Character of Submarine Groundwater Discharge (SGD) at the Coast of Mallorca Island

Preliminary results of geochemical tracers indicated that submarine groundwater discharge (SGD) is prevalent around the Mallorca coast, Spain; SGD would be an important source of trace metals and phosphorus to the coastal ocean. During September 2006, vented benthic chambers (a.k.a. seepage meters) were used at four diverse sites along the coast (Es Caragol, Porto Colom, Santa Ponça and Sóller) to directly measure SGD in order to investigate the characteristics of SGD. Mallorca is an example of a (Miocene) carbonate platform. The site at Sóller on the northeast coast is located on a wide stretch of beach in an embayment surrounded by part of the high relief of the Tramuntana Range; the other three sites were in areas of relatively low relief. The average rate of groundwater seepage at the four sites ranged from 3 to 19 cm d-1, being highest at Es Caragol, a wide embayment on the southern coast of Mallorca. All sites were characterized by an irregular, spatial distribution of seepage which was dominated by low values, less than 15 cm d-1 in most locations, but punctuated by high values (up to 65 cm d-1) in discrete locations. This characteristic was best represented by the observations at Sóller, where one device documented a consistently higher SGD, averaging 52 cm d-1, while the rest of the study area had an average SGD of 6 cm d-1. Such an irregular distribution would be expected to characterize fractured rock or karstic aquifers. There was little or no tidal modulation of the SGD which suggested to us that the driving forces adjusted, for the most part, to the tidal elevations in open water. In addition, the water seeping across the sediment-water interface showed little or no dilution from the ambient, open-water salinity, in fact, in one case, the salinity of the SGD was greater than ambient sea water. SGD at these locations appeared to be dominated by the recirculation of sea water through the aquifer, driven by oceanic processes. This work was supported by the Balearic Island Government (Govern de les Illes Balears, Econom Hisen I Innov; Núm: 8171/2006: 6014/2007).