Application of Pollution Load Indices, Enrichment Factors, Contamination Factor and Health Risk Assessment of Heavy Metals Pollution of Soils of Welding Workshops at Old Panteka Market, Kaduna-Nigeria

Pollution of the environment may occur through the industrial and commercial activities of man. This happens when substances resulting from human activities enter the environment. The environment is said to be polluted when the concentration of these substances attain levels that may cause discomfort and/or harm to man, fauna, and fl ora of his environment. The pollution of the environment has been found to result from man’s determination to match desire with Abstract

production through the establishment of various industries with the potentials to pollute the environment. Industry, big or small, is a source of pollution of water, soil, and air [1].
The sizes of workshops vary but the typical medium-sized workshop occupies about 5 ha of land area [2]. Activities conducted in these shops are typical of metal fabrication workshops and invariably involve working with solders, metal fi lings, and other materials that contain heavy metals unto bare soil. Lead (Pb 2+ ), for example, is known to come from the use of leaded gasoline whereas Cu 2+ and Cd 2+ from tyre abrasion, lubricants, industrial and incinerator emissions [3,4]. The source of Ni 2+ and Cr 6+ in welding workshop is believed to be due to corrosion of vehicular parts [5], Akhter & Madany [5] and Fergusson & Kim [6] and chrome plating of some motor vehicle parts [7]. The phenomenon contributes signifi cantly to the pollution of the urban environment. This makes the study of welding workshops soil important for determining the origin, distribution, and level of heavy metal in urban workshop surface environments. However, the quantitative data on heavy metal concentrations, their contamination levels, and their pollution sources have not been systematically gathered and intercompared. Therefore, this study focuses on heavy metal ions contamination in urban welding workshop soils. The sources, concentrations, pollution levels, sample collection and analytical tools of heavy metals are elucidated in this study; moreso, it is very mportant to assess and monitor the concentrations of potentially toxic heavy metals ions in different environmental soils as regards what constitute occupational hazard to man.
The climate of the study area; wet season is characterized by torrential rainfall from May to October, while the dry season is November to April [8]. The natural vegetation cover is tropical grassland of the Northern guinea savannah type with short scattered trees interspersed with tall grasses. Urbanization has

Soil sampling
Twenty-two soil samples were collected during May 2016 from different depths with an interval of 0-5 cm and 5-10 cm.
The 1 kg of each soil sample was collected using a stainless steel spade and a plastic scoop; all samples collected were stored in sealed polythene bags and transported to the laboratory for pre-treatment and analyses Figure 2.

Chemical analysis
The soil samples were dried, mechanically in the laboratory, the soil samples after air drying at room temperature, were sieved with nylon mesh (2 mm). The <2 mm fraction was ground in agate and pestle and passed through a 63-micron sieve.
Soil samples were analyzed for heavy metals. Furthermore, soil samples were digested by taken 2 g each, weighed into a beaker using an analytical balance (Mettler AE160), 50 cm 3 of concentrated Nitric acid (HNO 3 ), and 1 cm 3 Perchloric acid (HClO 4 ) were measured and added to the already weighed soil sample. The mixture was digested by boiling gently on a hot plate. After digestion, the sample was evaporated to dryness and the residue mixed with 0.1M HNO 3 and fi ltered into a 100 cm 3 fl ask using Whatman No.1 fi lter paper [9]. The blank determination was also carried out. Metals in the fi nal solutions were determined using (variant model AA650FS), Atomic Absorption Spectrometer (AAS). Standard stock solutions for all the elements were prepared in the laboratory following the procedures as described in Omoniyi, et al. [10]. The glassware used was made fromi borosilicate, which was washed several times with liquid soap, rinsed with distilled water and then soaked in 10 % HNO 3 solution for 24 hours [11]. Thereafter, they were washed with distilled water and dried in Memmert drying oven at 80 0 C for 5 hours [11].

Contamination assessment methods
The assessment of soil enrichment can be carried out in many ways. The most common ones are the index of geoaccumulation and enrichment factors [12]. In this work, the index of geoaccumulation (Igeo) and Enrichment Factor (EF) have been applied to assess heavy metals ions (Cr 6+ , Cu 2+ , Cd 2+ , Pb 2+ and Ni 2+ ) distribution and contamination in the welding workshop samples within Old Panteka market, Kaduna Figure   3.
A quantitative measure of the extent of metal pollution in the studied soil was calculated using the geoaccumulation index proposed by Muller [13], as shown in Table 1. This index (Igeo) of heavy metal concentration pollution is calculated by computing the base 2 logarithms of the measured total concentration of the metal over its background concentration using the following mathematical relation [14]: where Cn is the measured total concentration of the element n in the soil fraction, Bn is the average (crustal) concentration of element n in shale (background) and 1.5 is the factor compensating the background data (correction factor) due to lithogenic effects [15]. gave the following interpretation for the geoaccumulation index: Igeo<0 = practically unpolluted, 0<Igeo<1 = unpolluted to moderately polluted, 1<Igeo<2 = moderately 2<Igeo<3 = moderately to strongly polluted,  Contamination Factor (CF): CF is a quantifi cation of the degree of contamination relative to either average crustal composition of a respective metal or to the measured background values from geologically similar and uncontaminated area as shown in Table 2 [16]. It is expressed as: Where; Cm is the mean concentration, while Bm is the background concentration of metal either from literature (average crustal abundance) or directly determined from a geologically similar area. CF in this study was considered as: CF < 1 -Low contamination factor 1 < CF < 3 -Moderate contamination factor 3 < CF < 6 -Considerable contamination factor 6 > CF -Very high contamination factor [17].

Pollution load index:
This was determined using the equation below as described by Tomlinson, et al. [18], was evaluated with the expression: Where; C f is the contamination factor of each metal obtained by the ratio of the concentration of each metal in soil to that of the metal in background soil or groundwater; π is the geometrical mean operator; n is the number of metals investigated in each sample as shown in Table 3.
Enrichment Factor: Enrichment Factor (EF) has been employed for the assessment of contamination in various environmental media by several researchers as shown in Table   4. Its version adapted to assess the contamination of various environmental media is as follows:     EF>40 extremely high enrichment [19].
The enrichment factor, due to its universal formula is relatively simple and easy tool for assessing enrichment degree and comparing the contamination of the different environment.

Human health risk assessment
Health risk estimation includes the identifi cation of exposure pathways, which is the course a chemical takes from a source to an organism [20] and an exposure route, the way a chemical comes in contact with a receptor (i.e., by ingestion, inhalation, dermal contact, etc.). In this study, ingestion of soils contaminated with metals was considered as the main pathways for risk assessment. The health hazard to human adults and children from metals was derived after hazard quotient (HQ) estimation. HQ is the measure of the magnitude of exposure potential or a quantifi able potential for developing health effects after an averaged exposure period. The potential for non-cancer effects was evaluated by comparing the estimated average daily dose (mg kg -1 d -1 ) of the metal with the reference dose (RfD) (mg kg -1 d -1 ). The total health hazard was derived simply by summing the HQ values of all the metals. This total HQ is referred to as the Hazard Index (HI). kg; adults,70 kg), and AT is the averaging time (EF × ED days). RfD is the reference dose for individual metal (mg kg -1 day -1 ) [21].

Distribution and enrichment of metals
The Enrichment Factor (EF) of Cu 2+ , Cd 2+ , Pb 2+ and Ni 2+ concentration in the soil as shown in Table 4  The highest CF was observed in Pb and the least in Cr 6+ at 0 -5 cm depth ( gives a summative indication of the overall level of heavy metal toxicity in a particular sample was also presented in Table 3. The result showed that the highest PLI at both depths were recorded at Gulubi Junction (GUJ) study sites and the lowest PLI at the control site. All study sites had their PLI > 1 and the control site recorded PLI = 1. Based on the PLI grade standard by [25], results showed pollution for the study sites and no pollution for the control site as shown in Table 3.

Human health risk estimates
Health risk assessment was based on the assumption that humans exposed to metals through soils may suffer harmful effects. We assume that human adults and children are exposed     which was below the RfD [21].
The total health hazard index (HI) for adults and children ranged between 1.91E+03 to 1.26E+04 with mean value of 7.47E+03 and between 6.17E+03 to 6.51E+04 with mean value of 3.87E+04, respectively. These estimated higher values of HI were all above the acceptable safe risk level (HI ≥ 1), indicating high risk to human adults and children from the studied metals through soil ingestion (Table 6).
The total health hazard index (HI) for adults and children ranged between 9.81E+02 to 9.93E+03 with mean value of 5.95E+03, and between 5.09E+03 to 5.15E+04 with mean value of 3.08E+04, respectively. These estimated higher values of HI were mostly above the acceptable safe risk level (HI ≥ 1), indicating high risk to human adults and children from the studied metals through soil ingestion (Table 7).
Based on the analysis of variance (ANOVA) test at p<0.05 level of confi dence, there was signifi cant difference in the concentration of metal ons in the soils of the study areas as compared to that of control site. This may refl ect the level of pollution within the sampling locations (Table 8).

Conclusion
Overall, the results of the analyses revealed that soil samples within the vicinity of the welding workshops were heavily polluted by Cr 6+ , Cu 2+ , Cd 2+ , Pb 2+ and Ni 2+ . This was due to the activities within these areas that generated a lot of wastes, ranging from scrap metals to used solders and electrodes which contaminated the soils with heavy metals.
Similarly, the contamination indices indicated a signifi cant degree of contamination which suggests anthropogenic origins and confi rmed the effects of welding activities within these areas. These showed heavy metal ons concentrations in the soil samples from welding workshops as a source of pollution.
These results imply that pollution of the environment by welding workshops has human health and ecological risks.

Funding
The cost associated with the collection, analysis, and interpretation of data in this manuscript was the responsibility of the corresponding author.

Availability of data and materials
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