Sixteen physicochemical parameters including four major and ten trace metals were analyzed for seven top soil and six top stream sediment samples collected around the vicinity of cement factory in Mekelle, Ethiopia. Water and aqua regia extraction techniques were deployed to determine the water soluble and near total concentration of the metals in the samples. Water extraction results verified the magnitude of loading of the metals from anthropogenic sources. The mean values of the water soluble metals are found to exceed the reference values except for Ca and Zn in both lithogenic media indicating the severity of pollution. Spatial distribution of the metals suggested that cement factory dust and traffic emissions represent the most important pollutant sources for the investigated area. The mean proportion of the major and trace metals in the water soluble phase of soil can be put on a descending order as: Mg (57.76%) > Fe > Ca > K > Na > Mn (3.77%) and Mo (53.17%) > Cu > Co > As > Ni > Zn > Pb > Cr (5.02%) respectively. The order of major metals is almost reverse in the case of the stream sediment with slight difference in the order of the trace metals. Factor analysis has revealed that the first factor represents the contribution of metals (Mn, Cr, As, Co, Zn, Na, K, Cu, Pb and Ni) from local anthropogenic activities, whereas the second factor represents the contribution of metals (Na, Fe, Cu, Pb, Ni, Ca, As, Co, Zn and Mo) from both lithogenic and anthropogenic origins. Third factor consists of Ca, Mo, Mg, and Fe from geogenic source of the local geology. It is spatially evident that the cement dust emission has an impact on the soil’s Cr, Co, Mo and Ni content which drastically decreases downstream. The spatial pattern of Cu, Pb, As, Zn and Co shows significant association with the cement dust and loadings from the traffic movement on the road.
Dust emitted from cement factories falls on plants and soils with consequent modification of soil properties through reactions between the contents of the dust and water molecules to form various products. This causes soil degradation with respect to changes in soil structure, exchangeable bases and cation exchange capacity affecting the soil fertility [
In addition to getting an insight into heavy metal speciation and bioavailability, characterization of soil’s metal content helps to gain knowledge of the source of contamination and associated environmental and health risks that serve as basis for remediation of contaminated soil. The pathway of heavy metals to plants is mainly via adsorption and/or absorption which refer to binding onto the surface and penetration into the inner matrix of soil minerals respectively. In investigating the effects associated with major and trace metals that result from natural or manmade activities, the bio-available fraction of the total elemental content in soil and sediment must be identified. This is because total concentrations do not necessarily explain the bioavailability of metals in sediment and soil. Water soluble fraction consists of metals which are comparatively labile, highly mobile and potentially bioavailable [
Factor analysis is one of the statistical techniques that help in interpretation of geochemical data by revealing the factors responsible for the overall variability pattern. Factor analysis technique enables reducing the data to small set of factors that represent the underlying pattern of relationship among the metals providing information on the strength of a particular metal in a factor (loading), the amount of the total variability of each metals explained in a factor (communality) and the amount of total data explained in each model (eigen values in percent). The factors to be selected should have Eigen values greater than unity. The first few factors that account for most data variability are to be used in interpreting the natural or/and anthropogenic sources of metals.
The main purpose of this research is to examine the degree and extent of the impact of dust emission from a cement factory and vehicular movement on the distribution of selected major and trace metals in soil and stream sediments in the downwind and downstream vicinity. Thus this paper presents geochemical analysis and factor analysis results to reveal the metals’ loading on the water soluble portions of soil and stream sediment materials due to natural and/or anthropogenic causes. The cement industry under consideration has been in operation since 2000. The factory is suited on the foot of Messebo hill which encompasses the major raw materials for cement production. The pollution assessment in this paper would help to discriminate the existing major and trace metals in soil and stream sediments which can possibly be attributed to the dust emission of the factory.
The study area is located at about 10km NW of Mekelle City close to the Mekelle-Hagerselam asphalt road (
Soil and stream sediment sampling was conducted in the western vicinity of Messebo cement factory in March, 2012 (
the digested samples and shaken for 2 minutes. The resulting solution was filtered by adding distilled water till the filtrate gets 50 ml volume before cation and trace metals’ measurements were done by respective atomic absorption spectrometry instruments. In both cases, ICP multielement standard solution IV UN-2031 was used. On the other hand, the 10-gram subsample of each sample was treated with de-ionized water in a 1 g/10 ml soil/water ratio, shaken for 2 hours and filtered prior to analysis using the same instruments to determine the cations in water soluble phases. Repeat analyses on three randomly selected samples were performed and reagent blanks and standards were used to assure quality control of analysis. The precision and bias were generally <10%. For comparison with the geochemical background values of metals, five uncontaminated surface soil samples were collected from undisturbed area at about 6 km distance from the study area where anthropogenic activities are minimal and are believed to represent geological background with reference to heavy metals. These samples were digested with aqua regia in the same procedure as the main samples and the analysis results were used for comparison purposes as reference to assess the pollution of the study area.
The geochemical analytical datasets for the water and the difference between aqua regia and water extraction were statistically treated using SPSS version 16.0 software. Such subtraction approach is supposed to enhance the metals concentration attached to water soluble, amorphous and crystalline/primary rocks mineral components. In order to closely examine the distribution, relationships and possible sources of the metals in the water soluble and water insoluble phases of soil and stream sediment minerals, separate factor analysis were conducted for fourteen variables in both datasets. The factor analyses were performed by using principle component analysis (PCA) and computing the Eigen values. The rotation of principle components was carried out by the varimax method. The factor loadings, which can be regarded as combination between the elements, were then computed after rotating the original principal component solution. In this study, factors with Eigen values greater than 1 were retained and only variables with loadings greater than 0.60 and highest value were considered as having significant association under a particular factor.
The result of geochemical analysis of the major and trace metals of the soil and stream sediment samples are summarized in
The distribution of the metals in the lithogenic media of the study area is assessed by examining the relative proportion of the total concentrations in the water soluble and residual (difference between aqua regia and the water soluble values) portions. These two phases are believed to explain the natural and/or anthropogenic attributes to the source and mobilization of the associated cations. All the fourteen cations under consideration exist in the residual (water insoluble) phase for more than half of their total concentration as determined by aqua regia extraction. Manganese, chromium and lead have higher distribution in the residual phases while magnesium exhibited the highest concentration proportion in the water soluble phase. The proportion of the mean values of the major and trace metals in the water soluble phase is found to have a decreasing order as: Mg > Fe > Ca > Na > K > Mn and Ni > Mo > Cu > Co > As > Zn > Pb > Cr respectively (Figures 3 and 4). It is well perceived that metals might undergo crystallization hindering their water solubility due to the fact that in cement dust calcium silicate and calcium aluminates are present in the form of colloidal gels. The crystallization and solidification develop a hard crust-causing low yield crops by affecting the mobility of the nutrient metals in soil [
The water soluble and residual proportions of metals in both soil and stream-sediment media generally showed similar mean values of Ca, Fe, Cr, Pb, As and Zn as well as Co, Zn, Mo respectively (
It is evident that all the metals in the near total analysis (aqua regia extraction) are above the background values used as reference to assess the pollution status of the study area. It is also possible to note that the mean values of the water soluble extraction of all the metals is higher
than the reference values except for Ca and Zn in both lithogenic media. However, K, Mn, Cr, and Pb have low mean values in the water soluble portions close to the reference values (Figures 5(a) and (b)). Looking at the residual values, the stream sediments revealed to have higher Mg, Fe, Cu and As than the soil samples while K, Mn, Cr, Pb, Co and Mo showed comparable values in both media. On the other hand, the soil’s residual fractions have higher Na, Ca and Ni contents (
Major metals presence in water extraction depends on
the presence of salts hosting the cations. The water soluble portion of trace metals refers to the detached cations from their crystalline mineral structure or to anthropogenic loadings of the metals [
The result of the factor analysis for the water extraction and residual data is presented in Tables 3 and 4. The loadings of greater than 0.60 in the initial and rotated matrix are marked bold in the tables. The first factor components of water extraction data accounted for 33.52% of the total variance respectively, while having high loadings on the elements Mn, Cr, As, Co and Zn. About 42.66% of the total variance is accounted for by the first factor in the residual data with higher loading of Na, K, Mn, Cr, Cu, Pb and Ni in the lithogenic media. This indicates the influence of local anthropogenic activities such as the cement factory and traffic movement [20-22]. It was noted that Cr and Pb in both media as well as Mn in the soil media have lowest water soluble proportions but are grouped under the anthropogenic factor. Moreover, the association of Ni and Pb with K was previously found to likely be due to anthropogenic factors [
The second factor for water soluble data with 22.29% and for the residual data with 24.49% of the respective variances was composed by Na, Fe, Cu, Pb and Ni as well as Ca, As, Co, Zn and Mo loadings respectively. This factor implies high loadings of both natural and anthropogenic sources [
Extraction Method: Principal Component Analysis. Rotation Method: Varimax with Kaiser Normalization. *Rotation converged in 7 iterations. **4 Components extracted.
Extraction Method: Principal Component Analysis. Rotation Method: Varimax with Kaiser Normalization. *Components extracted. **Rotation converged in 9 iterations.
anthropogenic activities, where as factor 2 represents the contribution of metals (Na, Fe, Cu, Pb, Ni, Ca, As, Co, Zn and Mo) from both lithogenic and anthropogenic origins. Factor 3 consists of Ca, Mo, Mg, and Fe from geogenic source of the local geology.
Spatial distribution of the metals suggested that cement factory dust and traffic emissions represents the most important pollutant source for the investigated area (Figures 6 and 7). The soil samples collected from the north western side of the cement factory towards which the wind predominantly blows is characterized by comparable water soluble values of Na, K and Mn with stream sediment down stream side of the main road (Figures 6(a), (b) & (f)). The farm soil has higher Na and Mn in the residual as well as higher Ca and Mg in the water soluble values that decreases down stream. This spatial trend is accompanied with comparable Ca and Fe and increasing Mg residual values downstream implying the effect of cement dust on the soil and doleritic rocks on the bank of the creek (Figures 6(a), (c)-(f)).
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With regard to the trace metals, the soil has higher Cr, As, Co and Ni in the residual and lower Cr and As in the water soluble fractions than the stream sediment does. The farm land has higher Mo in water soluble and comparably lower Cu and Pb in both fractions relative to the stream sediments (
Based on the results of this research, it can be concluded that the source of emission on the study area not only comes from the cement factory, but may be influenced by emissions of the trucks transporting cement and the general traffic flow on the main road. The soil on the farm land investigated is heavily contaminated with Cr, Co, Mo and Ni while the stream sediment in the creek below the main road is moderately polluted with Pb, Cu and Zn. On the other hand, according to the factor analysis results, both water soluble and insoluble phases of the samples revealed anthropogenic factor for Mn, Cr, As, Co, Zn, Na, K, Cu, Pb and N and geogenic factor for Ca, Mo, Mg, and Fe from both lithogenic and anthropogenic origins while Na, Fe, Cu, Pb, Ni, Ca, As, Co, Zn and Mo are defined by factor of both natural and human induced sources of metals. The sever contamination is evident to the western and north western direction towards which the wind predominantly blow from the cement factory and near the main road decreasing in the down stream direction. The research has clearly determined that the anthropogenic activities have clear impact on the physicochemical characteristics of the lithogenic environment through the dust emission.
The author wish to express his deep gratitude to the geochemical laboratory staff of Earth Sciences Department in Mekelle University for diligently and carefully analyzing the soil samples.