Groundwater flow in mostly homogeneous coastal aquifers has been widely studied recently. Salt water with tidal fluctuation leads to transient variable-density flow in a sloping aquifer-ocean interface with a mixing zone affected by e.g., aquifer heterogeneity, tidal amplitude and density contrasts. In particular, the tidal fluctuation is able to change the salt distribution, contaminant travel time and pathway through the aquifer. The tidal range is controlled by many transient factors which act to change the tidal amplitude over time. The research on contaminant transport in these zones, especially with aquifer heterogeneity, a sloping land-ocean boundary and tidal forcing remains limited. To quantify the effect of different tidal amplitudes etc on the contaminant flows, a numerical modelling study is developed. With tidal amplitude increasing, the upper saline plume becomes larger, which leads to the increase of contaminant travel time and changes of the pathway. In layered heterogeneous aquifer system, low-permeability layer in the tidal mixing zone reduce the salt distributions and contaminant concentration discharge to the ocean, while increasing contaminant travel time compared to the unconfined homogeneous aquifers.