Impact of hydroclimatic extreme events on nutrient concentrations in the Rhine and Meuse Rivers in the Netherlands.

Abstract

Poor surface water quality threatens human water use and ecosystem health. Climate change and the associated higher frequency of hydroclimatic extreme events will further impact water quality. Although the effects of hydroclimatic extreme events on surface water quality are widely recognised, the impacts of events on nutrient concentration in rivers are less known. The objective of this study is to assess the effects of hydroclimatic extremes (i.e. droughts and floods) on river nutrient concentrations of the Rhine and Meuse Rivers in the Netherlands. Additionally, the extent to which relevant water quality thresholds (EQS standards), according to the EU Water Framework Directive, are violated during extremes is determined. Nutrient concentration, discharge, and water temperature data from seven monitoring stations in the Rhine and Meuse Rivers spanning the 1980 – 2023 period are investigated. Six different nutrient forms were included in the analysis, which can be divided into a number of steps. To begin, the statistical C-Q relationship between nutrient concentrations and discharge is investigated. Nutrients forms behave differently in the two river basins. In the Rhine, especially positive C-Q relationships are observed, while negative relationships are observed in the Meuse River. Ammonium, nitrite, orthophosphate, and water temperature have an inverse relation with discharge in the Meuse River. Other nutrient forms are more linearly related to discharge. Next, the impacts of floods and droughts on the river water quality are determined by a comparison of concentrations between average and extreme conditions. The results indicate that nutrient responses in the Rhine are influenced by mobilisation. All nutrient forms increase in concentration during floods possibly due to higher erosion and mobilisation, whereas all forms decrease in concentration during drought by reduced mobilisation. In contrast, most of the responses of nutrient forms in the Meuse appear to be more governed by dilution. Especially ammonium, orthophosphate, nitrite and organic nitrogen decrease in concentration during floods by dilution of constant point sources from wastewater treatment plant effluents and increase in concentration during drought due to limited dilution. In both rivers, nitrate shows distinct behaviour, increasing in concentration during floods and decreasing during droughts. The nitrate response seems to be driven by mobilisation from large agricultural diffuses sources. In addition to dilution and mobilisation, nutrient concentrations are influenced by additional processes such as release from sediments, accumulation, and temperature-affected processes (nitrification and denitrification). In addition, the spatial upstream-downstream effect is significant in the Meuse, resulting in a higher residence time of the river water and therefore increased dilution. The impacts of hydroclimatic extreme events on the water quality of the Rhine are of lower magnitude overall, potentially due to the source of the river water (snow- and rainfed). Even under average conditions, nutrient concentrations in both river basins already exceed the water quality standards set by the European Union Water Framework directive (EU WFD). Although concentrations of some nutrient forms can decrease during extreme weather, hydroclimatic extremes mostly intensify nutrient concentrations, increasing violation of the EQS standards and deterioration of the water quality. Surface water quality management should be improved by reducing the emission of nutrient main pollution sources (e.g. agriculture, industries, and households). Future research should focus on improving modelling approaches to predict impacts of hydroclimatic extreme events on the surface water quality and on improving the identification of hydroclimatic extreme events.