Spatial Distribution of Heavy Metals in Contaminated Agricultural Soils Exemplified by Cr, Cu, and Zn

Some paddy rice fields in central Taiwan had been exposed for more than 20 years to irrigation water containing metal. This study was carried out to evaluate the spatial distribution of heavy metal in soils exemplified by Cr, Cu, and Zn. Four sites were studied, and each site was divided into a regular grid of 5 m×5 m in the sampling process. At the surface, total metal concentrations were not normally distributed, showing a rather positively skewed profile. The concentration of Cr, Cu, and Zn away from the irrigation channel inlet, around 20 m, decreased by approximately more than 40 % with respect to that at the entrances. Findings revealed that a linear relationship between the logarithm of the metal concentration and the irrigation distance for each site was found. Vertically, the content of Cr, Cu, and Zn in soil decreased in two stages with respect to depth. The first stage was from the surface (0–15 cm depth), to the bottom horizon (15–30 cm depth) and the second stage was from the bottom horizon to the deeper horizon (30–45 cm depth). The highest concentrations were observed at the irrigation inlets, and the concentration decreased with increasing distance from the irrigation inlet until 40 m. The lowest concentrations were found at the irrigation outlets. A linear relationship between the logarithmic metal concentration and irrigation distance for each site was found. In the case of soil exposed to irrigation water containing metal, the patent of water flow on the soil surface and in the subsurface was a determining factor in the redistribution of Cr, Cu, and Zn within the sites studied.