Indicator Assessment

Nutrients in freshwater in Europe

Indicator Assessment
Prod-ID: IND-8-en
  Also known as: CSI 020 , WAT 003
Published 03 Sep 2013 Last modified 11 May 2021
29 min read

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  • Average nitrate concentrations in European groundwaters increased from 1992 to 1998, but have declined again since 2005.
  • On average, the nitrate concentration in European rivers declined by 0.5 mg N/l over the period 1992 to 2011 (20% relative to the average concentration), reflecting the effect of measures to reduce agricultural inputs of nitrate as well as improvement in wastewater treatment.
  • Average orthophosphate concentrations in European rivers have decreased markedly over the last two decades- On average concentrations declined by 0.08 mg P/l between 1992 and 2011 (72% decrease relative to the average concentration). Also average lake phosphorus concentration decreased over the period 1992-2011 (on average by 0.008 mg P/l, or 27% relative to the average concentration). The decrease in phosphorus concentrations reflects both improvement in wastewater treatment and reduction in phosphorus in detergents.
  • Overall, reductions in the levels of freshwater nutrients over the last two decades primarily reflect improvements in wastewater treatment. Emissions from agriculture continue to be a significant source.

Updated with 2018 data and text revised accordingly

Average concentrations of nutrients in European groundwater and surface waters (1992-2011)

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Nitrate in groundwaters: There was a slight increase in average annual mean nitrate concentration in European groundwaters from 1992 to 1998. Since 2005 the concentrations have declined again, and in 2011 the mean concentration had almost returned to the 1992 level.

Nitrate in rivers: At the European level nitrate concentrations have declined by 0.5 mg N/l on average,or 20% compared to the average concentration over the period.

Agriculture is the largest contributor of nitrogen pollution, and due to the EU Nitrate Directive and national measures the nitrogen pollution from agriculture has been reduced in some regions during the last 10-15 years, this reduced pressure is reflected in lower river nitrate concentrations.

Phosphorus in rivers. The average concentrations of orthophosphate in European rivers more than halved over the period 1992-2011. In many rivers the reduction started in the 1980s. The decrease is due to the measures introduced by national and European legislation, in particular the Urban Waste Water Treatment Directive [1], which involves the removal of nutrients. Also the change to use of phosphate-free detergents has contributed to lower phosphorus concentration.

Phosphorus in lakes. During the past few decades there has also been a gradual reduction in phosphorus concentrations in many European lakes. As treatment of urban wastewater has improved, phosphorus in detergents reduced, and many waste water outlets have been diverted away from lakes, phosphorus pollution from point sources is gradually becoming less important. However, diffuse runoff from agricultural land continues to be an important source of phosphorus in many European lakes. Moreover, phosphorus stored in the sediments can keep lake concentrations high and prevent improvement of water quality despite a reduction in inputs.

Nitrate concentrations in groundwater between 1992 and 2011 in different geographical regions of Europe.

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Present concentrations per country

See also WISE interactive maps: Nitrates in groundwater by country

Groundwater nitrate concentrations primarily reflect the relative proportion and intensity of agricultural activity. In 2011, 20 out of 32 countries had groundwater monitoring stations with average concentration above the threshold Groundwater Quality Standard of 50 mg NO3/l as laid down in the Groundwater Directive (2006/118/EC) [2]. Belgium and Spain had the highest proportion (more than 20%) of groundwater stations with average concentration above the standard, but there was also a high proportion (10-20%) of groundwater stations above the standard in Austria, Cyprus, Czech Republic, Denmark, Germany and the Netherlands. Groundwater nitrate concentrations were generally low (most or all groundwater stationsless than 10 mg NO3/l) in Albania, Bosnia and Herzegovina, Croatia, Finland, Iceland, Lithuania, Norway, Serbia and Sweden.

Trends in groundwater nitrate concentration (see also Fig. 1)

Looking at individual groundwater bodies (GWBs) there is wide variation in trends, with 28% of the GWBs showing significantly decreasing nitrate concentrations since 1992 (an additional 3% showed a marginally significant decrease), while 24% of the GWBs showed significantly increasing concentrations (an additional 3% marginally significant). The countries with the highest proportions of GWBs with significant decreasing trends were the Netherlands, Portugal and Slovakia.Along with Slovenia these countries also had the largest absolute (4.6-9.1 mg NO3/l on average) and relative (21-58 % compared to the average concentration) decrease over the period.

Geographical region time series and trends (Fig. 2)

There is marked variation in groundwater nitrate concentrations between different geographical regions of Europe. In Western Europe the concentrations are high, and the levels have been fairly stable over the whole period, with similar proportions of decreasing and increasing trends, and about half the GWBs with no significant trend. The other regions are represented by far fewer GWBs. The results show that Northern Europe is at the other end of the scale compared to Western Europe, with low concentrations. But as for Western Europe the levels have been fairly stable over time. In Eastern Europe the average concentrations started declining after 1996, but increased again after 2003. However, after a marked decline between 2010 and 2011, the levels are currently (2011) at about the same level as the start of the time series, and about 10 mg NO3/l lower than in Western and Southeastern Europe. In Southeastern Europe (only represented by Bulgaria) the concentration levels were high in the period 1997-2000. Disregarding this peak, there is still an overall increasing trend, with levels now slightly higher than in Western Europe. However, as for the other regions, the proportions of significantly increasing and decreasing trends were similar.

Nitrate concentrations in rivers between 1992 and 2011 in different geographical regions of Europe.

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Nitrate concentrations in rivers between 1992 and 2011 in different sea regions of Europe.

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Phosphorus concentrations in rivers (orthophosphate) between 1992 and 2011 in different geographical regions of Europe.

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Phosphorus concentrations in rivers (orthophosphate) between 1992 and 2011 in different sea regions of Europe

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Phosphorus concentrations in lakes (total phosphorus) between 1992 and 2011 in different geographical regions of Europe.

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Present concentrations per country 

See WISE interactive maps: Mean annual Nitrates in rivers 

Rivers draining land with intense agriculture or high population density generally have the highest nitrate concentrations. Rivers with nitrate concentrations exceeding 5.6 mg N/l are found predominantly in northwest France and southeast UK and northern Spain. However, a high proportion (more than 20%) of rivers with concentrations exceeding 3.6 mg N/l are found in many other countries, particularly in Belgium, the Czech Republic, Denmark, Germany and Luxembourg. Rivers in the more sparsely populated Northern Europe and mountainous regions generally have average concentrations less than 0.8 mg N/l.

Trends in nitrate concentration (see also Fig. 1)

Overall there has been a significant decrease in river nitrate concentrations at 38% of the stations (an additional 5% marginally significant), while there has been a significant increase at 10% of the stations (an additional 42% marginally significant). The countries with the highest proportions of river stations with significant decreasing trends are the Czech Republic, Denmark, Germany and Slovakia. Across Europe as a whole, the rate of improvement is still slow, reflecting the continued significance of agricultural nitrogen emissions. The Czech Republic, Denmark, Germany had the largest average decrease over the period (1.1-3.4 mg N/l, or 33-69% compared to the average concentration), while Bulgaria and Sweden also had large relative decrease (41 and 40 %, respectively).

Geographical region time series and trends (Fig. 3)

There is marked variation in river nitrate concentrations between regions, with Western Europe rivers having 2-3 mg N/l higher concentrations than Northern Europe, on average, and the remaining regions being somewhere in between. Except for the increasing trend in Southern Europe (0.6 mg N/l over the period on average), nitrate concentrations are generally decreasing (by on average 0.1, 0.3, 0.7 and 0.7 mg N/l over the period for North, East, Southeast and West, respectively)).

Sea region time series and trends (Fig. 4)

Nitrate concentrations in rivers vary markedly between the sea regions of Europe. The average nitrate concentration in rivers draining to the Greater North Sea is currently around 1-2 mg N/l higher than that in rivers feeding the the Black Sea, the Celtic Seas, Bay of Biscay and the Iberian Coast, 2-3 mg N/l higher than the rivers feeding the Baltic Sea and the Mediterranean Sea and around 3.5 mg N/l higher than that of rivers draining to the Arctic Ocean. The difference compared to the other sea regions was even larger at the beginning of the time series.

The Greater North Sea is the only sea region where there is a marked decreasing trend (1.0 mg N/l decrease over the period on average and 60% significantly declining trends). However, both for the Arctic Ocean, the Baltic Sea, the Black Sea and the Mediterranean Sea regions there are markedly more significant decreasing than increasing trends, but here the number of not significant trends is larger. For the Celtic Sea, Bay of Biscay and the Iberian Coast region there is virtually no trend, with equally many significantly increasing and decreasing trends, and a majority of the stations (64%) having no significant trend.


Present concentration per country

See also WISE interactive maps: Mean annual Orthophosphate in rivers & Mean annual Total Phosphorous in lakes

Relatively low concentrations of phosphorus in rivers and lakes are found in e.g. Northern Europe (Norway, Sweden, and Finland), the Alps, and the Pyrenees and Scotland, predominantly reflecting regions of low population density and/or high levels of wastewater collection and treatment. In contrast, relatively high concentrations (greater than 0.1 mg P/l P) are found in several regions with high population densities and intensive agriculture, including: Western Europe (southeast UK, the Netherlands, Belgium, western Germany, northernFrance), Southern Europe (southern Italy, central Spain and mid-Portugal), Eastern Europe (Hungary, Slovakia, Czech Republic, Poland), and South-Eastern Europe (Bulgaria, Former Yugoslav Republic of Macedonia, Serbia, Kosovo under UNSC Resolution 1244/99, Romania, Serbia, Turkey). Given that phosphorus concentrations greater than 0.1-0.2 mg P/l P are generally perceived to be sufficiently high to result in freshwater eutrophication, the observed high values in some regions of Europe are of particular concern.

Trends in phosphorus concentration (see also Fig. 1)

Average concentrations of orthophosphate in European rivers have decreased markedly since 1992. At 48% of the river stations there has been a significant decline in orthophosphate concentration since 1992 (an additional 6% marginally significant), while there has been a significant increase at only 7% of the stations (an additional 2% marginally significant). For lakes there has been a significant decline in total phosphorus concentrations since 1992 at 31% of the stations (an additional 7% marginally significant), while there has been a significant increase at 11% of the stations (an additional 2% marginally significant). This decrease reflects the success of legislative measures to reduce emissions of phosphorus such as those required by the Urban Waste Water Treatment Directive [1] (UWWTD).The countries showing the strongest decreasing trend (in terms of % significant decreasing trends, relative and/or absolute decrease over the period) were Austria (rivers only), Belgium (no consistent lake series), the Czech Republic (no consistent lake series), Denmark (lakes only), France (no consistent lake series), Germany (lakes only), the Netherlands (no consistent river series), Switzerland and the UK (rivers only).These are all countries with a large proportion of the population (more than 80%) connected to wastewater treatment (to a large extent tertiary), andexcept the Czech Republic and Switzerland all these countries were supposed to comply with the UWWTD by 2005 [3]. There are also other countries with high treatment levels (all also with compliance deadline in 2005), but where this has been the situation for some time, thus expecting less trend in phosphorus concentrations (Finland, Sweden) or where there are no consistent river or lake phosphorus dataseries (Greece, Italy, Spain).

Sea region time series and trends (Fig. 6)

Orthophosphate concentrations are generally lowest for rivers draining to the Baltic Sea and in particular the Arctic Ocean. For the other sea regions the concentration levels are currently fairly similar, but at the start of the time series, concentrations were highest for rivers draining to the Greater North Sea and the Celtic Seas, Bay of Biscay and the Iberian Coast. There are more significant decreasing than increasing trends in all regions, but the strongest decreasing trends are found in the Greater North Sea (54% significant decreasing trends, 0.13 mg P/l decline over the period on average) and the Celtic Seas, Bay of Biscay and the Iberian Coast regions (68% significant decreasing trends, 0.12 mg P/l decline over the period on average).

Geographical region time series and trends (Fig. 5 and Fig.7)

Northern Europe has markedly lower river orthophosphate concentrations than in the other regions of Europe. The same pattern is seen for lake total phosphorus concentrations. River orthophosphate concentrations have decreased in Eastern (54% significant decreasing trends, 0.06 mg P/l decline over the period on average) and Western Europe (64% significant decreasing trends, 0.12 mg P/l decline over the period on average). In Northern Europe there has hardly been any changes, while in Southeastern Europe the concentrations are highly variable, giving no clear trend overall.

Lake total phosphorus (Fig. 7) shows a similar strong decrease for Western Europe (53% significant decreasing trends, 0.04 mg P/l decline over the period on average), but the trend may be leveling out from 2005. There is virtually no trend in lake total phosphorus in Northern and Eastern Europe.

The difference between lake and river data for Eastern Europe is partly caused by the inclusion of a number of Czech and Slovak stations in the rivers dataset, with predominantly negative trends. Overall, the trend statistics for Eastern Europe lake total phosphorus indicates mainly decreasing trends (58% significant), but this decrease is hardly visible from the mean concentration time series (0.003 mg P/l decline over the period on average).


[1] The Urban Waste Water Directive (UWWD): Council Directive 91/271/EEC concerning urban waste-water treatment.

[2] The Groundwater Directive : Directive 2006/118/EC of the European Parliament and the Council of 12 December 2006 on the protection of groundwater against pollution and deterioration

[3] EEA Core Set of Indicators CSI024 Urban waste water treatment

Supporting information

Indicator definition

This indicator shows concentrations of phosphate and nitrate in rivers, total phosphorus in lakes and nitrate in groundwater bodies. The indicator can be used to illustrate geographical variations in current nutrient concentrations and temporal trends.


The concentration of nitrate is expressed as milligrams of nitrate per litre (mg NO3/l) for groundwater and milligrams of nitrate-nitrogen per litre (mg NO3-N/l) for rivers.

The concentration of phosphate and total phosphorus are expressed as milligrams of phosphorous per litre (mg P/l).


Policy context and targets

Context description

The environmental quality of freshwater with respect to eutrophication and nutrient concentrations is an objective of several directives. These include: the Nitrates Directive (91/676/EEC), aimed at reducing nitrate pollution from agricultural land; the Urban Waste Water Treatment Directive (91/271/EEC), aimed at reducing pollution from sewage treatment works and certain industries; the Industrial Emissions Directive (2010/75/EU), aimed at reducing emissions from industry to air, water and land; the Convention on Long-range Transboundary Air Pollution and the National Emission Ceilings Directive, aimed at reducing air pollution to, inter alia, avoid eutrophication of surface waters from air pollution; and the Water Framework Directive, which requires the achievement of good ecological status or good ecological potential of surface waters by 2015, unless exemptions are applied. The Water Framework Directive also requires the achievement of good chemical and good quantitative groundwater status by 2015 as well as the reversal of any significant and sustained upward trend in the concentration of any pollutant. In addition, the Drinking Water Directive (98/83/EC) sets the maximum allowable concentration for nitrate of 50 mg NO3/l. It has been shown that drinking water in excess of the nitrate limit can result in adverse health effects, especially in infants less than two months old. Groundwater is a very important source of drinking water in many countries and is often used untreated, particularly from private wells.

Among the key principles of The Seventh Environment Action Programme of the European Community 2014-2020 are the 'full integration of environmental requirements and considerations into other policies', 'better implementation of legislation' and 'more and wiser investment for environment and climate policy'. This could result in a more intense application of agri-environmental measures to reduce nutrient pollution of the aquatic environment, e.g. in the Common Agricultural Policy, which after the reform in 2013 has an even stronger focus on sustainable farming and innovation. Reducing nutrient pollution from agriculture is also an important aspect of the European Green Deal and the ‘Farm to Fork’ Strategy. Other action points in the European Green Deal are also related to reducing nutrient pollution, e.g. ‘Zero pollution action plan for water, air and soil’ and ‘Measures to address the main drivers of biodiversity loss’.


This indicator is not directly related to a specific policy target. The environmental quality of surface waters with respect to eutrophication and nutrient concentrations is, however, an objective of several directives:

  • Drinking Water Directive (98/ 83/EC) — maximum allowable concentration for nitrate of 50 mg/l.
  • Surface Water for Drinking Directive (75/440/EEC) — guideline concentration for nitrate of 25 mg/l.
  • Nitrates Directive (91/676/EEC) — requires the identification of groundwater sites/bodies where annual average nitrate concentrations exceed or could exceed 50 mg NO3/l.
  • Urban Waste Water Treatment Directive (91/71/EEC) — aims to decrease organic pollution.
  • The UN Sustainable Development Goals also include qualitative targets related to freshwater nutrient pollution. The most relevant targets are 6.3 on improving water quality and 6.6 on protecting and restoring water-related ecosystems, but also 6.1 on safe drinking water and several other targets with a more indirect coupling to nutrient pollution.

Related policy documents

  • COM (2001) 31 final. Environment 2010.
    Environment 2010: Our future, our choice, 6th Environmental Action Programme, Communication from the Commission to the Council, the European Parliament, the Economic and Social Committee and the Committee of the Regions. COM (2001) 31 final.
  • Council Directive (91/271/EEC) of 21 May 1991
    Council Directive of 21 May 1991 concerning urban waste water treatment (91/271/EEC)
  • Council Directive (91/676/EEC) 12 December 1991
    Council Directive of 12 December 1991 concerning the protection of waters against pollution caused by nitrates from agricultural sources (91/676/EEC).
  • Council Directive 75/440/EEC
    Council Directive 75/440/EEC of 16 June 1975 concerning the quality required of surface water intended for the abstraction of drinking water in the Member States
  • Council Directive 96/61/EC (IPPC)
    Council Directive 96/61/EC of 24 September 1996 concerning Integrated Pollution Prevention and Control (IPPC). Official Journal L 257.
  • Council Directive 98/83/EC
    Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption
  • Environmental Health Action Plan for Europe (EHAPE)
    WHO/Europe programme on environmental health policy.
  • The Future We Want –Declaration of the UN Conference on Sustainable Development, Rio (2012)
    The Future We Want is the declaration on sustainable development and a green economy adopted at the UN Conference on Sustainable Development in Rio on June 19, 2012. The Declaration includes broad sustainability objectives within themes of Poverty Eradication, Food Security and Sustainable Agriculture, Energy, Sustainable Transport, Sustainable Cities, Health and Population and Promoting Full and Productive Employment. It calls for the negotiation and adoption of internationally agreed Sustainable Development Goals by end 2014. It also calls for a UN resolution strengthening and consolidating UNEP both financially and institutionally so that it can better disseminate environmental information and provide capacity building for countries.
  • Water Framework Directive (WFD) 2000/60/EC
    Water Framework Directive (WFD) 2000/60/EC: Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy.


Methodology for indicator calculation

Source of data

The data on water quality of rivers, lakes and groundwater in Waterbase are collected annually through the WISE SoE - Water Quality (WISE-6) data collection process. It includes data on nutrients, organic matter, hazardous substances and general physico-chemical parameters as well as for biological data for rivers and lakes. The WISE-6 data flow was new as of 2019, and has from 2020 replaced WISE-4. This reporting obligation is an Eionet core data flow. A request is sent to National Focal Points and National Reference Centres every year with reference to templates to use and guidelines. As of 2015, WISE SoE - Water Quality (WISE-6)supersedes Eurowaternet reporting.

The data in Waterbase is a sub-sample of national data assembled for the purpose of providing comparable indicators of the pressures, state and impact of waters on a European-wide scale. The data sets are not intended for assessing compliance with any European directive or any other legal instrument. Information on the sub-national scales should be sought from other sources.

Site selection

Data from all reported monitoring sites are extracted for the indicator assessment. Some data are excluded following the QC process (see the QC section below). The time series analysis is based on complete time series only (see Inter/extrapolation and consistent time series below). For groundwater, the time series are based on data for groundwater bodies, not individual monitoring sites.

The determinands selected for the indicator and extracted from Waterbase are:

  • for groundwater: nitrate,
  • for rivers: nitrate, total oxidised nitrogen and orthophosphate,
  • for lakes: total phosphorus.

For rivers, total oxidised nitrogen is used instead of nitrate when nitrate data are missing. If both are monitored at the same site and at the same time, nitrate values are given precedence. All values are labelled as nitrate in the graphs, but it is indicated in the graph notes for which countries' total oxidised nitrogen data are used.

Annual mean concentrations are used as a basis in the indicator analyses. Unless the country reports aggregated data, the aggregation to annual mean concentrations is done by the EEA. Countries are asked to substitute any sample results below the limit of quantification (LOQ) by a value equivalent to half of the LOQ before calculating the site annual mean values. The same principle is applied by the EEA.

The annual data in most cases represent the whole year, but data are used also if they represent shorter periods. Up until 2012, data could be reported at different temporal aggregation levels. Here, annual data have been selected, but if this was not available, seasonal data were selected according to a specific order of preference.

Quality control (QC)

An automatic QC procedure is applied when data are reported, including checking that the values are within a certain range defined for each determinand. Automatic outlier tests based on z scores are also applied, both to the disaggregated and aggregated data, excluding data failing the tests from further analysis.  In addition, a semi-manual procedure is applied, to identify issues that are not identified in the automatic outlier tests. The focus is particularly on suspicious values having a major impact on the country time series and on the most recently reported data. This comprises e.g.: 
• values not so different from values in other parts of the time series, but deviating strongly from the values closer in time;
• consecutive values deviating strongly from the rest of the time series (including step changes);
• whole time series deviating strongly in level compared to other time series for that country and determinand;
• where values for a specific year are consistently far higher or lower than the remaining values for that country and determinand.
Such values are removed from the analysis (both time series/trend and present state analysis) and checked with the countries. Depending on the response from the countries, the values are corrected, flagged as outliers or flagged as confirmed valid. Any response affecting the indicator analysis is corrected in the next update of the indicator.

Inter/extrapolation and consistent time series 

For time series analyses, only series that are complete after inter/extrapolation (i.e. no missing values in the site data series) are used. This is to ensure that the aggregated time series are consistent, i.e. including the same sites throughout the time series. In this way, assessments are based on actual changes in concentration, and not changes in the number of sites. For the trend analysis, it is essential that the same time period is used for the different sites, so that the results are comparable. However, the statistical approach chosen can handle gaps in the data series, so inter/extrapolation is not applied here. For rivers and lakes, 'sites' in this context means individual monitoring sites. For groundwater it means groundwater bodies, i.e. the basis for inter/extrapolation and selection of complete data series is groundwater body data series. Each groundwater body may have several monitoring sites, and in some cases the number of monitoring sites has changed over the years. This means that some of the complete data series for groundwater (after inter/extrapolation) are not truly consistent and must hence be regarded as more uncertain than the complete series for lakes and rivers. The purpose of choosing this approach is to increase the number of groundwater time series in the analyses.

Changes in methodology: Site selection and inter/extrapolation 
Until 2006, only complete time series (values for all years from 1992 to 2004) were included in the assessment. However, a large proportion of sites were excluded by this criterion. To allow the use of a considerably larger share of the available data, it was decided in 2007 (i.e. when analysing data up until 2005) to include all time series with at least 7 years of data. This was a trade-off between the need for statistical rigidity and the need to include as much data as possible in the assessment. However, the shorter series included might represent different parts of the whole time interval and the overall picture may therefore not be reliable.

In 2009, it was decided to inter/extrapolate all gaps of missing values of 1-2 years for each site. At the beginning or end of the data series, one missing value was replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values were replaced by the values next to them for gaps of 2 years and by the average of the 2 neighboring values for gaps of 1 year.

In 2010, this approach was modified, allowing for gaps of up to 3 years, both at the ends and in the middle of the data series. At the beginning or end of the data series, up to 3 years of missing values are replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values are replaced by the values next to them, except for gaps of 1 year and for the middle year in gaps of 3 years, where missing values were replaced by the average of the 2 neighboring values.

In 2018, this approach was slightly modified using linear interpolation for gap filling in the middle of the time series. Moreover, if data were available from 1989-1991 these were applied in the gap filling procedure, making it possible to interpolate instead of extrapolating at the beginning of the time series.

Only time series with no missing years for the whole period from 1992 or 2000 after such inter/extrapolation are included in the assessment. Even if the gap filling is not applied in the trend analyses, the same time series are used, for easier comparison of the time series and trend analysis results. The gap filling procedure increases the number of sites that can be included in the time series/trend analysis. Using also the shorter time period from 2000 allows the inclusion of more sites, making the data more representative. Still, the number of sites in the time series/trend analysis is markedly lower compared with the analysis of the present state, where all available data can be used.

Aggregation of time series 

The selected time series (see above) are aggregated to country and European level by averaging across all sites for each year.

Trend analyses

Trends are analysed by the Mann-Kendall method (Jassby et al., 2020) in the free software R (R Core Team 2020), using the wql package. This is a non-parametric test suggested by Mann (1945) and has been extensively used for environmental time series (Hipel and McLeod, 2005). Mann-Kendall is a test for a monotonic trend in a time series y(x), which in this analysis is nutrient concentration (y) as a function of year (x). The test is based on Kendall's rank correlation, which measures the strength of monotonic association between the vectors x and y. In the case of no ties in the x and y variables, Kendall's rank correlation coefficient, tau, may be expressed as tau=S/D where S=sum_{i<j} (sign(x[j]-x[i])*sign(y[j]-y[i])) and D = n(n-1)/2. S is called the score and D, the denominator, is the maximum possible value of S. The p-value of tau is computed by an algorithm given by Best and Gipps (1974). The tests reported here are two-sided (testing for both increasing and decreasing trends). Data series with p-value <0.05 are reported as significantly increasing or decreasing, while data series with p-value >= 0.05 and <0.10 are reported as marginally increasing or decreasing. The results are summarised by calculating the percentage of sites (groundwater body/river site/lake site) within each category relative to all sites within the specific aggregation (Europe or country). The test analyses only the direction and significance of the change, not the size of the change.

The size of the change is estimated by calculating the Sen slope (or the Theil or Theil-Sen slope) (Theil 1950; Sen 1968) using the R software. The Sen slope is a non-parametric method where the slope m is determined as the median of all slopes (yj−yi)/(xj−xi) when joining all pairs of observations (xi, yi). Here the slope is calculated as the change per year for each site. This is summarised by calculating the average slope (regardless of the significance of the trend) for all sites in Europe or a country. For the relative Sen slope (Sen slope %), the slope joining each pair of observations is divided by the first of the pair before the overall median is taken and multiplied by 100. Again, this is summarised for Europe or individual countries by averaging across sites. The Sen slope was introduced for this indicator in 2013.

The Mann-Kendall method or the Sen slope will only reveal monotonic trends and will not identify changes in the direction of the time series over time. Hence a combination of approaches is used to describe the time series: a visual inspection of the time series, describing whether the general impression is a monotonic trend, no apparent trend, clear shifts in direction of the trend or high variability with no clear direction; an evaluation of significant versus non-significant and decreasing versus increasing monotonic trends using the Mann-Kendall results; an evaluation of the average size of the monotonic trends using the Sen slope results.

Current concentration distributions:

For analysis of the present state, average concentrations are calculated over the last 3 years. Outliers and suspicious values are removed before averaging. In this case all groundwater bodies and lake and river sites can be used, which is a far higher number than those that have complete time series after inter/extrapolation. The 3-year average is used to remove some inter-annual variability. Also, since data are not available for all sites each year, selecting data from 3 years will give more sites. The average value thus represents 1, 2 or 3 years. The sites are assigned to different concentration classes and summarised per country (count of sites per concentration class).
• The classes defining values for nitrate are based on typical background concentrations in the different water categories and the legislative standard (11.3 mg N/l) and guide value (5.6 mg N/l).
• The classes defining values for phosphate (rivers) and total phosphorus (lakes) concentrations are based on typical background concentrations in the different water categories and on the range of concentrations found in Waterbase, and only give an indication of the relative distribution of concentrations of phosphorus in each country.

Methodology for gap filling

Methodology for gap filling is described above (under inter/extrapolation and consistent time series).

Methodology references



Methodology uncertainty

Nutrient conditions vary throughout the year depending on, for example, season and flow conditions. Hence, the annual average concentrations should ideally be based on samples collected throughout the year. Using annual averages representing only part of the year introduces some uncertainty, but it also makes it possible to include more sites, which reduces the uncertainty in spatial coverage. Moreover, the majority of the annual averages represent the whole year.

Nitrate concentrations in groundwater originate mainly from anthropogenic influence caused by agricultural land-use. Concentrations in water are the effect of a multidimensional and time-related process, which varies from groundwater body to groundwater body and is, as yet, less quantified. To properly evaluate the nitrate concentration in groundwater and its development, closely-related parameters such as ammonium and dissolved oxygen should be taken into account.

Data sets uncertainty

This indicator is meant to give a representative overview of nutrient conditions in European rivers, lakes and groundwater. This means it should reflect variability in nutrient conditions over space and time. Countries are asked to provide data on rivers, lakes and important groundwater bodies according to specified criteria. 

The datasets for groundwater and rivers include almost all countries within the EEA, but the time coverage varies from country to country. The coverage of lakes is less good. It is assumed that the data from each country represents the variability in space in their country. Likewise, it is assumed that the sampling frequency is sufficiently high to reflect variability in time. In practice, the representativeness will vary between countries. 

Each annual update of the indicator is based on the updated set of monitoring sites. This also means that due to changes in the database, including changes in the QC procedure that excludes or re-includes individual sites or samples and retroactive reporting of data for the past periods, which may re-introduce lost time series that were not used in the recent indicator assessments, the derived results of the assessment vary in comparison to previous assessments.

Waterbase contains a large amount of data collected throughout many years. Ensuring the quality of the data has always been a high priority. A revision of Waterbase reporting and the database-composition process took place in the period 2015–2017. This included restructuring of the data model and corresponding reporting templates; transformation of the legacy data (i.e. data reported in the past, for the period up until and including 2012); re-definition of specific data fields, such as aggregation period defining the length of aggregation in a year; update of the datasets according to correspondence with national reporters; recodification of monitoring site codes across EIONET dataflows; and connection of the legacy data time series with the newly-reported data in restructured reporting templates. Still, suspicious values or time series are sometimes detected and the automatic QC routines exclude some of the data. Through the communication with the reporting countries, the quality of the database can be further improved.

Rationale uncertainty

Using annual average values provides an overview of general trends and geographical patterns in line with the aim of the indicator. However, the severity of shorter-term, high-nutrient periods will not be reflected.

Data sources

Other info

DPSIR: State
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • CSI 020
  • WAT 003
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