The previous studies primarily identify the two types of saltwater encroachment processes like active SI (seawater intrusion) and passive SI (submarine groundwater discharge). Under passive SI, the hydraulic gradient inclines towards the sea, making density and fresh groundwater flow work in opposite directions. Under active SI, the hydraulic gradient inclines landward, and density and fresh groundwater flow work in the same direction, causing more aggressive salinization and wider mixing zones. Mechanical dispersion's effects in controlling the mixing zones in coastal aquifers have become the subject of ongoing debate in SI studies. In this study, numerical experimentations of scaled-tank and a field-scale model were used to explore the influence of longitudinal and transversal dispersivities, separately, on the width of the mixing zone under passive and active SI, in response to inland freshwater head decline. The results show that under steady SI conditions, increasing the longitudinal dispersivity value widened the lower part of the mixing zone while increasing the transversal dispersivity value increased the mixing zone's width both at the lower and the upper part of the saltwater wedge. The same phenomenon as steady-state SI results was observed for passive SI conditions even though with larger effects. While under active SI, both an increase of longitudinal and transversal dispersivities widened the width of the mixing zones but their quantitative impacts are larger compared to those in passive SI conditions. As an instance, when the transverse dispersivity is increased 50 times, the width of the mixing zone at the middle of the saltwater wedge is almost doubled compared to it in the passive case. Ultimately, it is found that under active circumstances, the shear effect of transverse dispersivity pushes the toe position back.