Agricultural soils irrigated with acidic mine water: acidity, heavy metals, and crop contamination

Agricultural soils irrigated with acidic mine water from the Guangdong Dabaoshan Mine, China, were investigated. The pH of the soils could be as low as 3.9. However, most of the mineral acids introduced into the soils by irrigation were transformed to insoluble forms through acid buffering processes and thus temporarily stored in the soils. Different heavy metals exhibited different fraction distribution patterns, with Zn and Cu being mainly associated with organic matter and Pb being primarily bound to oxides (statistically significant at P = 0.05). Although the mean of exchangeable Cd was greatest among the Cd fractions, there was no statistically significant difference between the exchangeable Cd and the oxide-bound Cd (the 2nd greatest fraction) or between the exchangeable Cd and the carbonate-bound Cd (the 3rd greatest fraction). It was also found that there were generally good relationships between the concentrations of various Zn, Cu, Pb, and Cd fractions and pH, suggesting that a major proportion of each heavy metal in the soils was mainly derived from the acidic irrigation water. The results also show that the crops grown in these soils were highly contaminated by heavy metals, particularly Cd. The concentration of Cd in the edible portions of most crops was far in excess of the limits set in China National Standards for Vegetables and Fruits and this can be attributable to the extremely high transfer rate of Cd from the soils to the crops under the cropping system adopted in the study area.