Geochemical mapping in Portugal: a comparison study

To interpret geochemical patterns at a European scale it is essential, as a first approach, to find natural features or anthropogenic activities that justify the patterns found at the national scale. This study suggests a methodology for the interpretation of stream sediment data, carried out at different scales. The information is presented at a local scale, at a nationwide scale and its relationship with FOREGS continental scale stream sediments data is discussed.
Basic information such as temperature, precipitation, lithology, geology, mineral occurrences, population density by NUTS V, topography, land use from CORINE and GIS derived layer are used as interpretation tools. Maps displaying element concentration are presented as FOREGS interval suggestions. Maps displaying geoaccumulation index values are also presented.
For elements such as Cr it is possible to observe that, at a nationwide scale, near Porto and near Lisboa, the anomalous concentrations are probably related to anthropogenic activities, while in NE of the country the anomalous concentrations are strongly associated to ultramafic rocks. On the FOREGS mapping, due to the wide scale of sampling, it is only possible to detect the lithological influence.
U concentrations are high in the stream sediments of the Central Iberian Zone (CIZ), where several significant uranium deposits exist. In this region, granitic rocks are the predominant lithology, some of them being naturally enriched in U. This expected geochemical anomalies found in the nationwide scale mapping can also be observed on the FOREGS sampling.
At local scale, smaller anomalies can be detected, both natural and anthropogenic, such as mineralised vein enrichments, small anthropogenic influences downstream from towns, etc.
Similarities can be found in methodology of interpretation in the various scales, such as, at nationwide scale using local scale data interpretation and at FOREGS data interpretation using nationwide scale data.
Interpretation of FOREGS data itself is difficult at a nationwide level, as expected for small countries such as Portugal. Stream sediment media for some chemical elements, reflects wide watersheds and material input transported by watercourses, and indicates that it is necessary to study areas that exceed national limits. For the Portuguese case, the FOREGS data will get a new meaning if the Iberian Peninsula is interpreted as a whole.