The European Union Nitrates Directive (Council Directive 91/676/EEC concerning the protection of waters against pollution caused by nitrates from agricultural sources) aims to curb the introduction of excessive levels of nitrates into surface waters and groundwaters from agricultural fertilizers and waste. Excessive nitrate levels cause undesirable ecological changes in water and are a factor in harmful algal blooms. They also have adverse public health implications. The directive required Member States to carry out monitoring of surface waters and groundwater, identify nitrate-polluted waters, and designate vulnerable zones (i.e. zones draining into nitrate-polluted waters) by 20 December 1993. Action programmes for such zones were required to be established by 20 December 1995 in order to control nitrate pollution from agricultural sources. Member States have the option of applying these action programmes throughout their territory. Member States were also obliged to adopt codes of good agricultural practice; these are a binding part of action programmes.
Researchers from the University of Minnesota looked at strategies on how to reduce nitrate in rivers and streams more effectively. In their study, published in 2018 the journal Nature Geoscience, they found that wetlands mitigate nitrate pollution in rivers and streams five times more effectively than land-based mitigation approaches. In conducting the study, the research team used water samples gathered for more than four years from more than 200 waterways within intensively managed, 17,000-square-mile Minnesota River basin. In addition, they used geo-spatial data about land use in the watershed. The team managed to isolate the effect of wetlands on stream and river nitrate concentrations within large watersheds. The researchers also found that the reduction of nitrate as a result of temporary wetlands, such as riparian floodplains and wetlands that are not connected to the river network by surface water, was measurable and was greatest in high stream flows, when such characteristics are hydrologically connected to surface water.