MODELLING OF TRANSIENT RIVER - AQUIFER EXCHANGE USING PRESSURE HEAD AND HEAT MEASUREMENTS: THE HYPORHEIC ZONE'S DIMENSION

Water exchange processes in the floodplain of a lowland groundwater-surface water system are studied on the basis of a study site near Freienbrink, NE Germany. The surface water boundaries of this site are formed by an oxbow and the current bed of the river Spree, section Muggelspree. Surface and ground water pressure heads and water temperatures were collected in 12 piezometers and 2 recording stage gauges of a 300 m long transect throughtout a one-year-period. Because of water level fluctuations alteration of infiltration and exfiltration occurred. Due to clogging of the oxbow bed with an organic silt layer of different thickness the hydraulic contact between the oxbow and the adjacent aquifer is partially marginal. These features are described quantitatively using SUTRA in order to simulate coupled ground water flow and heat transport. As the main result, the mass transfer between the aquifer and both river sections can be quantified; mostly, groundwater is infiltrating into the surface waters with flow rates between 0.002 and 0.004 l s(-1) m(2) (150 to 350 l m(2) per day). Short periods of surface water exfiltration into the aquifer do not exceed 10 days and the flow rates are in the same range. Results of heat transport simulations generally confirm the transient subsurface flows and allow estimating influence of surface water infiltration into the riparian zone.