Identification of 3-dimensional flow pattern in a local quaternary gravel aquifer system using environmental isotope studies

The study area is situated in the Rhine-Graben about 15 km north of Karlsruhe (Germany). The river Rhine forms the western boundary of the area of investigation, which extends to about 25 km2. The in situ quaternary porous aquifer is used as reservoir for gravel extraction. On the other hand this aquifer represents important potable water resource for the whole region, which implies serious conflicts. Isotope hydrological investigations (stable isotope 2H, 18O and 3H) essentially contribute to a better insight into this complex hydrological system. The approximately 30 to 40 m deep quaternary gravel aquifer is partly subdivided in an upper and a lower part by fine grained layer like sandy and silty lenses (quaternary aquiclude). Hydrochemically the water of the upper part of the aquifer hardly differs from that of the lower one. But in respect to their environmental isotope contents the two parts of the aquifer are clearly different. Different hydraulic pressure conditions exist between the two parts of the aquifer with a spatial distribution over the investigation area, which induces exchange processes within the aquifer. With the help of isotope analyses these local situations can clearly be demonstrated and the in situ interactions monitored. The complex interaction within the aquifer and between surface water and ground water can be described by interpreting the environmental isotope data in connection with the hydrochemical investigations. The study shows that in the area under investigation along the river Rhine water infiltrates in both the upper and the lower part of the aquifer up to an amount of almost 100%. The different surface waters (small creeks and artificial lakes) take over the drainage function of the ground water in the area. The general flow pattern is essentially disturbed by active gravel pits and in addition by pumping wells of local water supply. The lake water, marked by a typical isotope signature, could be clearly indicated in several observation wells. These results give a distinct picture of the three-dimensional flow pattern. Besides the exchange rate of lake water to ground water, in one special case the ground water velocity could be estimated