Accumulation patterns of heavy metals in forest mosses from the south-west region of Nigeria

Environmental Pollution (Series B) 11 (1986) 67-78

Accumulation Patterns of Heavy Metals in Forest Mosses from the South-West Region of Nigeria P. C. O n i a n w a , S. O. Ajayi & O. O s i b a n j o Department of Chemistry, University of Ibadan, Ibadan, Nigeria

& A. E g u n y o m i Department of Botany, University of Ibadan, Ibadan, Nigeria

ABSTRACT The accumulated levels of the metals, Pb, Cd, Cu, Ni, Zn, Mn and Fe were determined in forest mosses from south-west Nigeria. Distinct gradients were observed only with Pb and Zn, and a small zone of ~andd~ contamination with Cu was identified in parts of the east and the southeast. This contamination was possibly a result of the use of copper-based fungicides and pesticides in cocoa farms in the area. The levels of the metals are compared with those in forest mosses in some other parts of the worM.

INTRODUCTION The use of mosses as indicators of atmospheric pollution by heavy metals is now well established. Apart from the monitoring of emissions from local 'hot spots' such as the vicinities of industrial complexes and mines (Burkitt et al., 1972; Lee, 1972; Cameron & Nickless, 1977; Ellison et al., 1976; Beckett et al., 1982), mosses have also been used for extensive regional surveys in various parts of the world. These include the studies of 67 Environ. Pollut. Ser. B 0143-148X/86/$03.50 © ElsevierApplied SciencePublishers Ltd, England, 1986. Printed in Great Britain

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the Maritime provinces of Canada (Kevin, 1983), Central and South Sweden (Ruhling & Tyler, 1969), the north-eastern United States of America (Groet, 1976), England (Lee & Tallis, 1973), Norway (Steinnes, 1977), and New Zealand (Ward et al., 1977), among others. A few studies, such as those of Ruhling & Tyler (1971, 1973) and Gydesen eta/. (1983), are of international dimensions. The technique has proved rapid and reliable in identifying metal emission and deposition gradients over vast areas. In addition, it is comparatively much cheaper than the use of standard automatic samplers which usually involve prohibitive costs. This makes the moss analysis method particularly suitable for use in developing countries such as Nigeria. Mosses are abundant in Nigeria, and an earlier survey (Onianwa & Egunyomi, 1983) has shown the local species to be suitable for biomonitoring atmospheric metal pollution. In the present investigation, an intensive study of a major part of the south-west region of Nigeria was undertaken. This involved an area of about 120 km radius with Ibadan as a central focus (Fig. 1). Ibadan, with an estimated population of 1.8 million in 1976, is a major commercial and administrative centre and the largest city in Nigeria. Relatively big towns (1976 population, 100 000-500 000) in the study area include Oyo, Ogbomosho, Oshogbo, Ilesha, Ife, Ondo, Ijebu-Ode, Shagamu, Abeokuta, Iseyin and Ilaro, while the others are mostly smaller towns (population < 100 000) and villages. The moss samples were collected during the periods August-September 1981, September-October 1982 and July-August 1983, from forests and outskirts of towns and villages in the study area to avoid localised contamination from inhabited areas. They were analysed for the metals Pb, Cd, Cu, Ni, Zn, Mn and Fe, with a view to determining the background levels for this region, and identifying accumulation and deposition gradients, if any.

MATERIALS AND METHODS The mosses were obtained as composites of each species in delimited areas at the outskirts of most of the towns and villages. More than one species were obtained at most of the sites, and these included acrocarpous ones like Barbula larnbarenensis, Bryum coronatum, Calymperes sp., Hyophilia crenulata, Octoblepharum albidum and Syrropondon armatus,

70

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and pleurocarpous species like Erythrodontium barteri, Leucodontopsis geniculata, Pinatella sp., Thuidium gratum, Racopillium orthocarpoides, Rhacopilopsis trinitensis, Stereophyllum sp. and Trichosteleum sp. The same sites and species were sampled during each of the sampling periods. The samples were air-dried, picked free of debris and oven-dried at 80 °C. About 1.0 g dried ground portion was predigested overnight in 25 ml HNOa: HC10 4 acid (Zasoski & Burau, 1977). This was followed by a programmed temperature heating on a Tecator digestion block for about 24h, up to 180°C. The warm digest was diluted to 25.0cm 3 with distilled water and subsequently analysed by air-acetylene flame atomic absorption spectrophotometry (Varian AAS), using metal standards prepared in 11% perchloric acid. The accuracy of the results from the AAS analysis was validated in a comparative analysis of representative moss samples by three other techniques: X-ray fluorescence spectroscopy, anodic stripping voltammetry and direct current-plasma atomic emission spectrophotometry, in various laboratories in Sweden. There was good agreement between the AAS values and those achieved by the other techniques (results to be published).

RESULTS AND DISCUSSION A summary of the results for the entire area is presented in Figs 2 and 3, while the average levels for a few of the sites are given in Tables 1 and 2. The metal levels did not change significantly from 1981-83, and the average values for the period are used for describing the patterns. Also, the levels did not vary significantly from one moss species to another at any particular sites, except that, with Pb, the pleurocarpous species accumulated slightly higher levels than the acrocarpous types. Lead Lead derived from automobile exhaust constitutes the most important pollutant in the study area due to the substantial increase in automobile traffic in recent times (Oluwande, 1979). The lowest levels of 5-20/zg g and < 10 pg g-~ in pleurocarpous and acrocarpous species, respectively, were obtained in samples from some of the remotest parts of the study area. These were around the villages of Ago-Are, Okuku, Okaka, Ipapo, etc., in the north-west, Igbo-Ora, Eruwa, Olokemeji in the west and

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the effects of diffuse emissions from the various activities within these bigger settlements, as well as their proximity to the busiest inter-city and inter-state highways (Fig. 1). These levels were much lower than those obtained in mosses from highly polluted regions of Sweden, Denmark, Germany and the north-eastern USA, but are comparable with the average levels of 30-50/~gg-1 in Hylocomium splendens in Norway (Table 3). Zinc The pattern of accumulation of zinc in the mosses was similar to that of lead, except that the differences between pleurocarpous and acrocarpous species were insignificant. Some of the zinc was derived from additives to local automobile diesel oils, in addition to other sources, such as windblown dust. The lowest levels of 10-40/~gg -1 were found at the sites

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which had the least Pb levels (Figs 2 and 3). At the other sites, the levels of Zn were within a wide range of 40-100ttgg -1. These higher levels compare well with those obtained in Sweden and the north-eastern USA (Table 3), but are much lower than the 350-400 pg g- 1 obtained in parts of north-west Germany.

Copper Copper levels in most parts of the region were 10-30 #g g-1 (Fig. 3). The levels in the remote 'background' areas identified in the north-west, south° •¢¢est, west and north-east, were not lower than in the other areas. No important sources of emission of Cu into the atmosphere are known in this region. An unusual elevation in Cu levels was observed consistently during the three years in samples from some parts of the east and southeast. This ranged from 100-1000 #gg-1, and typical average levels at some sites such as Oshogbo, Ilesha, Ife, Asewele, etc., are given in Table 1. The source of this contamination is not readily explained. It is possible that the use of some Cu-based fungicides and pesticides in many farms in this area may contribute to this contamination. The background levels of 10-30 pg g-1 are slightly higher than were obtained in mosses from Sweden, Poland and Canada, but compare well with those from West Germany and the USA (Table 3).

Cadmium The cadmium concentrations were still at a background level of 0.05-0-20 #g g-1 in all parts of the region (Fig. 2). No gradients were discernible, although a few erratically high and unexplainable levels were obtained in some vicinities, but were regarde~l as outliers. These background levels are lower than those obtained in parts ofindustrialised Europe and the USA where emissions from primary industries are significant. The Nigerian levels are comparable with those found in Polish parks, Greenland and the Maritime provinces of Canada (Table 3).

Nickel The nickel levels ranged from 3 to 15/~g g - 1 in all parts of the region, and no gradients were discernible. These levels compare well with the highest levels obtained in mosses from Sweden (1968) and the north-eastern USA

Heavy metal accumulation in Nigerian mosses

77

(Table 3). Levels in Denmark, Poland, Canada and, currently, in Sweden, were lower than the Nigerian values, while the 1980 levels of 40 #g g- 1 in parts of West Germany were higher.

Manganese and iron The levels of manganese (100-800 gg g-1) and iron (2000-15 000 gg g - l ) in Nigerian mosses varied widely and were the highest reported in the literature. No gradients in the distribution of these metals within the region were discernible. These high levels were comparable with those in topsoils from parts of the region (Ali, 1978). It is possible that most of the manganese and iron in the moss samples was derived via wind-blown dusts. In view of the very high and wide-ranging levels of these metals in the mosses, it may be unsuitable to use the moss analysis method for monitoring changes in the deposition of these metals. Comparison of available data on wind and wet precipitation for parts of the region with the metal accumulation gradients did not show any correlation. This is possibly because the pollutants were released in small amounts at low altitudes, and were not transported over very long distances due to the filtering effects of vegetation. It is encouraging to observe that, despite the rapid industrialisation in Nigeria during the last decade, the levels of the most important metal pollutants in this study, Pb and Cd, are still much lower than in most parts of the developed world.

REFERENCES Ali, E. H. (1978). Trace and macronutrient distribution in some of the basement complex soils of south-western Nigeria. MPhil dissertation, University of Ibadan, Nigeria. Beckett, P. J., Boileau, L. J. R., Padovan, D. & Richardson, D. H. S. (1982). Lichens and mosses as monitors of industrial activity associated with uranium mining in northern Ontario, Canada. Part 2. Distance dependent uranium and lead accumulation patterns. Environ. Pollut., 4, 91-107. Burkitt, A., Lester, P. & Nickless, G. (1972). Distribution of heavy metals in the vicinity of an industrial complex. Nature, Lond., 238, 327-8. Cameron, A. J. & Nickless, G. (1977). Use of mosses as collectors of airborne heavy metals near a smelting complex. Water, Air, Soil Pollut., 7, 117-25. Ellison, G., Newham, J., Pinchin, M. J. & Thompson, I. (1976). Heavy metal content of moss in the region of Consett (North East England). Environ. Pollut., 11, 167-74.

78

P. C. Onianwa, S. O. Ajayi, O. Osibanjo, A. Egunyomi

Grodzinska, K. (1978). Mosses as bioindicators of heavy metal pollution in Polish national parks. Water, Air, Soil Pollut., 9, 83-97. Groet, S. S. (1976). Regional and local variation in heavy metal concentrations of bryophytes in the north eastern United States. Oikos, 27, 445-56. Gydesen, H., Pilegaard, K., Rasmussen, L. & Ruhling, A. (1983). Moss analysis used as a means of surveying the atmospheric heavy-metal deposition in Sweden, Denmark and Greenland in 1980. NationalSwedish Environmental Protection Board--Bulletin, snv pin 1670, 1-44. Kevin, E. P. (1983). Heavy metal and sulphur concentration in Sphagnum magellanicum in the Maritime provinces of Canada. Water, A Jr, Soil Pollut., 19, 341-9. Lee, J. A. (1972). Lead pollution from a factory manufacturing anti-knock compounds. Nature, Lond., 238, 165-6. Lee, J. A. & Tallis, J. H. (1973). Regional and historical aspects of lead pollution in Britain. Nature, Lond., 245, 216--18. Oluwande, P. A. (1979). Automobile exhaust problems in Nigeria. Ambio, 8, 26--9. Onianwa, P. C. & Egunyomi, A. (1983). Trace metal levels in some Nigerian mosses used as indicators of atmospheric pollution. Environ. Pollut., 5, 71-81. Ruhling, A. & Tyler, G. (1969). Ecology of heavy metals--A regional and historical study. Bot. Notis., 122, 248-59. Ruhling, A. & Tyler, G. (1971). Regional differences in the deposition of heavy metals over Scandinavia. J. appl. Ecol., 8, 497-507. Ruhling, A. & Tyler, G. (1973). Heavy metal deposition in Scandinavia. Water, Air, Soil Pollut., 2, 445-55. Steinnes, E. (1977). Atmospheric deposition of trace elements in Norway studied by means of moss analysis. Kjeller Rep., KR-154, 1-13. Tyler, G., Bergvist, B., Ruhling, A. & Wiman, B. (1983). Metaller i skogsmark-deposition och oms/ittning. Naturvdrdsveket--Meddelande, snv pm 1692, 1-65.

Ward, N. I., Brooks, R. R. & Roberts, E. (1977). Heavy metals in some New Zealand bryophytes. Bryologist, 80, 304-12. Zasoski, R. J. & Burau, R. G. (1977). A rapid nitric-perchloric acid digestion method for multi-element tissue analysis. Comm. Soil Sci. Plant Anal., 8, 425-36.