Regional analysis of groundwater nitrate concentrations and trends in Denmark in regard to agricultural influence

Abstract

Abstract. The act of balancing between an intensive agriculture with a high potential for nitrate pollution and a drinking water supply almost entirely based on groundwater is a challenge faced by Denmark and similar regions around the globe. Since the 1980s, regulations implemented by Danish farmers have succeeded in optimizing the N (nitrogen) management at farm level. As a result, the upward agricultural N surplus trend has been reversed, and the N surplus has reduced by 30-55 % from 1980 to 2007 depending on region. The reduction in the N surplus served to reduce the losses of N from agriculture, with documented positive effects on nature and the environment in Denmark. In groundwater, the upward trend in nitrate concentrations was reversed around 1980, and a larger number of downward nitrate trends were seen in the youngest groundwater compared with the oldest groundwater. However, on average, approximately 48 % of the oxic monitored groundwater has nitrate concentrations above the groundwater and drinking water standards of 50 mg l −1 . Furthermore, trend analyses show that 33 % of all the monitored groundwater has upward nitrate trends, while only 18 % of the youngest groundwater has upward nitrate trends according to data sampled from 1988-2009. A regional analysis shows a correlation between a high level of N surplus in agriculture, high concentrations of nitrate in groundwater and the largest number of downward nitrate trends in groundwater in the livestock-dense northern and western parts of Denmark compared with the southeastern regions with lower livestock densities. These results indicate that the livestock farms dominating in northern and western parts of Denmark have achieved the largest reductions in N surpluses. Groundwater recharge age determinations allow comparison of long-term changes in N surplus in agriculture with changes in oxic groundwater quality. The presented data analysis is based on groundwater recharged from 1952-2003, but sampled from 1988-2009. Repetition of the nitrate trend analyses at five-year intervals using dating of the groundwater recharged in the coming years and a longer time series of the nitrate analyses can reveal the evolution in nitrate leaching from Danish agriculture during the past 10 yr. Similar analyses can be carried out to compare with other regions internationally.

Spatial distribution of soils determines export of nitrogen and dissolved organic carbon from an intensively managed agricultural landscape

“…The sandy nature implies a faster drainage of water and nutrients through the soil and a better aeration that favours mineralization and slows down denitrification (Scheffer and Schachtschabel, 2002). Therefore, and in correspondence with earlier Danish studies (Kronvang et al, 2008;Hansen et al, 2012), sandy soils may be the explanation for the higher NO − 3 concentrations in the northern stream water.…”

“…As pointed out in many studies (Böhlke and Denver, 1995;Ruiz et al, 2002a;Worrall and Burt, 2001), soils are able to store N, leading to poor correlations between N losses from agricultural soils and headwater quality. Consequently, N leaching is affected by past land use and management practices (Hansen et al, 2011(Hansen et al, , 2012, which varied a lot in this area during the past 300 yr (Caspersen and Fritzbøger, 2002), but a further study of this aspect was out the scope of the present study. Furthermore, the NO − 3 stored in soils can be released into sub-surface and groundwater after some time.…”

Management, regulation and environmental impacts of nitrogen fertilization in northwestern Europe under the Nitrates Directive; a benchmark study

“…They found that the frequency of downward nitrate trends in groundwater samples clearly increased with lower recharge age, providing proof that younger groundwater responds fastest to decreasing trends of SNB. Hansen et al (2012) further found that nitrate concentration decreased significantly more in areas with a high livestock density. Reported nitrate concentrations in Germany are higher than in the other northwestern EU member states because sampling is restricted to agricultural soils and focused on polluted regions.…”

Potential for biological nitrification inhibition to reduce nitrification and N2O emissions in pasture crop–livestock systems

“…Some of the reactive N excreted from intensive livestock systems (i.e. urine and feces) is recycled through agricultural systems in a limited way, results in N-saturation hotspots (farm soil NO 3 2 levels often reaching 300 to 400 kg N/ha per year), causing NO 3 2 contamination of groundwater Hansen et al, 2012). A major portion of the reactive N excreted from livestock systems is not recycled effectively through modern production systems as N source (WagnerRiddle et al, 2007;Centner and Newton, 2008;Schlesinger, 2009).…”