Dimensioning groundwater discharge

Excessive nutrient inputs and the resulting eutrophication are major challenges for surface water quality. As groundwater is the main diffuse nutrient source, its contribution must be quantified over time. Using the eutrophic Elbe River as a case study, this project examined spatial and temporal patterns of diffuse groundwater discharge and its impact on benthic and planktonic eutrophication.

Groundwater discharge along a 450 km free-flowing section of the German Elbe was identified using hydraulic head gradients. Discharge volumes were estimated via a multi-method approach: flux balance, inverse geochemical and tritium modelling, and Darcy calculations [1]. Discharge varied widely, with effluent conditions mostly from early summer to autumn and inverse gradients in spring and winter. The highest discharge likelihood was found in upland areas, decreasing downstream. Notably, cropland drainage channels in the lowlands contributed significantly, while groundwater added around 40 m³/s during low flow. To place these fluxes in a temporal context, a lumped parameter model was applied. Tracers (³H/³He, SF₆, CFCs, and ¹⁴C) indicated groundwater ages of 0–41 years [2]. This young system is partly denitrified and shows minor mixing with older water. It suggests that nutrient concentrations will decline in the future, as the fertiliser peak from the GDR era has passed.

Groundwater-Affected Eutrophication

Nutrients from groundwater can stimulate benthic algae directly, and, once in the water column enhance planktonic algae, particularly under phosphorus (P) limitation. This study tested groundwater effects on benthic algae via field experiments and modelled its impact on planktonic algae in the lower Elbe. Ecological effects were analysed using local (biofilm) and regional (phytoplankton) parameters - key indicators of eutrophication [3]. Biofilms were recorded seasonally on artificial substrates across river stretches influenced differently by groundwater. P limitation effects on plankton under low flow were assessed using stoichiometric modelling based on in-river algae and nutrient data.

Findings show that biofilms are mainly shaped by seasonality, aquifer type, and groundwater flux. Groundwater impact is highest in autumn in loose rock aquifers and in summer across all types. Major influencing factors include temperature, conductivity, turbidity, and nutrient levels. Regarding phytoplankton, groundwater-borne P contributes 1.5 t/day during low flow across the 450 km stretch. This translates to 46 t/day of additional particulate organic carbon under nutrient-limited conditions, significantly increasing eutrophication potential.

Literature:

[1] Zill, Julia, et al. "A way to determine groundwater contributions to large river systems: The Elbe River during drought conditions." Journal of Hydrology: Regional Studies 50 (2023): 101595.

[2] Zill, Julia, et al. "Will groundwater-borne nutrients affect river eutrophication in the future? A multi-tracer study along the Elbe River." EGUsphere (2025): 1-23. Under review

[3] Zill, Julia, et al. "Contribution of groundwater-borne nutrients to eutrophication potential and the share of benthic algae in a large lowland river." Science of The Total Environment 951 (2024): 175617.