Drivers of water level fluctuations and hydrological exchange between groundwater and surface water at the lowland River Spree (Germany): field study and statistical analyses

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Hydrological exchange processes in the floodplain of a lowland groundwater-surface water system with emphasis to the hyporheic zone are studied on the basis of an experimental site surrounded by an oxbow and the current bed of the river Spree. Water levels were collected in 12 piezometers and 2 recording stage gauges of a 300-m long transect throughout a 1-year-period. Due to clogging the hydraulic contact between the oxbow and the adjacent aquifer is marginal. Fluctuating river gauges cause an alternation of infiltration and exfiltration. Most of the time groundwater flux is directed into the river Spree while infiltration events are usually short. The largest infiltration event in the 1-year-period reached about 4 m into the aquifer. On the basis of a principal component analysis two major processes controlling water level fluctuations in the aquifer are identified. The first component responsible for about 70% of the fluctuations stands for the dampening and delay of fluctuations of the river gauge spreading into the aquifer. The second component explaining about 20% of the fluctuations stands for groundwater recharge due to precipitation events. In lower reaches with extended floodplains, fluctuations of groundwater levels are mainly controlled by river gauges and thus, the hydrological conditions in upstream reaches. In contrast local precipitation events are of minor importance and upslope groundwater level fluctuations are completely unimportant. A cross correlation of the times series of the different piezometers was used to calculate the time lag of pressure fluctuations at different locations and resulted in a propagation of the pressure fluctuations of about 1550 m d−1 while the effective velocity of the groundwater was about 1000 times smaller. Fast water level fluctuations should not be misinterpreted as fast flow velocities and special care is necessary in flow modelling of such data sets since most groundwater flow models cannot cope with a fast propagation of pressure head fluctuations.

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