Use of finite mixture distribution theory to determine the criteria of cadmium concentrations in Taiwan farmland soils

In the 1980s, the Taiwan Environmental Protection Agency investigated heavy metal concentrations in farmland soils and tried to establish the background and standard for heavy metal content of soils in Taiwan. Traditionally, Taiwan's standard for heavy metal pollution of soils was determined by the toxic effect on plant growth and the potential hazard to consumer health, without analyzing background values. However, local scientific experiments were not sufficient to support policy making in soil pollution criteria. The objective of this study was to use finite mixture distribution to fit sampling results and classify Cd concentrations based on 918 representative Taiwan farmland soils. Three models-Single Log-Normal Distribution Model (SLDM), Double Log-Normal Distribution Model (DLDM), and Triple Log-Normal Distribution Model (TLDM)-were examined in this study, and the results showed that TLDM was more appropriate to describe the Cd distribution in soils than either DLDM or SLDM. The probability distribution of Cd concentrations could be classified into three categories: virgin, affected, and polluted soils, with respective proportion of 80.13%, 19.45%, and 0.42%. The criteria of separation were 0.14 mg/kg to separate virgin and affected soils, and 0.64 mg/kg to separate affected and polluted soils.