An innovative procedure to assess multi-scale temporal trends in groundwater quality: Example of the nitrate in the Seine–Normandy basin, France

•Large and heterogeneous dataset: how to improve the assessment of trends.•Punctual and regional assessment of groundwater quality.•Homogeneity and nonparametric tests to determine recent groundwater quality trends.•We modify geostatistical approach to create spatial unit for regional trend analysis.•Improving the management of groundwater in compliance with the WFD requirements.