Concentrations are expressed as a median of annual mean concentrations. Up to three-year gaps of missing values have been interpolated or extrapolated. Only complete series with no missing values after this interpolation/extrapolation are included. The number of time series/river stations is shown in parentheses. The trend for 1992 to 2010 for each of the ecological quality classes has been linearly extended to 2027 — or when the concentration level becomes negative.
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