Ground water impact assessment for the proposed Sasol PFS at KSIA
Geological setting:
The regional geology in the vicinity of the KSIA is characterised by Quaternary aged unconsolidated
sedimentary formations of the Berea Formation which are underlain by the Permian aged
Pietermaritzburg Formation which are intruded by Jurrassic aged dolerite sill and dyke structures.
The Pietermaritzburg Formation bedrock is expected to out crop in some parts of the KSIA property
(Durban Geological Map- 2930 Durban, 1988). Based on the information obtained from the
geotechnical study undertaken by Davies Lynn & Partners the footprint area of the PFS can be
characterised as comprising of engineered fill material (resulting from the construction of the airport
and surrounding infrastructure) which comprises an interlayered sequence of residual clays (derived
form Shale/silty sandstone) and Berea Red moderately clayey sand to sandy clays up depth 4.5 to 5
m below the present level top of the asphalt layer covering the proposed PFS site.. Based on the
DCP result these material are mostly form of stiff in consistency (DLP Report N7638), June 2011).
Hydrogeological setting
The hydrogeology of the area comprises of unconsolidated to consolidated sediments of the Berea
Formation at shallow depths <10m, which in turn is underlain by interlayered sandstone shale
sequence of the Pietermaritzburg Formation. Hence ground water is controlled by the relatively
high permeability Berea Formation at shallow depths and lower permeability in the Pietermaritzburg
Formation. However the permeability of the Pietermaritzburg Formation is affected by the presence
of fractures and dolerite sills and dykes in the bedrock which will increase permeability if these
characteristics are present.
Based on the information obtained from the hydrogeological map (Groundwater Resources of RSA,
Sheet 2 1st Edition 1995, DWAF), the depth to the regional ground water table is expected to be 20
to 30m below ground level. Perched aquifer conditions are not unexpected and could occur at
shallower depths. The probability of drilling successful boreholes with a yield of >2L/s is 30 to 40 %
suggesting that ground water is unlikely to be a primary water resources in the area. The
groundwater quality is expected to be good with an electrical conductivity of <70 mS/m.
Risks of contamination of the groundwater and surface water
Although the geological formations have sufficient attenuation potential due to the clayey nature of
the unconsolidated and weathered bedrock, the possibility that the bedrock could be fractured and
intruded by dolerite is high thereby increasing the potential for migration of contamination into the
regional aquifer and possibly entering surface water bodies (e.g. uMdloti River). Hence the risk of
contamination of the underlying aquifer depends on the underlying rock formations i.e. high risk if
the underlying formations are fractured and shallow ground water conditions occur and low risk if
shallow formations have a low permeability i.e. clayey and the bedrock is not fractured with no
preferential flow paths to the regional aquifer and surface water bodies.
Impact Assessment
The potential for impacts on the ground water and related surface water bodies as a result of the
operation of the PFS at this facility depends on the nature of the underlying aquifer as described
above and the nature (frequency and volume) of the spill. The impact will be high in the event of a
major spill that is undetected for a long time and highly permeable underlying conditions. The
impact will be low if for relatively small spill events (<20L) and for low permeability aquifer
conditions i.e. high clay content and unfractured bedrock. Based on the geological setting and
information derived from the DL&P site investigations it would seem as that the site conditions in
the area where the underground storage tanks will be installed consists of engineered fill that
comprises of clayey materials with a firm to stiff consistency suggested naturally high attenuation
potential and no natural or artificial flow paths to the regional aquifer in the event of a spill leak
event that is timeously detected. Hence the potential for impacts on the regional aquifer are
considered low to medium and these can be lowered further to a low impact by installing adequate
housekeeping, monitoring and wet stock management systems as recommended in the DLP report
and the storm water management reports.