Investigation of distibution pollution and source of heavy metals in the agricultural soil of Bardsir Basin (Kerman Province)

The geochemical characteristics of materials in the environment are related to the chemical properties of the sources. In other words, the concentration of heavy metals in the soil depends on the type and chemistry of the parent rocks, which has formed the soil through the weathering processes and has led to different concentration of heavy metals in the soils. In addition to the composition of the parent rocks, a variety of natural and man-made factors are effective in increasing element concentration in the environment which causes intense pollution. Therefore, in short, the earth has a direct impact on human health through the food chain (eating and drinking) and the inhalation of dust and gases. Thus, the main purpose of this study is investigation and monitoring the arsenic, antimony, lead and cadmium concentrations and plotting the map of pollution distribution in the agricultural soil of Bardsir catchment.

Position of sampling points is determined according to expert judgment based on previous research, topographic maps, geology, satellite images and field study. These were selected to obtain the suitable distribution and zoning map of the study area. Also, characteristics identify the effect of geology on the pollution of the study area. 105 samples of composed agricultural soils were collected by averaging method from less than 10 cm in depth to prevent the potential effect of anthropogenic pollutants. Samples were sieved with a 2 mm sieve and particles smaller than 200 mesh were sent to the laboratory for 4-element chemical analysis by ICP-MS method. Statistical analysis was performed to measure the concentration of heavy metals, index of geo-accumulation, degree of pollution, risk potential factor and ecological risk.

The results show that agricultural soils are polluted by arsenic and antimony (14% in terms of arsenic and 29.5% in terms of antimony), more than allowable levels in some parts of the study area. Statistical analysis also verified that only As and Sb show low to intense pollution while the contamination and risk of Cd and Pb are low. The As zoning map shows that the contaminated agricultural soils are located in the center and north (basin outlet) of the study area. This may be related to the irrigation with polluted river, which is located in the downstream of volcanic outcrops, or to the synergy of different polluted waters at the outlet of the area or flood irrigation. These can transfer the dissolved pollutants from upstream to agricultural land. The Sb concentration zoning shows that the agricultural soils with the highest pollution are located in the upstream and near outcrops including south, west, east, and center of the study area, which is due to the low solubility of antimony compared to arsenic.

The overall results indicate that the agricultural soil is polluted to arsenic and antimony in some areas. Evaluation of the origin of these elements showed that the pollution has mostly geogenic resources and is derived from alteration and Cenozoic volcanic outcrops while anthropogenic pollution showed a small contribution to pollution. The transport and re-precipitation of heavy metals is controlled by dissolution- precipitation and adsorption-desorption reactions, and its transporting is controlled by oxyhydroxides of these elements in the study area.