Paula Álvarez-Iglesias

Redox status and heavy metal risk in intertidal sediments in NW Spain as inferred from the degrees of pyritization of iron and trace elements.

Mariculture is an important economic activity in shallow marine areas of the Rías Baixas (Galicia, NW Spain). The maintenance of high product quality requires surveillance of environmental quality, including the risk of metal toxicity. In this study the redox status of intertidal sediments in the Bay of San Simón, and the risk of toxicity posed by their As, Cd, Cr, Cu, Mn, Ni, Pb and Zn contents, were evaluated by determination of operationally defined reactive, silicate-bound, organic and pyrite-related fractions of these elements and of Fe. The large silicate-bound fractions of most of these metals indicate their lithogenic origin; the main exception is Pb, which in all respects exhibits singular behaviour associated with its predominantly anthropogenic origin in a ceramics factory. In sediments with larger fine-grained particle contents, which are oxic only in the top few centimetres, greater proportions of the trace elements are present as sulphides or associated with sulphide minerals: the degree of pyritization of Fe (DOP) is 46% overall, and the pyrite fraction of some elements doubtless increases at the expense of the reactive fraction, most overall degrees of trace metal pyritization (DTMPs) lying in the range 10-50%. A decline in pyrite genesis at depths below about 18 cm in these sediments is attributed to the exhaustion of organic matter susceptible to metabolization by sulphide-generating bacteria. In coarse-grained, oxic sediments the oxidation of sulphides makes pyrite-related fractions very small, and reactive and silicate-bound fractions are negatively correlated; reactive fractions associated with Fe-Mn oxyhydroxides are large, and DOP and DTMPs are low (generally much lower than in mud-rich sediments), except for Pb. Most of the elements studied are mainly present in forms that are neither bioavailable nor potentially bioavailable, and so do not constitute a significant environmental threat. However, the high DTMPs of Cu and Pb indicate significant potential bioavailability, which should be taken into account in evaluations of environmental quality and the risk to bivalve cultures.

The degree of trace metal pyritization in subtidal sediments of a mariculture area: application to the assessment of toxic risk.

Four 2-3m sediment cores were taken at the sites on the periphery of mussel raft concentrations in the subtidal zone of the inner Ría de Vigo (Galicia, NW Spain) with a view to evaluate the potential risk to mariculture from sediment-borne trace elements (As, Cd, Cr, Cu, Mn, Ni, Pb and Zn). The distribution of each of these elements in reactive, organic, pyrite and silicate-bound fractions was determined at 64 samples, and these data were used to calculate the degree of trace metal pyritization (DTMP) of each metal/metalloid. In the top 10-20 cm, relatively oxic conditions led to As, Cd, Cu, Pb and Zn having large reactive fractions due to their association with Fe and Mn oxyhydroxides. At lower levels, anoxic conditions favoured by intense diagenesis led to the precipitation of trace metals and metalloids as sulphides, with or without association with pyrite. Particularly large pyrite fractions in the 20-100 cm layer are attributed to the organic matter of this layer being more marine in origin than that of deeper sediments. DTMP was greatest for Cu and As, and least for Pb, Zn and Cr. The risk of trace element toxicity in the event of disturbances instituting oxic conditions in these sediments is discussed.

Diagenesis and anthropogenesis of metals in the recent Holocene sedimentary record of the Ría de Vigo (NW Spain)

To contribute to establishing a historical baseline for future evaluations of the environmental quality of the Ría de Vigo (NW Spain), we report metal contents determined in eight gravity cores obtained at representative locations in 1990 - before the installation of any wastewater treatment plants - and in fractions thereof that were obtained by the BCR sequential extraction method. The results suggest that early diagenesis had been influenced by both hydrodynamic conditions and organic matter input (the latter especially in areas devoted to mariculture), and that anthropogenic inputs had also affected the concentrations and between-fraction distribution of Cu, Pb, Zn and other metals in surficial sediments.

Low-level gamma spectrometry for pollution assessment in San Simón Bay (Vigo, Spain)*

A gamma spectrometer with HPGe detector of 50% relative efficiency and 1 cps total background has been dedicated to the measurement of an intertidal sediment core from a coastal environment at the Ria de Vigo (Spain). The area is affected by lead pollution and the source identification needs of a precise dating of the sediment core. Such a precise dating requires the measurement not only of the radionuclides directly involved in time calculation, as 210 Pb and 226 Ra, but also of ancillary radionuclides which inform about the dating model to apply and about the validity of its time estimation. Gamma spectrometry with Ge detectors performs a simultaneous measurement of the full content in γ-emitters of the sample. However, its use is limited by its high spectral background. We present the characteristics of our low- level background gamma spectrometer and also of Galea, the computing tool for the expert analysis of natural radionuclides. Both make possible to get the proper experimental results to reach a suitable dating. The results allowed us to detect a change in the sedimentation dynamics in the area under study, to verify the impact of lead pollution in the 210 Pb level, to obtain a sedimentation rate by using the CF:CS model with a suitable correction factor and, finally, to validate the sediment dating.

Trace Metals in Shallow Marine Sediments from the Ría de Vigo: Sources, Pollution, Speciation and Early Diagenesis

The maintenance of the environmental quality of coastal environments requires a good knowledge on them, identifying their potential problems and considering the different anthropogenic activities that take place in these settings. A great proportion of the suspended material that arrives to these environments is incorporated to the bottom sediments. Sedimentation is favoured due to the abrupt changes that are produced in the physico-chemical parameters (pH, Eh, salinity, etc.) by the confluence of continental and marine waters. Their sediments constitute environmental archives, recording trace element inputs to the marine environment. Metal concentrations in these sediments usually surpass in several orders of magnitude those existing in the adjacent water column and in the interstitial waters (Tessier & Campbell, 1988). Their analysis allows covering different objectives, such as studying the spatial and temporal history of pollution of a particular place (Zwolsman et al., 1993), detecting pollutant sources (Dassenakis et al., 1996) and evaluating their potential effects to the organisms (Fichet et al., 1998). Sediments can act as a secondary source of pollution by resuspension or dissolution (early diagenesis) processes. Their study needs a multidisciplinary approach, taking into account the different physicochemical processes operating in the study area and considering the relationships between metal concentration, mineralogy, grain-size and metal sources. All these variables are reviewed in this chapter, showing examples of sediments from a shallow transitional environment: the Ria de Vigo (NW Spain).

Factors Controlling the Incorporation of Trace Metals to Coastal Marine Sediments: Cases of Study in the Galician Rías Baixas (NW Spain)

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 Spains 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