Kaolinite deposition and clogging of moving streambeds under losing and gaining flow conditions

Clay deposition in streambed sediment can cause partial or complete clogging of the streambed. It was shown that clogging reduces the hyporheic exchange flux (HEF) between the water column and the streambed and can negatively affect stream ecosystems. For example, by reducing the fluxes of nutrients to benthic microorganisms. It has been shown that flow from the stream towards the groundwater (losing) or in the opposite direction (gaining) also affects clay deposition patterns; however, this has only been investigated experimentally under stationary bedform conditions. Here, we investigated the dynamics of clogging during moving bedform conditions and under losing or gaining fluxes. We conducted a series of experiments in a 640 long and 29 cm wide flume packed with sand (D50 = 270 μm). The flume is equipped with a drainage system that can simulate losing or gaining conditions at prescribed flux. We conducted experiments under two different losing fluxes and two gaining fluxes (10 and 20 cm/day), while stream velocity was constant at 29 cm/day.  During the experiments, kaolinite was added as a discrete series of pulses. Each pulse was added after the kaolinite deposition stabilized (2 - 4 days). HEF and the vertical hydraulic conductivity were quantified before each kaolinite addition by salt tracer test, and the “falling-head” methods, respectively. Morphodynamic properties of the bed were measured using high-frequency acoustic doppler sensor and time-lapse photometry timeseries. The kaolinite deposition rate was measured with a turbidity sensor, while core samples were taken at the end of the experiment to analyze the vertical deposition patterns. Preliminary results showed that HEF and hydraulic conductivity in losing conditions decreased from their initial value by 95% and 37%, respectively, while in gaining conditions, HEF decreased by 72% and hydraulic conductivity by 23%. We also observed that under gaining conditions, most of the clay was deposited at the upper part of the sediment (in the moving fraction). In losing conditions, kaolinite was also found deeper in the bed and below the moving fraction of the streambed. In all cases, most of the kaolinite mass was deposited at a depth of less than five cm. The results show a significant effect of stream-groundwater interactions on HEF and on suspended particle deposition in situations where the bed is under movement. Therefore, the quantification and prediction of clay deposition patterns in streams with strong interactions with groundwater has to be included in models that predict clogging and transport processes in streams.