Environmental Risk Assessment of Metal Contamination of Agricultural Soils along Major Roads of Two Peri – Urban Areas in Nasarawa State, North

. This research focused on the level, pollution, and ecological risk assessment of selected heavy metals from agricultural soils at major roadsides in Nasarawa Eggon and Doma areas. Composite soil samples were collected at 0–20 cm depth, homogenised, and assayed for metal (As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, U and Zn) levels using the X–ray fluorescence technique. Pollution and toxicity of the studied metals were determined by evaluating enrichment factor (E f ), geo–accumulation index (Igeo), contamination factor (C f ), degree of contamination (C d ), pollution load index (PLI), and ecological risk assessment (ERA). The result showed that concentrations of As, Cd, Cr, Cu, Fe and Ni were lower than the average shale values, except for As of site N2 in the Nasarawa Eggon area and Fe of site D4 in the Doma area. The E f suggests a significant anthropogenic contribution to the presence of As, Co, Hg, Mn, Pb and Zn in some of the sampled sites. The Igeo indicated that Nasarawa Eggon sampled soils were polluted by Hg and Pb while Hg and Co polluted those of Doma. The C f values revealed that most of the soils were contaminated with Hg in addition to As, Co, Fe, Mn, and Zn in some sites. All sites have low C d except N2 and N4 (Nasarawa Eggon) and D2 and D4 (Doma). However, PLI showed that only sites N2 and D4 were polluted. ERA revealed that As, Co, and Hg posed potential ecological risks ranging from low to a significant level in different sites. Sites N2, D2 and D4 showed a moderate ecological risk index. Therefore, this study showed significant anthropogenic inputs from automobile emissions and human activities to the pollution of agricultural soils along major roads and the pollution could pose negative health implications to human and animal health. assessment revealed that As and Hg pose moderate to significant potential negative implications to human and animal health in some of the sampled sites in Nasarawa Eggon while As, Co and Hg showed similar ecological risk in some of the sampled sites in Doma. The potential ecological risk index (RI) showed that metal pollution in sampled site N2 of Nasarawa Eggon and D2 and D4 of Doma can cause serious negative health implications for humans and animals in such an environment. Thus, the study suggests higher vehicular/human activities in Nasarawa Eggon than the Doma study area and they had a remarkable contribution to the presence of the investigated metals in the nearby agricultural soils.


INTRODUCTION
Soil pollution by metals (especially heavy metals) is a global phenomenon which results from natural or anthropogenic sources. However, the authorities in Nigeria give little attention to the associated health and physiological risks to humans and animals. Metals are unique environmental pollutants given their ubiquity and distribution in various environmental A c c e p t e d M a n u s c r i p t matrices [1]. Natural sources of metals depend on the geological substrates and the processes that form the soils [2]. Metals are concentrated and transformed into various products via anthropogenic processes. These processes often lead to much higher concentrations of different chemicals than those naturally present in the environment. According to De-Miguel et al. [3], the categories of anthropogenic sources of heavy metals can be generally classified as; urban elements, natural elements and elements of a mixed origin, implying that some metallic pollutants undergo geochemical reactions which alter their significant features. Heavy metals act antagonistically disrupting trace elements in the body, inhibiting, and competing with protein and enzyme for binding sites and impairing the immune system. Unachukwu and Agomuo [4] reported that non-communicable diseases (NCDs) such as cardiovascular disease, diabetes mellitus, cancer, renal diseases, liver failure etc., may be associated with heavy metal toxicity and are probably more pronounced through occupational exposures [5].
Nasarawa Eggon and Doma towns can be described as peri-urban or semi-urban areas which are the administrative headquarters of their respective Local Government Areas in Nasarawa State, north-central Nigeria. These towns are surrounded by rural areas and are predominantly agricultural areas with majority of the inhabitants known to be farmers and traders. Nasarawa Eggon town is located along the ever-busy Lafia-Abuja road. This major road is characterized by high automobile/vehicular and human traffic due to its proximity to Nigeria's Federal Capital Territory, Abuja as well as the Nasarawa State capital, Lafia.
Commuters (public and private) stopover at this town to buy agricultural produce (tubers such as yam and potatoes, fruits etc.) and food, while inhabitants of surrounding rural areas visit the town to carry out minor business transactions and also board vehicles to Abuja and other destinations. Drivers equally stopover in the town to repair and refuel their vehicles as well as pick passengers. Doma town is along busy Lafia-Rukubi road. It is a major road characterized by a high volume of automobile traffic arising from its linkage to Rukubi where Africa's largest rice mill is located and also, a major route to rural agricultural farms in Doma where largescale irrigation farming is practised all year round, using water from Doma dam. Thus, the focal towns are located on the high-way characterized by heavy vehicular and human traffic which results in high human activities [6].
The busy nature of Nasarawa Eggon and Doma major roads make them attractive to sundry human activities, ranging from indiscriminate citing of automobile parks and workshops (which are centres of sundry activities ranging from automobile servicing, panel beating, spraying, etc.), markets, business centers (typing, photocopying, printing, cybercafé, Niajabet stations, movie or football viewing center etc.), food vending spots, gas stations, primary health A c c e p t e d M a n u s c r i p t care centres, etc. Most of the businesses on these major roads (Nasarawa Eggon and Doma) are powered by generators due to a lack of power supply from the National grid. Their activities generate different kinds of wastes (gaseous, liquid and solid) which comprise automobile/vehicular and generator emissions, medical wastes, electronic waste (such as computers and their accessories, TV sets, fax machines, cell phones, telephones, photocopy machines and printers, dryers, wireless devices, video cameras, chips, motherboards, cathode ray tubes and peripheral items, kitchen equipment, electronic toys, washing machines, refrigerators etc.), automobile workshop wastes (damaged vehicle parts, car paint dust, engine oil, grease etc.) faeces, urine, garbage, sludge from sewage etc [6].
The unregulated disposal of waste in these towns results in indiscriminate and open dumping of these generated wastes (which are the surrounding agricultural soils) which wind up in landfills and informal dumps where they decompose and release toxic metals into the environment. Long-term dumping of municipal wastes can influence soil properties and productivity at municipal waste dump sites but still may be used for farming provided that ecotoxicological risks associated with its usage are continuously assessed and controlled [7].
The concentration of these toxic metals in the environment has significantly increased due to increased human activities through emissions from industrial plants, thermal power stations, waste disposal, soil amendments and fertilization, vehicular traffic and road infrastructure [8], mining, smelting and refining of ores. Heavy metals such as cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), manganese (Mn), nickel (Ni) and zinc (Zn) are often contained as additives in some lubricants and gasoline are non-degradable in the soil. Some of them have been classified as priority pollutants by United State Environmental Protection Agency (USEPA). At the moment, very few technologies, such as soil washing and bioremediation, are available to treat these mixed wastes [9]. Edori and Kpee [10] reported that heavy metals can be generated from faeces and urines of mammals which are either deposited in the agricultural soil or washed into the nearby agricultural soil.
High levels of heavy metals in soils do not necessarily reflect anthropogenic influence but instead may be of a diagenetic origin or grain size effects [11]. Since metals from both natural and anthropogenic sources accumulate in soils, it is often difficult to determine what fraction of soil's metal load comes from which source. A crucial step for pollution assessment of soil is to establish the expected natural background concentrations [12], from which various approaches can be used to quantify anthropogenic inputs. Thus, this study is aimed at quantifying the level of selected metals in agricultural soils along major roads of Nasarawa Eggon and Doma and evaluating their origins (natural and anthropogenic), contamination level A c c e p t e d M a n u s c r i p t and toxicity.

Description of Study Areas. Nasarawa Eggon (NE) town is in the Local Government
Area (LGA) of Nasarawa State, Nigeria and lies between latitudes 8°33" and 8°52" N and between longitudes 8°14" and 8°39" E. The climate of NE falls within the tropical savannah climate with two clearly marked seasons; wet and dry. It has a mean temperature of 15.6 °C and 26.7 °C with annual rainfall between 1317 mm and 1450 mm [13].
Doma town is in Doma LGA of Nasarawa State, Nigeria and located at latitude 8 0 24'N and 8 0 5'N and longitude 6 0 E and 6 0 30'E of the Greenwich meridian. The entire LGA is generally characterized by a low land area about 100-200 m above sea level although there is a kind of spatial variation in the surface area [14].
The ground positions of the sampling points in both Nasarawa Eggon and Doma study areas were referenced with a handheld global positioning system (GPS) unit (Garmin GPSMAP 76) and are shown in Figure 1.

Soil Pollution Assessment.
The metal contamination, as well as ecological risk of the soil samples in the study areas, were assessed using enrichment factor (Ef), geo-accumulation index (Igeo), contamination factor (Cf), degree of contamination (Cd), pollution load index (PLI), ecological risk (Er) and potential ecological risk index (RI) models.

Enrichment factor.
The enrichment factor (Ef) was evaluated by normalizing the metal concentration in the sample with respect to a reference metal [15]. The most common reference elements are Sc, Mn, Ti, Al and Fe [16]. The Ef of the metals/metalloids in the soil samples at both studied areas were calculated using Eq.1.
Where Cn(sample) is the concentration of the investigated metal "n" in the study site, metal (usually the average shale value of the reference metal). In this study, Fe is the most naturally abundant element in all the studied soils and thus, was used as the reference metal while the average shale value described by Turekian and Wedepohl [17] was used as the reference value.

Geo-accumulation index.
The geo-accumulation index (Igeo) values for the determined heavy metals were evaluated using the formula proposed by Muller [18] as shown in Eq. 2.
Where Cn is the concentration of the heavy metal "n" in the soil sample; Bn is the geochemical background value for heavy metal "n" which is either directly measured in fossil argillaceous sediments of the area or adopted from literature (world average shale value); 1.5 is the correction factor for the variations in background value which could be due to lithogenic effects. The world average shale values of the studied heavy metals as described by Turekian and Wedepohl [17] and shown in Table 2; were used in this study. Geo-accumulation classes proposed by Muller [18] as reported in Table 1, was used for the interpretations of geoaccumulation index.

Contamination factor (Cf), Degree of contamination (Cd) and Pollution load index
(PLI). The extent of soil contamination by a certain metal was determined using the contamination factor (Cf). Cf is the ratio of the concentration of metal to the background value of the metal, as given by Eq. 3.
Where Cn is the concentration of the metal "n" in the sample, Cb is the background concentration of the metal "n". Nigerian Directorate of Petroleum Resources target values for metals in soils (Table 2) were adopted as the background values "Cb" [19]. The sum of all contaminant factors of the various metals is referred to as the degree of contamination (Cd) [20]. It was calculated using Eq.4.
Where n is the number of metals studied and Cfi is the contamination factor for metal "i" in the soil sample. The standard employed in the interpretation of contamination factor and degree of contamination values was adopted from the different classes proposed by Hakanson [21] as reported in Table 1.
Pollution Load Index (PLI) was calculated so as to obtain a proper assessment of the degree of contamination. The procedure (Eq. 5) by Tomlinson et al. [22] was used to calculate PLI in order to measure the overall level of metal toxicity at a particular sampling site.
Where n is the number of metals and Cf is the contamination factor. A PLI value under one indicates unpolluted soils or sediments; zero indicates perfection; a value of one indicates the presence of only baseline levels of pollutants and values above one would indicate progressive deterioration of the soil quality (i.e., polluted) [22]. The pollution load index was interpreted as reported in Table 1.

Ecological risk (Er) and Potential ecological risk index (RI).
Hakanson [21] proposed ecological risk factor (Er) as a quantitative expression of the potential ecological risk of a given contaminant (metal). It was calculated using Eq. 6.
Er is the single index of ecological risk factor and Tr is the toxic response factor suggested by Hakanson [21] which is presented in Table 2. The potential ecological risk index (RI) index reflects the general situation of pollution caused by the simultaneous presence of the twelve (12) metals and was calculated using the expression in Eq. 7 n is the number of the metals considered; Er and RI are the potential ecological risk factors of individual and multiple metals, respectively. The ranges of values used for the interpretation of the potential ecological risk factor [21] are reported in Table 1.    Arsenic (As) is naturally found in water, soil, and sediments [23]. The nature of the soil parent material appears to be the main factor that determines the As concentration in soils, although due to its low supergene mobility, the soils are slightly enriched in As compared with their soil parent rocks [24]. As is not only toxic but rated to be carcinogenic which causes skin lesions such as hyperpigmentation (especially on the trunk), keratosis on the palms and soles, chronic cough, crepitation in the lungs, diabetes mellitus, hypertension, and weakness [23]. In this study, As levels recorded in the sampled sites in Nasarawa Eggon (except N2 and N4) and
However, in Nasarawa Eggon, the level of As in sampled site N2 is above DPR [19] and average shale [17] reference values while in sampled site N4 (Nasarawa Eggon) as well as sampled sites D2, D4 and D5 (3.55-6.60 mg/kg) of Doma study area, As levels are above the DPR [19] value but below average shale [17] value. This indicates possible anthropogenic contribution to As level in sampled site N2 while lithogenic origin with little or no anthropogenic contribution could be the source of As in other sampled sites including N4, D2, D4 and D5. The range of As in this study is higher than those recorded by Edori and Kpee [9] and Fosu-Mensah et al. [25] for soils within selected abattoirs in Portharcourt, Nigeria and soils at Korle Lagoon area in Accra, Ghana; respectively. However, Li et al. [26] and Santos-Francés et al. [27] reported As concentrations higher than the findings of this study, for surface soils in electronic waste dismantling area and soils of the Andes mountain range, respectively. On the other hand, Cd is a toxic metal that occurs naturally at a low level in the environment [30] and can disrupt biological systems more than most toxic metals, at acute and chronic exposures through inhalation and ingestion [31]. Cr in the soils could be due to waste consisting of lead-chromium batteries, coloured polythene bags, discarded plastic materials and empty paint containers [45]. Cr (III) is not toxic but Cr (VI) is a carcinogen [46] whose acute toxicity is due to oxidation properties, hemolysis and A c c e p t e d M a n u s c r i p t AUTHOR ACCEPTED MANUSCRIPT -JOURNAL OF MULTIDISCIPLINARY APPLIED NATURAL SCIENCE organ failures [47]. Occupational exposure to Cr is mostly by inhalation, but gastrointestinal tract and skin can occur [48]. Cr (VI) corrodes skin and causes denaturation and precipitation of tissue proteins [49].  [57]. Mn is an essential microelement for both plants and animals. However, exposure to a high dose of Mn results in kidney failure, liver and pancreas malfunctioning low fetal birth weight and increased infant mortality [58].
Nickel (Ni) is absorbed through the lungs [59], gastrointestinal tract [60] and skin [61], but excreted in the urine [62]. Human exposure to nickel causes a variety of pathologic effects but its adverse health effects are dependent on exposure route (inhalation, oral or dermal) and classification based on systemic, immunologic, neurologic, reproductive, distorted developmental or carcinogenic effects, following acute (1 day), sub-chronic (10-100 days) and chronic (100 days or more) exposure periods [63]. In this study, Ni concentrations in the sampled sites of Nasarawa Eggon (except N7) and Doma study areas are below the instrumentation detectable limit and the reference values. Ni was significantly found only in site N7 (64.59 mg/kg) of the Nasarawa Eggon area which is above the reference value by DPR [19] but below that of the average shale [17] value. This implies that the presence of Ni in sampled site N7 is possibly due to anthropogenic origin such as leaching from metal scraps, runoff of paints/pigment waste, discharge from sewage and indiscriminate waste dumps etc.
Lead (Pb) is the most immobile element present in the soil [64]. A literature survey showed that Pb has several negative health implications on various organs and systems in all living species under experimental conditions, including the blood, reproductive, immune system and kidneys [65]. Pb is a highly neurotoxic agent that particularly affects the development of the  Brazil mine [69] and sediments of watersheds in Abiete-Toko gold district, Southern Cameroon [15].

Zinc (Zn) is a micronutrient essential for normal plant growth which is involved in various
metabolic activities of many organisms [70]. Human exposure to a high dose of Zn can cause many health disorders such as ataxia, depression, gastrointestinal irritation, haematuria, icterus, impotence, kidney and liver failure, lethargy, macular degeneration, metal fume fever, prostate cancer, seizures, and vomiting for Zn [71]. Result (Table 2) shows that Zn concentrations in the sampled sites in Nasarawa Eggon (except N4 and N10) and Doma are lower than the reference values of average shale [17] and DPR [19]. The level of Zn in N10 soil of Nasarawa Eggon is above the reference values while in sample N4, the Zn level was above the average shale [17] value but below the DPR [19] value. Similar to the trend of Pb concentration, higher Zn concentrations were also observed in Nasarawa Eggon sampled sites than in Doma. This supports the earlier suggestion of higher automobile/vehicular traffic as well as generator usage in the studied Nasarawa Eggon area than in the Doma area. Zn and Pb are products of gasoline combustion from automobiles and generators exhaust [72].

Soil Pollution Indices or Assessment.
In order to understand the quality of agricultural soils along major roads with high vehicular and human traffic, levels of metal pollution were evaluated using different techniques for environmental assessment.

Enrichment factor.
The enrichment factor (Ef) is generally used as an appropriate method to discriminate between natural and anthropogenic sources of metals [74] and to reflect the status of environmental contamination, based on the use of a normalization element in order to improve the variations produced by heterogeneous sediments [75]. The Ef values close to A c c e p t e d M a n u s c r i p t AUTHOR ACCEPTED MANUSCRIPT -JOURNAL OF MULTIDISCIPLINARY APPLIED NATURAL SCIENCE unity indicate crusted origin, those less than 1.0 suggest a possible mobilization or depletion of metals, whereas Ef > 1.0 indicates that the element is of anthropogenic origin [76].
The Ef values obtained in this study for the soil samples from the study areas are presented in Table 3 and Birth [77] categorization of Ef values (Table 1)   It is obvious from the results ( Table 3) that the investigated soils of Nasarawa Eggon and Doma are highly enriched with Hg and Mn, as well as Pb and Zn in Nasarawa Eggon only and Co in Doma only. This is an indication that in addition to natural sources of these heavy metals, there is a significant contribution from anthropogenic sources to the presence of these heavy metals in the soils [78]. Studies by Gibson and Farmer [79] and Sezgin et al. [80] established that vehicular emission, industrial production and weathered materials are the three main sources of heavy metals in urban areas. Consequently, the high Ef of Pb, Mn and Zn in the soils could be attributed to automobile/vehicular traffic because Sutherland [16] had reported that automobile/vehicular exhaust emissions significantly accentuate Pb accumulations in the air and soils. Automobile emission accounts for about 80 % of heavy metal pollution in Nigeria [81] and has been shown to contain Pb and Mn from gasoline as well as Zn from tyres [72].
Weathering of rocks, burning of fossil fuels (petroleum and coals) and municipal waste (containing e-wastes, automobile and automobile workshop wastes, and medical wastes) [39] in these study areas are the probable sources (lithogenic and anthropogenic) of Hg, Pb, Mn, and Zn, in the investigated soils. The literature revealed that anthropogenic sources contribute more to heavy metal pollution than natural sources [9][25]. The Ef values of As, Cd, Co, Cr, Cu, Ni and U, as well as Pb and Zn in Doma; suggest they were majorly derived from crustal material, or natural weathering process [82].

Contamination Factor (Cf), Degree of Contamination (Cd) and Pollution Load
Index (PLI). The values of Cf, Cd and PLI at the sampled sites of both study areas are summarized in Table 5. Applying Hakanson [21] classification (Table 1), the sampled sites in the Nasarawa Eggon area showed low contamination by Cd, Co, Cr, Cu, Fe, Pb, U and Zn (

CONCLUSIONS
This study successfully quantified selected metals in sampled soils of study areas