How catchment properties shape variation in groundwater- surface water interaction: Using geogenic silicate as a tracer in hydrological turnover research

Hydrological turnover (HT), i. e. gross gains and losses of water along a stream or river is highly variable, producing spatial exchange patterns influenced by local surface and groundwater levels, geology, topography, channel morphology and sediment structures. However, seasonal variations of discharge and catchment storages might be additional factors influencing the locally occurring fraction of streamflow subject to HT.

We studied these process interactions at a third order tributary of the river Mosel in Trier, Germany by measuring HT 133 times (in total 399 individual discharge measurements) at two different stream reaches (~500 m each) over a period of two years. The two reaches show a pronounced seasonality in drainage behavior and differ mainly in valley width.  The underlying, silicate rich Devonian schists and slates enable the use of silicate as a marker for prolonged contact with the underground. Hence, we took samples for silicate concentrations in stream water as well as in the near-stream groundwater, regularly (in total 270 samples). For this purpose, we installed three sets of two groundwater wells at each reach. The first well of each set was located directly at the stream bank and the second well in a distance of 3 m from the stream. Thus, we created snapshots of the boundary layer between ground- and surface water where turnover induced mixing occurs. The results show in accordance with literature a site specific negative correlation of HT with discharge, while reach scale net Q changes correlate with HT only at the upstream site which is characterized by steeper hillslopes compared to the downstream section. Analyzing reach specific variation of silicate concentrations between stream and wells suggests that in-reach silica variation increases with the decrease of hydrological turnover and vice versa. This relationship differs between the two reaches and shows significant seasonal effects. These findings are supported by the results of a delayed/base flow separation analysis for both reaches, which shows a faster drainage behavior and a less pronounced contribution of longer delayed groundwater sources for the narrow valley upstream site.

These results imply that besides the discharge-induced HT variability seasonal states of groundwater storages might be an additional control on the magnitudes of HT affecting physical stream water composition throughout the year.