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22 nd Salt Water Intrusion Meeting: Salt Water Intrusion in Aquifers: Challenges and PerspectivesTHE ROLE OF RECHARGE AND MIXING PROCESSES TO EXPLAINFRESH WATER LENS DYNAMICS BELOW A SMALL ISLANDCONDESSO DE MELO, M.T. 1 ; POST, V.E.A. 2 ; WATERLOO, M.J. 3 ;GROEN, M.M.A. 3vincent.post@flinders.edu.au1. Flinders University, School of the Environment2. National Centre for Groundwater Research and Training, Adelaide, AustraliaGPO Box 2100, Adelaide, SA 5001, Australia3. VU University, Faculty of Earth and Life Sciences, Amsterdam, The Netherlands;Abstract. During the 21st Salt Water Intrusion Meeting, Post et al. (2010) presented aconceptual model for the distribution of fresh and saline groundwater below a small sandyisland in the Ria de Aveiro Lagoon in Portugal. The conceptualization was based on fielddata, but relied heavily on numerical modeling as well. The numerical model indicated thatthere is outflow of relatively fresh groundwater near the low water line of the sloping beach,but, at the time, this could not be confirmed in the absence of field data. Here we present newdata that validate the model outcomes in a qualitative sense, and show that the outflowspredicted by the model indeed occur. The results indicate that the presence of a salinecirculation cell below a sloping beach, as well as a ‘tube’ of seaward-flowing, relatively freshgroundwater underneath, is likely to be a general feature of shallow coastal groundwatersystems, even where the recharge area is small. We also provide new data on the transientbehavior of the vertical salinity distribution as observed in multi-level observation wellsduring the summer and winter season, as well as measurements with a high temporal andspatial resolution of the movement of the transition zone during a tidal cycle. Thesemeasurements provide insight into the role of recharge and mixing processes in explainingthe observed shape and position of the transition zone between fresh and saline groundwater.Based on a comparison of these observations with numerical model outcomes, theappropriateness of current process descriptions of mixing in models is discussed.REFERENCES Post, V.E.A., M.J. Waterloo, M.M.A. Groen and M.T Condesso de Melo(2010) Field data and numerical simulation of a fresh water lens below a small island with astrong tidal regime. Procedings of the 21 Salt Water Intrusion Meeting, Azores, Portugal.Keywords: tides; islands; mixing166