The continuous exchange of water between surface water and groundwater is a key environmental process controlling the transport and the fate of nutrients, solutes and pollutants in river networks. The dynamics of the near-stream groundwater has a non-negligeable role on controlling flow direction and solutes exchange between the stream water with the adjacent groundwater, however it is rarely considered in solute transport experiments. Despite the amount of individual studies, we are still uncertain about how the physical processes controlling in-stream solutes transport change with different hydrologic conditions and how these processes can be inferred by modelling outcomes. In this project we investigated solute transport in a headwater stream reach via instantaneous (slug) solute injections. The study site is a 55 m long corridor downstream of the Weierbach experimental catchment (see Hissler et al., 2021). The stream channel is unvegetated and consists of deposited colluvial material and fragmented schists (up to 50 cm depth) with underlying fractured slate bedrock that sporadically forms the streambed. The average channel slope is 6% and a 50 cm step riffle sits between wells 7W1 and 7W2 (see Bonanno et al., 2021). The stream reach is divided in 11 sections that define the nomenclature of the groundwater observation well network (eg. section one is indicated by wells 1W1 and 1W2). The complete list of groundwater measurements has been published in a previous Zenodo dataset and can be found HERE. The tracer chosen for the experiments is chloride. For each experiment, we prepared an NaCl solution using 2 liters of stream water and a fixed mass of reagent-grade NaCl. We injected the solution in a turbulent pool at the beginning of the stream reach (right before section 1) to assure complete mixing in the stream water. Electrical conductivity was measured via portable conductivity meter (Multisonde WTW). Conversion between EC and chloride concentration has been deduced via EC-chloride concentration plots in laboratory where a fixed amount of NaCl solution with known concentration has been progressively added to a sample the stream water collected before the experiment. Every regression equation between EC and chloride concentration plot had a R2>0.998. The dataset includes 30 files of chronologically-numbered instantaneous injections. The instantaneous injections have been conducted from 6-Dec-2018 to 11-June-2021. Every file includes: > A map of the investigated stream reach; > WTW sensor location along the stream reach and their distance from injection point; > The amount of NaCl mass injected in the stream; > Pictures of the stream channel and streamflow; > Notes about presence of leaf packs; > Time and net chloride concentration [mg/l] for each sensor. All the experiments, data cleaning, sensor calibration, and conversion from EC to Cl- concentration have been conducted by Bonanno Enrico between 2018 and 2021 as part of the Ph.D. project HYDRO-CSI (PRIDE15/10623093). François Barnic, Laurent Gourdol, Jean François Iffly and Jérôme Juilleret calibrated the Multisonde WTW and provided the necessary training. Laurent Pfister and Julian Klaus managed the project and were responsible for the founding acquisition.

The authors acknowledge funding from the Luxembourg National Research Fund (FNR) through the project: HYDRO-CSI (PRIDE15/10623093).