Abstract: The objective of the present investigation was to evaluate some total heavy metal content such as Cd, Pb, Cr, Ni and Cu in kondok soils and to determine the relationships between the contaminants and other physico-chemical soil properties. Currently, Cd, Pb, Cr, Ni and Cu do not seem in toxic levels. Also, the relationship between total heavy metal contents and some soils properties that was investigated on 69 disturbed soil samples. Significant relations between total heavy metal contents and some soil properties were determined at p<0.01.
INTRODUCTION
Heavy metals are well known to be toxic to most organisms when present in high concentration in the environment (Giller et al., 1988). In the last decades, human activities have continuously increased the levels of heavy metals circulating in the environment (Ma and Rao, 1997). Anthropogenic activities such as agriculture, industry and urban life increase content of these elements in soils and waters (Alegria et al., 1991). Heavy metals can be also found in the parent rock from which soils have developed. The anthropogenic heavy metals are believed to be easily accumulated in the top soil (Baker, 1990; Samsoe-Petersen et al., 2002), causes in potential problems such as toxicity to plants and animals (Ma et al., 2002; Berti and Jacobs, 1996), accumulation in food chain, perturbation of ecosystem and adverse health effects (Forstner, 1985; Stalikas et al., 1997). Of all the natural compartments, soil is perhaps the most important because it receives heavy metals coming from different sources and simultaneously acts as a buffer to control the movement of these elements to other compartments (Gil et al., 2004).
Kondok Season River passes from Haftgel City in Khuzestan Province. In order to agriculture extension in this area, construction of a dam on this river was suggested by Khuzestan water and power authority. To prediction of dam storage water quality after construction, some studies was done on physical-chemical soils properties and heavy metals contents such as Cd, Pb, Ni, Cr and Co in soils by Sazabpardazan Consulting Engineering (2007).
The purposes of this study are: (1) to determine the heavy metal content of Kondoke soils and (2) to determine the relationships between the contaminants and other physico-chemical soil properties.
MATERIALS AND METHODS
Study Site
The study was carried out in Kondok area (Fig. 1) South
of Iran (49°18'-49°35'N, 31°25'-31°40'E) and has semi-arid climate
with temperatures ranging from 25.7 to 37.6 and annual mean precipitation is
300 mm based on 30 year period.
Soil Sampling and Preparation for Analysis
Soil samples were taken from 20 points, taking into considers different
soil depths (0-150 cm). Plant residues and roots were removed by hand and soils
were transferred to laboratory. Samples were kept at room temperature and the
analyzed.
Soil Characterization
The following basic soil characteristics were determined: Soil reaction
(pH) was measured in a suspension at a soil/water ratio of 1:2 according to
McLean (1982). CaCO3 was determined according
to the calcimeter method of Nelson (1982) and total soluble
salt was analyzed measuring the electrical conductivity of an extract of the
same mixture (Rhoades, 1982). The texture of the soil
samples was determined by the hydrometer method (Day, 1982).
Organic matter was determined by the Walkley-Black procedure (Nelson
and Sommers, 1982).
Determination of Heavy Metals
The soil sample were dried at 105-110°C for a total period of one day
soil samples were sieved using 0.074 mm for the sake of homogeneity. Weighted
samples were transferred into acid baths. The acid volumes used were 15 mL HCl
and 5 mL HNO3 for each sample. On a hot plate, the samples were heated
at 115-120°C. After observing reddish gas exit from the heated samples and
making sure that the prepared samples are dry, the samples were removed from
the hot plate.
| Fig. 1: | Location map of the sampling site |
A 10 mL HCL and HNO3 mixture was added to each sample. Finally concentrations of each sample were measured by atomic absorption spectrophotometer.
Statistical Analysis
Statistical analysis were performed using the statistical package for social
science (version 15, SPSS Ins., Chicago, IL, USA) program. Pearson’s correlation
coefficients and p-values were calculated for all possible variable pairs.
RESULTS
Soil Properties
Some physico-chemical properties of soil sample obtain from 65 sampling
point at 0-150 cm depths in the total of 20000 ha study area is shown in Table
1.
Soil texture was not similar for all samples and also clay, silt and sand contents of the 65 samples were 14-43, 35.3-65.3 and 2.4-42.4, respectively. Value of soil pH was in range from 7 to 8 which related to value of soil salinity. CaCO3 content was high in all samples due to their origin from lime parent material. The organic matter content ranged was low, 0.07-1.29% (Table 1).
Heavy Metal Concentrations
Cd content of the samples soils were 0.23-2 mg kg-1, soil toxic
limit is around 2.5-3.0 mg kg-1for Cd (Fabis, 1987). Toxic limit
of Pb, Ni and Cu in soil is 50 mg kg-1 (Fabis, 1987). Pb was 1.67-8.34
mg kg-1, Ni, 1-9 mg kg-1 and Cu 1.83-9.2 mg kg-1;
Toxic limit of Cr is 75 mg kg-1 in soil (Fabis, 1987). Cr content
was 0.83-7.50 mg kg-1. The descriptive statistics on the heavy metal
concentrations of the soil are shown in Table 2.
The Relationships Between Soil Heavy Metal Concentration and Some Soil Properties
Correlations analysis was used to establish relationships between total
concentrations of heavy metals and soils physicochemical parameters. Table
3 shows the relationship between Cd, Ni, Pb, Cr and Co concentrations correlations
coefficient and some soils properties.
Cd, Ni, Pb and Cr contents were positively correlated with the soil depth. Cd, Ni and Cr contents were positively correlated with the soil silt content. Visa versa, sand was negatively correlated with Cd, Ni and Cr. Clay content was not significantly related except Cd. Organic matter was correlated with the Ni and Cr. CaCO3 was found significant only for Pb. Although, pH limitation alterations, Cd, Pb and Co contents was negatively correlated with it.
| Table 1: | Descriptive statistics on selected soil physical and chemical properties (n = 65) |
| Table 2: | Heavy metal concentration of soil samples (mg kg-1) |
| Table 3: | Correlation coefficients of relations between heavy metal contents (dependent variable) and soil depth, clay, loam, sand, organic matter, CaCO3, pH and calcium content of the soils (independent variab |
| **Significant at the %1 level, *Significant at the %5 level | |
DISCUSSION
Soil texture in this area was generally medium to heavy. It is strongly affected by parent soil material. The organic matter content was low due to soil high temperature and decomposition high ratio of this matter. The pH was in range 7-8 in soil different levels and when salinity is low, it is more than 8. CaCo3 content was high due to more content CaCO3 in soil parent material.
In all samples concentrations of heavy metal do not seem in toxic levels. Therefore these materials won’t be caused pollution in water reservoir in future.
According to correlations analysis Cd, Ni, Pb and Cr contents were positively correlated with the soil depth. This probably may be due to high organic matters contents in top soil. Clay content was not significantly related except Cd. This probably is due to kind of clays.
CONCLUSION
The results from this study showed that Cd, Ni, Pb, Cr and Co concentrations do not seem in toxic levels. Extractable contents of heavy metals are highly correlated to physico-chemical characteristics of the soils. Compared to other elements Cd were more significantly and correlated with soils properties. Accumulation of these elements increased in the upper soil layers. Probably, higher organic matter contents of the upper layers tended to this tendency. Sand was significantly and negatively correlated with Cd, Ni and Cr. This observation concurs with the findings in other studies (Aydin et al., 2004; Tarackgtoglu et al., 2006). Except Cd, none of the other heavy metal was not correlated with clay content. This is not according to other studies (Aydin et al., 2004; Tarackgtoglu et al., 2006) may be due to the kind of clay. Soil stability was low in various soil moisture conditions. This related to exist of clays such as chlorite and polygorscite and etc. The approved of this subject need to mineralogy by X-Ray. But, by attention to low soils CEC this subject may be approved. In most of the time soils CEC were lower than half soil’s clay percent and this approved the presence of minerals with low CEC.
ACKNOWLEDGMENTS
The authors should gratefully acknowledge the Research and Standards Office for Irrigation and Drainage Networks of Khuzestan Water and Power Authority (KWPA) for their financial support and assistance during the study and field visits. A.M developed the concept and M.A and K.T conducted the field study.