Nitrogen in surface water

The total quantity of nitrogen compounds in surface water declined by 45 percent between 1990 and 2021, from 158 million kilograms of nitrogen (N) to 87 million kilograms. The main sources of nitrogen in surface water are run-off and leaching from agricultural land and natural areas (which together account for nearly 60 percent), residual discharges from the sewerage system and waste water treatment plants (20 percent) and atmospheric deposition (17 percent, excluding North Sea deposition). All three are indirect sources, with nitrogen entering surface water through soil and groundwater, sewage water and the air, respectively. The discharge of nitrogen from industry and households directly into surface water, for example, now plays only a marginal role (5 percent).

The figures for atmospheric deposition, overflows and stormwater drains for 2021 have not been calculated, and therefore these figures are replicated from the 2020 figures.

Variation in nitrogen pollution from agricultural and natural land

Most sources show a gradual decrease in the concentration of nitrogen compounds in surface water during the period 1990-2021, but the situation is different with respect to run-off and leaching from agricultural land and natural areas. This is the process by which nitrogen compounds – originating from livestock manure or fertilizers – are washed out of the soil by rainwater (run-off) or via groundwater (leaching), and then end up in surface water (rivers, lakes and reservoirs). The same process occurs with phosphorus compounds.

The presence of nitrogen from these sources in groundwater is determined primarily by the quantities of fertilizer applied to agricultural land each year, but also by differences in annual precipitation levels. In dry years, run-off is significantly lower than in wetter years, even though the same amount of nitrogen may be present in the soil. Nitrogen pollution due to run-off and leaching from agricultural land and natural areas decreased by 22 percent between 1990 and 2021.

Less nitrogen pollution in treated waste water due to European requirements

The source ‘sewerage system and water treatment’ actually consists of a number of different sources: discharges from waste water treatment plants, stormwater drains, sewage overflow and (until the mid-1990s) untreated sewage. Of these sources, the most important source of nitrogen pollution are (residual) discharges or effluents from waste water treatment plants. In the Netherlands, the sewage from virtually all households and businesses ends up in one of the country’s 313 waste water treatment plants. Households are a particularly important source of nitrogen compounds in sewage: human urine and faeces, as well as cleaning products, contain significant concentrations of nitrogen compounds. Most nitrogen compounds (and other pollutants) are removed from sewage in the treatment plants, but there is always some residual discharge. Between 1998 and 2006, stricter European regulations meant that the water authorities that run the treatment plants had to reduce these residual discharges significantly. This was achieved through technological modifications in the water treatment process. As a result, between 1990 and 2021, the quantity of residual nitrogen discharged was reduced by 63 percent. The treatment efficiency for nitrogen compounds rose from 52 percent in 1990 to 85 percent in 2021, at the national level.

In addition to the discharge of treated wastewater, untreated stormwater sometimes also ends up in surface water. This is generally only moderately polluted. The Netherlands has two types of sewerage systems: mixed sewerage systems and separated systems. Mixed sewerage systems have emergency outlets for the release of stormwater during heavy rainfall, known as overflows. Waste water released through these overflows ends up in surface water. In separated sewerage systems, the rainwater that collects on roads and streets or on rooftops is kept separate from other wastewater and discharged into surface water through stormwater drains. The water in stormwater drains may be moderately polluted, but is much cleaner than wastewater from households and businesses.

Both types of sewerage systems lead to the discharge of additional nitrogen compounds into surface water. In 2021, such discharges amounted to 2.6 million kilograms of nitrogen, or nearly 3 percent of the total nitrogen in surface water in the Netherlands. The volume of nitrogen discharged through stormwater drains more than doubled between 1990 and 2021 as separated sewerage systems have become more common. Overflow discharges were reduced by half during the same period due to improvements to the country’s sewerage systems, such as increased storage capacity.

Atmospheric deposition

Over one sixth (17 percent) of the nitrogen in surface water can be attributed to nitrogen deposition into surface water directly from the atmosphere (including both dry and wet deposition). The original source of these nitrogen compounds is emissions from agriculture, industry and traffic into the atmosphere, both in the Netherlands and in neighbouring countries. The figures presented in this dossier refer only to nitrogen compounds that are deposited directly into fresh surface water (inland) plus a narrow strip of the sea along the coast. Atmospheric deposition into the North Sea (open water) is not included.

Other sources of nitrogen pollution

Other sources of nitrogen pollution in surface water include direct discharges from industrial plants, agriculture and nature (excluding the types of leaching and run-off mentioned above), a small number of households that are not connected to the sewerage system, and other sectors.

Direct discharges from industry contributed more than 8 percent of the total nitrogen present in surface water in 1990. Efforts to prevent such discharges had reduced that share significantly to 2.2 percent by 2021. Agriculture and natural areas still contribute to the direct discharge of nitrogen into surface water through greenhouse horticulture, droppings from waterfowl, farmyard run-off and manure that ends up in ditches accidentally when it is being spread onto farmland. Farmyard run-off occurs when nutrients (such as nitrogen compounds) present in silage stored on farms is carried away into surrounding surface water by rain water.