Nitrogen concentrations in flow separated river water

Nitrogen (N) loads and concentrations have been successfully reduced in most Danish streams during the last 30 years. Thereby also reducing the impact of the main driver of marine eutrophication in Danish coastal waters. However, the trend in N-loads and concentrations vary substantially among the monitored streams. The understanding of this variation are of great importance and interest for the evaluation of measures implemented to combat N eutrophication and for forecasting of effects of further measures.

River hydrographs can be split into base flow and quick flow components and the N concentrations in these two components can, thereafter, be calculated. The N concentration in the two components varies over time showing both longer term and seasonal variation. The quick flow component typically having a high variation reflecting present days leaching of N from fields and this strata has been significantly reduced during the last 3 decades due to a more sustainable farming practices.

During base flow conditions, stream water typically holds less nitrogen due to N removal in groundwater. Reductions in agricultural nitrogen leaching over the past three decades has reduced concentrations in the quick flow component and reduced the load to ground water aquifers. As groundwater aquifers are often large with a capacity of several years of recharge, the response in base flow N-concentrations is expected to be slow compared to the response in quick flow. The low response of the N-concentrations in base flow have implications on the rate of change of the river concentrations and consequently riverine N-loads to coastal waters. In some cases, the base flow N-concentration might still be influenced by the larger N-leaching of the past (1960-1990).

We have analyzed a national data set for developments in N-concentrations during base flow and quick flow. The data set covers the in country range in catchment size, land use and geology. The data set spans 29 years covering the period 1990 – 2018. In addition, measurements from a few streams monitored for a longer period have been included in the analyses