Transitional coastal environments such as the Galician Rias in the Atlantic coast of NW Spain are densely populated areas. Their environmental problems are highlighted by the conflicting interests of different economic sectors: extensive mariculture activities are located in its waters and intertidal zone; shipbuilding, carbuilding, canning and other industries compete with tourism on their shores; and dairy farming is the main agricultural activity in its surrounding hills and hinterland (Vilas et al., 2008). As a result, the management of the coastal zone is highly complex and it is difficult to balance quality of coastal waters with economic activities. For instance, in the Ria de Vigo, the southernmost of the Rias Baixas, wastewater treatment plants were not installed until the 1990s, and in spite of regional environmental legislation (Lei 8/2001), their capacity was still insufficient in 2005 when the European Court of Justice found Spain guilty of failure to fulfill its obligations under the Article 5 of the Council Directive 79/923/EEC on the quality required for shellfish waters (Case C-26/04 ECJ). This case was closed following Spain's submission of a pollution-reducing programme specifically pertinent to shellfish waters; success of this plan will depend critically on the behaviour of the sediments on the ria bottom. Galician Rias experience seasonal upwelling, which increases marine productivity. This promotes the deposition of high organic matter contents in the bottom sediments and contributes to the observed intense sedimentation rates of 1-6 mm yr-1 (Alvarez-Iglesias et al., 2007; Rubio et al., 2001). Current levels of trace metals (Prego & Cobelo, 2003) in sediments of these Rias have caused a significant concern by local and European authorities in the last ten years, especially in relation to the application of the Water Framework Directive (WFD), aimed to ensure that all waters reach “good status” by 2015. Some of these studies (Alvarez-Iglesias et al., 2003; Belzunce-Segarra et al., 2008; Rubio et al., 2000a) showed that the highest concentration of trace metals occurs in the muddiest surficial