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24, chemin de Borde Rouge –Auzeville – CS52627
31326 Castanet Tolosan CEDEX - France

Dernière mise à jour : Mai 2018

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Soils and geology

The geological bedrock of the catchment of Kervidy-Naizin is composed of fissured and fractured upper Proterozoic schists. The soils, developed into a loamy material derived from weathered schists and eolian Quaternary deposits, exhibit facies variations, which are locally dominated by silt, clay or sanstone materials. The mineralogical composition of schist, as determined from drill cutting analysis, include (in decreasing relative proportion): quartz, muscovite, chlorite, K-feldspar and plagioclase (Pauwels et al ., 1998).

The soil horizons whose total thickness may be highly variable at the scale of the catchment (from 35 to roughly 5 m) comprise a large number of secondary mineral phases including illite, smectite, kaolinite, various Fe-oxides and Fe-oxi-hydroxides (hematite, goethite,...) and Mn oxides. The soils are organized following the topography with well-drained ”Luvisols” in the upland areas while in bottomlands poorly-drained domains consist of "Luvisols –Redoxisols" and "Luvisols – Redoxisols dégradés" resulting from an increasing development of hydromorphy and degradation in originally well-drained horizons due to an excess of water of topographical origin. These soils are acid (pH from 5 to 7 in surface, from 4.6 to 6.7 in the regolith).

The content of organic matter varies from 2.5-6.5% in the upper layers to 0.3-2% in deeper zones and in the regolith. Organic layers are 30-40 cm deep. The hydraulic conductivity is around 10-5 m/s in upslope soils to 10-6 m/s in bottolands. The weathered zone is 1 to more than 30 m deep and its hydraulic conductivity is between 4.10-5 and 2.10-6 m/s. The conductivity of the geological bedrock is estimated around 3.5 10-6 m/s (Pauwels et al., 1996).