Using contrasting tracers to characterize groundwater dynamics under a prolonged drought in the lowland catchments in the North German Plain

Groundwater, as the key strategic reserve in times of drought, is sensitive to climate change, especially unconfined, shallow aquifers. Frequent and prolonged drought provides an urgent impetus to improve understanding of groundwater dynamics and its residence times in drought-sensitive areas where water and food security are threatened. The Demnitzer Mill Creek catchment is a long-term environmental observatory typical lowland of North German Plain where streams are dominated by groundwater, however its groundwater recharge and dynamics remain poorly constrained. We applied water table observations, isotopic (δ18O, δ2H, 3H), hydrogeochemical, and geophysical investigations to characterize the spatial and temporal patterns of groundwater recharge in a shallow, unconfined aquifer system. Long-term groundwater levels showed a declining trend since 2011, which accelerated after 2018 resulting in increasingly intermittent seasonal streamflow. Geophysical surveys and groundwater monitoring indicated that shallow water tables (typically <3 m deep) in low to moderate permeability surficial deposits are generally recharged during winter, leading to higher groundwater – surface water connectivity in riparian alluvial aquifers, which is the first order control on streamflow generation. This was supported by similar geochemical characteristics of groundwater and streamflow. Water stable isotopes indicated a high damping in groundwater with a bias towards winter precipitation and direct recharge. Although 3H dating showed that the age of shallow groundwater was young (~5 years) and generally similar to streamflow, estimates had high uncertainty and some deeper groundwater was free of 3H. Such multiple approaches help understand changes in groundwater recharge and dynamics during droughts and contribute to the development of sustainable land and water management strategies for groundwater systems that are sensitive to climate change.