VOL 1 No 1 ART 9

A study of trace heavy metal levels in Warri soils and vegetables, Southern Nigeria.

A.E. Ihenyen

Department of Geology,

University of Benin

Benin City, Nigeria

A.E. Aghimien

Chemistry Division

Nigerian Institute for Oil Palm Research,

Benin City, Nigeria

Abstract

Concentration and distribution of 9 elements: Fe, Mn, Zn, Cu, Cr, Cd, Ni, V and Pb in 29 soil samples and in spinachs were, investigated in urban warri. Metal concentrations in the soil were generally (4-74ppm) except Fe, which had some high values between 108-231 ppm. Concentration of elements fell into the following ranges: Mn: 2.12-25.76 ppm; Zn:0.2-1.88 ppm; Cu: 10.53-15.10 ppm; Cr: 0.12-0.86 ppm; Cd: 1.20-3.51 ppm; Ni: 0.02-3.89 ppm; V: 0.2-1.25 ppm and Pb: 0.42-1.5ppm. Soils in the built up area contained the highest concentrations of heavy metals. Three correlation levels were observed:- In-group 1, Cr, Zn, Ni, and V strongly correlated with each other. In group 2 Fe, Zn, Mn, Cu, & Ni displayed positive correlations of different levels with each other while group 3, Pb and Cd did not correlate with any metal. Cluster analysis produced 4-distinct region-specific clusters corresponding to different levels of pollution. The result of assay for heavy metals on the home-grown edible vegetable Celosia argental L.,(Lagos spinach) indicated that heavy metals contained in them, did not show glaring differences from those obtained in the natural background level estimates, although, minor bioaccumulation was established.

Keywords: Heavy metals, soils, vegetables, Warri, Nigeria.

Introduction

Soil is one of the repositories for anthropogenic wastes. Biochemical processes can mobilize them to pollute water supplies and impact food chains. Trace heavy metal contamination in the soils is a major concern because of their toxicity and threat to human life and the environment (Purves 1995). Heavy metals such as Cu, Cr, Cd, Ni, and Pb are potential soil and water pollutants.

Heavy metals studies have been conducted in soils with differing levels of anthropogenic influences such as in urban cities (Ihenyen 1998a, 1998b), industrial areas (Parry et al. 1981, Culbard et al. 1983), automobile traffic (Lagerwerff & specht 1970, Garcia-miragaga 1984), mining activities (Culbard & Johnson 1984) and in sediments of inland waters (Ihenyen 1992).

Few data exist on heavy metal concentration in many highly populated and industrialized cities in Nigeria. Most Nigerian cities have inefficient and/or non-existent drainage system leading to contamination of immediate surroundings and the ground water with trace heavy metals. Heavy metal studies are necessary to evaluate both soil/sediment and groundwater contamination. This becomes necessary because most fruits and vegetable supplies come from gardens within these cities. Also most of the municipal water supplies come from boreholes, most of which are shallow wells.

Elemental background levels in soils from U.K. (Thornton 1982), in Holland (Edelman & Bruin 1986) and India (Kuhad at al 1989) have been investigated. These countries were concerned with long-term health effects arising from industrial activities.

The biogeochemistry of the edible vegetable: Celosia argenta L. (Lagos spinach) was used in estimating the level of bioaccumulation (if any) of the heavy metals studied in this work. It is a fast growing crop which is popular in south western Nigeria owing to the soft texture of its leaves. Its origin is in West Africa (Badra 1991). It is not cultivated as vegetable on a large scale outside Nigeria. However, it can be found in neighbouring Republic of Benin. Lagos spinach, popularly called “green vegetable” is eaten by yorubas more than any other ethnic group in Nigeria who call it sokoyokoto.

The objective of this study was to determine the levels of trace heavy metals in soil and home-grown vegetables against estimated background values.

The Study Area

The area studied is the metropolitan city of Warri in the Niger Delta area of Southern Nigeria, (Fig.1). It is a medium sized town characterized by extensive petroleum

exploration, production, and petrochemical activities. It is located on the top of the coastal plain sand (Benin Formation). The Benin Formation is the upper -most units in the Niger Delta Basin and is predominantly made up of over 90%sand with isolated clay/shale intercalations. The sands are coarse-grained, poorly sorted and sub angular-well round. They are white or yellowish-brown and contain thin lignite streaks and wood fragments. The sands are of continental to deltaic plain origin. The sands and stones may represent point-bar deposits, channel fill and natural levees whereas the shale may be interpreted as back swamp deposit or ox-bow fills.

In Warri the sands are white. The sediments are of continental to deltaic plain origin. The Benin Formation is about 2000m thick around Warri and thins towards the Delta margin. It is from Oligogene -Recent.

The Benin formation is the best water-bearing unit in southwestern Nigeria with enormous ground water storage and recharge of over 6.63 x 10⁸m³annually (Oteze 1981). Most of the municipal water-needs in Warri come from borehole tapping the aquifer system. The aquifer is therefore potentially vulnerable to infiltrations of toxic contaminant. In the study area, mean annual rainfall is about 422mm/year (Federal Meteorological Services, Oshodi, Lagos).

The city of Warri has a population of about 300,000 inhabitants. It is about the third largest city in the south-south geographical zone in Nigeria. Rainfall occurs from April to October while the dry season lasts between November and March.

Like many towns in Nigeria, Warri lacks drainage system. Nearly all the road networks (both major and minor) lack roadsides drains. As a consequence, surface run-offs contaminated with municipal wastes drain directly into the associated gardens and farmlands. Municipal wastes contain among others, spent motor oils from the numerous over-aged and over-used automobiles and from the scattered numerous roadside mechanic workshops and effluents from many small-scale industries. The result is that surface sediments in and around metropolitan Warri have a dirty organic-rich appearance with unbearable stench.

Data collection and analysis

Twenty nine soil samples and 6 samples of Celosia argentea L.(Lagos spinach) collected in April 1999 were used in this study (Fig.1). The samples were collected from various locations to cover industrial, commercial and residential areas. The background level estimation was determined with soils and vegetables collected from unimpacted area in Ugboroke and mechanic village (samples 24 and 25 (soils) and samples 2 and 4 (vegetables))

The collected soil samples were air-dried and sieved into coarse and fine fractions. From the fine fractions less than 63µ, 0.5g were digested with 5ml conc. HNO₃ (Analar) until the solution became slurry. It was then cooled to room temperature and dilute to 50ml with deionised water, filtered and the filtrate stored in precleaned polyethylene storage bottles. No attempt was made to distinguish between exchangeable, carbonate, Fe-Mn oxide, organic and residual fractions. The dried vegetable samples were similarly digested with conc. HNO₃ (Analar), after samples were separately pulverized to fine fraction.

Trace metal concentrations of each fraction was analysed by Atomic Absorption spectrophotometry using “Buck scientific 200A” by flame atomization. Quality assurance was guaranteed through double determinations and use of blanks for correction of background and other sources of error. The spatial distributions of the metals in Warri urban soils were evaluated using multivariate statistical techniques of Ludwig and Reynolds (1988).

Results

Table 1 presents the results of heavy metal analysis in Warri soils. The values of heavy metals in some home grown vegetable in the city are shown in Table 2.

Metal concentrations in soils were low. Low values may be explained by the soil texture which allows a high degree of flushing during heavy rains.

Correlation coefficient (Table 3) and multivariate Q-cluster analysis (Table 4) of the elements studied were calculated from data on Table 1. The cluster analysis produced four distinct groups. Cluster 1 is made up of six stations (23, 24,25,26,27,28); cluster II comprised 4 stations (4,5,6,7). Cluster III consisted of 8 stations (8,10,11,15,18,20,21,29) and eleven stations (1,2,3,9,13,14,16,17 & 22) made up cluster IV. These clusters are interpreted to represent different levels of pollution.

Discussion

Heavy metal concentrations were generally low except Fe which showed a range from 4-74ppm with a few high values between 108-231 ppm. The control stations contained low Fe levels (1.34 and 6.73ppm). The correlation coefficient in Table 3 showed that Fe was positively correlated with Cu. The topsoil in Warri is composed of “Coastal Plain Sand”, a member of the Benin Formation. The soil is devoid of tropical lateritic cover which seemed confirmed by low Fe content in the control stations. In this study, Fe contents in excess of 36 ppm was observed along heavy traffic routes. Wear and tear of motor steel components is inferred to account for high values.

The concentrations of Zn in soils ranged from 0.2 ppm to a high of 1.88ppm. Sediments in the southern part of the city contained the highest concentrations which ranged from 0.89 -1.88ppm. Sample stations included 1,2,6,7,8,9,11,14 and 17 obtained from the built-up section of the city in which both commercial and industrial activities are high. The northern sector of the city is less developed and sparsely populated; correspondingly, it contained less than 0.68ppm in soils. Lagerwerff and Specht, (1970) inferred Zn to come from additives, which form components of some Lubricating oils while Forstner and Muller (1970) ascribed it to household wastewaters. The low Zn concentrations in the northern section of the city seemed to reflect the low human activities. Comparison with levels obtained in the control stations (24,25), showed contamination of soils in the built-up sector in southern Warri.

The concentrations of Cu similarly showed highest values in samples in the built-up area south of Warri. They ranged from 10.53-15.10ppm in comparison with 1.20 ppm and 1.45ppm obtained in the control stations. Copper concentrations were much lower (2.15 –8.76ppm) in the less built-up northern sector. Results here indicated that Warri urban soils were contaminated with Cu in comparison with background level estimations (1.20.1.45ppm).

In Warri urban soils, no clear trend was observed in Cr distribution except in stations 2, 3, 12,14 and 17 located in the south where levels exceeded the concentration ranges 0.20-0.48ppm. Background level estimates were 0.21 & 0.19ppm respectively. The low concentrations of Cr in Warri soils cannot easily be explained. One reason may lie in the low utilization of Cr in industries on the one hand coupled with low levels of Cr in the control stations (samples 24 &25). Soils in Warri seemed less impacted with Cr.

Cadmium in Warri soils ranged between 1.2 and 2.59ppm. Higher values (3.24-3.51ppm) were observed along raods subjected to traffic holdups (stations 2,3,18,21 &22). The control stations contained 1.25 and 1.89ppm. Cadmium is contained in motor oils and car tyres (Lagerwerff, 1970). The rather high occurrence of Cd in Warri soils seemed attributable to exhausts, and wear and tear from the numerous over-aged motor vehicles on Warri urban roads, and from the natural high Cd contents in soils. Some level of imparment was noted in Warri soils.

High values of Ni (0.42-3.89ppm) were generally observed in the southern part of Warri compared to considerably lower (<0.30ppm) values in Warri north. In a previous study (Ihenyen 1998) established a relationship between Ni-enrichment and high traffic volume. Traffic density is high in the south. The high Ni levels observed in Warri south is attributed to traffic volume.

The distribution of V is similar to Ni. They have similar geochemical behaviour during soil formation (Kabata- Pendias et al 1992). Soils in the north and northwest with comparatively lower human activities, were observed to contain low V concentration (<0.88ppm). This contrasted with considerably higher levels in southern Warri (1.04-6.66ppm). Contamination of soils with V particularly in southern Warri urban was also observed.

Lead distribution pattern also followed V and Ni. Lead showed higher levels in soils in southern Warri (> 0.42ppm) than in the north and northeast. (<0.35ppm). Motor vehicles run solely on leaded gasoline. Consequently, the elevated Pb in soils in the south in excess of the estimated background levels (<0.17ppm) is suggested to higher traffic volume. Soils in Warri south showed impairment with Pb.

The result of study of heavy metal levels in Warri urban soils showed a decreasing trend in heavy metal concentrations from the south to the north. Antropogenic activities were highest in the south. Evaluation of the heavy metal pollutional status in soils of city of Warri showed a worrisome trend. The metal Fe, Zn, Cn, Cd, Ni, V, and Pb were observed to show some levels of impairment.

Correlation among Trace metals

The 9 metals were grouped according to correlation levels (Table 2). In group 1, the elements Cr, Zn, Ni and V strongly correlated with each other. Those in group 2 (Fe, Zn, Mn, Cu, Ni) displayed positive correlations of different levels with each other. Cadmium and Pb are in group 3. Cadmium did not correlate with any metal while Pb only correlated with Cu. Elements of group one have similar ionic radii and exhibit similar geochemical behaviour during soil formation (Kabata-Pendias et al 1992). Group 2 have larger ionic radii than those in group 1. They were probably associated with clay fractions. Concentrations of Pb showed the lowest among the 9 elements analysed. Lead concentrations appeared evenly distributed in Warri soil samples. It showed least correlation with other metals. Cadmium also appeared evenly distributed but showed no correlation with other metals. Cadmium and Pb have similar ionic radii and therefore exhibited similar geochemical behaviour too.

Cluster analysis

The cluster analysis produced is represented in Table 4 and the spread shown in figure 1. Cluster I consisting of 6 samples stations were obtained in the northern sector of the city. This group contained the lowest heavy metal concentrations. This sector of the city is least developed and also has a low population density and low traffic volume. Cluster II contained 4 sample points located in the south-south. Population density and anthropogenic activities are higher than in cluster I. There was a marked increase in concentrations of all metals studied, thus reflecting the higher commercial and industrial levels.

Cluster III made up of 8 stations overlapped the areas where most commercial activities take place. Vehicular activities were also highest here. With the exception of Fe, Zn, and Cu, metal concentrations were higher in soils than in cluster II. Cluster IV which consisted 11 sample locations contained the highest concentrations of all elements studied except Fe. These stations were located in the part of the city where population and industrial activities were highest coupled with a high traffic density.

Vegetables

Heavy metals levels in the vegetables did not show any significant difference with the natural background levels (Table 2). Minor bioaccumulation of elements were however noticeable in vegetables from southern Warri urban (no 1, 3, 6). The values in vegetables grown in the less stressed northern sector had values close to the natural background levels.

Results indicated minimal bioaccumulations based on the comparison of results obtained with the background level estimation. Consumption of home-grown vegetables in Warri urban is thus considered not to be hazardous to health.

The low concentration levels of heavy metals in Warri soils may in part be attributed to the porous sandy soil texture and heavy rainfalls which allow a high degree of surface flushing. Sands and loamy marine sediments are especially prone to leaching due to sandy textures (Myers et al 1990). Also the Benin formation has a high hydraulic conductivity (Oteze 1981). A comparison of the heavy metal concentrations in soils and vegetables considered in this study with the background level from the control stations (24 & 25, soil) and (no.2 & 4, vegetables) in areas remote from anthropogenic influences indicated low level impact in soils and minor bioaccumulations in vegetables particularly in the southern part of the city where both industrial and commercial activities are high.

There are no reported cases of anomalous concentrations of heavy metals in groundwater in urban Warri although impairment with some metals was noted by Ihenyen (2002). The situation however calls for concern because of the absence of efficient drainage system.

Conclusion

The result of the study of heavy metals in the soil and on the edible vegetable: Lagos spinach in metropolitan Warri showed some impairment and minor bioaccumulation respectively particularly in the southern sector. Anthropogenic activities are high in this section of the city. The results showed low level of heavy metal accumulation and bioaccumulations. A high degree of leaching and flushing of heavy metal during heavy rains may in part be responsible for the low levels.

The interpretation of cluster analysis of heavy metal concentrations in sediments categorised sediments pollution into four-region-specific cluster representing different levels of pollution.

Acknowledgement

This study was financed with a grant of the University of Benin Research and Publication Committee (URPC) 1/96. This is gratefully acknowledged. The anonymous reviewer is also acknowledged for his suggestions.

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