Impacts of groundwater level management on geochemical processes at an agricultural site with shallow groundwater

Jörg Steidl, Ottfried Dietrich, Christoph Merz

Abstract The water and nutrient cycles of agricultural areas with shallow groundwater tables determine whether these systems act as sources or sinks within landscape‑scale budgets, which strongly depend on prevailing water‑management conditions. Developing sustainable land use systems that minimize environmental impacts therefore remains a major challenge. To improve our understanding of how different water‑management strategies influence these processes, lysimeter studies were conducted in the wetland region of the Spree catchment in Brandenburg, Germany. Three modern weighable groundwater lysimeters were operated under prevailing hydrological and geochemical boundary conditions, each managed with distinct water level targets: two with deeper, seasonally fluctuating levels reflecting current regional practice, and one with consistently near‑surface levels representing a sustainable management objective. Dissolved solutes were sampled at three depths using suction cups, and vegetation growth was monitored. Geochemical datasets were analyzed using principal component analysis, with principal component loadings interpreted as indicators of dominant geochemical processes. Principal component scores were subsequently linked to soil‑water, groundwater level, and grassland variables using a random forest approach to evaluate how water‑level control affects hydraulic and geochemical dynamics. The results reveal a complex interplay of processes and challenge the assumption that constant near-surface water levels alone ensure stable geochemically reducing conditions. Downward transport of solutes during winter highwater periods emerges as a key mechanism for substance removal. Moderate summer drawdowns, less than 50–60 cm, do not impair stable geochemical reducing conditions, as the system exhibits robust redox behaviors without persistent oxidation. However, winter recovery to near-surface water levels remain essential.