VARIATION IN NUTRIENT LEVELS OF THE GROUNDWATER IN THE UPPER RHINE ALLUVIAL FORESTS AS A CONSEQUENCE OF HYDROLOGICAL REGIME AND SOIL TEXTURE

The Rhine in the French alluvial plain has been deprived of the main p arts of its hooded areas. One of the consequences is the high level of nitrate in groundwater due to intensive agriculture except under fore sted areas. The purpose of this study is to show the role played by th e forest ecosystem in influencing groundwater quality in relation to t he hydrological regime, vegetation and soil characteristics. Phosphate and nitrogen levels in groundwater are compared in three forested sec tors. Two are on a calcareous substrate, one of which is still flooded with summer high waters, whereas the other has not been hooded for 30 years. The third sector is on a decarbonated substrate which is flood ed with high water in winter or early spring. In all the sectors phosp hate, nitrate and ammonia are very low in the groundwater as compared to the surface water. Nitrate concentrations in groundwater differ wid ely between the three forested sites, whereas ammonia and phosphate re main 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.