Floodplains play an important role in the hydrological cycle. They serve as buffer zones where water and solute exchange and heat transfer take place between the shallow groundwater (GW) and surface water (SW). Presented research focuses on quantifying the water flux between the shallow aquifer and river, which is often characterized by a high temporal and spatial variability.
A multi-method approach is used to measure the fluxes in three distinctive Belgian catchments (Zwarte Beek, Mombeek and Dijle). As GW head observations do not directly allow for quantifying the exchange flux, other state variable observations such as heat, hydrochemistry and isotopic tracers, are monitored to quantify the fluxes. Multiple temperature lances are installed in the catchments, which measure riverbed temperatures at multiple depths every 15 minutes. Point-in-time riverbed temperature measurements were carried out along the river channel to capture the spatial differences of the exchange flux. Based on these temperature data, the calculated fluxes have an order of magnitude varying from 10-7 to 10-6 m/s during observation period. The flux direction, representing either losing or gaining river, changes not only within the time of year but also along the longitudinal river profile. Besides the heat, a number of other tracers have been deployed, including major ion concentrations, electrical conductivity, stable isotopes (2H and 18O), and radon (222Rn). In total, 74 GW and river water samples have been collected during seasonal field campaigns. Major ion concentrations show that both river water and shallow GW belong to the calcium-dominated water type in all catchments. Nevertheless, the dominant anions vary from catchment to catchment and display a seasonal shift, e.g. from CaHCO3 type in summer/autumn to CaMIX type in winter in the Mombeek and Dijle catchments. Stable isotopic values of the autumn campaign show a narrow range: δ2H between −49.5‰ and -42.9‰ and δ18O between –7.4‰ and -6.4‰. Nearly all samples are isotopically depleted since they fall below the global and local meteoric water lines, suggesting a meteoric origin and evaporation effect for both GW and SW. Radon surveys indicate that its activities, especially in shallow GW, show large seasonal variations (from autumn to winter) in the Dijle catchment. A couple of SW samples from all three catchments show elevated radon activities which is most likely the result of GW exfiltration into the river. A quantitative analysis of these field approaches and preliminary interpretations will be presented and discussed during the conference.