Assessment of aquifer vulnerability using 2D resistivity profiling Atle Dagestad and Einar Dalsegg; Geological Survey of Norway
The landscape in Norway is dominated by a post-glacial mountain terrain incised by deep narrow valleys. The geology is dominated by relatively unweathered Precambrian-Caledonian crystalline hard rock with an unevenly distributed cover of glacial, periglacial, marine and fluvial superficial deposits. Aquifers of importance for larger water supplies are mainly found in glaciofluvial and fluvial deposits along the valley bottoms. The exploited aquifers are normally shallow (10-20 metres deep) with a thin unsaturated zone and are recharged by a combination of river bank and precipitation infiltration. As population, infrastructure and agriculture are also concentrated along these same valley bottoms, the aquifers are normally potentially vulnerable to anthropogenic contamination.
The Geological Survey of Norway (NGU) has been involved in several projects aimed at assessing aquifer vulnerability. A clear geological conceptual model is essential in these studies: different geophysics methods, in combination with exploratory drilling, are widely used to construct such models. In the past few years NGU, has increasingly used 2D geoelectrical resistivity profiling in the context of such hydrogeological investigations. The utility of this method has been proven in various geological settings, especially in areas where sand/gravel aquifers are overlain by low resistivity deposits of marine silt and clay. Unlike the commonly-used ground penetrating radar (GPR), 2D resistivity offers good penetration through low resistivity deposits, and may therefore constitute an effective mapping tool in lowland areas of Norway where fine-grained marine deposits are common. In the context of aquifer vulnerability, the technique allows one to map the presence of low-permeability protective cover above aquifer units, and also permits an assessment of the continuity of such cover. The technique’s high penetration capacity, together with its ability to distinguish different sediment types, suggests that it may also prove a powerful tool in mapping deeper aquifer horizons within valley hydrostratigraphy.