Nitrate response in Danish groundwater to agricultural N management and regulation since 1940

The presentation focuses on the Danish approach for mitigation of groundwater nitrate pollution caused by agriculture addressing both the approaches being developed to elucidate trends and the actual results. Historically, the increasing import of synthetic fertilizers and animal feed from the 1940s until 1975 caused an increasing leaching of nitrate into the groundwater. Since 1985, the Danish government has adopted national action plans and mitigation measures, including tools such as maximum animal stock density, better handling, and utilization of nitrogen in manure, maximum N-quota for specific crops, and the use of catch crops. A clear effect of this one-size-fits-all national general nitrogen regulation was seen until 2000 after a trend reversal of nitrate in oxic Danish groundwater from the mid-1980s1. The trend reversal is also reflected in the nitrogen surplus and use efficiency, and a higher prosperity of the society2. Since 2016 focus has been on developing cost-efficient geographically targeted mitigation measures as a supplement to the general regulation. The idea is that the local N regulation of a field is adjusted to the natural denitrification potential of the subsurface3. However, after 2016 the groundwater nitrate pollution tends to increase because farmers have been allowed to use more fertilizer for improving their economic conditions.
Drinking water with high levels of nitrate is a global problem linked to a variety of health problems including birth defects and cancer. In Denmark all drinking water originates from groundwater, and nitrate is not being removed by treatment at the waterworks. Results from a large epidemiological Danish study on the health effects of colorectal cancer on nitrate in drinking water found a 15 % higher risk of colorectal cancer when comparing the 1/5 parts of the population having the lowest nitrate (NO3-) concentrations (< 1.27 mg/l) to the 1/5 parts of the Danish population having the highest concentrations (≥ 9.25 mg/l) at public waterworks4. These results were recently used in a health-economic study demonstrating that the Danish society could save more than $300 million a year by reducing nitrate concentrations in drinking water and at the same time avoid 127 cases of colorectal cancer annually5.

1. Hansen et al., 2011. Trend Reversal of Nitrate in Danish Groundwater – A Reflection of Agricultural Practices and Nitrogen Surpluses since 1950. Environmental Science & Technology. https://doi.org/10.1021/es102334u

2. Hansen et al., 2017. Groundwater nitrate response to sustainable nitrogen management. Scientific Reports. https://doi.org/10.1038/s41598-017-07147-2.

3. Hansen et al., 2021. Assessment of complex subsurface redox structures for sustainable development of agriculture and the environment. Environmental Research Letters. https://doi.org/10.1088/1748-9326/abda6d

4. Schullehner et al., 2018. Nitrate in drinking water and colorectal cancer risk: A nationwide population-based cohort study. International Journal of Cancer, 143 (1), 73-79. https://doi.org/10.1002/ijc.31306

5. Jacobsen et al., 2024. Health-economic valuation of lowering nitrate standards in drinking water related to colorectal cancer in Denmark. Science of the Total Environment. https://doi.org/10.1016/j.scitotenv.2023.167368.