Poster, International Symposium Soil processes under extreme meteorological conditions, Bayreuth: 25.02.2007 - 28.02.2007
Abstract:
Riparian wetlands represent the transition zone between the unsaturated zone, groundwater and surface water. The influence of the upslope areas on the chemical composition of surface waters is often predominated by chemical transformation processes in the riparian wetlands, even though they represent only a minor fraction of the catchment (Hooper 2001, Gburek et al. 2000). A change of the chemical dynamics in this part of the catchment, e.g. due to increasing intensity and frequence of extreme hydrological conditions (extended draughts, heavy rainstorms) will have an direct impact on the chemical composition of the surface waters. We studied this relationship in a riparian wetland site in the Lehstenbach catchment, South Germany. The temporal and spatial variability of the chemical composition in groundwater was investigated monthly using a quite dense network of piezometers. Whereas we observed only slight differences of chemical parameters (electrical conductivity, pH) in space, they varied significantly from sampling date to sampling date, indicating changes in the chemical species available for solute export. As shown by temporal high resolution measurements, electrical conductivity of groundwater decreases rapidly during precipitation events, even in a depth of one meter. Within the saturated zone, oxic event water seems to be mixed with anoxic pre-event water, supplying oxygen to the anoxic groundwater and substrates for the subsequent formation of reduced species. The intensitiy of redox processes in the riparian wetland thus depends on the frequence of precipitiation events. Solute export from the wetland was highly non linear and occured mainly during heavy rainstorms after long dry periods and during snowmelt in spring, as shown by time series of electrical conductivity of runoff. The type of solutes exported depended on the hydrological boundary conditions, i.e. the intensity and the frequence of precipitation events and the position of the groundwater table in the riparian wetland when rainfall occured. During heavy rainstorms after long dry periods (low groundwater table), oxidised species like NO3 were exported from the riparian wetland. Long periods with high groundwater table implicated the accumulation of reduced species and thus the mobilisation of Fe(II) and P. The export of iron occured continuously during high water periods independent of the intensity of precipitation events. However, the export of P exhibited a high intensity of rainstorms and snowmelt events.