Abstract: The effects of automobile and municipal solid wastes on soil mercury (Hg) distribution were studied in soils of a toposequence at Nekede, Southeastern Nigeria in 2005. Three pedons representing 3 physiographic units of the site namely Upper Slope (US), Lower Slope (LS) and Foot Slope (FS) were aligned and dug along a transect. Results show that heaviest concentrations of Hg were reported in epipedal and subsurface horizons in LS land unit. Highest values of Hg concentration was found in the surface layers in all land units. Minimal Hg concentrations were recorded in the FS soil classified as Fluvaquentic Eutropept (Eutric Fluvisol). Soil organic carbon, CEC, pH and texture had significant correlations with Hg using the available data hence reliable predictors of Hg distribution and availability. There is need to evaluate uptake of Hg by common crops of the study area since epipedal contents were above critical limits.
INTRODUCTION
Heavy metal inputs to the soil include those from commercial fertilizers, liming materials, sewage sludges, atmospheric depositions (Sensesi et al., 1999) and repeated applications of organic manures, fungicides and pesticides (Li et al., 1997; Moore et al., 1998; Hans et al., 2000). Nonetheless, these heavy metals are naturally occurring (Ojanuga et al., 1996) and possibly from weathering of rocks but their concentrations in the pedosphere increase due to anthropogenic inputs (He et al., 2004).
One great source of these heavy metals in Southestern Nigeria is motor vehicle servicing centres popularly called mechanic villages. In urban and peri-urban towns of this sub-region, automobile wastes are disposed unhygienically, leading to excess accumulation of heavy metals, such as cadmium chromium, nickel, lead and mercury (Onweremadu et al., 2007). A major activity of these automobile service stations is deforestation and encroachment into nearby farms, rivers and other natural resources which often lead to soil erosion and alteration of mercury (Hg) levels (Fostier et al., 2000). Again, in these automobile stations vegetal debris resulting from deforestation are burnt as a diffuse source (Roulet et al., 1999). All these influence the accumulation and distribution of Hg especially in the study site with a steep to undulating river slope. Soil Hg, accumulation may be short-term or long (Brabo et al., 2003) but essentially enhances the transport and leaching of neurotoxic heavy metals, such as mercury (Farella et al., 2001) to soil and water resources.
At the foot slope of the toposequence, dry season vegetable farming takes, involving fluted pumpkin (Telfairia occidentalis), Amaranth (Amaranthus cruentus L.) and several species belonging to the Solanaceae. At the upper and middle slopes of the toposequence, rainfed arable farming is practiced. It is feared that Hg may be absorbed by these crops which are readily consumed by the teeming urban population. Again, fish farming is common in the study area and there is high tendency of Hg accumulation. In similar riparian communities, mercury contamination of fish has been reported (Sampaio da Silva et al., 2005) and such Hg exposure is dangerous to human life (Brabo et al., 2000) when concentrations exceed maximum soild dump sites and deforestation permissible limits. Based on the above, this study investigates the distribution of Hg in soils receiving automobile wastes at toposequence in Owerri Southeastern Nigeria. It is hypothesized that these automobile depositions, solid wastes and deforestation increase soil Hg content and distribution in the ecosystem.
MATERIALS AND METHODS
Study Site
Nekede is a town within Otamiri watershed in southeastern Nigeria and lies
between latitudes 5°101 55.51011 and 5° 25110.12011N
and longitudes 6°45125.11011 and 7° 051
06.21011 E. The northeastern part of the town juts into Owerri municipality,
Southeastern Nigeria. Proximity to Owerri Urban has led to the urbanization.
This part of Nekede despite its fragile and rugged physiographing. A large expanse
of the site is occupied by automobile servicing enterprises, who discharge wastes
on adjoining farms and wetlands. The major geological material in the study
area is coastal plain sands (Benin formation) of the Oligocene-miocene era.
It is a humid tropical environment, with an average annual rainfall of about
2500 mm and temperatures are high, changing only slightly during the year (27-29°C).
The vegetation is highly altered by anthropogenic activities, such as farming,
fishing sand mining, deforestation for fuel wood and establishment of automobile
service stations. Varying forms of agriculture are practiced in response to
sloping terrain that finally juts into the Otamiri River. Generally slash and
burn system of clearing farms is practiced and soil fertility regeneration is
still by bush fallowing. Rainfed agriculture is common but during dry seasons
(3 months), farmers move towards the foot slope proximal to the river to for
dry season vegetable production.
Field Sampling
A transect was established from soils nearest to the automobile service
station 50 m away from the centre towards the Otamiri River in Imo State, Southeastern
Nigeria. From the origin of the transect to the foot slope of the topography
is about 1500 m. Three profile pits were dug along the transect, representing
3 identified physiographic units, namely Upper Slope (US) Lower Slope (LS) and
Foot Slope (FS). Soil profile pits were described according to FAO (1998) procedure.
Soil samples were collected based on morphologically identifiable pedogenic
horizons. In each profile pit, 5 soil samples were obtained, giving a total
of 15 soil samples for the investigation. These soil samples were air-dried
and sieved using 2 mm sieve preparatory to laboratory determinations. Three
core samples were collected in each horizon, giving a total of 45 core samples
for bulk density analysis (Table 1).
Laboratory Analyses
Bulk density was measured by core method of Grossman and Reinsch (2002)
while particle size distribution was determined by hydrometer method (Gee and
Or, 2002). After equilibrating for 30 min, soil pH was estimated in water, with
a soil-liquid ratio of 1:2.5, using a Beckman Zeromatic pH meter (Hendershot
et al., 1993). Soil Organic Carbon (SOC) was determined by combustion
at 840°C (Wang and Anderson, 1998). Total Nitrogen (TN) was estimated by
microkjeldahl method (Bremner, 1996). Cation Exchange Capacity (CEC) was determined
by ammonium acetate leaching at pH 7.0 (Blakemore et al., 1987).
| Table 1: | Site characteristics |
Total mercury (Hg) levels were measured with the Cold Vapour Atomic Fluorescence (CVAF) technique as described by Pichet et al. (1999). In this method, 250 mg of sieved soil was digested in a 10:1 nitric and hydrochloric acid mixture and heated before injection into spectrophotometer.
Classification
Soils were classified using USDA Soil Taxonomy (Soil Survey Staff, 2003)
and FAO/UNESCO legend (FAO, 1998).
Data Analyses
Analytical data were subjected to correlation and regression analysis using
SAS computer programme (SAS Institute, 2001).
RESULTS
Soil Properties
Soils are deep and sandy, with US and LS being sandier (Table
2). Values of bulk density in LS were higher than the rest of other physiographic
units (Table 2). Argillation is most prominent in US and least
developed in FS soils. Soils are strongly (LS soil) to moderately acidic (US
and FS Soil). Soil organic fractions (SOC and TN) decreased with depth in all
pedons except in FS where this consistency was broken with Bg2 having
16.0 and 1.3 g kg-1 of SOC and TN, respectively when compared with
10.0 and 1.0 g kg-1 values of SOC and TN, respectively in Bg1.
Higher values of SOC were obtained downslope. Cation exchange capacity values
were generally low (CEC < 6.9 cmol kg-1) and this agrees with
the findings of Igwe (2001) in his study of a Nigerian floodplain in Southeastern
Nigeria. Heavy concentrations of Hg were recorded the surficial horizons in
all pedons of the study site (Table 3). Heaviest concentrations
were found in LS soil (0.26-1.88 mg kg-1).
Soil Mercury and Soil Properties
Highly significant negative correlations (p = 0.01, n = 15) were established
when Hg was related with SOC and sand while clay content had a significant positive
relationship with soil Hg (r = 0.758; p = 0.05, n = 15) (Table
4). There was strong correlation between Hg and CEC in soils of the study
site (r = -0.834; p = 0.05 n = 15) as well as between Hg and pH (r = 0.826;
p = 0.01’ n = 15). However, there was no significant relationship between
Hg and BD.
| Table 2: | Physical properties of soils |
| Table 3: | Chemical properties of studied soils |
| CEC = Cation Exchange Capacity, SOC = Soil Organic Carbon, TN = Total Nitrogen, Hg = Mercury | |
| Table 4: | Relationship between soil Hg and selected soil properties (n = 15) |
| SOC = Soil Organic Carbon, CEC = Cation Exchange Capacity, BD = Bulk Density, **: Significant at p = 0.01, *: Significant at p = 0.05, NS = Not Significant | |
| Table 5: | Classification of soils |
Soil Classifications
Available soil data (Table 2 and 3)
plus field information show that soils exhibit properties of Typic hapludult
(Dystric Nitisol), Fluventic Dystrudept (Dystric Fluvisol) and Fluvaquentic
Eutropept (Eutric Fluvisol) for US, LS and FS physiographic land units, respectively
(Table 5).
DISCUSSION
Soils are generally sandy although US and LS physiographic units were sandier. The implication of this in terms of soil Hg distribution is that translocation of Hg from epipedal to sub-surface horizons is rapid since sandy textures are associated with poor retentively (Bouma, 1991). This could by the reason for very high concentrations of Hg in LS soil profile pit (Table 3) despite the high Bulk Density (BD) values. But in FS soils with higher clay contents, high Hg values were limited to epipedal horizons. Bulk density values were highest in LS and FS soils. This could be attributed to the weight of the automobile and municipal solid wastes impacting on soils in LS soils. Seasonal flooding of the Otamiri River could be the reason for high BD in FS soil. High BD implies lower porosities, suggesting poor translocation of soil Hg across horizons with high values. However, movement through pores could be affected by pore size distribution, pore continuity and tortuosity (Eynard et al., 2004) and chemical nature of the metal (He et al., 2004). Slow movement of mercury across these subsurface horizons may serve as watershed to protect groundwater form contamination by agrochemicals and/or other pollutants (Ezeaku and Anikwe, 2006). Least pH values were found in LS soils and this could be responsible for high Hg values in the profile pit since most heavy metals and trace elements become more available and biotoxic at low to very low pH values.
Soil CEC was generally low, suggesting minimial availability of exchange sites for the adsorption of soil Hg. This means that more soil Hg will be available for translocation within the pedosphere for onward movement to surface and groundwater bodies. This could exceed the maximum permissible limits in Nigerian waters (FEPA, 1988). The same reason is true for low SOC values especially in US soils since high SOC content implies more exchange sites for adsorption of Hg on the soil micelle. High epidedal concentration especially in LS is attributed to heightened dumping of automobile wastes as well as municipal solid wastes on the physiographic land unit by the State Environmental Projection Agency. There is high possibility of increased transportability of Hg from LS to FS soils due to slope effect and these will eventually move downstream where the metal causes entrophication and poisoning of aquatic life (Giesler et al., 2005), Similar findings were reported by Mainville et al. (2006) in the Andean Amazon, Napo River Valley in Ecuador. With the exception of US, soils of the study site are Inceptisols, implying that these soils are still young and further weathering and pedogenesis can filter the pedons of the current Hg contamination.
CONCLUSIONS
The study has revealed poor retentivity of Hg due to sandiness of soil textures and very high possibility of surface and ground water Hg pollution. Soil Hg had the highest concentrations on surficial horizons, portending great danger to most arable crops and human health. There is need for modelling using the soil parameters that significantly correlated with Hg. Again, popular crops in the study site should be assessed to ascertain the levels of bioavailability.