Delineating the saltwater wedge in two carbonate aquifers in Mallorca ( Spain )

Delineating the actual location and shape of the freshwater-saltwater transition zone is a prerequisite for long-term prediction of groundwater quality in coastal regions. Two sites with saltwater intrusion problems in Mallorca are currently investigated within the framework of the European research project ALIANCE. Whereas the extent of the saltwater intrusion is well known and easy to understand for one of the sites (Campos), the saltwater intrusion problem of the second site (Pollensa) is far more complex and much less understood. The Campos site is located in the south of the island in Miocene reefal units. The aquifer is affected by saltwater intrusion over an estimated surface area of 150 km. The Pollensa site is situated in the north-eastern part of the island in a karstified and fractured limestone of Jurassic Age. FIRST FIELD CAMPAIGN The first field campaign in February 2002 consisted mainly of logging existing wells for electrical conductivity and temperature. A total of 22 holes were logged in both the Campos and the Pollensa site. An important aspect of these existing wells is that all of them are equipped with long screened interval, usually over the entire saturated thickness of the aquifer. The conductivity profiles at the Campos site were more or less as expected from the hydrogeology. Fresher water overlies saltwater with a conductivity of 53000 μS/cm at 25°C, which is the conductivity of the Mediterranean Sea (Figure 1, right profile). At the Pollensa site, conductivity profiles were harder to interpret. The average conductivity profile recorded showed a very straight line of lower conductivity (1500-2000 μS/cm) in the upper part of the screened interval, followed by a steady increase to about 7000 μS/cm in the lower part of the screened interface (Figure 1, left profile). A separation of freshwater and saltwater could not be observed. It seemed unlikely that this reflects the natural groundwater situation, as these boreholes are all close to the main recharge area of the island, the Serra de Tramuntana. We hypothesised that recordings at the north-eastern site were affected by ambient flow within the boreholes, caused by different piezometric heads within the long screened well sections. The fully screened boreholes are supposed to shortcircuit different fractures, respectively high transmissive zones with different hydraulic heads, at various depths. Consequently, the natural formation fluid distribution is no longer visible or even no longer existent. Insight into the real saltwater wedge extension is therefore difficult to obtain with such a classical approach. TESTING-WHILE DRILLING OPERATION For this reason a testing-while-drilling procedure was developed and applied during drilling of the first test hole in September 2002, called MP1 (Mallorca Pollensa 1), to a depth of 100 m. Sulphorhodamine B tracer was added to the drilling fluid with an average concentration of 200 ppb to allow for back-calculation of the groundwater The Second International Conference on Saltwater Intrusion and Coastal Aquifers — Monitoring, Modeling, and Management. Mérida, Yucatán, México, March 30 April 2, 2003