The amount and variety of waste material have increased with the growing of technology and population. Increased vehicular traffic and emissions from refuse dumps are major contributors to air pollution and a matter of growing importance in many city centres. Of the priority pollutants, heavy metals cause adverse effects on the environment. Normative limits and international guidelines indicate the maximum levels for a number of individual pollutants in air samples. However, no restrictions have yet been given in many Nigerian cities. Municipalities have often suffered from smoke emanating from refuse dumps, which makes air quality sometimes very low.
There are numerous human activities which result in the release of toxic materials to the atmosphere. The identity of these sources has been established in most cases but their quantitative importance is rarely determined. For example, Okuo and Ndiokwere (2005) reported that in Warri (south-south Nigeria), the major sources of elemental pollution were due to re-entrained soil, automobile exhaust, residual oil combustion, petroleum activities, refuse and biomass incineration.
Indiscriminate dumping of refuse can influence soil physicochemical properties, but can still be used for farming provided the risks associated with its usage are continuously assessed and controlled. The introduction of metal contaminants into the environment could result from various sources; a few of which are application of sewage materials and leaching of garbage (Forstner and Wittmann, 1983). The impact on man would be felt if these metals enter into the food chain and accumulate in living organism (Moore and Ramamoorthy, 1984; Altundogan et al., 1998).
Waste handling facilities are lacking in many highly populated areas in most developing and under developed countries due to cost and lack of proper planning. This results in the discharge of household sewage and refuse into the environment untreated. Most waterways passing through urban centre are used as dump sites with open gutters leading from household direct to the waterways. A lot of toxic materials are leached by rain into soil and waterways from such dump sites. Waste amended soils have been reported to have high organic matter content (Anikwe, 2000). Soil organic matter influences the degree of aggregation and aggregate stability and it can reduce bulk density and increase total porosity and hydraulic conductivity in heavy clay soils (Anikwe, 2000).
Continuous disposal of municipal wastes in soil may increase heavy metal concentration. Heavy metals may have harmful effect on soils, crops and human health (Smith et al., 1996). However, Voutsa et al. (1996) maintain that there is generally not strong relationship between the concentration of heavy metals in soils and plants, because it depends on many factors such as soil metal bioavailability, plant growth and metal distribution to plants parts.
In contrast to automatic monitoring techniques, the study of bioindicator organisms can reveal the biologic impact of pollution over a geographical and temporal scale, depending on the selected species and approach.
In this study mosses and lichens have been recognized as suitable biomonitors
or bioaccumulators for air pollution (Bargagli, 1998; Bargagli et al.,
2002) and terrestrial invertebrates are used for monitoring air and soils (Dallinger,
1994; Bamgbose et al., 2005). In this study a moss F. capsularis
was used to monitor trace metal pollution resulting from burning refuse dumps
in the Zaria, Nigeria.
MATERIALS AND METHODS
Collection and treatment of samples: This study was carried out in the city of Zaria and Kaduna in northern Nigeria, where five locations were chosen in each case. Four dump sites were sampled in Zaria and another four dump sites from Kaduna. In each of these sites composite samples of soils were collected seven meters within the vicinity of the dump sites (K1, K2, K3 and K4; Z1, Z2, Z3, Z4). Soil samples were also collected from uncontaminated areas in Zaria and Kaduna. The samples were air-dried, ground and sieved using 300 μm sieve. The moss plants identified as Funaria capsularis at the Biological Sciences Department, Ahmadu Bello University, Zaria, Nigeria, were collected from building walls near four major roads Zaria. Control samples were also collected from uncontaminated sites, which are essentially residential. They were scooped with plastic spoon and transferred into polythene bags. The samples were rinsed first with tap water and then with distilled water to remove any attached soil particles. They were then air-dried, ground and stored for analysis.
Chemical analysis: The soil and moss samples were digested by standard methods described in Awofolu (2005). The digested samples were analyzed for metals (cadmium, copper, iron, lead and zinc) using Atomic Absorption Spectrophotometer (SOLAR 32) at the chemistry laboratory of National Research Institute for Chemical Technology, Zaria, Nigeria and expressed as micrograms per gram dry weight. The Cation Exchange Capacity (CEC) was determined and calculated using method described by Agbeni (1995).
RESULTS AND DISCUSSION
The values obtained for the different dumpsite samples in Kaduna were varied. In all the cases the pH of the dump sites were found to be higher than pH of the control site. Of the dump sites analyzed, K2 had the highest pH (Table 1). The pH of soil samples collected in the vicinity of the dump sites were also found to be higher than that obtained for the control site. The pH of soil affects all soil its properties i.e., chemical, physical and biological (Brady and Weil, 1999). Even though the pH of soil may be dependent on several factors such as chemical activities taking place in it, it appears that leaching or runoffs from these dump sites could have contributed to the relatively higher pH observed in these soil samples (Table 1). This means that dump sites could impact on the characteristics of soil in close proximity to them, which in turn affect plant growth because pH has been widely accepted as exerting a controlling influence on the availability of micro-nutrients to plant (Sanders, 1982). The increase in pH of the dump sites may be because of high organic matter which tend to buffer the soil by preventing excessive pH changes due to release of exchangeable cations during mineralization of organic matter (Woomer et al., 1994).
Organic matter is the reservoir of essential and non-essential elements for
plant growth and development; hence increased organic matter may lead to increased
soil productivity (Anikwe and Nwobodo, 2002). The control site had the lowest
organic matter content of 1.96%, while the site K2 had the highest
organic matter content of 7.10% of all the dump sites investigated (Table
1). The organic matter content of the soils in the vicinity of these dump
sites was found to be higher than that of the control sites. The high organic
matter content of the dump sites may be attributed to the decaying municipal
wastes. The relatively higher organic matter content in soils around the dump
sites also gives an indication that the source of organic matter is the dumpsite.
K2 samples with the highest organic matter content also showed highest
concentration for cadmium, copper, lead and iron, with the exception of zinc-these
metals are capable of being retained in the organic phase of the soil.
|| pH, OM, CEC and metal ion concentration in soil samples from
dump sites in Zaria and Kaduna, Nigeria
||Pearson correlation coefficient metals and other parameters
analyzed in soils (Kaduna)
||Pearson correlation coefficient metals and other parameters
analyzed in soils (Zaria)
|| Heavy metal concentration in Funaria capsularis (μg
correlation coefficient showed that there is a positive correlation between
organic matter and metal concentrations (Table 2).
Cation Exchange Capacity (CEC) which is a measure of the sum total of the exchangeable cations that a soil can absorb was relatively higher in the dumpsite samples. Again the K2 sample had a value more than two times the value obtained from the control samples i.e., 8.60 and 18.80 cmol kg1, respectively (Table 1). The higher value of CEC obtained for the soils around dump sites compared to the control sites again showed that the dump sites could affect the properties of the soils in close proximity to them. The highest value of CEC obtained in K2 was expected because soils with high pH and high humus content normally have high cation exchange capacity (Brady and Weil, 1999). K2 with the highest CEC also had the highest concentration of Cd, Cu, Pb and Fe with the exception of Zn in the dump sites soil. The high cation exchange capacity of the dump sites may also be attributed to decomposition of municipal wastes, which yield more of the exchangeable bases thereby raising the fertility status of the dump sites.
Zaria control samples showed relatively low pH values of 6.50, while Z3
showed the highest value of 9.00 (Table 1). The values here
were higher than the values obtained from sites in Kaduna. The dump sites had
higher pH values when compared with soil samples collected in the vicinity of
the dump sites and control sites. This suggests that the dump sites in Zaria
may also affect plant growth like the samples collected from Kaduna. The values
for the soil samples collected from around the dump sites were found to be higher
than the control site. This again suggests that the relatively high pH observed
in these areas is as a result of leaching from the dump sites into the surrounding
soils. Soil pH showed similar correlation with all analyzed metals with exception
of Cd in soil (Table 3).
The organic matter content of the control site was the lowest (3.44%) while that of Z3 was the highest (7.40%) (Table 1). The highest and lowest values here were also higher than the values obtained for sites in Kaduna. The organic matter content of the dump sites was higher than control sites. The higher level obtained for soils around dump sites compared with that of the control sites, showed that the dump sites had effects on the soil. The high organic matter content of the dump sites may be attributed to the use of the sites for dumping of municipal wastes. Site Z3 with the highest organic matter content also showed highest concentration of Cu, Pb, Zn and Fe with the exception of Cd. This contrasts site K2 with highest organic mater showing highest concentration of all metals analyzed with the exception of Zn. Organic matter showed high positive correlation with all metal analyzed in the soils with exception of Cd, which showed low correlation (Table 3).
The results of the study showed that Cation Exchange Capacity (CEC) had highest value in site Z3 (28.00 cmol kg1) while control site had the lowest value with 10.20 cmol kg1 (Table 1). The higher value obtained in the soil collected in the vicinity of the dumpsite compared to the control sites may be as a result of the effect of dump sites has on it. The highest value of CEC obtained in Z3 was not unexpected because soils with high pH and high humus content also have high CEC (Brady and Weil, 1999). This is result is similar to that obtained for site K2 where the high CEC showed high concentration all metals except of Zn.
The result obtained in the analysis Funaria capsularis revealed the
presence of cadmium, lead, zinc, copper and iron in the samples (Table
4). Cadmium concentration were found to be 0.0487, 0.0246, 0.0326, 0.0101
and 0.0000 μg g1 for sites M1, M2, M3,
M4 and the control site, respectively. The highest value of 0.0487
μg g1 was obtained from site M1, while 0.0000 was
obtained as the lowest concentration from the control site. The values obtained
from M1, M2, M3 and M4 were higher
than the 0.005 μg g1 limit given by USEPA (1993), but lower
than 1 μg g1 limit by FEPA (1991) and 1-3 μg g1
limit by EC Council (1986) directive 86/278/EEC. The values were lower when
compared with the range of 0.10-0.58 given by Sardans and Penuelas (2005) in
a similar study using the moss Hypnum cupressiforme.
The result showed copper concentration of 1.3781, 1.3856, 1.4092, 1.0267 and 0.0726 μg g1 for sites M1, M2, M3 and M4 and the control site, respectively. 1.4092 μg g1 was obtained here as the highest concentration from site M3, with 0.0726 μg g1 was obtained as lowest from the control site. The concentration in sites M1, M2, M3 and M4 were higher than the 1 μg g1 limits by USEPA and FEPA, but lower than the 50-140 limits by EC Council directive 86/278/EEC. The values were lower than the 20-115 μg g1 range given by Sardans and Penuelas (2005) in a study in which the Moss Hypnum cupressiforme was used.
Lead concentration were found to be 0.1151, 0.9560, 0.1092, 0.0986 and 0.0026 μg g1, respectively for sites M1, M2, M3, M4 and the control sites. The highest value of 0.9560 μg g1 was obtained from M2, while the lowest concentration of 0.0026 μg g1 was obtained from the control site. The values obtained were higher than zero limited given by USEPA, but lower than the 1 μg g1 limit given by FEPA. The values obtained here were lower than the 5-57 μg g1 range given by Sardans and Penuelas (2005) in a study using the moss Hypnum cupressiforme. The high level of lead may be as a result of the busy nature of the road.
Zinc concentration was found to be 1.0498, 0.9826, 1.2361, 1.5782 and 0.0098 μg g1 for site M1, M2, M3, M4 and the control site, respectively. The highest concentration of 1.5782 μg g1 was obtained from M4 and the lowest of 0.0098 μg g1 was obtained from the control site. The values for M1, M3 and M4 were all higher than 1 μg g1 limit by FEPA, but lower than the 5 μg g1 limit by USEPA. The values obtained here were lower than the 30-180 μg g1 range given by Sardans and Penuelas (2005) in a study using the moss Hypuum cupresiforme.
Iron concentration was found to be 0.9153, 0.8451, 0.5682, 0.6281 and 0.0005 μg g1 for M1, M2, M3 and M4 and control site, respectively. The highest value of 0.9153 was obtained from site M1 while the control site had the lowest with 0.005 μg g1. The value obtained in sites M1, M2, M3 and M4 were higher than the 0.3 μg g1 limit by USEPA, but lower than 20 μg g1 limit given by FEPA.
The result revealed that all the sites had values of soil pH, soil organic
matter and cation exchange capacity higher than the unpolluted sites.
The level of cadmium, copper, lead and iron obtained in some sites were above
the limits given by some international environmental protection agencies, but
the levels of zinc obtained were less than these limits. The results also revealed
that the constant burning of these dump sites could contribute significantly
to the concentration of these trace metals in the environment and this could
possibly enter into the food chain as revealed by the high concentration observed
in Funaria capsularis.