Influence of airborne dust on the metal concentrations in crop plants cultivated in a rooftop garden in Seoul

Plants grown in urban areas are likely to be exposed to heavy metals from industrial activities and traffic. In particular, Seoul, the capital city of Korea, is affected by seasonal winds from northwest China, named the Asian dust wind, which contains various heavy metals and probably causes heavy metal deposition on plants growing in rooftop gardens in Seoul. Growing media and plants from 18 rooftop gardens distributed throughout Seoul were examined in this study to determine the magnitude of heavy metal (cadmium (Cd), copper (Cu), chromium (Cr), nickel (Ni), lead (Pb) and zinc (Zn)) exposure in the urban agricultural environment. The heavy metal levels in all of the growing media examined were below the Korean standard limits of soil, although the Cu and Zn levels were elevated due to excessive use of livestock-based composts. Heavy metal concentrations in plants were also comparable with the natural levels. Compared to unwashed plants, washing reduces the hazard quotient of heavy metals by 17-28%. This was due to deposition of airborne dust on the plant surface. Cu, Pb and Zn deposited on the plant surface were attributed to the urban environment such as industrial activities and traffic, while Cr and Ni were attributed to the Asian dust wind carrying the corresponding heavy metals from China to the northwest of Seoul.