Submarine groundwater discharge on the algarve shelf: Case study of albufeira-ribeira de quarteira aquifer system

Abstract

In the Algarve region (S of Portugal) occur several coastal aquifer systems which geometry and structure of the water bearing formations, groundwater flow, and water balance, are favorable to the existence of submarine groundwater discharges (SGD) on the shelf. In the aquifer system of Albufeira-Ribeira de Quarteira, one of these coastal systems, several freshwater discharges were identified on the Algarve shelf up to 5.6 km from the coastline. Two submarine springs closer to the coast (120 m south from the intertidal springs of the Olhos de Água beach) were selected, among others, for sampling and field parameters monitoring such as electrical conductivity and temperature by divers. The discharges were characterized from the hydrogeochemical point of view and compared with the characteristics of the onshore aquifer system. SGD have a Cl-Na facies, with seawater fractions of 25 % and 29 %, electrical conductivity of 9000 µS/cm, temperature of 19.8°C, pe of 6.65 and 6.91, pH of 6.34 and 7.16, being close to equilibrium in relation to carbonates, and a source of nutrients for seawater contributing with nitrate, ammonium and silica. The perception that aquifer systems do not end on the coastline lead us to new challenges in its management, including redefinition of the reservoir, sustainable onshore abstraction, offshore reserves and maintenance of coastal and marine dependent ecosystems.