Temporal variability of water quality of submarine groundwater discharge in Ubatuba, Brazil

Abstract

Measurements of electric resistivity of the seabed and of the electric conductivity of submarine groundwater discharge (SGD) collected in seepage meters have been made to evaluate temporal variability of the origin and quality of submarine groundwater discharge in Ubatuba, Brazil. A diurnal variation of SGD conductivity was found under the condition of semi-diurnal tidal changes over a period of 4 days. Both the ground resistivity and the conductivity of the discharged groundwater indicated that the freshwater–saltwater interface moved towards offshore at high tide, i.e. in the direction opposite to commonly made observations. SGD comprised a combination of Submarine Fresh Groundwater Discharge (SFGD) of terrestrial origin, and of Recirculated Saline Groundwater Discharge (RSGD) of marine origin. The maximum of the terrestrially derived fraction SFGD/SGD was found at a distance of 50 m offshore. A lower SFGD/SGD ratio was found closer to shore, where the highest SGD flux was measured. SGD conductivity and ground resistivity displayed a diurnal cyclicity at semi-diurnal tidal water level variations, indicating that tidal water level fluctuations may not be the primary driver of SGD flux, but preferential flow in volcanic hydrogeologic setting at Ubatuba.