Can bioturbation be responsible for thicker freshwater lenses than expected in littoral environments?

Abstract

A study of hydrological and lithological characteristics that allow the development of freshwater lenses in littoral ridges underneath clay sediments of the coastal plain was undertaken, considering as a case of study the freshwater lenses located in the coastal sector of Samborombón Bay (Argentina). Geological characteristics were studied at a regional scale, as well as detailed sedimentological and ichnological aspects. Thickness of the lens and the freshwater–saline water interface were determined by salinity measurements in exploration wells located at different depths. Results obtained suggest that the infiltration of rainwater into the littoral ridges causes the formation of freshwater lenses that deepen below the clayey layers of the coastal-plain deposits. The key to answering this question are the crustacean burrows, which regionally occur in the fine sediments of the coastal plain. The burrows create an interconnected system of galleries and tunnels filled with sandy-silt material that favours seepage. Because the alternation of coastal-plain deposits and littoral ridges is a frequent situation throughout the world, the data provided by the present work do not represent just a simple case study; on the contrary, they show that biological structures promote bigger freshwater lenses in coastland environments, thus increasing water reserves.