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Concentrations are expressed as median of annual average concentrations. Data are from representative river stations. Stations that have no designation of type are assumed to be representative and are included in the analysis. Consistent time series trends have been calculated using only stations that have recorded concentrations for each year included in the time series (see Methodology section for further details). Consistent time series then averaged for the 3 time periods 1992 to 1995, 1996 to 1999 and 2000 to 2002.