Analysis of the interaction between surface water and groundwater using gaseous tracers in a dynamic test at a riverbank filtration intake

We discuss a study designed to elucidate the genesis and inflow conditions at riverbank filtration wells located on a mountain river. This article seeks to identify the most important drivers of spatio-temporal dynamics of water flow in the hyporheic exchange zone, in natural conditions and conditions disturbed by the water abstraction. In our study we try to contribute to further understanding the dynamics of groundwater mixing with river water in the hyporheic exchange zone. We focus on understanding river/aquifer interactions at the scale of reach of an intake, especially the unidirectional water flows induced by water abstraction. To understand these issues, a two-day field hydrogeological experiment was conducted based on a pumping test of increasing intensity. At each pumping stage, groundwater and river samples were collected to determine the concentration of noble gases, CFCs, SF6, stable isotope content, and the chemical composition of the water. The study results indicate a short pressure propagation time (of the order of several hours) between the intake and the river, which already results in the inflow of water from the river-aquifer mixing zone at low rates of water abstraction by the intake. As pumping rates increase, the conditions of mixing and inflow of water to individual wells stabilize or approach stabilized conditions in less than 1-2 days. The share of river water in the water flux flowing into the intake does not exceed 12%, whereas the ratio of river water to groundwater in the mixing zone is estimated at 1 to 7 or more. The range of the river-aquifer mixing zone may reach up to 170 m from the river and this range can be identified with a good approximation as coinciding with the range of the zone of hyporheic exchange in the study area.