Exchange between a river and groundwater, assessed with hydrochemical data
Abstract. We describe the chemical composition of groundwater from an alluvial granular aquifer in a valley fill flood plain (River Thur Valley). The aim of this work is to investigate the chemical processes, which are necessary to manage groundwater quality during river restorations. The river flows along this valley and is mostly downwelling on its way, indirectly through an unsaturated zone in the upstream part, and directly through the water-saturated bed in the downstream part. River Thur has been channelized with barriers for more than a century. Since 1992, the authorities started to restore sections of River Thur with river-bed enlargements. Several wells near the river draw groundwater, which is partly to mainly recharged by bank filtration. Many alpine and perialpine rivers have a total mineralization in the order of 200–400 mg L−1 and the chemical composition of the main constituents is of a Ca-Mg-HCO3-(NO3) type. The groundwater composition changes with increasing distance from the downwelling river and with increasing depth in the aquifer. The groundwater body consists of a mixture of groundwater from the seepage of precipitation and from river-bank filtration. The main difference between river water and groundwater results from the microbial activity in river-bed and bank materials. This activity leads to a consumption of O2 and to a higher partial pressure in CO2 in the groundwater. A spatial distribution of different groundwater types was established with geochemical mapping of concentration data, and with fluid logging. Criteria for the distinction of different groundwater compositions are the distance of a well from the river and the subsurface residence time of the groundwater to reach this well. Some of the groundwater has a very short residence time in the subsurface of days to weeks, and some of months to years. Groundwater in the River Thur valley aquifer is not pristine. The land use in the flood plain and the seasonal and climatic conditions (e.g., hot dry summer 2003) result in alterations of the natural geochemical groundwater composition. Time-series measurements of nitrate (from agricultural impact in the catchment) and chloride showed temporal changes in groundwater composition. Bed enlargements in river corridors can lead to a reduction of groundwater residence times and to an increase of riverbank filtration in nearby wells, which increases their contamination risk.