Assessing paddy soils contaminated by cadmium in Guilan province using geostatistical methods and GIS

Accumulation of heavy metals in soil has resulted in several environmental problems in different parts of the world. High concentrations of these elements in soils may lead to high absorption by plants and this is the main route into the human food chain and animal and biological cycles. Consequently, investigations are necessary to find out how accumulation of these elements can be prevented that creates soil and ecosystem contamination. Among heavy metals, cadmium is one of the most dangerous due to its high mobility in biological systems and ability to accumulate in plant tissues without causing any visible symptoms. Considering the high mobility of Cd in acid soils and the optimal growth of rice in such soils, and according to the role of rice in public health, this study was conducted to assess of the contamination of paddy soils with Cd in Guilan Province. For this purpose, 100 soil samples were randomly taken from 0-20 cm depths using GPS devices from different paddy farms. The soil samples were then passed through a 2-mm sieve after drying. The total cadmium concentration and some soil characteristics such as EC, pH, bulk density, particle density and soil organic matter were then measured. The spatial distribution of cadmium was investigated using different geo-statistical methods including IDW, Spline, and Kriging with a GIS software. The results of the soil physical and chemical analyses showed that the average pH of the soils and saturated electrical conductivity were 6.48 and 1.59 ds/m, respectively, indicating no salinity hazard for rice production. The sand, silt and clay contents of soil samples were between 4 to 35, 28 to 49 and 39 to 65, respectively, so the soil textures were in the range of clay loam, silty clay and clay. These soil textures were moderately heavy to heavy and so are suitable for rice cultivation. The average amount of organic matter was 1.78 and the total Cd concentration before cropping varied between 0.65 to 1.40 and, after harvest, was 1.97 to 11.05. From the geo-statistical methods studied, the ordinary Kriging with spherical model was selected as the best method. According to the results obtained, part of the study area becomes contaminated after harvesting, based on the Kelly index which can show the effect of chemical fertilizers use and incorrect land management, and so the use of these fertilizers should be carried out with care.