Will groundwater-borne nutrients affect river eutrophication in the future? A multi-tracer study provides evidence

Abstract. Groundwater can be a major source of nutrients and contaminants to river systems in agriculturally active areas with significant implications for water quality and ecosystem health. The Elbe river in eastern Germany, characterised by Cretaceous aquifers upstream and Quaternary aquifers downstream, is located in areas of intense agricultural activity and is therefore vulnerable to nutrient fluxes. This study investigates the time scales of diffuse groundwater-borne nutrients entering the river using multi-environmental tracers (³H/³He, SF₆, CFCs, ¹⁴C). By applying lumped parameter models, we concluded on time scales of groundwater flow from recharge to the river ranging from 0 to 41 years, with infiltration occurring predominantly after 1985. Our results highlight a young groundwater system with measurable denitrification and minimal to moderate admixtures of older water fractions clearly discernible with helium. This suggests that the legacy of nutrient inputs from intensive fertilisation during the GDR period (1945–1989) has already peaked, with groundwater-borne nutrient concentrations expected to decline over 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.