Arsenic concentration in selected soils around Abeokuta, southwestern Nigeria

Arsenic in Soil and Groundwater Environment P. Bhattacharya, A. B. Mukherjee, J. Bundschuh, R. Zevenhoven, R. H. Loeppert (Editors) Trace Metals and other Contaminants in the Environment, Volume 9, 257–267  2007 Elsevier B.V. All rights reserved ISSN 0927-5215 / doi 10.1016/S0927-5215(06)09009-6

Chapter 9 Arsenic concentration in selected soils around Abeokuta, southwestern Nigeria Adewole M. Gbadebo Department of Environmental Management and Toxicology, College of Environmental Resources Management, University of Agriculture, P.M.B 2240, Abeokuta, Ogun State, Nigeria

Abstract The objective of this study was to investigate and compare the concentration level of As content in two distinct soil types: one of which was derived from limestone sedimentary rock environment and the other of which was derived from the basement complex rock environment. To achieve this goal, a total of 55 composite soil samples were collected at a maximum depth of 15 cm in each site. Arsenic contents of the soil samples were analyzed using atomic absorption spectrometer after acid extraction. The As contents of the analyzed soil samples were correlated to some soil parameters such as granulometric fractions of the soil (i.e., sand, silts and clays), organic matter and pH. These factors have been found to be responsible for the variation in the As contents of the two soil types. The soils of sedimentary origin have been found to be richer in As contents than the soils of basement complex origin.

9.1

Introduction

Arsenic is a priority pollutant as stated in Water Quality Standard Handbook (USEPA, 1994). It has also been declared by National Research Council (1999) as a human carcinogen. According to Centeno et al. (2002), epidemiological and clinical studies reported in the medical literature have confirmed the role of As in the introduction of cancers of the skin, which include keratosis, squamous cell carcinoma and basal cell carcinoma. Apart from these skin lesions, other internal lesions that have been epidemiologically linked with As exposure include angiosarcoma of the liver, lung cancer and bladder cancer (Tsai et al., 1999),

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gastrointestinal manifestations which include noncirrhotic portal hypertension (Nevens et al., 1990), liver angiosarcoma (Neshiwat et al., 1993) and hepatic or splenic enlargement and hepatocellular carcinoma (Centeno et al., 2002). Arsenic (As) is widely distributed in nature and is one of the several elements that are generally toxic to biological species (Levinson, 1974). Arsenic constitutes a serious environmental problem due to its complicated chemical stability (Vaughan et al., 1992). Natural sources of As release into the environment include magmatic and hydrothermal deposits, basement complex and volcanic deposit (Sax, 1974). Its average content in the Earth’s crust is 1.5 mg/g. Also, anthropogenic sources of As in the environments include burning of As-rich coal, mining and smelting industry (Adriano, 1986; Azcue and Nriagu, 1995; Manz and Castro, 1997; Loredo et al., 1999; Savage et al., 2000; Centeno et al., 2002). Others include the use of agrochemicals: pesticides such as lead arsenate (PbAsO4) and calcium arsenate (Ca3AsO4) (Sax, 1974); insecticides such as sodium arsenate (Na2AsO4) (Bishop, 1972) and herbicides such as thioarsenites (H3AsS3). The median value of As elemental composition in uncontaminated soil is 6.0 mg/g (Bowen, 1979). Contaminated soils from various sources, however, contain significant levels of As. Worldwide emission of As into soil in 1983 was estimated as 82  106 kg/year (Nriagu and Pacyna, 1988), but the new estimation by Pacyna and Pacyna (2001) indicates that the world As emission in the 1990s has decreased by 38.88%, e.g., 5011 t/year. Due to the potential health effects of As to biota and humans and the fact that soil is a major route of this metalloid to both plants and animals, its presence in soil environment is of major concern. For this reason, this study was aimed at providing a contribution to a database on the As level of the soil types derived from two distinct geological environments. This chapter also discusses the relationship of As with controlling factors like granulometric fractions, pH and organic matter.

9.2

The study area

This study was carried out on soils developed from sedimentary and basement complex parent materials near Abeokuta, southwestern Nigeria. The sedimentary parent material falls within longitudes 380 E and 3150 E and latitudes 6510 N and 7020 N, while the basement complex material is delineated by longitudes 3260 E and 3400 E and latitudes 790 N and 7140 N within Ogun State region of the southwestern part of Nigeria (Fig. 9-1). The areas generally fall within the tropical region, with a mean annual rainfall ranging from 1500 to 2000 mm and mean annual temperature ranging from 27C to 35C (Oguntoyinbo et al., 1983; Onakomaya et al., 1992).

259

Arsenic concentration in selected soils around Abeokuta, southwestern Nigeria

3°05′

3°20′

N

EXPLANATION State Capital Well sites Basement Complex Environments Sedimentory Environment

OYO STATE

REPUBLIC OF BENIN

7°15′

UNAAB Farms

Aiyetoro

ABEOKUTA Igbogila Lapeleke Itori Ilaro

Iyana Egbado Ewekoro Elebute Akinbo Papalanto

ONDO STATE

6°50′

LAGOS

0

20

40

STATE

50 km

Fig. 9-1: Map of Ogun State in southwestern Nigeria with the geological environments and the study area

Specifically, the locations for soil sampling in the sedimentary parent material area were Itori, Ewekoro, Elebute, Iyana Egbado and Papalanto. For the basement complex area, the locations were the University of Agriculture Abeokuta (UNAAB) farmlands. According to Jones and Hockey (1964), the sedimentary portion of the study area has been classified in descending order of age from bottom to top into Abeokuta Formation, Ilaro Formation, Coastal plain sand and recent sediments. The major lithologic units of these formations are limestone as basal units overlain by shale and clay members. Similarly, the basement complex portion of the study area comprises of crystalline rocks, such as gneisses, older granites and pegmatites. The soils in the region where the study areas are located have been observed by Adetunji (1991) and Akanni

260

A. M. Gbadebo

(1992) to be mostly influenced by parent materials with other factors like climate, vegetation and topography playing significant roles. The soils of the sedimentary portion of the study area according to Gbadegesin (1992) have been broadly grouped into ferruginous tropical soils, while that of the basement complex portion have been broadly classified as ferralitic soils.

9.3

Materials and methods

Surface soils (0–15 cm) were taken at both the sedimentary and basement complex portions of the study area. The samples were collected with 2.5 cm diameter hand auger (a stainless steel screw). Each sample comprised a composite of three subsamples taken across 1  1 m2. The samples were air-dried at 25C for 72 h and disaggregated, passed through <2 mm sieve and used for the determination of pH using de-ionized water in the ratio of 1:2.5 (Ball, 1964), soil texture analysis and organic matter content using Walkley–Black dichromate method (Hesse, 1971). The samples were further ground to a fine powder (<180 mm) using ceramic mortal and pestle. The finely milled soils were digested using aqua regia (3:1 HCl:HNO3) (Alloway, 1990). Arsenic concentration in the soil samples was determined in the soil extracts by atomic absorption spectrometer. The relationship between As and soil parameters in both sedimentary and basement complex environments was analyzed through an ANOVA test.

9.4

Results and discussion

The results of the soil properties and As concentration in the study area are shown in Tables 9-1 and 9-2. The particle size analysis indicated that the soils from the two geological environments are generally sandy, with the soils taken from the sedimentary environment being less drained than the sampled soils from different parts of UNAAB farms. The mean pH values of the soils were in the range of 7.2–7.3 for the soils of the sedimentary environment and 6.1– 6.6 for the soils of the basement complex environments. Thus, the soil reaction in the sedimentary portion of the study area is slightly alkaline and also very slightly acidic in the neighborhood basement complex environment (mean pH as low as 6.6). The slight alkalinity in the soils of the sedimentary environment was due to the presence of limestone in this region. Soils developed from limestone are expected to be alkaline in reaction as indicated in these results. The observed trend is that of decreasing pH or alkalinity with distance away from the limestone-rich sedimentary environment to the limestone-

Grain size analysis

Organic matter (%)

Arsenic concentration (mg/kg)

pH

Sample location

No. of samples % Sand % Silt % Clay Range

Mean

Range

Itori

3

90.1

7.2

2.2

3.9–4.8

4.4

7.14–7.30 7.24

35.0–50.0

39.5

Ewekoro

5

89.0

6.9

4.1

3.8–4.6

4.1

7.27–7.32 7.30

24.0–83.0

31.5

Elebute

4

89.2

6.8

4.0

3.9–4.5

4.2

7.14–7.38 7.20

15.1–60.5

37.0

Papalanto 3

87.9

7.2

4.9

4.3–4.7

4.3

7.23–7.29 7.20

5.0–21.0

26.5

Iyana Egbado

88.6

6.9

3.5

3.9–4.4

4.2

7.21–7.30 7.28

10.0–18.6

14.8

4

Mean Range

Mean

Arsenic concentration in selected soils around Abeokuta, southwestern Nigeria

Table 9-1: Range and mean values of As and other parameters in soils from sedimentary environment

261

262

Table 9-2: Range and mean values of As and other parameters in soils from basement complex environment Arsenic concentration (mg/kg)

Sample location

No. Grain size analysis Organic matter (%) of samples % Sand % Silt % Clay Range Mean

Range

Mean Range

Mean

COLERM Farm

25

pH

67.2

16.8

16.5

1.2–1.8

1.4

6.60–6.70

6.40

5.5–34.2

16.2

UNAAB Farm A 4

53.2

21.9

26.4

0.4–0.6

0.4

5.99–6.17

6.06

9.0–15.0

11.3

UNAAB Farm B

3

63.2

18.6

18.1

0.7–0.9

0.6

5.30–6.95

6.13

10.0–14.3 12.5

UNAAB Farm C

3

71.4

15.0

13.6

1.1–2.0

1.7

6.39–7.02

6.60

0.0–33.0

16.2

A. M. Gbadebo

Arsenic concentration in selected soils around Abeokuta, southwestern Nigeria

263

free basement complex environment in the study area. Similarly, the slightly acidic characteristic of the soils developed from the basement complex of the study area is considered normal for the soils of tropical regions. However, this relatively low pH (6.6) may be an onset for severe acidic conditions in this region where agricultural practices are supported with the use of fertilizers. The mean values of organic matter content in the sedimentary portion of the study area ranged from 4.1% to 4.4%, while the values ranged from 0.4% to 1.7% in the basement complex. These values which have been considered low for most soils in Korea (Kim, 1985) have been rated medium in the sedimentary portion of the study area and very low to low in the basement complex for the tropical soils (Landon, 1984). The very low to low organic matter contents of the UNAAB soil farmlands are possibly due to the paucity of vegetation on the soils (Myung et al., 2002), since the areas were more subjected to intensive farming annually by the University community. The ‘total’ As concentrations in the surface soil samples from the two distinct geological environments are shown in the tables above. Significant As concentrations were found in the various sites of the sedimentary environment, compared with the sites of the basement complex environment. The average values in the sedimentary environment range from 14.8 to 39.5 mg/g, while the average values for the basement complex environment range from 11.3 to 16.2 mg/g. These values have been found to be within the world-recommended range of 1.0–73.0 mg/g for soils (Frink, 1996) and far higher than the Korean guideline for As of 6.0 mg/g in agricultural land (DOE Korea, 1996). These values of As in this study are close to the spatial distribution of As in the soils of Aragon (Spain) where the calcisols (i.e., the soils developed on the sedimentary rocks) indicated median As content in the range of 6–7 mg/g (Navas and Machin, 2002). The relatively high concentration of As content in the soil of sedimentary environment may be as a result of limestone mining activities in this region, which probably have aided the atmospheric deposition of this metalloid on the surface soil. This supports the claim that soils around mining areas are likely to have elevated concentrations of As. Soils in the vicinity of gold mines have been reported of As enrichment as high as between 393 and 2500 mg/g (AmonooNtizer et al., 1996; Hyo-Talk et al., 1999), while the soil around the coal mining areas of Durham has been observed to have As concentration as high as 380 mg/g (Lord, 1999). These mining sites are not only repository of As, but also serve as point source of As pollution to the surrounding soils of the farmlands. Jones and Jarvis (1981) have recognized that there are a number of physical and chemical properties of soils affecting As mobilization – immobilization processes. These include soil pH, cation exchange capacity (CEC), organic matter content, soil granulometric fractions and lithology.

264

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Table 9-3: ANOVA test of significant difference of the parameters in both sedimentary and basement complex environments Parameters/variables

Sum of squares

Mean df square

F



% Sand

Between groups 1412.320 1 Within groups 184.642 7 Total 1596.962 8

1412.320 26.377

53.543

0.000

% Silt

Between groups 272.568 Within groups 26.127 Total 298.696

1 7 8

272.568 3.732

73.026

0.000

% Clay

Between groups 494.018 Within groups 94.462 Total 588.480

1 7 8

494.018 13.495

36.609

0.001

Organic matter

Between groups 22.969 Within groups 1.220 Total 24.189

1 7 8

22.969 0.174

131.846 0.000

pH

Between groups 1.991 Within groups 0.195 Total 2.186

1 7 8

1.991 71.540 2.783  10-2

0.000

1 7 8

555.458 57.700

0.017

Arsenic concentration Between groups 555.458 Within groups 403.902 Total 959.360

9.627

These factors except the CEC have been considered in this study. Table 9-3 gives a summary result of the ANOVA test of the relationship between the As and these controlling factors in the soils of both the sedimentary environment and the basement complex environment. The test indicated that there is a significant difference within each of the location and also between the two locations in terms of As concentration (i.e., p > 0.017). This significant difference is more pronounced within the sedimentary and basement complex environments and between the two locations for the particle size analysis, organic matter content and pH at the level of p > 0.00. Arsenic has a complex chemical behavior and can bond with many different elements and react differently depending on the physico-chemical properties of the water content of the soil, making the prediction of its stability rather difficult (Frost, 1967). This probably explains the relatively low content of As metal in the basement complex of the study area.

Arsenic concentration in selected soils around Abeokuta, southwestern Nigeria

9.5

265

Conclusion

The soils developed from the limestone-rich sedimentary environments of the study area are slightly alkaline and have higher As concentration when compared with the slightly acidic soils developed from the gneissic-rich basement complex environment. This As level in the sedimentary environment of the study area has been favored by the lithologic sequences of shale and clay members on top of the basal limestone units, organic matter content and granulometric fractions of fine sand, silt and clay. The limestone quarrying activity in the area supported the release of cement dust which served as binding material for the metal contents of the soil. The absence of the cement dust and low organic matter content in the basement complex of the study area, in addition to the intensive farming activities by the University community, were probably responsible for the comparatively low As level in this sector of the study area. The area must have also suffered from soil loss resulting from intensive precipitation and runoff.

Acknowledgments Many thanks to Dr Prosun Bhattacharya who reviewed the extended abstract of this paper, Prof. Roger Finlay and other members of ICOBTE committee for the ICOBTE Scholarship awarded to me for the presentation of this paper at the 7th International Conference on the Biogeochemistry of Trace Elements Uppsala, Sweden, 2003.

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