Field trials of ground conductivity methods for mapping aquitard "windows"
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Date
2000
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University of New Brunswick
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.