Quantification of groundwater recharge is one of the most important tasks in hydrogeology. Many methods of recharge estimation, which can be applied in various spatial and time scales, have been proposed. The main goal of the present work is to identify groundwater recharge at an experimental site on an outwash plain, using different techniques: analysis of water table fluctuations, field tracer experiment and numerical modeling of the unsaturated zone flow.
The study area is located in Bory Tucholskie region in northern Poland. It represents a typical outwash plain setting, dominated by fluvio-glacial sands with locally occurring clay layers and lenses. The depth to groundwater table is about 6 to 10 m. Continuous measurements of weather data, soil water content, electric conductivity and groundwater table position have been carried out at the site since April 2017. Water content in the vadose zone is measured in four soil profiles, at four depths in each profile. In order to capture the influence of vegetation cover, two profiles were located on open grassland and two in pine forest. The tracer experiments were carried out in two profiles as pulse injections of chlorides applied on the ground surface. The sensors of electric conductivity placed at various depths allowed to track the tracer. Monitoring wells were installed close to each of the four profiles, in order to measure groundwater level and electric conductivity in the saturated zone. The measured range of fluctuations of groundwater table was 0.4 to 0.75 m. Numerical simulations of flow in the vadose zone were carried out using HYDRUS-1D computer program, based on one-dimensional form of the Richards equation. In this work we will present a comparison of the results obtained from the analysis of water table fluctuations, tracer tests and numerical modeling. The preliminary results show that groundwater recharge in the investigation area is about 75 to 130 mm/year.
The work is funded by National Science Centre (NCN), Poland (grant 2015/17/B/ST10/03233 "Groundwater recharge on outwash plain").