Poster, AGU Fall Meeting 2016, San Francisco, USA: 2016-12-12 - 2016-12-16
Abstract:
Ecosystem functioning of streams heavily depends on nutrient and pollutant fluxes between the stream and the adjacent groundwater. To study potential reactions, we have to estimate the residence time of water exchanges through the streambed and bank sediment. These exchanges within the hyporheic zone have already been thoroughly investigated. However, most previous studies assumed steady-state conditions, despite the fact that the magnitude and timing of riverbank exchanges are highly dynamic. In this study, we estimate residence times of riverbank exchange under transient conditions at a restored river reach in Switzerland. In the stream and in two adjacent observation wells (in 1 m distance to the stream), we continuously analyzed dissolved gas concentrations (O2, N2, O2, Ar, He, Kr, Ne) with a portable mass spectrometer for five months on a 30 m river reach. Additionally, we continuously measured electric conductivity, water tables, and water and air temperatures at all sampling points. At the observation wells we also employed slug tests to estimate the hydraulic conductivity of the investigated stream reach. The obtained time series of tracer data reveals how residence times depend on changes in the hydraulic connectivity of the stream and the adjacent groundwater. Changes in the hydraulic state are induced by (i) different groundwater pumping rates of nearby groundwater abstraction wells, (ii) increased river discharge and (iii) subsequent changes in the hydraulic conductivity of the streambed as a result of unclogging the streambed after floods. Our results contribute to existing knowledge in this research area by identifying non-stationary processes such as the unclogging of the riverbed after flood events. In order to test our hypotheses, our next step is to use our experimental data to constrain a numerical model.