|dc.description.sponsorship||This thesis is part of the project that the South African government and
research institutions had initiated to study the fate and the transport of the
LNAPLs contaminant in groundwater. The aim of the dissertation is to
characterize the geology and hydrogeology of the Joint Fuel Depot site in East
London, as well as to characterize the state of groundwater which is exposed
to LNAPLs contamination from the tank’s leakage. This characterization has
lead to the drawing of a geological and hydrogeological conceptual model of
Site characterization has been initiated as the first step of the investigation to
provide detailed information on the geological and hydrogeological conditions
of the site. Such information is important as it leads to a better understanding
of the position and flow of LNAPLs in the sub-surface and how the
groundwater is affected.
The electrical resistivity tomography (ERT) was selected as one of the
techniques to delineate the geological units, to locate the aquifers as well as
weathered and fractured zones which are considered as preferential pathways
of groundwater and contaminants.
The literature review and site description had a permit to acquire relevant
information on the previous use of the ERT techniques in LNAPLs site
characterization and on the local geological and hydrogeological setting. ERT
techniques have proved to be applicable and appropriate for this investigation.
ERT is a 2-D electrical resistivity technique which uses an automatic multielectrode
instrument to inject a current into the ground through two
electrodes, and which measures the voltage drop at two other electrodes. The
injection of a current and the measurement of the voltage drop are
sequentially repeated along a line of several electrodes to produce a 2-D
resistivity distribution of the subsurface.
Like any other geophysical methods, the ERT results need to be
complemented by other field testing results to permit an integrated
interpretation which leads to the complete characterization of the LNAPLs site.
The field testing includes soil testing, groundwater testing, borehole logs and
groundwater levels. The soil testing provided the initial soil water content, soil
particle distribution and the spatial distribution of volatile organic carbons on
the site. Groundwater testing evaluates the presence of petroleum
hydrocarbons in the saturated zones. Borehole logs gave the nature and
thickness of geological units and water level data allowed to determine the
direction of the groundwater on the site.
The field testing results were very useful in ERT interpretation. They have
been used to compare to the contrasts revealed in the resistivity model
sections provided by the ERT surveys. This integrated interpretation has lead
to establish, firstly, that the Joint Fuel Depot site is underlain by four major
geological units including the sand, clayey sand, clay and dolerite sill; and
secondly, that there is evidence of LNPALs contaminant on the surface of the
From the integrated interpretation of the results, it could be concluded that
the ERT survey has proved to be applicable in the LNAPLs site
characterization. It is recommended that the ERT survey be used for threedimensional
resistivity distributions for a more accurate delineation of LNAPLs
plumes and that a time-lapse survey be considered to monitor the changes
and progress of LNAPLs contaminant in the subsurface.||en_ZA