Tidal dynamics and implications for coastal aquifer pollution vulnerability ranking.

Empirical-based vulnerability ranking of coastal aquifers to ocean-emanating influxes at tidal frequencies is not in public domain. Dynamic pattern analysis of coastal aquifers in microtidal and mesotidal settings of the Nigerian coast shows groundwater level oscillation to be mostly sinusoidal as their contiguous surface water body counterparts. This, in concert with sinusoidal conductivity change patterns, suggests periodic intrusion and expulsion of ocean-borne contaminants out of the aquifers at tidal frequencies. The contrasting water level oscillation amplitudes, water level rise/fall rate and flood/ebb duration ratios form the basis for a simple conceptual model of coastal aquifer vulnerability ranking outlined. The asymmetric water level rise rate and flood duration as well as very high tidal cycle-mean groundwater to surface water conductivity ratio of microtidal aquifers suggest poor flushing potentials and thus higher long-term vulnerability to ocean-borne pollutants when compared with the mesotidal counterparts.