Nitrates et orthophosphates continuent de baisser dans les rivières

None of the 6 major hydrographic basins in mainland France show an upward trend over this period. The most significant declines are observed in Adour-Garonne and Rhône-Méditerranée-Corse. The Artois-Picardie and Rhine-Meuse basins show the least pronounced declines, while Loire-Brittany and Seine-Normandy are close to the overall average trend.

• Over 2015-2017, average nitrate concentrations in the 55 catchment basins vary from 1 to 32 mg/L. Half exceed 14 mg/L and one quarter exceed 21 mg/L. • Four Seine-Normandy catchment basins have particularly high average concentrations (> 20 mg/L). They also show a chronic deterioration trend (index up 11 to 50% between 1998 and 2017). • In contrast, seven Western catchment basins have similarly degraded conditions (15-25 mg/L) but tend to improve over the long term (index down 11 to 50%).

• At the metropolitan scale, the orthophosphate index drops by 37% between 1998 and 2017. It declines by over 40% in four major basins and increases slightly in Rhine-Meuse (+2%). • In total, 48 catchment basins show a declining index.

• Unlike nitrates, concentration variability is lower: most catchment basins fall between 0.1 and 0.2 mg/L and are considered in "good chemical status" for this parameter under the Water Framework Directive. • Overall, the range of average inter-basin concentrations is 0.02 to 0.64 mg/L over 2015-2017. • The most degraded conditions are found in 2 catchment basins in Artois-Picardie and Rhine-Meuse (> 0.4 mg/L), with one showing a declining index and the other rising. The least degraded conditions are in 3 Rhône-Méditerranée-Corse basins (< 0.05 mg/L). • > Nitrates and phosphates in Europe: The 2018 report from the European Environment Agency indicates nitrate concentrations in European watercourses decreased 20% between 1992 and 2015, and phosphate concentrations by approximately 50%. • > Methodology • > Spatial and temporal coverage: Data for this study are extracted from the Naïades database of the water information system: www.eaufrance.fr. They cover the period from 1998 to present. Overseas departments are not included in the study. • > Analysis results are processed at the scale of 6 major metropolitan hydrographic basins and their subdivision into 55 catchment basins. • > Each catchment basin comprises connected hydrographic sub-sectors. Each sub-sector corresponds to a physical watercourse basin or a group of small watercourses with a homogeneous outlet. • > Monitoring data: Measurement points of monitoring networks are selected if: they have at least 1 water sample taken per quarter (total of at least 4 samples per year); they are present for at least two consecutive years. • > Concentration evolution index calculation: Annual average concentrations are calculated for each measurement point. They enable biannual evolution coefficients to be calculated at catchment basin scale. • > When a coefficient is missing from a time series, it is replaced by the weighted moving average of the 2 preceding and 2 following coefficients. For example, if a value is missing at position i, it is replaced by the average of positions i-1 and i+1 weighted by 0.5, and positions i-2 and i+2 weighted by 0.25. • > The basin index is then calculated by chaining coefficients from a base of 100, set in 1998. The metropolitan index is the average of catchment basin indices weighted by their area. • > The rate of change over the study period is calculated by linear regression of the index against years.