Heavy metal pollution of soils along North Shuna-Aqaba Highway, Jordan

Int. J. Environment and Pollution, Vol. 22, No. 5, 2004 597

Copyright © 2004 Inderscience Enterprises Ltd.

Heavy metal pollution of soils along North Shuna-–Aqaba Highway, Jordan

F.M. Howari∗Faculty of Science, Geology Department, UAE University; PO Box17551; Al-Ain, UAE

Y. Abu-RukahDepartment of Earth and Environmental Sciences, Yarmouk University,Irbid, Jordan

P.C. GoodellGeological Science Department, University of Texas at El Paso, ElPaso, Texas 79968, USA

Abstract: Attention to heavy metal contamination associated with highways ormotorways has risen in the last decades because of the associated healthhazards and risks. The present study analysed the metal content in soil samplesof one of the main highways along the western part of the Jordanian border, theNorth Shuna–Dead Sea–Aqaba Highway. The metals analysed were Pb, Zn,Cd, Co and Ni. In the samples collected, the recorded average concentrationswere as follows: 40 ppm for Ni, 5 ppm for Cd, 79 ppm for Zn, 79 ppm for Pb,and 25 ppm for Co. The average concentrations of Cd, Pb, and Co are higherthan the average natural background values of heavy metals. The geo-accumulation index of these metals in the soils under study indicated that theyare uncontaminated with Ni, Zn, and Co and moderately contaminated with Cdand Pb. In all of the investigated locations, the study found that concentrationsdecreased with depth. The cluster statistical analyses and pollution load indexwere used to relate pollution to land use or highway conditions. Two maintrends were identified: (i) higher concentrations were located near intersectionsclose to the urban areas in the Jordan Valley, in association with junctionscontrolled by traffic lights and check points, and (ii) lower concentrations werefound to the southwest in areas of mainly barren landscape close to the DeadSea and Aqaba.

Keywords: border, heavy metals, Jordan Valley, motorway, soils.

Reference to this paper should be made as follows: Howari, F.M., Abu-Rukah,Y. and Goodell, P.C. (2004) ‘Heavy metal pollution of soils along NorthShuna-–Aqaba Highway, Jordan’, Int. J. Environment and Pollution, Vol. 22,No. 5, pp. 597–607.

∗ Author for correspondence.

598 F.M. Howari, Y. Abu-Rukah and P.C. Goodell

598

1 Introduction

Amongst the range of contaminants that may be found in soils, potentially toxic elementsor heavy metals are of particular interest for a number of reasons. First, they show atendency, under normal circumstances, to accumulate in soils and have a long persistencetime because of the interactions with particular soil components (Charlesworth and Lees,1999a and b; Chon et al., 1995; Ellis and Revitt, 1982). However, it is possible thatenvironmental change may result in conditions that could result in dramatic releases ofstored metals, the so-called ‘chemical time-bomb’ effect. Secondly, they are ubiquitous insoils and arise from both natural and anthropogenic sources, with pathways includinginheritance from the parent rocks, application of wastes, and local and long-rangeatmospheric and fluvial deposition of emissions from industry and mining (Forstner andWittman, 1983; Harrison et al., 1981; Howari and Banat, 2001; Abu-Rukah, 2001).

In particular, interest in the levels of contaminants associated with highways ormotorways has risen in recent decades, in the light of the impact of high levels of lead inthe blood of children living in urban areas (Day et al., 1975; Harrison et al., 1981;Hamilton, 1984). There is a likelihood of this being caused by inadvertent hand–mouthcontamination caused while the child plays in a city street or in a neighbourhood adjacentto motorways (e.g. Charlesworth et al., 2003). Heavy metals may come from manydifferent sources in urbanized areas, one of the most important being vehicle emissions(Harrison et al., 1981; Kelly et al., 1996; Li and Liu, 2001).

Akhter and Madany (1993), Fergusson et al. (1980) and Ellis and Revitt (1982) noteda trend for higher concentrations of metals to be found on streets where traffic was morelikely to undergo stop–start manoeuvres, such as at traffic lights. Soils borderinghighways often have high contents of heavy metals, and many studies on street dust havefocused on elemental concentrations and source identification (e.g. Hamelton et al., 1984;Wong and Mak, 1997). Almost all of those studies reveal that deposition of metals ishighest close to the roadside, and the contamination was found to diminish rapidly withdistance from the road.

The present study investigated the metal content of roadside soils along important andmain national highways in the Jordan Valley (JV), the North Shuna–Dead Sea–AqabaHighway (Figure 1). The study of heavy metals along this highway is important becausethe surrounding areas in the valley have been used since ancient times as an importantagricultural area. The valley is divided into three parts: north, middle, and south. Thenorthern part extends from Tebris Lake to Deir Alla, passing through North Shuna (thispart borders the highway investigated), the middle JV starts from Deir Alla to the DeadSea, and the southern JV extends from the Dead Sea to the Gulf of Aqaba. The climate,soils, and water resources vary widely among the three parts. The average annual rainfallis about 280 mm at Deir Alla, and 35 mm at Aqaba. Irrigation in the JV consumes about340 million cubic metres per year.

The elements Cd, Pb, Ni, Zn, and Co could become toxic when present in anomalousconcentrations. Thus those can be used to calculate the pollution load index (PLI)suggested by Salmons and Forstner (1984) and used in this study. The objective of thepresent study was to quantify the metal pollution in the southern JV along the NorthShuna–Dead Sea–Aqaba Highway using a statistical method, the index of geo-accumulation (I-geo) and the PLI. In addition, cluster analyses were applied to the data inorder to establish similarities between sampling locations along the highway, withreference to the metal content.

Heavy metal pollution of soils 599

599

L1

L2

L3

L4

L5

L6

L7

L8L9

L10

39O 55038O 45037O 35036O 25035O 150

300

39O

300

39O

200

32O

100

31O

00

30O

S audiaA r abia

SyriaIraq

Figure 1 Location map of the investigated area.

2 Materials and method

2.1 Samples collection

A total of 30 soil samples, taken from depths of 0–20 cm, were collected from tenlocations. Soil samples were collected from ten transects perpendicular to the traffic flowon the North Shuna–Dead Sea–Aqaba Highway. This highway has a length ofapproximately 400 km from North Shuna to Aqaba, with an estimated average traffic flowof 400 vehicles per day, mostly trucks and microbuses. For each transect, samples weretaken from three depths, 0–5 cm, 5–10 cm, 10–20 cm. The sampling sites were located atdistances of 0 to 15 m from the roadside.


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2.2 Geochemical analyses

All samples were oven-dried at 70–100 °C, and gently crushed with a rolling pin todisaggregate the samples but not to break down the grains themselves. The concentrationsof metal ions were measured with a PX Unicam Sp Atomic Absorption (AA)spectrophotometer. Soil samples (0.2 g) were digested with 25% HCl, and 2 ml of thesolution were further digested with 25% HNO3. From this solution, 2 ml were taken anddigested with 40% HF. A 5 ml aliquot was placed in a water-bath for 2 h, and 50 ml ofH3BO3 was added. This solution was diluted with 40 ml of distilled water, and theconcentrations of Pb, Zn, Ni, Co and Cd were determined using the atomic absorptionspectrophotometer.

In order to evaluate the data in more detail the pollution load index (PLI) were used,where the PLI of elements was calculated according to the formula (Salmons andForstner, 1984):

5 ZnNiCoPbCd CFCFCFCFCFPLI ××××=

where PLI is the pollution load index and CFCd, CFPb, CFCo, CFNi and CFZn are thecontamination factors for cadmium, lead, cobalt, nickel and zinc. The contaminationfactor is the concentration of a metal divided by the background value (CF = X/BG,where X is the metal concentration and BG is the metal background value). Backgroundvalues used here are the standards reported by Turekian and Wedephol (1961) andAlloway (1989).

To be able to define the extent of pollution, it is necessary to establish the naturalbackground value for the soils, and enrichment may then be defined as the differencebetween the measured value and the background. A quantitative measure of the extent ofmetal pollution in the investigated area was calculated using the geo-accumulation indexproposed by Müller (1979). It is expressed as:

geo-accumulation index = log2Cn/1.5Bn

where Cn = measured concentration, Bn = background value (average shale) of element nand 1.5 = background matrix correction factor.

The geo-accumulation index consists of seven grades (0 to 6), indicating variousdegrees of enrichment above the background values and ranging from unpolluted to veryhighly polluted sediment quality.

3 Results and discussion

3.1 General trend

The average concentrations of the present metals of concern in sampling sites in relationto the natural background values are presented in Table 1, which shows the wide range ofvalues found for each metal (Figures 2 and 3). Comparison of data shows that the averageconcentrations of the investigated heavy metals were higher than their correspondingvalues in the Earth’s crust as well as higher than those of average world soils. Changes inconcentration of heavy metals with depth can vary significantly within soil profilesthrough a number of biological and chemical processes. The concentrations of these


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metals tend to decrease with increasing soil depth (Figure 3). The calculated I-geo (geo-accumulation index) values for the collected samples are given in Table 2 and remain ineither class 0 or class 1, which indicates that the investigated samples are uncontaminatedto moderately contaminated; they are uncontaminated with Ni, Zn, and Co and moderatelycontaminated with Cd and Pb.

Table 1 Mean values of the concentrations (ppm) of metals in the investigated roadside soils incomparison with an average crustal abundance and average world soils.

Soils of the study areaAveragecrustal

abundance(Wedephol

, 1978)

Averageworldsoil

(Alloway1989)

L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 Av.

Pb 12.5 25 122 74 67 71 56 46 84 68 67 45 70

Cd 0.2 0.35 18 6 10 5 5 30 6 5 4 5 9

Zn 70 90 100 150 80 210 160 30 50 105 105 140 113

Co 25 15 39 35 38 31 33 39 22 28 30 30 32

Ni 75 40 33 50 49 50 91 29 14 80 63 96 56

0

50

100

150

200

250

L1 L2 L3 L4 L5 L6 L7 L8 L9 L10

Locations

Con

cent

ratio

ns (p

pm)

Ni Cd Zn Pb Co

Figure 2 Variations of heavy metal concentrations along the investigated highway.

Table 2 Measure of metal pollution in the investigated soils.

Metal Soil Average shale I-geo class Designation of soil quality

Ni 40 68 0 Uncontaminated

Cd 5 0.3 1 Moderately contaminatedPb 79 20 1 Moderately contaminatedZn 79 95 0 UncontaminatedCo 25 20 0 Uncontaminated


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Figure 3 Vertical variations of heavy metal concentrations along the investigated highway.

3.2 Lead

Lead is a naturally occurring bluish-gray metal found in small amounts in the Earth’scrust. In the environment, much of it comes from human activities including burning fossilfuels, mining, and manufacturing. Because of health concerns, lead from gasoline, paintsand ceramic products, and pipe solder has been dramatically reduced in recent years.When lead is released to the air, it may travel long distances before settling to the ground.Once lead falls onto soil, it usually sticks to soil particles.


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Lead critical concentrations in soils are between 100 and 400 ppm, and the globalmeasured lead in surface soils is estimated as 25 ppm (Table 1). In the study area, theaverage concentration of lead is about five times greater than the corresponding values inthe world soils. The first location has the highest concentration of lead, which is 122 ppm.There are several major sources of lead at this location, but the most important and majorsource is the combustion of lead-containing gasoline additives. In addition, this location isan outlet for the catchment areas of north Jordan, and thus receives potential lead load viarunoff. As much as 24% of petrol lead is retained in the car, but the remaining 76% isemitted to the atmosphere. About 50% is deposited while 26% remains in the ambient air(Habous, 2002). The actual concentration of lead along the highway sides is variable anddepends on various factors, such as site, traffic factors, prevailing wind and humidity(Audat, 2000; Jiries et al., 2001).

3.3 Zinc

Zinc is one of the heavy metals essential for life. It acts biologically as a catalytic orstructural component of numerous enzymes involved in energy metabolism and intranscription and translation (Alloway, 1990; Forstner and Wittman, 1983). Zinc entersthe air, water, and soil as a result of natural processes but mostly as a result of humanactivities, such as mining, purifying of zinc, lead, and cadmium ores, steel production,coal burning, and burning of wastes. In air, zinc is present mostly as fine dust particles.Inhaling large amounts of zinc (as zinc dust or fumes from smelting or welding) can causea specific short-term disease called metal fume fever.

The concentration of zinc in the investigated soils ranged from 30 to 210 ppm (Table1). The concentrations of zinc in some locations were high, for example, location 2 hadconcentration of 150 ppm, and location 4 had 210 ppm. In these locations oil spills wereevident. According to Jiries et al. (2001) and Ellis and Revitt (1982), zinc may be derivedfrom the mechanical abrasion and oil spills of vehicles, and the high concentrations inthose locations may therefore be related, in part, to traffic movements.

3.4 Cadmium

Cadmium has no biological functions and is characterized by its high toxicity.Environmental pollution by cadmium has been rapidly increasing in recent decades as aresult of the rising consumption of cadmium by industry (Alloway, 1990). Itsconcentration in soil is derived from both natural and anthropogenic sources. Naturalsources include underlying bedrock or transported parent material. The major factorsgoverning cadmium speciation, adsorption and distribution in soils are pH, solubleorganic matter content, hydrous metal oxide content, clay content and type, presence oforganic and inorganic ligands, and competition from other metal ions (Howari and Banat,2001).

The average natural abundance of cadmium in the Earth’s crust has most often beenreported from 0.1 to 0.5 ppm, but much higher and much lower values have also beencited depending on a large number of factors. Igneous and metamorphic rocks tend toshow lower values, from 0.02 to 0.2 ppm, whereas sedimentary rocks have much highervalues, from 0.1 to 25 ppm. Fossil fuels contain 0.5 to 1.5 ppm cadmium, but phosphatefertilizers contain 10 to 200 ppm cadmium (Adriano, 1986). However, the normal soil


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content of cadmium ranges between 0.01 and 2 ppm, and the average concentration wasreported to be 0.35 ppm. The critical concentration of cadmium lies between 3 and 8 ppm(Alloway, 1990; Adriano, 1986). The concentration in the investigated samples rangedfrom 4 to 30 ppm, with an average of 9.2 ppm, which is nearly equal to the criticalconcentration in soils. The presence of cadmium is believed to be associated with tyrewear, as indicated by Ellis and Revitt (1982).

3.5 Cobalt and nickel

Cobalt is a natural earth element present in trace amounts in soil and plants, and hence indiets. It usually occurs in association with other metals, such as copper, nickel, manganeseand arsenic. Natural sources of cobalt in the environment are soil, dust, seawater, volcaniceruptions and forest fires. It is also released to the environment from burning coal and oil,from car, truck and airplane exhausts, and from industrial processes that use the metal orits compounds. The average concentration of cobalt in soils throughout the world is8 ppm. Toxic effects on plants are unlikely to occur below soil cobalt concentrations of 40ppm. However, the concentration in the soil is not the only factor determining toxicity.

Nickel is also a naturally occurring element that exists in nature mainly in the form ofsulfide, oxide, and silicate minerals. Because it is ubiquitous, humans are constantlyexposed to nickel in various amounts. It has been shown to be an essential element incertain microorganisms, animals, and plants. The generally held view is that nickel isprobably an essential element for humans as well.

Geochemically, cobalt and nickel are similar; they exhibit oxidation states of 2+ and3+. The normal cobalt concentrations in soil range from 31 to 38 ppm, and theconcentration in the studied roadside soil ranges from 22 to 39 ppm (Table 1). The criticalnickel concentration in soil is around 100 ppm, and its concentration in the studied soilsranged from 14 to 96 ppm. Locations 1–6 showed relatively high concentration of cobalt,and Locations 1–5 and 8–10 showed relatively high concentrations of nickel. In additionto the aforementioned anthropogenic sources of metals in the study area, the content ofnickel and cobalt in those locations may be partly related to the local geology, whichshows the presence of some cobalt- and nickel-bearing parent materials, including basalts,phosphate, and bituminous limestone.

3.6 Pollution load index and cluster analyses

Figure 4 shows the values of PLI for the different locations sampled. There is a slightdecrease in the values of PLI toward Aqaba. North Shuna (e.g. location 1) exhibited thehighest PLI values, and in the vicinity of the Dead Sea (e.g. location 6) and Aqaba (e.g.location 10) the values were lower.

To construct similarities between the sampling stations along the highway, single-linkage cluster analyses were performed. The resulting dendogram (Figure 5) reveals thatthe highway can be divided into two distinct regions. The first group of clusteredlocations are in the vicinity of urban areas, such as North Shuna, that are characterized bythe presence of checkpoints, traffic lights, road junctions or intersections (e.g. locations 1and 5). The second group can be discriminated into two subgroups. The first groupcontains locations that border slightly urban or slightly agricultural areas, in the vicinity ofthe Dead Sea or Deir Alla. The second subgroup includes locations that border barrenlandscape, after the Dead Sea and close to Aqaba. This discrimination is mainly based on


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field observation, highway conditions, associated sources of pollution, PLI values andmetal content, which coincide with the aforementioned discrimination. The presentedresults and method enable the classification of the sampling locations along North Shuna–Dead Sea–Aqaba Highway in terms of pollution load, as related to associated land andhighway conditions.

0

100

200

300

400

500

600

700

800

L1 L2 L3 L4 L5 L6 L7 L8 L9 L10

Location

Pol

lutio

n Lo

ad In

dex

PLI

Figure 4 Pollution load index calculated for different locations along the investigated highway.

6 7 8 9 10 11 12 13 14 15 16 17 18 19

Linkage Distance

L1

L2

L3

L4

L5

L6

L7

L8

L9

L10

Location

Figure 5 Single-linkage cluster analyses of the investigated sites.


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4 Conclusion

The average total metal concentrations were determined in the roadside soils to be79 ppm for lead, 79 ppm for zinc, 5 ppm for cadmium, 25 ppm for cobalt and 40 ppm fornickel. Those concentrations were higher than the maximum concentrations of thecorresponding elements in the average world soil. The pollution lead index and clusteranalyses indicate that concentrations in roadside soils are highest in North Shuna, whereroad junctions are controlled by traffic lights and checkpoints. Further to the southwest, inareas of mainly barren landscape close to the Dead Sea and Aqaba, the PLI values werelower. Cluster analyses showed that the degree of pollution varied according to thedifferent metal contents in soils as well as different conditions of the highwayinvestigated.

5 Acknowledgement

We gratefully acknowledge the Department of Earth and Environmental Sciences,Yarmouk University, for technical assistance in analysing the collected samples.

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Heavy metal pollution of soils along North Shuna-Aqaba Highway, Jordan

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