Detailed Regional Hydrogeophysical Investigations - The Solbjerg Case

In cultivated areas with growing ground water abstraction there is an increasing need to map the vulnerability<br>of the aquifers and to delineate the aquifer systems circumstantially in order to obtain sufficient detailed and<br>reliable hydrogeological models. These models are demanded for the set-up of hydraulic frameworks used to<br>estimate the effects of abstractions on drawdown, on surface waters and on mobilization of unwanted ground<br>water qualities. These initiatives are required to ensure that future water supply can provide the same good<br>quality of drinking water as today.<br>A large number of the aquifers in Denmark are composed of Quaternary sand and gravel deposited in deep<br>valleys eroded in Tertiary clays and covered by an inhomogeneous moraine clay cap. It is generally observed<br>that variations of hydraulic conductivity of these formations are closely related to variations of electrical<br>formation resistivity.<br>In recent years the application of geophysical mapping of aquifers using electrical methods has increased<br>significantly in Denmark. By joint interpretation of spatially dense measured data a detailed and reliable<br>delineation of aquifers and their vulnerability is carried out.<br>The Municipal Water Supply of Aarhus has performed investigations to increase the number of abstraction<br>sites to provide supplementary capacity. A large area at the village of Solbjerg south of Aarhus has been<br>investigated using detailed geophysical mapping. The methods applied are the transient electromagnetic<br>method, and PA-CEP electrical profiling. The presented case demonstrates how the aquifer systems are<br>delineated and the vulnerability estimated from the results of the investigation.