CAT: An integrated geological, geophysical, geochemical and isotope hydrological approach to study the marine-ground water interaction

Full text: The Coastal Aquifer Test Field (CAT) is situated in northern Germany between the Elbe and Weser estuaries. Here the Leibniz Institute for Applied Geosciences (GGA) and its research partners study the interaction between ground water and seawater in a geologically complex and anthropogenically influenced environment. The site extends 50 km. in the N-S and 30 km. in the E-W direction. The geology is dominated by quaternary glacial and periglacial deposits of till, silt, gravel and sand. Topography is generally flat, reaching 40 m. above MSL where coarse glacial material is found at the surface, called 'Geest'. Postglacial fine clastic tidal flat, lagoonal and peat deposits are found in very flat areas, few meters above and below sea level, called 'Marsch'. Outside the dikes recent fine classic tidal flat sediments are deposited by the North Sea. On the Geest ridge ground water infiltrates and flows towards the Marsch areas which are drained by artificial ditches and towards the tidal flats. The seawater-freshwater interface is found close to the surface in the Marsch, sometimes off shore in the tidal flats and reaches deeper than 100m in the recharge regions of the Geest. The drinking water supply of the cities Bremerhaven and Cuxhaven and several smaller communities are based on the deep fresh ground water resources beneath the Geest. Two helicopter borne surveys were conducted during the years 2000 and 2001. Frequency domain electromagnetic data were inverted to electrical resistivities. The data identified the saltwater-freshwater interface and gave a three-dimensional insight into the geology. The lateral extent of a freshwater lense in the tidal flats of the North Sea was mapped for the first time. This survey was ground truthed with vertical electrical soundings, seismic transects and samples. It constrained the positions and hydraulic properties of glacial channels created during the quaternary by the 'Elster' and later glaciations. Several exploration boreholes were drilled and various geological, geophysical and hydraulic tests were applied at these boreholes. Sediment samples for geochemical investigations were taken and five multilevel wells were installed, some of them at the saltwater freshwater interface. The talk concentrates on the isotope hydrological investigations using D, 18O, Tritium, 3He, CFCs, SF6, 85Kr, 39Ar, 13C, 14C and 4He on a total of more than 170 samples. Age-depth profiles in the ground water from T/3He and 85Kr quantify the magnitude of ground water recharge in the infiltration areas. CFCs and SF6 only allow age estimates, due to contamination from industrial areas and a military airport, and due to anaerobic degradation in the marshes. Tritium in the fresh water springs of the tidal flats shows a fast component of ground water outflow towards the North Sea. 13C and 14C analysis in ground waters dated by T/3He and 85Kr gives new insight into the geochemical and isotopic evolution of the total dissolved inorganic carbon (TDIC). In this complex geochemical system with timescales for ground water flow in the order of several centuries to a few millennia the weakness of radiocarbon ground water dating becomes evident. It was tried to address this problem by the use of 4He and 39Ar. A future goal of the studies is the development of a three dimensional numerical model for the entire ground water system, simulating coupled density dependent ground water flow and transport. The model will be calibrated by geophysical and isotope hydrological methods. For the vicinity of the city of Bremerhaven in the southern part of the study area, such a model is in the calibration process. (author)