2.4.5. Enrichment Factor (EF)

Enrichment factor (EF) is an effective tool to evaluate the pollution degree of heavy metals driven by anthropogenic or geologic forces in sediments [45,46,47]. It is a normalization method suggested by Sinex and Helz [48] and is used in standardizations of the acquired heavy metal content in sediments with respect to a reference metal that is either Fe or Al [49]. In this study, Fe is used as a reference metal for geochemical normalization. The EF is calculated by the following formula:

where C_n is the measured concentration of heavy metal, C_background is its corresponding background value and Fe_n and Fe_background are the concentrations of Fe in the sample and its corresponding background value, respectively. The sediments could be divided into six classes based on the EF value: EF < 1.5, no enrichment; 1.5 < EF < 2, slight enrichment; 2 < EF < 5, moderate enrichment; 5 < EF < 20, severe enrichment; 20 < EF < 40, highly severe enrichment; and 40 < EF, extremely severe enrichment [50].