Abstract
Cases of contamination by metals in the water wells of the Cavado Valley in north-west Portugal can be attributed to the heavy leaching of clay soils due to an excess of nitrogen resulting from the intensive use of fertilisers in agricultural areas. This work focuses on the natural weathering characteristics of soils, particularly the clay material, through the study of samples collected near the River Cavado. Samples taken from various sites, after physico-chemical characterisation, were subjected to clay dissolution tests, using sodium salts of different ionic forces, to detect the relationship between certain physico-chemical parameters of water, such as pH, nitrate, chloride and sulphate content, in the dissolution of clay and the subsequent extraction of such cations as Al, Fe and K. In acidic sandy clay soils, the mineralogical composition of which was characterised by a predominance of quartz, micas, kaolinite and K-feldspars, decreases of the clay material/water pH ratio increases dissolution of the micaceous and K-feldspars phases. The presence of nitrates in the aqueous solution apparently advanced the extraction of all three cations Al, Fe and K. The specific surface area of the clay material showed a significant correlation with the main kinetic parameters of cation extraction.
Highlights
The natural weathering of soils constitutes an important geological process for humans, since it promotes soils genesis and contributes to the formation of economically important mineral deposits such as kaolinite, laterite, bauxite, bentonite, charcoal and petroleum (Gomes, 1988)
Each sample was prepared for analysis using the following procedure: (a) about 10 g of each sample was dried at 105°C during 24 h; (b) the sample was ground it in ball mill, during 20 min. obtaining particles size < 0,6 mm; (c) 4 g of mixture
In the latter groups the contents of Ba and Sr probably corresponds to the presence of K-feldspar, whereas the other constituents show concentrations related to micas
Summary
The natural weathering of soils constitutes an important geological process for humans, since it promotes soils genesis and contributes to the formation of economically important mineral deposits such as kaolinite, laterite, bauxite, bentonite, charcoal and petroleum (Gomes, 1988) This is a complex phenomenon, of interdisciplinary understanding, covering knowledge of several areas such as pedology, hydrology, biology, geochemistry and physics. In environments of greater depth, research into the development of geothermal energy and the deposition of radioactive wastes has produced an extensive database. Despite these advances, the perception of measures, quantifying and the modelling atmospheric water flow, through soil vegetation in satured and unsatured zones, is still in a relatively early stage of development (Saether & Caritat, 1997). It is imperative to develop parameters for the elucidation and
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