has been linked to still births and defects; it can move to the fetus from the placenta and is transferable from mother to child during breastfeeding.Other important sources of arsenic exposure include coal burning, use of arsenic in pesticides, consumption of contaminated foodstuffs and exposure to wood preserving arsenicals.Water for domestic applications has permissible limit of 0.01 mg/L as recommended by environmental regulatory bodies in Nigeria (NSDWQ, 2007) as well as the World Health Organization (WHO, 2004).The stream water of the study area was not contaminated with As, water from ten villages in the area had mean concentration of As below the safe limit of 0.01 mg/L recommended by the Nigerian drinking water quality standards (NSDWQ, 2007).The value recorded in the control samples indicates contamination from use of agrochemicals.Copper had the highest mean concentration in all the sites both during dry and wet seasons, this might be due to its association with gold ore satellite materials such as bournonite (PbCuSbS 3 ), tennantite [(Cu, Fe, Zn,) As 4 S] and the more common ore formed with iron and sulphur, chalcopyrite (CuFeS 2 ).This element is also a major constituent of some important farm chemicals such as CuSO 4 commonly used as herbicides in cocoa plantations of these agrarian communities.All the stream water samples obviously had lower concentration value compared to the 1 mg/l safe limit recommended for Cu in water meant for domestic purposes (USEPA, 2007).The stream water in all the areas will also be suitable if considered for use in small scale livestock watering as the 5 mg/L target water quality range (TWQR) for Cu was not exceeded (DWAF, 1996), however, using the water for irrigation purpose might be a problem as the TWQR of0 to 0.2 mg/L for the metal was clearly exceeded in all stream water samples from all the  villages regardless of the season.Long-term exposure to copper can cause irritation of the nose, mouth and eyes and it causes headaches, stomachaches, dizziness, vomiting and diarrhea.Intentionally high uptakes of copper may cause liver and kidney damage and even death, though, the carcinogenic effect of the essential element has not yet been determined but all the above symptoms call for remediating these sites.
In a similar trend, the concentration of iron recorded in all the stream water samples and controls for both types of seasons were all below the recommended limit by the Nigerian and world regulatory bodies except those from Oke-Ora village.The lower mean concentration values of Fe observed during the dry season in some of the areas might be due to reduction in the influx of oxide and clay minerals associated with runoff during wet season.Clay minerals have a large specific surface area that favors the adsorption of ions (Espeby and Gustaffson, 2001) while the stream waters in the areas will be safe as well as Fe contamination is concerned because of the lower mean concentration than the 0.1 mg/L limit, the waters will however not be suitable for use in livestock watering as it is above the 0 to 10 mg/L TWQR (DWAF, 1996) for iron in water for that purpose.Water contaminated with excess Fe may have taste and develop aesthetic problems thus preventing their usage for domestic purpose.Fe occurs in minerals such as arsenopyrite FeS 2 , FeAs, FeAsS, chalcopyrite CuFeS 2 , Pyrite FeS 2 and tennantite [(Cu, Fe, Zn,) As 4 S] which are all associated with mining of gold.Zn is known to be less toxic to humans (Ayenimo et al., 2006).It is widely available in the environment and is an essential element for human health.People with too little zinc in the body system can experience loss of appetite, deceased sense of taste and small, slow wound healing and skin sores.Zinc shortages can even cause both defects.Pollution of stream sediments with heavy metals has been an important issue of environmental concern.Many toxic metals are released from both anthropogenic and natural origin where they get attached to suspended materials and finally get settled on to the sediment harming important microorganisms and indirectly affecting humans.Though, some metals are essential to living organisms, above certain levels, they become toxic to the biota (Wendy, 2005).
The results of heavy metal analysis of the sediments showed a generally higher mean values in the dry season.Cd content in the stream sediment was elevated in two villages from the studied area in the dry season, while Pb content was lower in all sediment samples for both seasons.Mean concentration of Fe in the sediment was clearly above recommended values in 75% of sampling population in the dry season and only in two villages in the wet season.Fe is a major element closely associated with gold mining.Higher values of this metal in the dry season are an indication of the effect of the mining activity on the contamination of stream While Pb and Cu are found in galena (PbS) and bournonite (PbCuSbS 3 ); Fe, As and Zn occurs in pyrites (FeS 2 ), arsenopyrites (FeAsS) and tennantite (Cu, Fe, Zn) As 4 S), Zn metal is associated with sphalerite (ZnS).
Table 4 shows the enrichment factors for the metals in the sediment samples for both dry and wet seasons.The table revealed the metals Cd, Cu and Mn to be more enriched in the sediment samples in the dry season while for Pb, two of the villages have higher EF in the wet season.For Cr, seven villages have their EF values higher in the dry season.However, almost all the villages, except Abasobo, Sabo and Igun, have their EF values higher in the wet season.Enrichment factor is used to estimate anthropogenic effect on sediment by measuring pollution as a ratio of the metal enrichment in the sample above the concentration of the metal in the reference sample.The determined heavy metal is normalized with respect to reference sample such as Fe, Zn Al, Zr, Sc or Ti (Mediolla et al., 2008;Blaser et al., 2000;Schiff and Weishberg, 1999;Schropp et al., 1990) and is calculated as (Loska et al., 2008) Where C m and C Fe is the concentrations of metal and Fe in both the sediment and control samples, respectively, using Fe as the normalizing reference metal.Generally, except for Zn, all the metals have their factors lower than 1, a value accepted to indicate no enrichment while an EF value of 2 indicates minimum enrichment (Sutherland, 2000); this may be an indication of leaching and depletion of those elements higher than their accumulation (Uduma and Jimoh, 2014).The EF values of Zn are however higher than 1 in Oke-Ora, Alaba oke and Okutu-Omo in the wet season, indicating minor enrichment.

Conclusion
The higher concentrations of heavy metals found in the stream water and sediments samples in the study area had been associated with gold ore and the use of agrochemicals in the farming activities.There is a need for proper and continuous monitoring of the study area to safeguard the lives of the villagers.Authorities should set up a small scale mining industry in this area with proper environmental monitoring and evaluation as well as a standard environmental impact assessment program to be put in place in order to prevent outbreak of diseases and control the activities of the illegal mining.

Figure 1 .Figure 2a .
Figure 1.Different uses of stream water in the area: (A) Drinking, (B) Laundry, (C and D) Sieving to concentrate dug pit, (E) Production of palm oil from palm fruit.

Figure 2 :Figure 2b .
Figure 2: Map of the Studied are showing the Sampling Points

Figure 3 .
Figure 3. Seasonal Variations of Cd, Pb, As, Cu, Zn and Fe in Stream Waters in the mining area.
: EF = [C m in sediment ÷ C m in control sample] / [C Fe in sediment ÷ C Fe in control sample]
* (Values are expressed as a mean of triplicate analysis).

Table 2 .
Mean variation of As, Cd, Pb, Cu, Fe and Zn (mg/l) in stream waters in the mining area.

Table 3 .
Mean variation of Cd, Pb, Cu, Fe, Mn, Cr, and Zn (mg/Kg) of stream sediments in the study area during dry and wet seasons.