keywords: Geo-accumulation Index (Igeo), Contamination Factor (CF), Pollution Load Index (PLI), Ecological Risk Index
Environmental contamination of heavy metals has become a serious global issue. Studies on heavy metals source in soils were said to be either natural source or anthropogenic activities. Anthropogenic sources include mining operations, metal processing and smelting operation, chemical production, factory emissions, farming operations, rapid industrialization and urbanization. While the natural sources include factors such as volcanic eruptions and continental dusts, as well as soil parent materials and pedogenetic processes; causing worries over their possible impacts on human health and the environment. Factors influencing the toxicity of heavy metals include the chemical specie, dosage and exposure pathway. This paper review the sources of heavy metal, it toxicity and give detail interpretation of 19 soil contamination modeling indices equations such as the Geo-accumulation Index (Igeo), Contamination Factor (CF), Pollution Load Index (PLI), Metal Pollution Index (MPI), Enrichment Factor (EF), Degree of Contamination (DC), Ecological Risk Index (ERI), Potential Ecological Risk Index (PERI), Percentage Bioavailable and Non-bioavailable Fractions, Transfer Factor, carcinogenic and non-carcinogenic risks, Hazard Quotient, Hazard Index, Cancer Risk (CR), Lifetime cancer risk (LCR), Daily Intake of Heavy Metal (DIM) and Health Risk Index (HRI). In addition, this review addresses the basis of their applications and aid in decision making.