Distribution pattern of the heavy metals in stream sediments of the Baycheh-Bagh area, northwest of Zanjan

In order to investigate the concentrations of heavy metals in stream sediments of the Baycheh-Bagh area, northwest of Zanjan province, 131 samples were collected from intended stations. These samples after dissolution by four acid method, were analyzed by ICP-OES. To determine the type of spatial distribution of data, two important spatial statistics, i.e. global Moran’s I and Anselin local Moran’s I were employed. The average concentrations of As, Bi, Cd, Ni, Pb, Sb and Cu in stream sediment samples were 21.83, 0.32, 0.35, 35.57, 68.96, 37.67, 1.96 and 82.71 ppm, respectively which are higher than the average of crust whereas the average concentration of Co is 20.30 ppm. The obtained results indicate that the heavy metals in the study area has a spatial structure and distributed in cluster form.According to the Local Moran’s I results, the most contaminated area in the study area was detected in western part of the Baycheh-Bagh abounded mine and located on the present polymetallic vein-veinlet mineralizations and their related alterations. Analysis of obtained data and investigation of their relationship with the geological rock units of the study area show that concentration of the studied elements have geogenic source and is related to the mineralizations and alterations.

Heavy metals are one of the most important sediment contaminations and are important in low concentrations due to their in solubility and their physiological effects on organisms and human (Zare Khosh Eghbal et al. 2012). In an aqueous environment, sediments are the final destination of heavy metals accumulation and can act as a source of water pollution. On the other hand, mineral resources, especially metal deposits, contain significant amounts of heavy metals and potentially toxic elements. Contact of surface and underground water with the host rocks, ore mineralizations and altered rocks releases various elements, provides a large amount of heavy metals and potentially toxic elements in downstream deposits and surrounded soils. This fact highlights the importance of environmental studies of stream sediments. Spatial statistics is one of the basic concepts of modern sciences in monitoring and analyzing environmental data that focuses on the spatial relationship of data.
Spatial analysis of environmental data is crucial factor in spatial statistics. The first step in spatial analysis is identifying the data dispersal structure. Regarding to the importance of monitoring the spatial changes of environmental data and considering the distribution, arrangement and behavior patterns of variables in environmental topics, this study attempts to investigate the pattern of heavy metals dispersion in the stream sediments of the Baycheh-Bagh area, located in the northwest of Zanjan.

In order to evaluate the environmental contamination of heavy metals in the Baycheh-Bagh area, 131 samples were selected from intended stations. In this step, about 500 gr of stream sediments from a depth of 5-15 cm was selected from each station. After crushing and milling the samples, the obtained powders were dissolved by the four acid solutions and were analyzed by the ICP-OES at Environmental Science Research Laboratory, University of Zanjan. The studied elements in this research include As, Sb, Cd, Pb, Zn, Ni, Co, Bi and Cu. After receiving the analytical results from the laboratory, the data were processed statistically. For this purpose, the statistical parameters such as mean, median, mod, standard deviation, skewness, Kurtosis, range, minimum and maximum were analyzed for the nine mentioned elements using SPSS 22 software. Furthermore, Kolmogorov-Smirnov test was used to examine the distribution of the normal data. In the absence of normal data, the logarithm transformation method was used to normalizing them. To determine the amount of stream sediments contamination to heavy elements in the study area, global Moran’s I and Anselin local Moran’s I were employed. Determination of the degree of dispersion or clustering of features, global Moran’s I was used.
 

  The Global Morans’s I spatial autocorrelation was used to determine the spatial distribution of the data. The results of this analysis indicate that all metals have a strong correlation and cluster distribution. The Moron index for all metals is above 0.9 and among them the strongest and weakest correlation is for Bi and Cd elements, respectively. Accordingly, it can be concluded that the studied heavy metals have a spatial structure and are distributed in cluster form. In other words, the data tend to be concentrated in study area and have been influenced by almost the same factors. The results of the local Moran’s I analysis showed that the Bi and Pb have the highest and lowest high-high points, respectively, and the Co and Pb have the highest and lowest low-low points, respectively. According to the local Moran’s I statistics, the most polluted region of the study area was detected in the western part of the Baycheh-Bagh mine which are located on the polymetallic vein-veinlet mineralizations and related alterations. These metals include As, Bi, Cd, Co, Sb, Zn and Pb which have sheared between high-high points. The analysis of the samples from the stream sediments shows that the source of the concentration of the studied elements is geogenic (resulted from geological processes) and is related to mineralizations and alterations.