- ~~~~~~~~~~ Abstract. The Rhine in the French alluvial plain has been deprived of the main parts of its flooded areas. One of the consequences is the high level of nitrate in groundwater due to intensive agriculture except under forested areas. The purpose of this study is to show the role played by the forest ecosystem in influencing groundwater quality in relation to the hydrological regime, vegetation and soil characteristics. Phosphate and nitrogen levels in groundwater are compared in three forested sectors. Two are on a calcareous substrate, one of which is still flooded with summer high waters, whereas the other has not been flooded for 30 years. The third sector is on a decarbonated substrate which is flooded with high water in winter or early spring. In all the sectors phosphate, nitrate and ammonia are very low in the groundwater as compared to the surface water. Nitrate concentrations in groundwater differ widely between the three forested sites, whereas ammonia and phosphate remain relatively constant. Nitrate levels were the lowest in the sector with silty/clayey soils, whereas they were largely influenced in the loamy soil by hydrological conditions, whether flooded or not.