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Methodology

Annual mean concentrations are used as a basis in the indicator analyses. The aggregation to annual mean concentrations is done by the EEA, unless the country has reported aggregated data only.

Automatic quality control procedures are applied both to the disaggregated and aggregated data, and data failing certain tests are excluded from further analysis. In addition, a semi-manual procedure is applied, focusing on suspicious values having a major impact on the country time series and on the most recently reported data. This comprises e.g.:

• outliers;
• consecutive values deviating strongly from the rest of the time series;
• 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 and checked with the country.

For time series analyses, only complete series after inter/extrapolation are used. This is to ensure that the aggregated time series are consistent, i.e. including the same sites throughout. Inter/extrapolation of gaps up to 3 years are allowed, to increase the number of available time series. At the beginning or end of the data series 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 linearly interpolated. The selected time series are aggregated to country and European level by averaging across all sites for each year.

Trends are analysed with the Mann-Kendall method  in the free software R , using the wql package. This is a non-parametric test suggested by Mann (1945) and has been extensively used for environmental time series . 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 size of the change is estimated by calculating the Sen slope . Absolute and relative Sen slopes are summarized across Europe and countries by averaging. For the trend analysis the same time series as for the time series analysis are used, but without gap filling.

For analysis of the present state, average concentrations are calculated across the last 3 years with data. In this way data from far more groundwater bodies and lake and river sites can be used than in the time series analysis. 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 gives more sites. The sites are assigned to different concentration classes and summarised per country (percentage of sites per concentration class). The purpose of the analysis is to compare the distribution of concentrations among countries. The class boundaries are thus mainly selected to represent the range of concentrations and are neither linked to targets or goals of specific policies nor to national based thresholds. The only exception is the threshold of 50 mg NO₃/l (11.3 mg NO₃-N/l), which is related to the maximum allowable concentration for nitrate in the Drinking Water Directive (2020/2184) the Groundwater Directive (2006/118).