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

The concentration of five soil heavy metals ions (Cr⁶⁺, Cu²⁺, Cd²⁺, Pb²⁺, and Ni²⁺) was measured in eleven sampling sites along Old Panteka market Kaduna from two different depths. These chemical elements in the samples were determined using atomic absorption spectrometer. The assessment of heavy metal pollution was derived using the Enrichment Factors (EF) and geoaccumulation index (Igeo). This study revealed that the soil is predominantly polluted by Pb²⁺Cu²⁺Cd²⁺>Ni²⁺>Cr⁶⁺ and Cu²⁺>Pb²⁺>Cd²⁺>Ni²⁺>Cr⁶⁺ metal ions at 0-5 cm and 5-10 cm depths respectively. As recorded the highest EF value of 29.63 and 20.54 for Pb²⁺ and Cu²⁺ at 0-5 cm and 5-10 cm depths respectively followed by Cu²⁺ (17.13), Cd²⁺ (10.07), Ni²⁺ (0.99) and Cr⁶⁺ (0.53) at 0-5 cm and Pb²⁺ (19.68), Cd²⁺ (12.47), Ni²⁺ (1.19) and Cr⁶⁺ (0.55) at 5-10 cm depths respectively, and the mean Igeo provided the same trend of pollution levels as in the case of the EF, which indicates that the highest level goes to Pb²⁺ (1.61) and Cu²⁺ (1.58) at 0-5 cm; Cu²⁺ (1.71) and Pb²⁺ (1.50) at 5-10 cm depths respectively, which exhibits unpolluted to moderately polluted. Meanwhile, Ni²⁺ recorded (0.15) and (0.22) at 0-5 cm and 5-10 cm depths respectively, while Cr²⁺ recorded (-0.07) and (0.08) also at 0-5 cm and 5-10 cm depths respectively, which illustrates that both of these metals vary from unpolluted to moderately polluted. The concentrations of Cr⁶⁺, Cu²⁺, and Ni²⁺ levels are below 0 at the control sites, which demonstrates background concentrations. Risk assessment results show high health risk to human adults and children due to metals’ exposure through contaminated soil ingestion.

Abbreviations

AAS: Atomic Absorption Spectrometer; LADD: Lifetime Average Daily Dose; HQ: Hazard Quotient; HI: Hazard Index

Introduction

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 flora of his environment. The pollution of the environment has been found to result from man’s determination to match desire with 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 filings, and other materials that contain heavy metals unto bare soil. Lead (Pb2+), for example, is known to come from the use of leaded gasoline whereas Cu²⁺ and Cd²⁺ from tyre abrasion, lubricants, industrial and incinerator emissions [3,4]. The source of Ni²⁺ and Cr⁶⁺ 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 significantly 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.

Materials and methods

Study area

The study area, situated in the northern part of Kaduna state between 10⁰23’ -10⁰43’N and i7⁰17’-7⁰37’E (Figure 1). 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 taken over the original vegetation of Kaduna. The soil is mainly sandy clay, which reduces infiltration and accelerates overland flow and erosion, particularly where the soil surface has little or no vegetation cover.

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 cm³ of concentrated Nitric acid (HNO₃), and 1 cm³ Perchloric acid (HClO₄) 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 HNO₃ and filtered into a 100 cm³ flask using Whatman No.1 filter paper [9]. The blank determination was also carried out. Metals in the final 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 % HNO₃ solution for 24 hours [11]. Thereafter, they were washed with distilled water and dried in Memmert drying oven at 80 0C 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 (Cr⁶⁺, Cu²⁺, Cd²⁺, Pb2+ and Ni²⁺) 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]:

lgeo = log2 (Cn/1.5Bn) 1.0

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

1

2

3

4< Igeo<5= strongly to extremely polluted and

Igeo >5 = extremely polluted.

Contamination Factor (CF): CF is a quantification 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:

CF = Cm/Bm 2.0

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:

$PLI={\left[{\text{π}}^{\text{n}}\text{i}\left({\text{C}}_{\text{f}}\right)\right]}^{1/\text{n}}\text{ 3}\text{.0}$

Where; Cf 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:

$EF=\raisebox{1ex}{$(\text{Cx}/\text{Cref})\text{sample}$}\!\left/ \!\raisebox{-1ex}{$(\text{Bx}/\text{Bref})\text{background}$}\right.\text{ 4}\text{.0}$

Where;

Cx = Content of the examined element in the examined environment

Cref = Content of the examined element in the reference environment

Bx = Content of the reference element in the examined environment

Bref = Content of the reference element in the reference environment

An element is regarded as a reference element if it is of low occurrence variability and is present in the element in trace amounts. It is also possible to apply an element of geochemical nature whose substantial amounts occur in the environment but has no characteristic effects i.e. synergism or antagonism towards an examined element. The contamination categories are recognized on the basis of the enrichment factor:

EF<2 states deficiency to minimal enrichment,

EF = 2-5 moderate enrichment,

EF = 5-20 severe enrichment,

EF = 20-40 very high enrichment and

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 identification 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 quantifiable 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). Recommended equations used for estimating ADD, HQ, and HI were from EPA [20]:

ADD (mg kg^-1 day^-1) = (Cs ×IR×F×EF×ED)/(BW×AT) 5.0

Hazard Quotient (HQ) = LADD/RfD 6.0

Hazard Index (HI) = HQ_cd+ HQ_cri + iHQ_cu i+ HQ_Ni i+ HQ_pb 7.0

where Cs is the metal’s concentration in soil (mg kg^-1), IR is the soil ingestion rate (adult, 100 mg day^-1; children, 200 mg day^-1), F is the unit conversion factor, EF is exposure frequency (365 days/year), ED is the lifetime exposure duration (children, 12 years; adults, 70 years), BW is the bodyweight (children, 27 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].

Results and Discussion

Heavy metal concentrations

Table 5 summarizes the minimum, maximum, mean and standard deviation of a number of metals ions (Cr⁶⁺, Cu²⁺, Cd²⁺, Pb2+, and Ni²⁺) in twenty-two soil samples collected at old Panteka market welding workshops soil of Kaduna State. A close look at Table 5 shows that the variability in the range of all the metal distributions as compared with their means respectively is an indication of pollution of the sample with those metals ions. The decreasing trend of averages of metal levels was as follows: Cu²⁺>Pb2+>Ni²⁺>Cr⁶⁺>Cd²⁺ mg/kg concentrations at both depths respectively.

Distribution and enrichment of metals

The Enrichment Factor (EF) of Cu²⁺, Cd²⁺, Pb2+ and Ni²⁺ concentration in the soil as shown in Table 4 are 17.13, 10.07, 29.63, 0.99 and 20.54, 12.47, 19.68, 1.19 at 0 – 5 cm and 5 – 10 cm depths respectively. Meanwhile, enrichment factor (EF) of Cr⁶⁺ at both depths recorded 0.53 and 0.55 at 0 – 5 cm and 5 – 10 cm depths respectively, which indicates that the soil is uncontaminated (deficiency to minimal enrichment) by Cr⁶⁺ metal. Since the soil samples have been taken from welding workshops with considerable high volume of metal works, heavy traffic rates, and motor vehicles burning leaded gasoline and spent engine oil from welding generator sets could be considered as the main sources of the lead in the soils of the study area. The behaviour of Cu²⁺ shows that the enrichment factors (EF) are 17.13 and 20.54 at both depths, whereas, the values of the geochemical index range from -0.63 to 1.58 at 0 – 5 cm and -0.63 to 1.71 at 5 – 10 cm respectively, indicating uncontaminated to slightly contaminated soil. Relatively higher values of Cu²⁺ concentrations in the analyzed soil samples reflect anthropogenic effects which might be as airesultiofiburningifossilifuel,iweariand teariofityresiand other metal filing emanating from welding activities taken place in the workshops [22]. The Enrichment Factor (EF) values between 0.5 and 1.5 indicates that the metals are entirely from the coastal materials whereas EF values greater than 1.5 indicates that the sources are most likely to be anthropogenic activities [23].

In order to have an idea about the levels of contamination of the soils of the various welding workshops clusters, data obtained were compared with the background values. The background value of an element is the maximum level of the element in an environment beyond which the environment is said to be polluted with the element [24].

The highest CF was observed in Pb and the least in Cr⁶⁺ at 0 – 5 cm depth (Table 2). A similar trend was observed in Cu²⁺ and the least also in Cr⁶⁺ at 5 - 10 cm depth. In general, the decreasing order of CFs of heavy metals was Pb2+ > Cu²⁺ > Cd²⁺ > Ni²⁺ > Cr⁶⁺ at 0 – 5 cm depth and at Cu²⁺> Pb2+ > Cd²⁺ > Ni²⁺ > Cr⁶⁺ 5 - 10 cm depths respectively.

On variation with depths, the CFs values generally increased down the soil profile. The very high values of Cu²⁺ and Pb2+ at both depths could be due to the influence of welding activities taking place at the workshops such as indiscriminate disposal of substances containing metals such as vehicles spare parts, smelting and so on. The PLI which represents the number of times by which the metal content in the soil exceeds the average natural background concentration, and 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 to metals through ingested soils all the days in a year during the life span. Risk was assessed by estimating the incremental lifetime average daily dose (LADD), hazard quotient (HQ), and hazard index (HI) for the selected metals.

LADD is the amount of pollutant intake per kg of bodyweight per day that is sufficient to cause adverse health effects when absorbed into the body over a long period of time. If the HQ for a chemical is equal to or less than 1, it is assumed that there is no appreciable risk that health effects will occur. A hazard index (HQs) <1 suggests that risks are not expected from any chemical, alone or in combination with others. The average daily dose (ADD) and hazard index (HI) for adults and children from selected exposure to metals through soil contact are presented in Tables 6,7.

The LADD of Cr⁶⁺, Cu²⁺, Cd²⁺, Pb2+, and Ni²⁺ through soil ingestion for human adults at 0 – 5 cm depth ranged between 1.43E-06 –i0.163 mg kg^-1 d^-1 (mean, 8.14E-02 mg kg^-1 d^-1), 0.0224 – 3.664 mg kg^-1 d^-1 (mean, 1.84E+00 mg kg^-1 d^-1), 0.0019 – 0.009 mg kg^-1 d^-1 (mean, 6.4E-03 mg kg^-1 d^-1), 0.166 – 1.7435 mg kg^-1 d^-1 (mean, 1.04E+00 mg kg^-1 d^-1), and 0.0134 – 0.2072 mg kg^-1 d^-1 (mean, 1.17E-01 mg kg^-1 d^-1) respectively. However, average LADD for children at this depth was 4.22E-01 mg kg^-1 d^-1 (range, 7.41E-06 – 0.844 mg kg^-1 d^-1), 9.563 mg kg^-1 d^-1 (range, 0.1161 – 18.893 mg kg^-1 d^-1), 0.033 mg kg^-1 d^-1 (range, 0.0098 – 0.04667 mg kg^-1 d^-1), 5.381 mg kg^-1 d^-1 (range, 0.8615 – 9.0403 mg kg^-1 d^-1), and 0.6065 mg kg^-1 d^-1 (range, 0.0693 – 1.0744 mg kg^-1 d^-1) respectively for Cr⁶⁺, Cu²⁺, Cd²⁺, Pb2+ and Ni²⁺ from soil ingestion. The LADDs of total metals for adults and children ranged from 2.04E-01 to 5.77E+00 mg kg^-1 d^-1 with mean value of 3.09E+00 mg kg^-1 d^-1, and from 3.09 to 2.99E+01 mg kg^-1 d^-1 withimean value of 1.60E+01 mg kg^-1 d^-1, respectively for children at this depth.

These average daily intakes (ADIs) were mostly above the recommended reference dose (RfD) values for Cr⁶⁺ (Cr⁶⁺ salt, 1.5 mg kg^-1 d^-1), Cu²⁺ (0.04 mg kg^-1 d^-1), Cd²⁺ (0.001mg kg^-1 d^-1), Pb2+ (0.00014 mg kg^-1 d^-1) and Ni²⁺ (0.02 mg kg^-1 d^-1) except for Cr⁶⁺ 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).

Similarly, the LADD of Cr⁶⁺, Cu²⁺, Cd²⁺, Pb2+ and Ni²⁺ through soil ingestion for human adults at this 5 – 10 cm ranged between 0.000143– 0.231043mg kg^-1 d^-1 (mean, 0.115664mg kg^-1 d^-1), 0.022857 – 4.923471 mg kg^-1 d^-1 (mean, 2.484593 mg kg^-1 d^-1), 0.001829 – 0.016043 mg kg^-1 d^-1 (mean, 0.009851 mg kg^-1 d^-1), 0.136929 – 1.369143 mg kg^-1 d^-1 (mean, 0.821501 mg kg^-1 d^-1), and 0.020257 – 0.243971 mg kg^-1 d^-1 (mean, 0.142243 mg kg^-1 d^-1) respectively. However, average LADD for children at this depth was 0.599741mg kg^-1 d^-1 (range, 0.000741 – 1.198 mg kg^-1 d^-1), 12.88307 mg kg^-1 d^-1 (range, 0.118519 – 25.52911 mg kg^-1 d^-1), 0.051074 mg kg^-1 d^-1 (range, 0.009481 – 0.083185 mg kg^-1 d^-1), 4.25963 mg kg^-1 d^-1 (range, 0.71– 7.099259 mg kg^-1 d^-1), and 0.737556 mg kg^-1 d^-1 (range, 0.105037 – 1.265037mg kg^-1 d^-1) respectively for Cr⁶⁺, Cu²⁺, Cd²⁺, Pb2+ and Ni²⁺ from soil ingestion. The LADDs of total metals for adults and children ranged from 0.182015 to 6.783671 mg kg^-1 d^-1 with mean value of 3.57385 mg kg^-1 d^-1, and from 0.943778 to 35.17459 mg kg^-1 d^-1 with mean value of 18.53107 mg kg^-1 d^-1, respectively for children at this depth.

These average daily intakes (ADIs) were mostly above the recommended reference dose (RfD) values for Cr⁶⁺ (Cr⁶⁺ salt, 1.5 mg kg^-1 d^-1), Cu²⁺ (0.04 mg kg^-1 d^-1), Cd²⁺ (0.001mg kg^-1 d^-1), Pb2+i(0.00014 mg kg^-1 d^-1) and Ni²⁺ (0.02 mg kg^-1 d^-1) [21].

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 confidence, there was significant difference in the concentration of metal ons in the soils of the study areas as compared to that of control site. This may reflect 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 Cr⁶⁺, Cu²⁺, Cd²⁺, Pb2+ and Ni²⁺. 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 significant degree of contamination which suggests anthropogenic origins and confirmed 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. The soil samples were high in Cr⁶⁺ (113.95 mg/kg;PUG & 161.73 mg/kg; PSR respectively at both depths) Cu²⁺(2550.53 mg/kg;GUJ & 3446.43 mg/kg; HMC respectively at both depths), Cd²⁺ (6.30 mg/kg;PUG & 11.23 mg/kg; PUG respectively at both depths), Pb2+ (1158.88 mg/kg;PSR & 853.68 mg/kg; HMC respectively at both depths), and Ni²⁺ (145.05 mg/kg;GUJ & 170.78 mg/kg; GUJ respectively at both depths) concentrations which were far above WHO maximum contaminant limit. The Enrichment and Contamination Factors, Pollution Load and the Geo-accumulation Indices values also showed that the soils in the study area were polluted with these heavy metals. A comprehensive study of the pollution/contamination indices of hazardous heavy metals ons shows that steps should be taken to minimize the impact of these elements on human health and the environment especially as.

To this end, intense torrential rainfall in this study areas get some of these ions leached into the soil and by extension into underground water; whereas some few impermeable once get wash-away into new by wells located within the vicinity of the workshops.

The authors wish to thank the Multi-User Science Research Laboratory (MUSRL) of the Department of Chemistry Ahmadu Bello University Zaria for providing technical assistance and AAS Analysis.

Authors’ Contributions

The design and experiments of this research from the measurement, sample collection, preservation, and analysis of heavy metals were performed by Jimoh Abdullateef; Edith.B. Agbaji, Victor.O. Ajibola and Mustapha.A. Funtua supervised and guided the work while MUSRL staff provided access to the laboratory where digestion and heavy metal analyses were carried out using AAS technique. All authors read and approved the final manuscript.

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

General data repository and workflow management/versioning connect to other services; free Open Science Framework.

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