Geoökologisches Kolloquium WS 2004/05

Alle Termine

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Anne-Catherine Pierson-Wickmann und Jérôme Molénat
Université de Rennes 1 und Institut National de la Recherche Agronomique, Rennes
Donnerstag, 04.11.2004 16:15 H6

Water and solutes transfer in catchments with high agricultural pressure

Brittany (western France) is in the first french and european regions for pig, dairy, poultry and vegetable production. This has resulted in a high polluting pressure on soils and water resources. In this environmental context, the CAREN (Centre Armoricain de Recherche en Environnement- Brittany centre for environmental research) has been studying three small catchments (Kervidy, Kerrien and Kerbernez). These three catchments were first equipped for water quantity and water quality monitoring as early as 1975 for the Kervidy-Naizin watershed and 1990 for the Kerbernez and Kerrien catchment. Since, each has been the subject of detailed agronomical, hydrological, pedological and biogeochemical studies designed to better understand and model how space and time variations in the pedo-geology, hydroclimatology, landscape and agricultural activities control water transfer processes and water quality at the scale of small watersheds (<5 Km2). These catchments are variable source area catchments where source area extensions are controlled by the groundwater depth in the bottom land.

The presentation will focus on two following points of the researches:
- the dynamics and the role of shallow groundwater on solutes, particularly nitrates, transfers in the cachment. The chemistry of the groundwater within the catchments exhibits a clear spatial pattern of anion concentrations. The upslope part has high nitrate concentrations and low sulfate concentrations, and in the bottom land, groundwater is less acidic, with lower nitrate concentration and higher sulfate concentration. The processes responsible for this pattern and the consequences of the pattern on the temporal variations in the stream water chemistry have been identified. However in spite of a good understanding of the origins of stream water, high uncertainties remain about the transfer times of water and solutes transported by water in the catchment and the linkage between the vadose zone and the groundwater is not understood well yet. Consequently current researches deal with the development of groundwater age dating methods and with solute transfers from vadose zone to groundwater.

- The impact of agricultural pressure on weathering process. The chemistry of stream- and ground-waters are largely influenced by the varied agricultural inputs, which can also induce variable effects on chemical weathering processes. First of all, chemical fluxes have to be estimated and corrected for atmospheric and agricultural inputs. The estimate of atmospheric inputs is based on local precipitations, while agricultural input estimate is based on a database of the various fertilizers used by local farmers over a 10-years period. The calculations lead to some primary conclusions on cation fluxes at the watershed outlet. Potassium, which is a major cation provided by agriculture, is almost consumed by local biosphere. The other cations (Mg, Ca, Na) display large fluxes, and much larger than those of small granitic catchments. Agricultural inputs seem to control and increase the chemical weathering by either (1) a preferential mineral dissolution, or (2) a release by leaching of cations from old agricultural inputs adsorbed on soil particles.


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