Carbonate redistribution and hydrogeochemical processes in two calcareous soils with groundwater in a Mediterranean environment

Summary The morphology of the soil is closely associated with water regime and redistribution of solutes. We studied in detail two calcareous soils with groundwater representative of the two main hydro‐geological regimes, one on the plateau and the other in the central depression, of an experimental catchment near Roujan in the South of France. We recorded macro‐and micromorphological features, and measured the level of the water table and the saturating solution. The morphological features were primarily those associated with carbonate redistribution. The forms associated with iron and manganese added information. The two groundwater systems behaved differently over 3 years. The temporary perched groundwater of the plateau was characterized by a small partial pressure of CO 2 (pCO 2 ) and a large pO 2 , with rapid lowering of the water table (4–6 cm d −1 ). The permanent groundwater of the depression, in contrast, had a larger pCO 2 and a small pO 2 . The water table fell more slowly (1–2 cm d −1 ). Higher up, rapid drying of the profile and release of CO 2 leads to over‐saturation of the soil solution with respect to calcite, favouring the formation of poorly crystallized micrites, which are characteristic of the plateau system. In the lower part of the catchment, persistent waterlogging leads to more marked anaerobiosis (large estimated pCO 2 ) favouring the development of large stable rhombohedral crystals characteristic of more stable hydrodynamic conditions. In both situations, iron and more particularly manganese redistribution agrees with the waterlogged state of these soils.