Geochemical analysis of surface sediments along the Portuguese Margin

Twenty-nine surface samples from the Portuguese shelf, recovered offshore from the mouths of the Ave, Douro, Lis and Mira rivers, were analysed using ICP-OES for selected major and trace elements, after total dissolution. Organic carbon, carbonate content and grain size were also determined. Five evaluation tools have been applied in order to compare the three study areas and to evaluate sediment geochemistry and other sediment compositional variability in the acquired samples: (1) empirical methods based on comparison with standard reference criteria, e.g. the NOAA sediment quality guidelines, (2) normalisation ratios using a grain-size proxy element, (3) "Gradient Method", plotting contaminant vs. organic matter or Al, (4) definition of a regional geochemical baseline from a compiled database, and (5) enrichment factors.
The evaluation of element and component associations indicates differences related both to the onshore drainage areas and to the environmental shelf setting. Despite the considerable variability in total metal contents indicated by our results, the sediment metal composition is largely of natural origin. Metal enrichments observed in the Mira area are associated with the drainage of mineralised areas rich in Cu, Pb, Zn, Fe and Mn. The near absence of human impact on shelf sediments, despite the vicinity to urban areas with high industrialisation levels, such as the Ave-Douro and Lis areas, is attributed to effective trapping in the estuaries and coastal zones, as well dilution with less contaminated sediments shelf sediments and removal with fine fractions due to grain-size sorting. The character of the contaminated sediments transported to these shelf areas is further influenced by grain-size sorting as well as by dilution with less contaminated marine sediments. The results obtained individually by the different methods complement each other and allow more specific interpretations.

本研究对葡萄牙陆架采集的29个表层沉积物样品开展分析，这些样品采自阿威河（Ave）、杜罗河（Douro）、利斯河（Lis）以及米拉河（Mira）河口外海域。样品经全消解处理后，采用电感耦合等离子体发射光谱法（ICP-OES）测定了选定的常量元素与微量元素含量；同时还测定了样品的有机碳、碳酸盐含量及粒度参数。本研究采用5种评价方法对采集的样品进行分析，以对比3个研究区域的沉积物特征，并评估沉积物地球化学组成及其他组分的变异情况：（1）基于标准参考准则的经验法，例如美国国家海洋和大气管理局（NOAA）沉积物质量指南；（2）以粒度替代元素为基准的归一化比值法；（3）“梯度法”：以污染物与有机质或铝（Al）元素为变量进行绘图分析；（4）基于汇编数据库构建区域地球化学基线值；（5）富集因子法。
对元素与组分组合特征的评价结果显示，沉积物差异既与陆上汇水区有关，也受陆架环境背景的影响。尽管本研究结果显示金属元素总含量存在显著变异，但沉积物中金属组成整体源自自然成因。米拉河海域观测到的金属富集现象，与富含铜（Cu）、铅（Pb）、锌（Zn）、铁（Fe）以及锰（Mn）的矿化区域的径流输入有关。尽管阿威-杜罗河与利斯河附近区域属于工业化水平较高的城区，但陆架沉积物几乎未受人类活动影响，这一现象可归因于河口与近岸带对污染物的有效截留、低污染陆架沉积物的稀释作用，以及粒度分选作用导致的细粒组分迁移移除。输入至上述陆架区域的污染沉积物的特征，进一步受到粒度分选作用以及低污染海洋沉积物稀释作用的影响。不同评价方法单独得到的结果可互为补充，从而可开展更具针对性的解释。