Field trials of ground conductivity methods for mapping aquitard "windows"

Abstract

The City of Fredericton relies almost exclusively on groundwater as a source for potable water. However, borehole logs indicate the absence of the protective clay aquitard that overlies the Fredericton aquifer in certain areas. These absences or "windows" may allow contaminants to enter the groundwater supply. Therefore, the Fredericton aquifer is potentially vulnerable to contamination from leaks or spills on the ground Clay, in general, has a cation exchange capacity (CEC), making it conductive. The clay overlying the Fredericton aquifer is more conductive than the sands surrounding it. Resistivity and ground-conductivity methods are two non-invasive geophysical techniques which are sensitive to electrical conductivity. These two techniques accurately, easily and economically image the absence or presence of clay near the ground surface in the Wilmot Park area. The apparent conductivities measured in areas near boreholes and where ground penetrating radar data indicate that the aquitard has been eroded are significantly lower in comparison to the apparent conductivities measured over areas where the clay layer is known to be intact at depths of approximately 5m. The "window" boundary was defined by performing ground conductivity surveys along an extensive grid. Subsurface stratigraphy was modelled using the WinSev5 resistivity depth sounding software and the Ghosh Spreadsheet. In addition, the depth to the top of the clay is estimated based on layered stratigraphic models.