Comparison of Soil Samples from Selected Anthropogenic Sites within Enugu Metropolis for Physicochemical Parameters and Heavy Metal Levels Determination

This study compared the physicochemical parameters and heavy metal levels in soil samples from selected anthropogenic sites within Enugu metropolis, Enugu State, Nigeria using standard analytical methods. Soil samples at depths (0 - 20 cm) and (20 - 40 cm) were collected from waste dump sites, metal scrap dumps, fuel filling stations and auto-mechanic workshops and analyzed for physicochemical characteristics and heavy metal levels. Atomic absorption spectrophotometer was used for heavy metal determination while conventional analytical methods were employed for physicochemical parameters evaluation of the soil samples. At soil depths 0 - 20 cm and 20 - 40 cm the respective mean range of pH, electrical conductivity, organic matter and organic carbon contents in the soil samples were, 6.33 - 6.74, 101.46 - 123.21 µS/cm, 6.41% - 8.35% and 13.73% - 16.14% for auto-mechanic workshops; 6.92 - 7.43, 56.46 - 60.02 µS/cm, 1.53% - 2.20% and 11.93% - 12.60% for fuel filling stations; 7.14 - 7.84, the sub-soils. The electrical conductivity values of top and sub-soil samples from the studied auto-mechanical workshops were above the recommended limits. At soil depths 0 - 20 cm and 20 - 40 cm, the respective mean range of Zn, Pb and Cd in the soil samples were 17.29 - 19.16 µg/g, 0.704 - 0.96 µg/g and 0.26 - 0.33 µg/g for auto-mechanic workshops; 4.13 - 4.88 µg/g, 0.21 -0.32 µg/g and 0.03 - 0.11 µg/g for fuel filling stations; 30.02 - 36.11 µg/g, 0.43 -0.48 µg/g and 0.15 - 0.19 µg/g for metal scrap dumps; 9.30 - 10.84 µg/g, 0.53 -0.60 µg/g and 0.38 - 0.45 µg/g for waste dump sites. The mean levels of Pb in soil samples from mechanic workshops and waste dump sites were above the recommended permissible limits for agricultural purposes. The study therefore indicated that these sites (auto-mechanic workshops and waste dump sites) could be major sources of Pb pollution to nearby farmlands, streams and the general environment. Plants grown on or around these sites may not produce high yields and could be severely contaminated with heavy metals which portend health danger to food consumers within the environment.


Introduction
Nigeria with a land area of approximately 950,000 km 2 is richly endowed with diverse resources. However, there are staggering environmental problems manifested in various forms to present a grim woes across the length and breadths of the nation. Pollution with different types of degradation such as erosion, loss of fertility and the continuing speed of urbanization are major threats to the sustainability of the environment and resources within it in Nigeria [1]. Human activities in urban areas largely contribute to the contamination of urban soils and this is a major environmental concern. Studies have shown that environmental pollution and its attendant problems on land, air and water qualities are severe now than ever [2]. There are several evidences to this fact, ranging from soil fertility loss, depletion of biodiversity, several health problems (those leading to metabolic disorder) and ecological problems.
The major causes of environmental pollution include indiscriminate discharge of wastes (solid and liquid) into natural habitats, unplanned siting of fuel filling station close to residential areas, indiscriminate dumping of wastes from auto-mechanic works, abattoirs and metal scrap outlets.
These practices are common and a disturbing trend in Nigeria, since there were well defined management protocols to regulate it. One of the major challenges confronting Nigeria as a country is solid waste management [3]. Solid wastes which are normally referred to as garbage or trash mostly contain, electronic appliances, plastics and motor oil and when undergoing decomposition or  [4]. For instance, scrap metals are an important part of municipal solid wastes which have monetary value in Nigeria.
According to [5] scrap yards are haphazardly sited in urban centres in Nigeria where all kinds of scraps from abandoned automobiles, machineries and electrical appliances are disassembled and stacked for future recycling purpose.
Many of these scrap materials are made up of contaminants that adversely affect the environment when not properly kept and managed [6].
Also, automobile emulsion is perhaps the greatest single source of contamination and it has been shown to contain lead, zinc, cadmium and nickel, the most important being lead from fuel and zinc from tyres and it accounts for about 80% of automobile pollution with heavy metals in Nigeria [7]. Repairs and servicing of automobiles and other types of machinery in mechanic workshops are sources of heavy metals in the environment. Automobile mechanic workshop wastes have been implicated for elevated concentrations of cadmium, chromium, copper, lead, nickel and zinc in the soil profile in the vicinity of automobile waste dumps in Nigeria [8].
Automobile used waste (oil) contains product sediment, water and metallic particles resulting from machinery wears, organic and inorganic chemicals used in oil additive that alters soil chemical characteristics and increase the heavy metal loads in the soil as a result of the constant and indiscriminate spewing on the soil surface of the workshops. Open dumps are generally unsanitary and constitute malodorous places in which disease-carrying vermin such as rats and flies proliferate [9]. Methane and other gases are released into the surrounding air and micro organisms decompose the solid wastes and fires pollute the air with acrid smoke and numerous volatiles. Liquids that ooze and seep through the solid waste heap ultimately reach the soil, surface water and ground water contaminating them with organic and inorganic pollutants.
Hazardous materials such as heavy metals, pesticides and hydrocarbons that are dissolved in these liquids often contaminate solid and water [10].
According to [8], the continuous disposal of municipal waste on the soil may lead to an increase in heavy metal loads in the soil and surface water that could become inimical to deep feeding plants. Heavy metals such as arsenic, cadmium, lead, chromium, nickel, cobalt and mercury are of concern primarily because of their ability to harm soil organism, plants, animals and human beings [10]. More emphatic are the untreated dumplings' that rapidly increase soil toxicity making such large area dump sites potentially hazardous for agricultural purposes.
According to [11], fuel filling stations sited very close to residential areas and agricultural farmlands are major sources of organic and inorganic pollution of these areas due to regular discharge of petroleum products these filling stations. Most pollutants from the filing station have been known to be persistent and bioaccumlative within the environment and the potential lifelong effects resulting from these pollutants on the inhabitants of the environment and workers in

Study Area
Enugu metropolis is made of the three local government areas, namely: Enugu Due to the peaceful atmosphere in the metropolis, a lot of economic activities are going on, hence anthropogenic sites such as auto-mechanic workshops metal scrap dumps, fuel filling stations, waste dump sites, hospitals and major and street markets are common.

Sample Location and Collection
Four soil samples each from four anthropogenic active sites (dump sites, metal scraps yards, fuel filling station and mechanic workshops) were selected across each of the local government areas that make up the metropolis. The choice of these locations was based on their proximity to residential neighborhoods'.
In each of these selected sites, soil samples were collected using soil auger at The soil samples were air dried ground and passed through a 2 mm sieve and stored in clean labeled plastic cans until analysis. Before each sampling, the surface debris on the soils was removed.

Physicochemical Analyses of the Soil Samples
Physicochemical parameters such as pH and electrical conductivity were determined using pH meter and conductivity meter respectively while organic matter and organic carbon contents were determined in the soil samples were determined using Walkley and Black wet oxidation procedures as described by [13] [14].

Heavy Metal Analyses
Exactly 5 g of the soil sample was weighed and 6ml of freshly prepared aqua regia (1:3) HNO 3 :HCl respectively was added and placed in a digestion block for about 30mins. It was allowed to cool and then filtered into a 100 ml volumetric flask with distilled water. The filtrate was analyzed for selected heavy metals (Zn, Pd and Cd) using Perkin-Elmer Analyst 300 atomic absorption spectrophotometer (AAS).

Contamination/Pollution Index (C/P) of the Metals
The contamination/pollution index of the studied metals in the soil samples was calculated using the scheme formulated by [15]; where A represents concentration of the metals in the soil and B represents target value.
Contamination/pollution index value greater than 1, defines pollution range but when it is less than 1, it defines the contamination range.

Statistical Analysis
Results obtained from all samples were subjected to descriptive (mean, standard deviation and ranges) statistics.

Quality Control
All glass wares used were previously soaked in 14% nitric acid for 24 hours to remove possible entrained metals, washed with detergent and rinsed with deionized water. Quality control was assured by the use of triplicates standard reference materials and procedural blanks.

pH
pH is an important soil property, having great effects on solute concentration and adsorption in the soil. pH affects the mobility of heavy metals in the soil. [16] stated that soil pH is correlated with the availability of nutrients to the plants. Results of Table 2 show that at soil depth 0 -20 cm and 20 -40 cm, the mean pH values of the soil samples were, 6.33 ± 0.11 and 6.74 ± 0.08 for auto-mechanic workshops; 6.92 ± 0.03 and 7.46 ± 0.01 for fuel filling stations; 7.14 ± 0.12 and 7.84 ± 0.06 for metal scrap dumps; 6.51 ± 0.05 and 6.81 ± 0.13 for waste dump sites.
Only the pH of auto-mechanic workshop soil samples at depth of 0 -20 cm was below the recommended permissible limits. The results of Table 2 for pH determination in the soil samples further show that the pH values of the soil samples increased with soil depth for all the investigated anthropogenic sites within Enugu metropolis. This observation therefore suggests that at the soil surface where anthropogenic activities are felt, soil chemical characteristics such as pH may have acidic values which lessen with soil depth occasioned by evaporation and oxidation of solutes at the soil surfaces.  [19] obtained lower pH values of 3.6 and 5.4 respectively in soil samples from auto-mechanic workshops in Abraka, Delta state than what was obtained at soil depth of 0 -20 cm and 20 -40 cm in the soil samples from auto-mechanic workshops in this study. The lower pH values in soil samples in the auto-mechanic workshops compared to the other studied sites could be attributed to anthropogenic activities in such environment such as spilling up of acid on the soil from discharged motor batteries and spilling of fuel on the soil during routine maintenance and repair activities. Again, the low pH values observed for soil samples in the mechanic workshops and waste dump sites could be as a result of a higher decomposition of organic matter in these sites which then releases carbon IV oxide that reacts with water to form carbonic acid, which eventually reduces soil pH. Table 2 show that at soil depths 0 -20 cm and 20 -40 cm, the mean electrical conductivity of the soil samples were, 123. 21  The soil samples from auto-mechanic workshops at depths of 0 -20 cm and 20 -40 cm and waste dump site at 0 -20 cm soil depth were above the WHO recommended permissible limits. The electrical conductivity values of the soil samples at the studied sites were found to decrease with depth. The studied sites had electrical conductivity values in the following decreasing order; auto-mechanic workshops > waste dump sites > metal scrap dumps > fuel filling stations. According to [20] the implication of high electrical conductivity in soils is that there is reasonable or significant presence of ions in such soils. The higher electrical conductivity values observed for soil samples from the auto-mechanic workshops within the metropolis could be attributed to reactions between some spilled acids from motor batteries and some metals from vehicle scraps leading to the formation of some soluble and ionizable inorganic salts in the soils. Equally the build-up of degradable wastes in the soils in auto-mechanic workshops and waste dump wastes could have accounted for their observed higher values of electrical conductivity than in other studied sites. [19]  [5] obtained a higher mean range of 870.00 -1480.00 µS/cm for electrical conductivity in soil samples in metal scrap yards in Benin City, Edo State than was reported in soil samples from the metal scrap dumps in this study.

Results of
The marked variation in the electrical conductivity values in soil samples in the studied anthropogenic sites could be ascribed to the variations in the cation exchange capacities in the soil samples and the differing rates at which metallic salts and organic matter complexes are formed.

Organic Matter (%)
Soil organic matter enhances the usefulness of soils for agricultural purposes. It supplies essential nutrients and has unexcelled capacity to hold water and absorb cations. It also functions as a source of food for soil microbes and thereby helps enhance and control their activities [10].
According to [2] soil organic matter usually acts as a store house or reservoir for most metals hence can influence their bioavailability in the soil. Results of Table 2 show that at soil depths 0 -20 cm and 20 -40 cm, the mean organic matter contents of the soil samples were, 8.34 ± 0.34 and 6.41% ± 0.51% for auto-mechanic workshops; 2.20 ± 0.14 and 1.53% ± 0.10% for fuel filling stations; 4.12 ± 0.09 and 3.81% ± 0.11% for metal scrap dumps; 10.75% ± 0.31% and 8.83% ± 0.19% for waste dump sites. The organic matter contents of the samples in the studied anthropogenic sites were also found to decrease with soil depth.
This therefore buttresses anthropogenic activities as mostly affecting the soil surface and decreased with soil depths owing to evaporation, oxidation and leaching effects of solute in the soil.
The soil samples at waste dumps sites were found to have higher organic matter contents and this was attributed to its harboring of a more increased level of biodegradable waste than the other studied anthropogenic sites.
The soil samples from the studied anthropogenic sites had organic matter contents in the following decreasing order; waste dump sites > auto mechanic workshops > metal scrap dumps > fuel filling stations.
The organic matter contents of the soil samples from all the studied anthropogenic sites were within the recommended permissible limits.

Organic Carbon (%)
Results of Table 2 show that at soil depths 0 -20 cm and 20 -40 cm, the mean organic carbon content of the samples were, 16.14 ± 1. 15

Zinc
Results of Table 3 show that at soil depths 0 -20 cm and 20 -40 cm, the mean ± 0.94 µg/g for waste dump sites. The mean levels of Zn in the soil samples from the studied anthropogenic sites were found to decrease with soil depth and this could be attributed to Zn mineralization by plants growing in the studied soils.
The mean levels of Zn in the soil samples from the studied sites were within permissible limits.
Soil samples from metal scrap sites were found to contain the highest mean levels of Zn of the four studied anthropogenic sites and this could be attributed  Zinc is involved in various metabolic activities of many organisms and is also one of the micro nutrients essential for normal growth, its increased level can cause many health disorders [19].

Lead
Results of Table 3  Lead is a cumulative poison and a toxic element at elevated levels, hence, its presence at toxic levels in agricultural lands portends great danger to agricultural yield and eventually the food chain process.

Cadmium
Results of Table 3 show that at soil depths 0 -20 cm and 20 -40 cm, the mean Cd levels in the soil samples were, 0.26 ± 0.03 and 0.33 ± 0.10 µg/g for auto-mechanic workshops; 0.08 ± 0.01 and 0.11 ± 0.03 µg/g for fuel filling stations;  [24] was found to be lower than Cd values in the soil samples from the waste dump sites in this study and in any case both studies were in agreement in that the mean Cd levels in soil samples from the two environments were within recommended limits for an agricultural soil.
[21] attributed the high levels of Cd in the soils of most waste dump sites in Nigeria to the dumping of poly vinyl chloride (PVC) materials, nickel-cadmium materials and food package materials.
The mean Cd levels of 1.50 µg/g in soils around metal scrap dumps in Agbor and Abraka in Delta State reported by [18] were higher than what this study obtained for Cd in soil samples from metal scrap dumps within Enugu metropolis.
Results of Table 4 show the contamination factor/pollution index of the  Table 1. This result therefore suggests that soils from the auto-mechanic workshops and waste dump sites would not be profitable for agricultural purposes and thus harvesting crops grown in these sites or manuring other farmland with soils from the sites could result to an increased exposure by people to Pb and other pollutants through the food chain process. Table 1 shows that both the top and sub soil samples from fuel filling stations and metal scrap dumps were slight to moderately contaminate with Cd while severe to very severe contamination with Cd was observed for soil samples from auto mechanic workshops and waste dump sites within the metropolis.
Although the soils from the investigated anthropogenic sites could not be said to be polluted with Cd but considering the toxicity associated with this metal especially to man, edible crops grown on soils from mechanic workshops and waste dump sites still poses a very serious health risk to animals and man.

Conclusions
The results of this study show that anthropogenic sites such as auto mechanic workshops and waste dumps sites adversely affect the soil characteristics especially for agricultural purposes while fuel filling stations and metal scrap dumps do the same but to a lesser degree. In these sites, the soil characteristic of the top soils was most affected than the sub-soils suggesting; therefore anthropogenic activities adversely affect the soil characteristics of the top soil than the sub-soil.
The top and sub-soil samples from auto mechanic workshops, fuel filling stations, metal scrap dumps and waste dump sites had Zn and Cd levels within permissible limits for agricultural purposes.
The levels of Pb in soil samples from mechanic workshops and waste dump sites were above the permissible limit and this therefore suggests these sites as potential sources of Pb pollution to the surrounding agricultural farmlands and streams.
The study therefore concludes that soil samples from especially waste dump sites and auto mechanic workshops may not aid agricultural yield and edible plants grown on soils from these sites pose great health danger to food consumers.