Use of integrated pollution indices in assessing heavy metals pollution in soils of three auto mechanic villages in Abuja

This research work targeted at the use of integrated pollution indices models of Pollution Index (PI), Average of Pollution Index (PIAvg), Pollution Load Index (PLI) and Nemerow Pollution Index (NIPINemerow) in assessing heavy metals pollution in soils of three auto mechanic villages of Abuja. Soil samples were randomly collected with a stainless hand auger to a depth range of 0 to 15 cm and were analyzed with atomic absorption spectrophotometer of model Unicam 969 Solar to determine the heavy metal contents in them. Mean concentration (mg/kg) of heavy metals from the results were observed to follow a decreasing order; Apo: Cu (7668) Zn (5360) Cr (1174) Fe (467) Pb (333) Ni (196) Cd (10.5); Kugbo: Zn(1587) Cu (1042) Cr (783) Ni (234) Fe (217) Pb (170) Cd (9.47) and Zuba: Zn(1190) Cr (767) Cu (512) Fe (279) Pb (250) Ni (127) Cd (10.4). Strong positive correlations exist between heavy metals which indicate same source of contamination, mutual dependence and identical behaviors. Results of integrated pollution indices showed that investigated soils have been polluted to various degrees ranging from low to high level pollution. This indicates deterioration of sites quality.


INTRODUCTION
One of the major fallout of industrialization and urban development is the release and accumulation of heavy metal containing wastes and other environmental pollutants in the environment (Du et al., 2013).Pollution of natural environment by heavy metals over the years are mainly from anthropogenic sources which include; traffic emission (Wei et al., 2010), industrial emission (Jiang et al., 2017;Ekeocha et al., 2017;da Silva et al., 2016;Du et al., 2015;Jiang et al., 2014), and agricultural processes (Arao et al., 2010).Heavy metals can exert their toxicity via dermal, skin contacts (exposure/ absorption), inhalation and ingestion which all have adverse effect on health of humans and organisms (De Miguel et al., 2007).Bioaccumulation of heavy metals in human tissue can lead to some health problems like: cellular and tissue damage, infertility, neurotoxic effects in children, circulatory and nerve tissues damage, breakage of metabolism route, heart and liver disease, kidney damage, chronic renal, cancer, lung infection, cardiovascular diseases, respiratory system disorders, skin and tooth decay among others (Jarup et al., 2013;Damek-Proprawa et al., 2003).Hence there is need to assess the contamination level and environmental effects of selected heavy metals in major auto mechanic villages of Abuja, bearing in mind the inflow of vehicles to these auto mechanic sites on daily basis for various vehicle maintenance.
With the development of ecological and exploration geochemistry survey, a great deal of data related to heavy metal concentration in soils and water sediments have been measured which can be used to assess the quality of ecological geochemistry environment.Many calculation methods have been presented to assess the environmental quality, such as pollution index (Gong et al., 2008) and Principle correlation (Cheng et al., 2007).Pollution index is a powerful tool for processing, analyzing and conveying raw environmental information to decision makers, managers, technicians and the public (Caeiro et al., 2005).This research was carried out in Abuja, North Central part of Nigeria in order to evaluate the contamination level(s) of various heavy metals in soils in selected auto mechanic villages in the area.Nevertheless, the research was not conducted as a regulatory sampling or as a result of an outbreak of any disease.The sole objective was to assess the levels of heavy metals in soils and to evaluate the degree of contamination of soils by heavy metals using various integrated pollution indices models.

Study sites, sample collection and analysis
The study was conducted in three major auto mechanic villages in Abuja namely; Apo, Kugbo and Zuba (Figure 1).The area is located in the North-Central part of Nigeria with geographical coordinates of latitude 9 40 ' N and 9 29 ' E. The city also shares a land border with six other states namely: Benue, Kaduna, Kogi, Nassarawa, Niger and Plateau States.Soil samples were randomly selected with a stainless hand dug auger to a depth range of 0 -15 cm with five of the soil samples drawn from each of the mechanic villages (Apo, Kugbo and Zuba).A control sample was also collected from a distance of 100 km were neither industrial nor commercial activities takes place.Sampled soils were homogenized, air dried; crushed and sieved in a mechanical sieve of mesh size 338 μm with make Endecott's Limited London, England, serial number 489494.Heavy metals concentrations in each sample were determined using atomic absorption spectrophotometer (Unicam 969 Solar) immediately after digestion of soil samples (George et al., 2013;Kouadia et al., 1987).The sampling and analyses were all done in the month of November, 2014.

Statistical analysis
Descriptive statistics were conducted to determine the average, range, mean and standard deviations of investigated heavy metals in soil samples.Person's correlations matrix was also performed to evaluate sources of heavy metals in soils of studied sites, the dynamic of the contamination and potential relationship among measured variables.Statistical analysis was done using IBM SPSS 16.0 software.

Integrated pollution indices assessment
Integrated indices are indicators used to calculate more than one metal contamination which are based on single indices and each kind of integrated index might be composed by the above single indices separately (Gong et al., 2008).Four integrated pollution indices: Pollution Index (PI), Average of Pollution Index (PIAvg), Pollution Load Index (PLI) and Nemerow Pollution Index (NIPINemerow) were used.

Pollution index
Pollution index is defined as the ratio of the metal concentration in the city to the background concentration of that metal (Wei et al., 2009).
Where Ci is the mean concentration of each of the investigated metal in soil or sediment drawn from at least five sampling sites and Cri is the background value of the metal.In order to unify the assessment results, reference values as provided by DPR (2002) were used as background values (Cr = 100, Fe = 5000, Ni = 35, Cu = 36, Zn =140, Cd = 0.8 and Pb = 85).The following terminology was used for the pollution index model: PI < 1, non pollution; 1 ≤ PI < 2, low level pollution; 2 ≤ PI < 3, moderate level of pollution; 3 ≤ PI < 5, strong level of pollution; PI ≥ 5, very strong level of pollution (Yang et al., 2011).

An average of pollution
An average of pollution index (PIAvg) model is an aspect integrated indices used to calculate more than one metal contamination and it's mathematical written as: Where Pi is the single pollution index of heavy metal i and m is the count of the heavy metal specie.Values of PIAvg < 1 indicates high quality soil and PIAvg > 1 indicates low quality soil.This type of pollution index was used to assess the quality of abandoned-minedtailings environment (Bhattacharya et al., 2006).

Pollution load index
Pollution load index is an example of root of the product of pollution index indices which is based on contamination factor of each metal in soil.This aspect of integrated pollution indices have been used to quantify pollution load of heavy metals in both soils and sediments.
The model also provides an easy and comprehensive means of assessing the quality of an investigated site (Waheshi et al., 2017;Goher et al., 2014).
PLI can be defined as; PLI = (PI × PI ×PI × ------PI ) equation ( 3) Where PI is pollution load index of individual metal described earlier and n is the number of investigated metals.The following terminology was used for the pollution load index: PLI < 1, perfection; PLI = 1, only baseline levels of pollutants present; and PLI > 1, deterioration of site quality (Tomlinson et al., 1980).

Nemerow pollution index
The formula of Nemerow pollution index contains the biggest monomial pollution index in the evaluation of parameter which illustrates the effect of the pollutant the highest pollution index on the environmental quality.In recent times, some researchers have applied Nemerow pollution index model in assessing the quality of soil in an environment (Jiang et al., 2014;Cheng et al., 2007).
Where, Pi is the single pollution index of heavy metal i; Pimax is the maximum value of the single pollution indices of the investigated heavy metal(s) and m is the count of the heavy metal species.The quality of soil were classified into five categories from Nemerow pollution index: NIPINemerow < 0.7, unpolluted, 0.7 ≤ NIPINemerow < 1.0, little pollution; 1.0 ≤ NIPINemerow < 2.0, slight pollution; 2.0 ≤ NIPINemerow < 3.0, moderate pollution; NIPINemerow ≥ 3.0, serious pollution (Cheng et al., 2007).Sum of pollution index can be mathematically defined as: Where Pi is the single pollution index of heavy metal i and m is the count of the heavy metal species.The sum of pollution index was widely used in soil and sediment quality assessment by heavy metal such as the degree of contamination and potential ecological risk index (Maanan et al., 2014;Håkanson et. al., 1980).Lacatusu (2000).

RESULTS AND DISCUSSION
Results of concentrations of heavy metal (mg/kg) in soil from investigated sites are presented in  mean values of heavy metals in all the sites were observed to have exceeded those from control, DPR background value and those of some international regulatory bodies (Table 1).They also exceeded those reported by some researchers (Mugoša et al., 2016;Kamunda et al., 2016).This indicates that the studied sites have been contaminated by heavy metals generated from various auto mechanic repairs carried out the studied sites.
Results of the pollution index (PI) shown in Table 3 reveal that in Apo auto mechanic village, 71.24% of investigated heavy metals: Cu, Zn, Cd, Cr and Ni were in the class of very strong level of pollution.Pb and Fe were also in the class of strong level and non-pollution.In Kugbo auto mechanic village, 71.24% of the heavy metals were in the class of very strong level of pollution leaving 14.28% of it to moderate and non-pollution respectively.57.14% of the heavy metals in Zuba auto mechanic village were also in the class of very strong level of pollution with 21.43% of the metals in the class of serious and non-pollution.High (PI Avg ) values (Table 3) recorded in the three sites depicts that soil in these sites have low quality due to contamination by heavy metals.
Results of pollution load index also revealed that Apo, Kugbo and Zuba sites had PLI values of 8.26, 3.99 and 3.59 which are all greater than 1 which indicate deterioration of site quality due to pollution.These values were also higher than those reported by Goher et al. (2014).

Conclusion
Different pollution indices models employed in the calculation of heavy metal pollution in soil samples from three auto mechanic villages of Abuja showed that mean content of these heavy metals (mg/kg) follow a decreasing order of: Apo site: Cu > Zn > Cr > Fe > Pb > Ni > Cd; Kugbo site: Zn > Cu > Cr > Ni > Fe > Pb > Cd and Zuba site: Zn > Cr > Cu > Fe > Pb > Ni > Cd.These values also exceeded those from control, background values of national and some international regulatory bodies.Results of correlation analysis indicated that some of the investigated heavy metals in all the sites are mutually dependence with identical behaviour and same origin.Integrated pollution indices assessment conducted reveal that soils in the investigated sites have been polluted to various degrees by heavy metals.These heavy metals could also be traceable to anthropogenic sources probably from various auto repairs done in the area which poses serious ecological risk to human, organisms and environment at large.

Figure 1 .
Figure 1.Geographical location of investigated Auto Mechanic Sites in Abuja.

Table 2 .
Pearson's correlation coefficient matrix for heavy metals in investigated auto mechanic villages.

Table 3 .
Values and categories of Pollution Index (PI), Average of Pollution Index (PIAvg), Pollution Load Index (PLI) and Nemerow Pollution Index (NIPINemerow)., mean concentration of heavy metals from at least five points; Cri, background value; PI; pollution index; np, non pollution; lp, low level pollution, mp, moderate level of pollution; sp, strong level of pollution; vsp, very strong level of pollution; PIAvg, average of pollution index; lq, low quality of soil; hq, high quality of soil; PLI, pollution load index; p, perfection; bp, baseline pollutants; dsq, deterioration of site quality; NIPINemerow, Nemerow pollution index; sp, slightly polluted; mp, moderately polluted; sp, seriously polluted. Ci