Distribution and Speciation of Heavy Metals in Soils around Some Selected Auto Repair Workshops in Oghara, Delta State, Nigeria

Soil contamination by heavy metals is a worldwide environmental problem. Hence determining the chemical forms of a metal in soils is important to evaluate its mobility and bioavailability. This study determined the distribution and speciation of some heavy metals (Fe, Cu, Zn, Pb and Cd) in soils around some selected auto repair workshops in Oghara, Delta State, Nigeria. Soil samples were collected with the aid of soil Augar within a depth of 0 – 15 cm from the vicinity of the four selected auto repair workshops in Oghara, Delta State, Nigeria. The control samples were taken from a site free from auto repair and commercial activities. The soil samples were assessed for some physico-chemical properties, total heavy metal concentration, chemical speciation, mobility and some metal assessment indices of the heavy metals as a function of soil properties. The mean concentration of Fe, Cu, Zn, Pb and Cd in all the sites analyzed were 550.54, 31.08, 36.15, 4.21 and 1.11 mg/kg respectively. Site B and the control had the highest and lowest total concentration of the five metals analyzed respectively. The levels of Cu were above the DPR target value in sites A and B, while the levels of Cd were above the target value in all the sites except in the control site. All the metals were found to be mostly concentrated in the residual fraction except Zn which was found mostly in the carbonate fraction. The mobility factors revealed that Zn is the most mobile element with an average mobility factor of 41.54% while Cd is the least mobile element with an average mobility factor of 16.51%. Contamination factors, index of geoaccumulation and pollution load index were also calculated. This study showed that mechanic workshop is one of the major sources of anthropogenic heavy metals concentration in the environment.


INTRODUCTION
It has been widely accepted that soil plays a key role in sustaining life in earth's ecosystems (Young and Crawford, 2004). The very survival of mankind is tied on its productivity as a medium for plants to grow (Kabata-Pendias and Mukherjee, 2007). Heavy metals emanating from anthropogenic Automobiles introduce a number of toxic metals into the environment. Also the wear of auto tires, degradation of parts, grease, peeling paint and metal in auto-catalysts are sources of heavy metal pollution (Pecheyran et al., 2000). This has led to elevated levels of heavy metals in automobile mechanic workshop soils (Ipeaiyeda and Dawodu, 2008;Iwegbue, 2007). This implies that water bodies (surface and ground water) within and away from the automobile mechanic workshops may equally be polluted with these metals due to continuous interactions between soil and water and the high dispersion rate (Nwachukwu et al., 2010). The fate of the various heavy metals and metalloids in the automobile mechanic workshops is of great concern because soil, water and dust in these areas may contain higher than average abundance of these elements, which may cause the formation of the more available forms of these elements (Adriano, 1992). In recent years there has been increased interest in the studies on speciation or chemical forms of heavy metals in polluted soils and sediments using sequential extraction techniques because these provide knowledge on metal affinity to soil components and the strength with which they are bound to matrix (Norvell, 1984). The use of sequential extractions, although time consuming, furnishes detailed information about the origin, mode of occurrence, biological and physicochemical availability, mobilization and transport of trace metals (Ure and Davidson, 2002). Sequential extraction procedures selectively extract metals bound by specific soil fractions with minimal effects on the soil components. In practice, sequential fractionation schemes have been suggested to identify element distribution with operationally defined soil pools (Amanda and Weindorf, 2010). As a result of ineffective law enforcement agencies to enforce existing environmental laws coupled with lack of stringency even when attempts are made to enforce, Nigerian citizens and indeed residents of Oghara and environs in Delta State continue to dump refuse and litter the environment indiscriminately with such toxic substances as condemned engine oil, car batteries from mechanic workshop and solid waste even on the streets. These heavy metals can become a threat to vegetation and animals and ultimately affects the quality of human life, Thus, it becomes imperative to assess the levels of physico-chemical properties, spatial distribution and chemical speciation of heavy metals in soil from auto-repair workshops in Oghara and its environs in Delta State, Nigeria in order to determine their potential hazards to humans.

II. MATERIALS AND METHODS Study Area
Oghara is a town in Ethiope West Local Government Area of Delta State, Nigeria and is located between latitude 5 0 35′1''N and longitude 5 0 51'16''E. the city has road intersections connecting Sapele to Warri and Benin. This study was conducted in four popular automobile workshops in within the town Oghara, site selection was based on the distance from one another, and all samples were collected within the range of latitude 5 0 55 l 54N to 5 0 57 l 11N and longitude 5 0 38 l 40E to 5 0 41 l 19E. Global positioning system (GPS) and ground reconnaissance were used for identification of sites and geo-referencing.  (Tripathi and Misra, 2012). All samples were air dried and ground to pass through a 2mm sieve and used for both physico-chemical analysis and fractionation experiment (Anegbe and Okuo, 2013).

Physico-chemical Analysis of the Soil Samples
The pH and the CEC were determined as described by Anegbe and Okuo (2013

III. RESULTS AND DISCUSSION
The physico-chemical properties of the soil samples at various sites are shown in Table 2. Soil pH is the most widely accepted parameter which exerts a controlling influence on the availability of micronutrients and heavy metals in the soil to plants (Igwe et al., 2005). The pH values of the soil samples from the automobile workshops were found to be in the acidic region (5. 10 -5.40) and lower than that of the control (6.40). Acidity controls availability, mobility and toxicity of heavy metal ions in the soils. Most metals tend to be less mobile in soil with high pH as they tend to form insoluble complexes . Electrical conductivity measures soil salinity. The electrical conductivities of the soil samples from the automobile workshops were all higher than that of the control. This indicates that movement of charge particles would be more at the workshops than that of the control because there are more soluble salts in the soil samples from the automobile workshops than the control (Karaca, 2004;Arias et al., 2005).  (Anegbe and Okuo, 2013). Soils with high sand content exceeding 70% will have weak surface aggregation and such soils will be porous and have high rate of water infiltration and air circulation (Gbadegesin and Abua, 2011). The nitrogen and phosphorus contents of the soil samples were both higher at the automobile workshop sites compared to the control. T-test was used to indicate significant difference between variables. Pvalues less than 0.05 were considered statistically significant.      Figure 6). The minor role of the organic fraction in the speciation of Cd noted in this present study is consistent with the low adsorption constant of Cd to organic matter (Yusuf, 2007

Mobility Factor
The operationally defined extraction sequence fractionates the heavy metals in the soil in the order of decreasing solubility. As a result, the exchangeable and carbonate (F1 + F2) fractions which are the early fractions, capture the most reactive and presumably the most mobile and bioavailable fractions (Salbu et al., 1998). The relative index of metal mobility was calculated as a mobility factor (MF) on the basis of the following equation (Kabala and Singh, 2001). Where; F1 = Exchangeable metal content fraction F2= Metal content bound to carbonate fractions F3= Metal content bound to Fe-Mn Oxide Fraction F4= Metal content bound to organic matter fraction

F5=
Residual metal content fraction. The results obtained from table 5 below showed high mobility factor of the heavy metals within an average range of 16.51% -41.54% for all the sites, which indicates a high lability and biological availability of the metals (Kabala and Singh, 2001; Anegbe and Okuo 2013). The 0.00% mobility of Pb observed in the control site indicate that the metal is not bio-available for plant uptake in that site. According to Wong et al. (2007), high mobility of metals in acidic sandy loam is due to low pH, low clay and low organic matter contents. This means that soil sample with low pH, low percentage of clay and low organic matter content retains fewer metals. Thus, more metals would be released into the soil solution.

Assessment of Metal Contamination Contamination Factor (CF)
The level of contamination of soil by metal is expressed in terms of a contamination factor (CF) calculated as: Where the contamination factor CF < 1 refers to low contamination; 1 ≤ CF < 3 means moderate contamination; 3 ≤ CF ≤ 6 indicates considerable contamination and CF > 6 indicates very high contamination.  contaminated with respect to Zn, and very highly contaminated with respect to Cd.

Index of geoaccumulation (Igeo)
Index of geoaccumulation (Igeo) was used to evaluate the heavy metal pollution by comparing current concentrations with reference (control) values as reported by Bentum et al. (2011). Where Igeo is Index of geoaccumulation of the metal, Cn is the measured concentration of the element in the sample and Bn is the geochemical background value. As reported in table 7, this index consists of seven scales (0-6) ranging from background concentration to very highly polluted. The interpretation of the results was made based on the scale above in comparison with control sample.