Will groundwater-borne nutrients affect river eutrophication in the future? A multi-tracer study along the Elbe River

Abstract. The Elbe River drains an intensely used agricultural area and cuts through a series of consolidated and unconsolidated aquifers with heterogeneous hydraulic properties and dimensions. For decades and as a result of overfertilization, particularly in the former GDR, the hosted groundwater transport nutrients into the river with serious implications for water quality and ecosystem health. As fertilization practices changed over time, nutrient loads in the groundwater recharge declined since the 1990s. This study investigates the residence time scales of groundwater along the river, as a measure to estimate the input periods of the associated nutrients entering the river using multi-environmental tracers (3H/3He, SF6, CFCs, 14C). By applying lumped parameter models, we conclude that the average ages of groundwater range from a few up to 41 years, with infiltration occurring predominantly after 1985. Our results identify a young groundwater system with measurable denitrification and minimal to moderate admixtures of older water fractions clearly discernible with 4He. That indicates, the groundwater that was recharged during the GDR period (1945–1989) at maximum fertilizer application has already run off, the nutrient concentrations in the groundwater have peaked and may continue to decline in the coming decades. These results are crucial for informing river basin management strategies aimed at mitigating eutrophication and protecting aquatic ecosystems. It provides valuable insights into the temporal dynamics of groundwater contributions to surface waters and their regional implications for sustainable resource management.