Aarhus University Seal / Aarhus Universitets segl

No. 270: Streams 2016 - Chemical water quality

Thodsen, H., Tornbjerg, H., Windolf, J., Bøgestrand, J., Larsen, S.E., Ovesen, N.B. & Kjeldgaard, A. (2018): Vandløb 2016 - Kemisk vandkvalitet og stoftransport. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 54 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 270


This year’s report presents topics on the nitrogen and phosphorus run off to Danish coastal waters. All data presented are collected in accordance to the National Monitoring Program for the Aquatic Environment and Nature (NOVANA).

Nitrogen and phosphorus 

The concentrations of nitrogen and phosphorus in streams have been markedly reduced since 1989. The main reason is reduced leaching from cropped areas resulting in a mean reduction of approximately 46 % for nitrogen due to several general regulations of farming practices (e.g. fertilization norms, seasonal regulation of manure application, catch-crops), whereas an approximate 42 % reduction in phosphorus is due to improved treatment of residential- and industrial waste water. However, concentrations of nitrogen and phosphorus in the streams are still approximately 3-4 and 2-3 times higher, respectively, than in unimpacted (reference) streams. 

Similar reductions in the total load of nitrogen and phosphorus from land to Danish coastal waters have been estimated for the period 1990-2016. The reductions are 45 and 63 % percent for nitrogen and phosphorus, respectively, if calculated as discharge weighted mean concentrations to take climatic differences between years into consideration. Considering the diffuse run-off of nitrogen alone, including leaching from cropped as well as uncropped areas and contribution of wastewater from scattered dwellings, the reduction is approximately 34%.  

In 2016, the total load from land was estimated to about 62 000 tons of nitrogen and 2300 tons of phosphorus. For most months, these amounts being significantly lower than the average for the period 1990-2015. This should be seen in the context that total run-off of water was 7% larger in 2016 than the yearly average for the period 1990-2015. 

As it is impossible to measure all water transported to the Sea, there is some statistical uncertainty on the estimated transports, as these needs to be based on both measurements of discharge and nutrient concentrations combined with model estimates for ungauged catchments.