SEASONAL TREND OF POTENTIALLY TOXIC TRACE ELEMENTS … · toxic elements in the geologic environment (Fordyce et al., 2000; Siegel, 2002). Bedrock geochemistry and several anthropogenic

SEASONAL TREND OF POTENTIALLY TOXIC TRACE ELEMENTS INSOILS SUPPORTING MEDICINAL PLANTS IN THE EASTERN

REGION OF GHANA

S. B. DAMPARE1,3,∗, Y. AMEYAW2, D. K. ADOTEY1, S. OSAE1,Y. SERFOR-ARMAH1, B. J. B. NYARKO1 and D. ADOMAKO1

1National Nuclear Research Institute, Ghana Atomic Energy Commission, P.O. Box LG 80,Legon-Accra, Ghana; 2Center for Scientific Research into Plant Medicine, P.O. Box 73,

Mampong-Akwapim, Ghana; 3Present address: Department of Earth Sciences, Okayama,University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan

(∗author for correspondence, e-mail: dampee [email protected], Tel.: +81-86-251-7881,Fax: +81-86-251-7895)

(Received 15 December 2003; accepted 10 August 2005)

Abstract. Investigation was conducted into the occurrence and extent of potentially toxic heavymetals and/or trace elements in soils from three study areas in the Eastern Region of Ghana namelyAbonse, Pepease and Mamfe. A protocol for soil sampling, preparation and analysis was developed toyield sufficient information on elemental concentrations in the soils. The soil samples were analysedfor As, Co, Cr, Cu, Sb, Th, U, V and Zn, using Instrumental Neutron Activation Analysis (INAA)coupled with conventional counting system. Concentrations of Co, Cr, Th and Zn positively correlatewith As, and regressions performed on Co-Zn, Co-Cr, Co-Th, Cr-Th and Zn-Cr values also showstrong correlations. The regressions suggest that these elements are geochemically associated andmight be enriched simultaneously. Though there are relatively high concentrations of Cu (maximum;46.3 mg/kg), Co (maximum; 9.17 mg/kg), Th (maximum; 15.1 mg/kg) and U (maximum; 2.38 mg/kg)in some of the soils compared with their levels in world average soils, the average concentrations ofthe trace elements in general are below or within levels in world average soils/uncontaminated soils.Also, the enrichment index computed in the soils indicates that all the study areas are not enriched withtoxic elements, particularly As, Cu, Sb and Zn, and the enrichment index values decrease in the orderof Pepease > Mamfe > Abonse. When the average composition of the soils was normalized to theaverage composition of the underlying rocks, the soils were depleted in the trace elements determined,indicative of background concentration values. Lack of correlations between soil characteristics suchas texture, organic carbon, organic matter, total nitrogen and available phosphorous, and most of thetrace elements suggests that the influence of these parameters on the distribution of the trace elementsis not important in the surface soils.

Keywords: enrichment index, Ghana, heavy metals, INAA, medicinal plant, soil, toxic elements

1. Introduction

Heavy metal pollution studies worldwide have mostly concentrated on the vicinityof urban and industrial areas. This is particularly due to the fact that moderate toabnormally high concentrations of heavy metals have been reported in the surface

Water, Air, and Soil Pollution (2006) 169: 185–206 C© Springer 2006

186 S. B. DAMPARE ET AL.

environment of such areas. Heavy metal pollution in and around urban and industrialareas is a direct result of emissions from motor vehicles, smelters, metal-processingworks, power stations, municipal waste incinerators and petroleum refineries. Min-ing activities, through milling operations coupled with grinding, concentrating oresand disposal of tailings, along with mill wastewater, and mine also provide obvioussources of metal contamination (Adriano, 1986). The impact of mining on the envi-ronment as regards heavy metal pollution has also received attention worldwide. Itis, therefore, not surprising that the degree and extent of heavy-metal pollution as aresult of human activities has been one of the main topics studied in environmentalgeochemistry (Plant and Raiswell, 1983).

Besides anthropogenic metal inputs, high levels of potentially toxic elementsof concern to both man and animals such as Cd and Mo, have been reported innatural soils and rocks (Lakin, 1972; Lund et al., 1981; Thornton, 1983; Kimand Thornton, 1993; Chon et al., 1996; Lee et al., 1998). Researches carried outby various environmental scientists have revealed that the occurrence and geo-graphical distribution of certain diseases could be correlated with the presence oftoxic elements in the geologic environment (Fordyce et al., 2000; Siegel, 2002).Bedrock geochemistry and several anthropogenic inputs are, therefore, the maincontrollers of heavy metal distribution in the earth’s crust (Bowie and Thornton,1984).

In Ghana, little has been done to investigate the levels of potentially toxic ele-ments in natural geochemical materials or environment. Background levels of traceelements coupled with data on plant uptake, toxicity, and fate and transport of traceelements in soils are important in policy decisions regarding the application rates ofwaste materials, inorganic fertilizers and pesticides to agricultural and/or forestedlands. Most heavy metal and trace element pollution studies have focused on goldmining areas (e.g., Amasa, 1975; Serfor-Armah, 1994; Boadu et al., 2001; Nyarkoet al., 2003).

The Eastern Region of Ghana is one of the key forested areas in the country.Scientists and herbalists usually use plants from this area for preparation of herbalmedicines for the management and treatment of various diseases. In the Abonse,Pepease and Mamfe areas of this region, Creptolepis sanguinolenta (Lindl.) Schtr.,an anti-malaria plant, is harvested by researchers of Centre for Scientific Researchinto Medicine (CSRPM) for curing malaria and typhoid fever. Contaminated soilscan influence the metal uptake by plants grown on them, though metal concentra-tions in plants vary with plant species (Adriano, 1986; Alloway, 1995). Abundanceof toxic (trace) elements in natural soils is also dangerous to the plants grown onthem. However, knowledge of elemental concentrations of soils on which theseplants grow is fairly limited.

The objective of this study was to investigate the levels of concentration of po-tentially toxic elements in soils from areas where plants were obtained for medicalpurposes. It was aimed at providing information on the concentrations of poten-tially toxic elements in non-contaminated forested lands thereby contributing to

SEASONAL TREND OF POTENTIALLY TOXIC TRACE ELEMENTS 187

development of database of potentially toxic elements in the geologic environmentof Ghana.

2. Study Area

2.1. GEOLOGY

The sampling locations of the study area are shown in Figure 1. The geologyof the Eastern region of Ghana comprises the late Proterozoic-Paleozoic VoltaianGroup (which forms a thick sedimentary cover in the eastern part of the WestAfrican Craton), the Togo Formation (which is part of the Precambrian MobileBelt), the intrusive basin-type Eburnean granitoid (Cape Coast granite complex) andthe Proterozoic Birmian Supergroup belonging to the West African Craton. Abonseand Mamfe areas are mainly underlain by quartzites and phyllites, which belong tothe Togo Formation. The Togo Formation, which trends in the northeast direction,originally consisted of alternating arenaceous and argillaceous sediments whichhave now been converted to phyllites, schists and quartzites except in few places,where unaltered shales and sandstones are seen (Kesse, 1985). The lithology of thePepease area falls within the Voltain Group, which mainly consists of sandstonesfollowed by shales and siltstones (Ahmed et al., 1977). Thin irregular bands oflimestone, calcareous and siliceous cherts and sedimentary breccias (Ahmed et al.,1977), red,green, purple, mauve and chocolate arkoses, mudstones and pebbly grits(Junner and Hirst, 1946; Affaton et al., 1980; Anan-Yorke, 1980) also occur in theVoltaian.

2.2. CLIMATE

The study area lies within the wet semi-equitorial climatic zone with two mainrainfall seasons (Dickson and Benneh, 1988). The mean annual rainfall rangesbetween 1250 mm and 2000 mm. The first rainfall maximum occurs from May toJune with the heaviest rainfall in June. The second rainy season is from Septemberto October. Mean monthly maximum temperature for Abonse, Mamfe and Pepeaseduring the period of sampling is respectively 33.6 ◦C, 29.8 ◦C and 29.8 ◦C, and meanmonthly minimum temperatures are 22.0 ◦C, 21.7 ◦C and 20.9 ◦C respectively. Meanmonthly relative humidity at 0600Hrs for Abonse, Mamfe and Pepease is 83%, 96%and 90% respectively, while mean monthly relative humidity at 1500Hrs is 69%,69% and 68% respectively.

2.3. SOURCES OF CONTAMINANTS

As has been indicated above, potentially toxic elements in the environment could benatural or derived from anthropogenic activities. Naturally, these elements could


Figure 1. Map of Eastern Region of Ghana showing the study area.

be introduced into soil via the breakdown of metal-rich parent materials by pe-dogenic processes as well as the presence of mineralization or geothermal fluids.Anthropogenic potential sources of heavy metal inputs to soil/agricultural landare numerous, including atmospheric deposition from industrial activities, sewagesludge, industrial wastes, livestock manures, composts, fertilizers and lime andagrochemicals. The effects of heavy metals in soils have been documented in liter-ature (e.g. Chuan et al., 1996; Steinmann and Stille, 1997; Sheppard et al., 2000;Antoniadis and Alloway, 2002; Nicholson et al., 2003; Aubert et al., 2004).


The sites under investigation are not in the urban environment and are far fromthe influence of heavy vehicular traffic and industrial activities. No mining activityhas occurred in the areas in the past and presently there are no mines located in orclose to any of the three sampling sites. Agricultural activities are not uncommonin the area. Although contaminants such as Cd, Cu, Zn, As and Pb arise fromindustrial and agricultural activities (Naidu et al., 2003), usage of chemicals inthese areas is either nil or insignificant. Contributions of trace elements to the soilsfrom anthropogenic activities seem to play minimal role in trace elements inputand/or enrichment in the soils, and that the main source of potentially toxic elementcontamination in the soils of the three areas is natural, resulting from weatheringof underlying rocks/ bedrock.

3. Methodology

3.1. SAMPLING AND SAMPLE PREPARATION

Sampling of soils was carried out at three-month interval in 2002 at the same sites,beginning from March to December to observe seasonal trend of toxic elements innatural soils from the study area. Soils were sampled at depth 10–20 cm using handauger and plastic trowel into clean polyethylene bags. Each soil sample compriseda composite of 10 samples taken within a 5 × 5 m square.

After air-drying in a dry and dust-free place below 25◦C for 5 days, thesoil samples were disaggregated and passed through 180 µm sieve (for finesand fraction) and were homogenized. Four replicate samples, 100 mg eachof each soil, were weighed into clean polyethylene foils, wrapped and heat-sealed. The sub-samples were packed into a 7 ml polyethylene capsule andheat-sealed. The elemental comparator standards used in this work were madefrom the plasma emission spectroscopy standard solutions (SPEX) (SeignioryChemical Products (SCP) Canada, Ltd.). The standard had a certified purityof >99.999% and had a concentration of 1000 ppm. A working stock so-lution of 20 µg/g and 50 µg/g were prepared from the 1000 ppm standardsolution by dilution. Of the working standard solution, 0.5 ml was pipet-ted into clean, 1.2 ml polyethylene vials using micropipette, half filled withfinely grounded sucrose (obtained from Koch Light Laboratories, USA). Twelvereplicate standards were prepared. Few drops of deionized distilled water(DDW) were added to form a homogeneous mixture and then dried underan infrared lamp before heat-sealing the cap. Standard reference materials,International Atomic Energy Agency (IAEA)-Soil-7 and Estuarine SedimentIAEA-405 were used as quality control for the INAA analyses. Six replicatesamples were prepared for these reference materials in the same manner as thesamples.


3.2. SAMPLE IRRADIATION AND COUNTING

All prepared samples and standards were irradiated using the Ghana ResearchReactor-1 (GHARR-1) facility at the GHARR-1 Centre of National Nuclear Re-search Institute (NNRI) of the Ghana Atomic Energy Commission (GAEC). Thereactor was operated at a half-full power of 15 kW and at a thermal neutron fluxof 5 × 1011 nc m−2 s−1. The capsules were sent into the inner irradiation sites ofthe reactor by means of a pneumatic transfer system, operating at a pressure of0.25 atms. The irradiation time, decay time and counting time were chosen accord-ing to the half-lives of the elements of interest. Irradiation times were 1 min, 1 h and6 h with corresponding cooling times of 5 min, 72 h and 30 days respectively, priorto counting. Samples and standards were irradiated and counted under identicalconditions and geometries.

Counting of irradiated samples was done using a PC-based gamma-ray spec-trometry system. Details of spectrometry system and analytical measurements aredescribed elsewhere (Dampare et al., 2005). The analytical photopeaks used for thedetermination of the various elements have been summarized in Table I.

Accuracy and precision of the analytical technique were assessed by the si-multaneous activation of standard reference materials IAEA Soil-7 and EstuarineSediment IAEA-405. Our values compared favourably with the certified values forall elements with bias less than 6%. The precision was calculated as a percent-age relative standard deviation (%RSD) of six replicate samples of the preparedmaterials, and it was less than 7%.

4. Results and Discussions

4.1. PHYSICOCHEMICAL CHARACTERISTICS OF SOILS

Physicochemical characteristics such as soil texture, pH, organic carbon, total or-ganic matter, total nitrogen and available phosphorous were measured for the soils,following standard procedures, and are shown in Table II. The soils can texturallybe classified as sandy loam and sandy clay loam. Abonse and Mamfe soils aretypically sandy loam while soils of Pepease are sandy clay loam in nature. Soils ofAbonse and Mamfe are low in organic matter content, less than 3%. Organic carboncontent in these soils ranges from 1.0% to 1.7%, and carbon/nitrogen (C/N) ratioranging from 10.0 to 11.4, indicating that the organic matter is well decomposed.Organic matter content of Pepease soils is slightly higher than that of Abonse andMamfe, ranging from 1.9% to 4.0%. Average percentage organic content is 1.9,and the organic matter is also well decomposed with C/N ratio of 10.0.

The available phosphorous content in the soils is very low, ranging from 0.0001%to 0.001%. Phosphate-bearing rocks and phosphate fertilizers are among the majorsources of phosphate in soils. The study area is not underlain by such rocks so it


TAB

LE

I

Rad

ionu

clid

esan

dir

radi

atio

nsc

hem

eus

edfo

rth

ede

term

inat

ion

ofel

emen

talc

once

ntra

tions

inso

ilsa

mpl

es.

Ele

men

tR

eact

ion

Prod

uctr

adio

nucl

ide

Hal

f-lif

eE

nerg

y(k

eV)

Irra

diat

ion

time

(Ti)

Dec

aytim

e(T

d)C

ount

ing

time

(Tc)

Cr

50C

r(n,

γ)C

r51C

r5127

.72

days

320.

16

h30

–40

days

5400

0s

V51

V(n

,γ)V

52V

523.

75m

ins

1434

.11

min

5m

ins

600

sC

o59

Co(

n,γ

)Co60

Co60

5.27

yrs

1173

.2,1

332.

56

h30

–40

days

5400

0s

Zn

64Z

n(n,

γ)Z

n65Z

n6524

3.9

days

1115

.66

h30

–40

days

5400

0s

Cu

65C

u(n,

γ)C

u66C

u665.

10m

ins

1039

.21

min

5m

ins

600

sA

s75

Cu(

n,γ

)As76

As76

26.3

2h

559.

11

h2–

4da

ys60

0s

Sb12

1Sb

(n,γ

)Sb12

2Sb

122

2.70

days

564.

21

h2–

4da

ys60

0s

Th

232T

h(n,

γβ

)Pa23

3Pa

233

27.0

days

312

6h

30–4

0da

ys54

000

sU

238U

(n,γ

)U23

9U

239

23.4

7m

ins

74.7

1m

in5

min

600

s


TAB

LE

II

Phys

icoc

hem

ical

char

acte

rist

ics

ofso

ilsfr

omth

eE

aste

rnR

egio

nof

Gha

na.

Ava

ible

phos

phor

ous

Org

anic

Org

anic

Tota

lTe

xtua

lSi

teM

onth

pH(m

g/kg

)ca

rbon

(%)

mat

ter

(%)

nitr

ogen

(%)

Sand

(%)

Cla

y(%

)Si

lt(%

)C

lass

Abo

nse

Mar

ch4.

05±

0.00

92.

75±

0.00

91.

14±

0.00

21.

97±

0.00

70.

11±

0.00

368

.00

21.2

410

.76

Sand

ycl

aylo

amA

bons

eJu

ne4.

25±

0.00

56.

88±

0.01

31.

69±

0.00

42.

92±

0.05

90.

15±

0.00

362

.24

27.2

410

.36

Sand

ycl

aylo

amA

bons

eSe

ptem

ber

4.10

±0.

005

4.43

±0.

007

1.30

±0.

003

2.25

±0.

007

0.13

±0.

003

71.2

620

.84

7.90

Sand

ycl

aylo

amA

bons

eD

ecem

ber

4.34

±0.

009

7.47

±0.

005

1.23

±0.

002

2.12

±0.

035

0.11

±0.

003

71.7

416

.28

11.9

8Sa

ndy

clay

loam

Mam

feM

arch

3.70

±0.

005

2.87

±0.

012

1.48

±0.

004

2.54

±0.

023

0.13

±0.

003

71.6

18.7

89.

62Sa

ndy

loam

Mam

feJu

ne4.

47±

0.00

96.

16±

0.00

91.

11±

0.00

21.

93±

0.03

80.

11±

0.00

379

.00

14.0

07.

00Sa

ndy

loam

Mam

feSe

ptem

ber

4.30

±0.

005

2.34

±0.

009

1.00

±0.

001

1.73

±0.

031

0.09

±0.

002

78.4

813

.98

7.54

Sand

ylo

amM

amfe

Dec

embe

r4.

34±

0.00

75.

72±

0.01

21.

33±

0.00

32.

29±

0.03

30.

12±

0.00

375

.12

16.2

68.

62Sa

ndy

loam

Pepe

ase

Mar

ch4.

12±

0.00

82.

53±

0.01

41.

10±

0.00

11.

89±

0.02

80.

10±

0.00

357

.00

37.1

25.

88Sa

ndy

clay

loam

Pepe

ase

June

6.63

±0.

005

6.41

±0.

005

2.34

±0.

007

4.03

±0.

010

0.23

±0.

003

42.7

638

.08

19.1

6Sa

ndy

clay

loam

Pepe

ase

Sept

embe

r6.

87±

0.00

71.

44±

0.00

72.

26±

0.00

73.

88±

0.07

10.

22±

0.00

342

.14

38.8

18.0

6Sa

ndy

clay

loam

Pepe

ase

Dec

embe

r6.

37±

0.00

55.

54±

0.00

91.

83±

0.02

83.

15±

0.04

00.

20±

0.00

348

.38

37.7

13.9

2Sa

ndy

clay

loam


not unexpected to observe low phosphorous content in the soils. The trend furtherindicates that phosphate fertilizers have not been applied to the soils.

Soils of Abonse and Mamfe are acidic with pH values ranging from 3.70 to4.47 whereas Pepease soils are mostly near neutral (Table II). Generally, there isno significant change in pH values between rainy and dry seasons. This indicatesthat no liming or fertilizers had been applied to the soils to influence the pH.

4.2. TRACE ELEMENTS

Total concentrations of As, Cu, Sb, Zn, Co, Cr, Th, U and V in soils from Abonse,Mamfe and Pepease are shown in Tables III–V respectively.

In the Abonse area, the average concentrations of the trace elements were mostlywell below levels in world average soils/uncontaminated soils reported by Bowen(1979).

With the exception of Cu whose concentrations are mostly relatively higher thanthe world average soils, the soils from Mamfe area also contain heavy metals andtrace elements below those in world average soils. Also relatively higher concen-trations of Cu, Co, Th and U, compared with their levels in world average soils, arecontained in some soil samples from Pepease area. Generally, there are no signif-icant differences in trace element concentrations in soils from the three samplingsites. This is probably due to the fact that the areas are underlain by rocks of similarcompositions.

It is also observed that none of the soils is enriched in the potentially toxicelements considering permissible levels of metals suggested by Kloke (1979). Ac-cording to Kloke (1979) and Kabata-Pendias and Pendias (1984), the permissible ortolerable level of an element is the approximate concentrations (or the threshold ofthe element’s concentrations) in soils above which crops produced are consideredas unsafe for human or animal health. Nevertheless, it is not advisable to interpretdata sets that involve analyses of multiple elements on the merit of single elementcontent, as most toxic element contamination in the surface environment is asso-ciated with a cocktail rather than one element. Consequently, Nishida et al. (1982)proposed a pollution index for Japanese river sediments, which many workers haveadopted for pollution studies. In this study, we have adopted the revised pollutionindex (called enrichment index) of Lee et al. (1998) for soils underlain by blackshales and slates in Korea. The enrichment index was calculated by averaging theratios of element concentrations to the hazard criteria, the permissible level. Inthis study, four elements (As, Cu, Sb, Zn) were selected for the computation ofenrichment index in the soils using the equation below:

Enrichment Index =∑

(Metal concentration in soil/Permissible levels for metal)

Numbers of metal

Enrichment Index = 1/4 (As/20 + Cu/100 + Sb/5 + Zn/300)


TAB

LE

III

Tota

lcon

cent

ratio

nsof

toxi

ctr

ace

elem

ents

inso

ilsfr

omA

bons

e(m

g/kg

).

Mon

thA

sC

uSb

Zn

Co

Cr

Th

UV

Mar

ch1.

90±

0.09

20.9

±1.

150.

55±

0.02

25.6

±1.

132.

70±

0.2

34.9

±1.

56.

79±

0.34

2.01

±0.

0926

.3±

1.7

June

2.54

±0.

1025

.2±

1.44

0.57

±0.

0250

.5±

2.83

3.48

±0.

337

.3±

1.5

6.11

±0.

341.

65±

0.06

80.8

±3.

7Se

ptem

ber

2.15

±0.

1028

.1±

1.61

0.59

±0.

0337

.2±

1.79

2.98

±0.

330

.1±

1.3

5.38

±0.

261.

53±

0.06

48.1

±2.

7D

ecem

ber

1.62

±0.

1018

.3±

0.71

0.46

±0.

0123

.3±

1.19

2.40

±0.

119

.3±

0.8

4.30

±0.

211.

82±

0.04

49.3

±2.

5N

orm

also

ila6

301

908

709

290

Perm

issi

bleb

2010

05c

300

5010

0–

––

a Bow

en(1

979)

.bK

loke

(197

9).

c Kab

ata-

Pend

ais

and

Pend

ais

(198

4).


TAB

LE

IV

Tota

lcon

cent

ratio

nsof

toxi

ctr

ace

elem

ents

inso

ilsfr

omM

amfe

(mg/

kg).

Mon

thA

sC

uSb

Zn

Co

Cr

Th

UV

Mar

ch1.

36±

0.06

29.6

±1.

230.

28±

0.01

46.5

±2.

233.

34±

0.2

22.0

±0.

96.

12±

0.30

0.85

±0.

0340

.0±

2.5

June

0.85

±0.

0435

.4±

1.80

0.19

±0.

0135

.2±

1.80

2.58

±0.

120

.5±

1.0

4.30

±0.

241.

27±

0.07

46.8

±2.

6Se

ptem

ber

0.98

±0.

0237

.6±

1.99

0.22

±0.

0125

.4±

0.91

3.07

±0.

222

.1±

1.0

4.38

±0.

211.

24±

0.05

51.1

±2.

7D

ecem

ber

1.13

±0.

0546

.3±

2.73

0.15

±0.

0134

.8±

1.98

2.49

±0.

113

.4±

0.8

4.47

±0.

181.

10±

0.04

55.7

±2.

8N

orm

also

ila6

301

908

709

290

Perm

issi

bleb

2010

05c

300

5010

0–

––

a Bow

en(1

979)

.bK

loke

(197

9).

c Kab

ata-

Pend

ais

and

Pend

ais

(198

4).


TAB

LE

V

Tota

lcon

cent

ratio

nsof

toxi

ctr

ace

elem

ents

inso

ilsfr

omPe

peas

e(m

g/kg

).

Mon

thA

sC

uSb

Zn

Co

Cr

Th

UV

Mar

ch2.

13±

0.04

36.9

±2.

140.

23±

0.01

50.0

±1.

062.

34±

0.1

35.6

±0.

58.

13±

0.18

1.04

±0.

0353

.7±

3.1

June

4.52

±0.

2032

.1±

1.72

0.35

±0.

0294

.0±

9.12

8.81

±0.

364

.1±

2.8

10.6

±0.

424.

78±

0.21

54.6

±2.

5Se

ptem

ber

3.58

±0.

1039

.7±

1.76

0.46

±0.

0283

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Figure 2. Correlation plots for As with (a) Co, (b) Zn, (c) Cr and (d) Th in soils from Eastern Regionof Ghana. ∗ Significant at p < 0.05.

Enrichment index is very useful in evaluating the degree of multiple metal en-richment or contamination in soils. An enrichment index greater than 1 indicatesthat, on average, element concentrations are above the permissible level, and any en-richment may be from anthropogenic inputs or natural geological sources (Nimickand Moore, 1991; Chon et al., 1996). In all the study areas, the enrichment indexis less than 1. This suggests that the study areas are not enriched with toxic ele-ments As, Cu, Sb and Zn. The enrichment index values of the three areas occur inthe decreasing order of Pepease (range: 0.18–0.24; mean: 0.22) > Mamfe (range:0.14–0.17; mean: 0.15) > Abonse (range: 0.11–0.17; mean: 0.14).

Scatter diagrams for element concentrations in the soils from the study areasare shown in Figures 2 and 3. Arsenic correlates well with Zn (r2 = 0.80), Cr(r2 = 0.89), Co (r2 = 0.78) and Th (r2 = 0.76) at p < 0.05. Very strongcorrelations exist between regressions performed on Co-Zn values (r2 = 0.87),Co-Cr values (r2 = 0.84), Co-Th values (r2 = 0.83), Cr-Th values (r2 = 0.85)and Zn-Cr values (r2 = 0.83) at p < 0.05. The regressions suggest a singlesource responsible for As, Co, Cr, Th and Zn disposition in the soils from the studyarea.


Figure 3. Correlation plots for element concentrations in soils from Eastern Region of Ghana.∗Significant at p < 0.05.

The trace element concentrations in the soils were normalized to their respectiveconcentrations in the bedrock in order to assess the contribution of trace elementsfrom the underlying lithology to the soil. Since phyllite is a better sampler of ele-mental composition than sandstone and due to lack of chemical data on the VoltaianGroup, an average phyllite composition of the Togo Formation (unpublished data)was used to normalize the samples. With the exception of Cu, which was slightlyabove and comparable to that of phyllite, the soils were depleted in all other trace


Figure 4. Average potentially toxic elements pattern in topsoils from the Eastern Region of Ghana,normalized to average phyllite composition from the Togo Formation (Unpublished data).

elements compared to phyllite (Figure 4). This indicates that the source of traceelements were mainly from the bedrock through weathering processes.

4.3. PHYSICOCHEMICAL CHARACTERISTICS AND DISTRIBUTION OF TOXIC

ELEMENTS IN FORESTED SOILS

Figure 5 shows seasonal trends of the toxic elements in soils from Abonse, Mamfeand Pepease respectively during the period of study. No significant variation in traceelement contents was found in soils from all the three sampling sites. This indicatesthat there were probably no external metal inputs to the soil from other sources apartfrom the underlying rocks or bedrock lithology. Elements are released from rocksinto the soil profile through weathering. Since this is a long-term process, obviouslyelemental concentrations in the soils would hardly change within 1 year duringwhich the study was carried out, provided the concentrations were not influencedby anthropogenic activities. Although the sampling period covered both dry seasons(i.e., March and December) when soils could be under reducing conditions, andrainy seasons (i.e., June and September)–oxidizing conditions, rainfall seemed tohave hardly affected elemental concentrations during the period of study. And thatthere was probably no significant flushing away or accumulation of elements byrain from place to place.

Metal mobility in soils is influenced by many factors including soil pH, organicmatter, soil type, cation exchange capacity (CEC), soil binding capacity and climate.The individual roles of these factors and/or soil parameters have been well capturedin literature (e.g. Antoniadis and Alloway, 2002; Naidu et al., 2003; Aubert etal., 2004; Sterckeman et al., 2004; Udom et al., 2004). Most of these factors areinterrelated and may also vary seasonally and/or temporally. Metal immobilizationis generally high in clay soils due to their high binding capacity. Loamy soils alsoretain metals better than sandy soils (Rieuwerts et al., 1998). The soils studied


Figure 5. Seasonal trend of toxic elements in soils from (a) Abonse, (b) Mamfe and (c) Pepease inthe Eastern Region of Ghana.

are mostly sandy loam and sandy clay loam, with little amount of organic matter.Considering the fact that metal mobility is high in sandy soils due to their lowbinding capacity, metal mobility will not be high in these soils of inferred moderatelybinding capacity; and hence the trend of metal variability observed in this study.

In order to ascertain factors that affect trace element distributions in the soils,Pearson correlation coefficients between trace elements and soil parameters weredetermined (Table VI). Lack of correlation of most of the trace elements with clay,silt and sand contents suggests that the trace elements are not specifically associatedwith grain size, and that these parameters are insignificant in the distribution ofthe elements in the soils. Sb is, however, positively correlated with clay and silt,and negatively correlated with sand, whereas Cu is positively correlated with sandand negatively correlated with clay. This observation suggests that Sb and Cu are

SEASONAL TREND OF POTENTIALLY TOXIC TRACE ELEMENTS 201TA

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respectively controlled by the clay and sand contents of the soils. Sb releasedthrough weathering of soil minerals is more likely to be retained in the soils, as clayand organic carbon usually decrease the probability of elements being leached out(Tack et al., 2004).

Also, there is no correlation between soil pH and the trace element contents.This is unexpected though, as most trace element contents usually increase withincreasing pH due to the fact that their solubility decrease with increasing pH(Adriano, 1986). The pH is positively correlated with clay, silt, organic carbon,total organic matter and total nitrogen, and negatively with sand. Similarly, the traceelements appear not to be associated with the organic carbon, total organic matter,total nitrogen and available phosphorous contents in the soils. This suggests that themeasured physicochemical parameters of the soils do not control the distributionof the trace elements and that other variables seem to affect their distributions.

4.4. TRACE ELEMENT PHYTOAVAILABILITY IMPLICATIONS

It has generally been assumed that some trace elements (e.g., Cd, Cu, Zn and Pb) areimmobile in managed agricultural soils (McBride, 1995). As total concentrationsof the trace elements examined were low, it appears there will be no appreciablephytotoxicity risk to plants growing on the soils. However, the phytoavailabilityof metals in soils is dependent on a number of factors including soil type, planttype, soil properties and the metal bioavailable concentrations. The bioavailabiltyof metals in turn is largely dependent on metal distribution between the solid andsolution phases. Such a distribution is controlled by soil processes cation exchange,specific adsorption, precipitation and complexation (Rieuwerts et al., 1998).

Rainfall plays an important role in bioavailabilty of metals. For example, Dowdyand Volk (1984) observed that when heavy sewage sludge were disposed on sandy,acidic soils with low organic matter content, and receiving high rainfall or irrigationwater, movement of heavy metals was likely to take place in the soils. Soil pH isacknowledged to be the principal factor influencing concentrations of soluble andplant available metals (Brallier et al., 1996). Metal solubility and pH are inverselyrelated, with solubility increasing at lower pH values and vice versa (Tills andAlloway, 1983b; Ram and Verloo, 1985, Chuan et al., 1996). Of the three sitesinvestigated, soils of Abonse and Mamfe are acidic whereas Pepease soils aremostly near neutral. This acidic nature of the soils may increase the bioavailabilityof metals to crop plants (Adriano, 1986). However, lack of correlation between mostof the trace element and pH suggest that pH might not be important in influencingthe phytoavailability of the elements.

Although total metal or trace element concentrations are very useful in ele-mental bioavailability studies, they do not necessarily correspond with metal nor-mally available for plant uptake. Determination of fractions of trace elementsin solution (i.e. bioavailable metal concentrations) of total elemental concentra-tions is, therefore, crucial in assessing the potential risks associated with elevated


elemental concentrations in soils as a result of natural weathering of rocks or throughanthropogenic activities.

5. Conclusion

Potentially toxic elements in the natural soils of Abonse, Mamfe and Pepease havebeen investigated. Instrumental neutron activation analysis (INAA) has been veryuseful in determining these toxic elements in the natural soils. This study has alsoevaluated seasonal trends in toxic trace elements geochemistry, with emphasis onunderstanding the behaviour of As, Cu, Sb, Zn, Co, Cr, Th, U and V in soils. Ourdata show no significant and systematic seasonal variations in soil toxic elementconcentrations, and that the toxic element concentrations in the soil were probablynot influenced by precipitation.

The enrichment index computed for the soils suggest that none of the soils isenriched with toxic elements, nevertheless, the enrichment index of soils decreasesin the order of: Pepease > Mamfe > Abonse. The regression analysis performedbetween element pairs suggest that As, Co, Cr, Th and Zn are geochemically asso-ciated and might have been enriched simultaneously.

The mean concentrations of the toxic elements As, Cu, Sb, Zn, Co, Cr, Th, Uand V in the soils are mostly within their respective levels in world average soilssuggested by Bowen (1979). Also, the levels of these toxic elements are withinthe permissible levels proposed by Kloke (1979) and Kabata-Pendias and Pendias(1984). Furthermore, the concentrations of the elements were mostly below theirrespective concentrations in the underlying bedrock, and are reflective of theirderivation from the bedrock lithology. Soil physicochemical characteristics such astexture, organic carbon, organic matter, total nitrogen, available phosphorous andpH seem not to have had any important influence on the distribution of most thetrace elements in the soils, indicated by lack of correlation between the parametersand the trace elements. It appears from the low total concentrations of the traceelements that the risk associated with consumption of crops and medicinal plantsfrom these soils could be minimal. However, such conclusions could be misleadingas the ability of plants to bioaccumulate metals and possibly other contaminantsvaries with the nature of plant species, the nature of metal species as well as durationof contact with the surface binding the metals. It is suggested that further studiesbe carried out on speciation of these potentially toxic elements in relation to metaluptake by the plants. Such research should be extended to other areas where plantsare usually harvested for the preparation of herbal medicine.

Acknowledgments

Financial support was provided to the second author, Mr. Y. Ameyaw, bythe Centre for Scientific Research into Plant Medicine (CSRPM) and Ghana


Studies Council, U.S.A. We are also grateful to the Reactor Manager and his teamof operators, and the technicians of NAA Laboratory of GHARR-1 Centre. Manythanks to Prof. H.K. Akoteye of University of Cape Coast for his suggestions.

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Documents

SEASONAL TREND OF POTENTIALLY TOXIC TRACE ELEMENTS … · toxic elements in the geologic environment (Fordyce et al., 2000; Siegel, 2002). Bedrock geochemistry and several anthropogenic