Content and bioavailability factors of soil heavy metals in mudflat coastal areas

The coastal zones with tidal mudflats have the most development potential for agriculture and industry in China. It is therefore vital to study the spatial patterns of soil heavy metals and to identify the intrinsic relationship between the spatial patterns of soil heavy metals and the natural and human driving factors in coastal regions. Using classical statistical and geostatistical methods, this study investigated the effect of land use types on the contents of total and bioavailable heavy metals (Pb, Cr, Cd and As) in surface soil not only to delineate the spatial distribution of these forms of soil heavy metals, but also to determine the relationship between contents of bioavailable heavy metals and basic soil physical and chemical properties. The study was conducted in a coastal area in Jiangsu Province in China-a rapidly developing industrial and agricultural region. Results indicated that soil environmental quality across the study area was generally good. The contents of soil Pb, Cr, Cd and As were lower than the accepted environmental quality evaluation standard (the first class), although there was an increasing trend of accumulation of each heavy metal in recent years. Also environmental risk and ecological toxicity of soil bioavailable Pb, Cr, Cd and As in the study area was not significant. Land use type affected total contents and bioavailable contents of Pb, Cr and Cd to different degrees. Greenhouse facility soils and greenery park soils had the highest contents of total and bioavailable Pb. Also soil contents of total and bioavailable Cr were high in residential and industrial areas, whereas greenhouse facility soils had the lowest total and bioavailable Cd contents. Land use patterns had little impact on total and bioavailable contents of As. In addition to land use type, biological absorption was another important factor affecting the contents of soil heavy metals in the study area. Total soil Pb, Cr and Cd and bioavailable Pb and Cr had significant directional trend across the study area. Spatial pattern of the contents of total and bioavailable soil heavy metals was generally influenced by various factors at different scales. The whole spatial distribution of soil heavy metal was a strip-like pattern due to large-scale tidal deposits. Patched local distributions were mostly controlled by small-range factors such as industrial emissions and human activity. Soil bioavailable Pb, Cr and Cd exhibited significant negative correlation with soil clay particle content, cation exchange capacity and pH. Because of the high content of fulvic acid of soil, a positive correlation existed between bioavailable soil Pb, Cr and Cd contents and soil organic matter content. Sol bioavailable As content was positively correlated with soil pH, but there was no significant correlation between soil bioavailable As and other soil physical and chemical properties. The correlation between bioavailability of soil heavy metals and soil basic physico-chemical properties was critical for developing appropriate management practices that control heavy metal pollution in coastal mudflat regions. The findings of this research provided additional scientific basis for source reduction, ecological activity passivation, pollution abatement and risk prevention of soil heavy metals in mudflat coastal areas.