Physical and Chemical Changes in Estuarine Salt Marsh Lands Due to Reclamation (Bay of Biscay, North Coast of Spain).

Many salt marshes on the European Atlantic sea board were drained during the 19th century and the first half of the 20th century and reclaimed for urban development or agriculture. In despite of this, claimed areas do not remain fully isolated from estuarine dynamics and tidal influence can be felt differently in accordance with topographic and geomorphological dynamics governing these areas. Drained salt marsh sediments suffer slow and continuous physico-chemical changes which caused develop of immature soil profiles and processes as desalination, acidification and oxidation. These transformations have given rise to serious environmental problems as development of acid sulphate soils, toxicity and solubilisation of metal cations and the release of large amounts of CO₂ into the atmosphere.
In order to know the transformations suffered by saltmarshes after reclamation we have studied the morphological, physical and chemical properties of 14 sediment cores sampled in natural mud flats and 31 profiles of soils developed over drained mud flats along four estuaries (Villaviciosa, San Vicente; Foz and Urdaibai estuaries) located at Bay of Biscay ( North coast of Spain).
In order to identify the reclaimed areas, natural and human elements in the estuaries have been mapped and zoned in accordance with a susceptibility criterion to flooding (flooded areas at every full tide and areas flooded only during equinoctial tides and river flooding). This flood-zoning was used as a framework within the sampling was designed.
Data coming from the study and analysis of sediments and soil profiles allow to assess a different evolution of the estuarine mud flats after drainage; although the same trend has been observed at the four estuaries studied, differences arising from the geological substrata along the whole river basin stress the magnitude of the processes, in which deep changes in both morphological and chemical properties have occur.