Aarhus University Seal / Aarhus Universitets segl

No. 452: Watercourses 2019 - Chemical water quality and substance transport.

Thodsen, H., Tornbjerg, H., Bøgestrand, J., Larsen, S.E., Ovesen, N.B., Blicher-Mathiesen, G., Rolighed, J., Holm, H. & Kjeldgaard, A. 2021. Vandløb 2019 - Kemisk vandkvalitet og stoftransport. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 74 s. - Videnskabelig rapport nr. 452
http://dce2.au.dk/pub/SR452.pdf

Summary

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

Nitrogen and phosphorus load to the sea

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 35% for nitrogen, whereas an approximate 38% reduction in phosphorus is found, mainly due to improved treatment of residential- and industrial wastewater. However, concentrations of nitrogen and phosphorus in the streams are still approximately 5 and 2 times higher, respectively, than in un-impacted streams.

Reduced loads of nitrogen and phosphorus from land to Danish coastal waters have been calculated for the period 1990-2019. The reductions are 45% and 69% for nitrogen and phosphorus, respectively, if calculated as discharge-normalised annual loads (TN) and discharge-weighted mean annual concentrations (TP). Considering the diffuse runoff of nitrogen alone, including leaching from cropped as well as uncropped areas and contribution of wastewater from scattered dwellings, the reduction is approximately 35%.

In 2019, the total load from land was estimated to about 74 000 tons of nitrogen and 2100 tons of phosphorus. For most months, these amounts were significantly lower than the average for the period 1990-2019. The normalised diffuse nitrogen load was calculated to 61 000 ton N compared to 50 000 ton N in 2018. The normalised mean diffuse load for the presiding five years (2014 – 2018) is 51 000 ton N, ranging between 50 000 and 53 000 ton N. The annual runoff of water was 361 mm compared to 325 mm as the average from 1990-2018.

Since it is impossible to measure the total load of water and nutrients to the sea, the estimated loads are affected by uncertainty, as these loads need to be based on both measurements of discharge and nutrient concentrations combined with model estimates for ungauged catchments.

In this year’s report, presented time series of total phosphorus loads have a dark background covering the period 2008-2015, as total nitrogen analyses from this period probably are analysed using a wrong method, supposedly underestimating the concentration.