Geophysical Assessment of Groundwater Protective Layers

Layers covering aquifers can play a crucial role in protecting groundwater from infiltrating contaminants pending on their spatial distribution and hydrological properties. Non-invasive geophysical characterizations of groundwater protective layers provide important subsurface information at low cost and are highly desirable for the management of risks associated with groundwater contamination and runoff prediction. Geophysics ultimately employs measuring soil physical properties, and yields the prospect of gaining information beyond the point scale usually employed by hydrologists, therefore greatly reducing the uncertainty, especially in highly heterogenic geological settings.Soil electrical resistivity is one important soil physical property that depends primarily on structure and volume of pore space, soil water content, mineralization and clay content, thus making resistivity methods a natural choice for hydrologic surveys. Two technologies, which are applicable in the field, are High Resolution Multielectrode Geoelectrics and GeoRadar. The added value of combining the two methods offers the perspective of developing a survey technology applicable under wider conditions than any of the two methods on its own. However, a simple conversion of geophysical data into hydrological information is not commonly practicable. In order to improve the understanding of the relationships between geophysical measurements and soil physical and hydrological properties, correlation functions were derived, which effectively demanded the measurement of both hydrological and geophysical parameters.The protective potential of groundwater covering layers against contaminants is quantified by the infiltration time a contaminated volume of water would need to percolate through a unit area. In this context both the coefficient of permeability and the thickness of such layers must be identified with high accuracy. While thickness and structure determination of layers is a common application with a high expertise, the determination of soil hydrologic properties proofs to be a new ground. The rational is based on the idea that the hydraulic conductivity depends significantly on the grain size distribution and in particular on the clay content of fine-grained soils; consequently special interest was deployed in clay content variations.The results completely meet the expectations and support the field observations. A linear correlation between field measured resistivity and lab measured clay content was found, manifesting a trend of lower resistivities with an increase in clay content. Moreover, we were able to substantiate a dependence of resistivity from the lab measured saturated hydraulic conductivity. Although the uncertainty of estimating the saturated hydraulic conductivity is significant, results will be shown that support the practical use of the method established in accurately predicting variations with a precision that could be in the order of magnitude. Cost effective Geoelectric and Georadar surveys provide additional fundamental information required by hydrologists and engineers by greatly reducing the uncertainty accompanied with point scale measurements at a comparable precision, thus ultimately providing a tool for assessing ground water hampering sediments for a variety of tasks