Modeling Cadmium Accumulation Trend in Wheat Farm Soils

Accumulation of heavy metals in soils causes many environmental problems worldwide. Therefore, evaluating soil heavy metals accumulation rate is crucial for preventing soil and environmental contamination. The objectives of this study were to model soil Cd accumulation trend and to assess soil contamination with cadmium (Cd) in wheat (Triticum sp) farms of Khuzestan province. Soil Cd accumulation rate was modeled using a stochastic mass balance model. In this model, long term information (27 years) on input parameters (agriculture, atmosphere), that cause Cd to enter into the soil, and output parameters (plant uptake and leaching), that influence Cd to exhaust from the soil were collected. The effects of input parameters uncertainties on model outputs based on different parameter distributions were evaluated using Latin hypercube sampling and stepwise regression. A number of 250 farms (within the total study area of 355 thousand hectares) were selected. In each region, regarding the area size and wheat farms distribution, compound soil (0-20 cm) and seed samples were collected during wheat maturity period. The soil Cd concentrations (total and DTPA-extractable) and Cd seed samples as well as some soil properties were measured. Spatial soil and seed Cd analysis were conducted by ordinary Kriging method. Based on actual distribution, the maximum soil Cd accumulation rate was calculated at Dezful region (14.38 gr ha-1 yr-1), while the minimum rate was obtained for Bagh-Malek (3.00 gr ha-1 yr-1) region. Regarding the uniform distribution scheme, the highest Cd accumulation rate was obtained at Behbahan region (56.30 gr ha-1 yr-1) and the lowest at Bagh-Malek (8.40 gr ha-1 yr-1). Analysis of model outputs reveled that the phosphorous fertilizers had main influence on the uncertainty of the stochastic model output. Modeling results using uniformly distributed parameters was not very sensitive to uncertainty of parameters and ignored most parameters. The results indicated that the total Cd concentrations in 95 percent, DTPA-Cd concentrations in 25 percent, and grain Cd in 95 percent of samples exceed 0.80, 0.10, and 0.20 mg kg-1, respectively. The total Cd concentrations were significantly correlated with TNV, P-available (p0.01), CEC, and organic carbon (OC) (p0.05). The DTPA-extractable Cd had significant correlation with OC, pH, and clay content (p0.05). The significant correlations were also obtained between grain Cd concentrations and OC, CEC (p0.01), and DTPA-extractable Cd (p0.05). The obtained Kriging maps indicated that total Cd at the south and the west, DTPACd at the east and the south, and grain Cd concentration at the east, at the west and some parts of south area were hot spots. It seems that the application of phosphorous fertilizers, rotation, soil properties, war contamination, and wheat variety are main factors responsible for high Cd concentration in seed and soil. It can be concluded the application of phosphorous fertilizers was the most important source of entering Cd to the soils of wheat farms