The occurrence and evaluation of LNAPLs contamination in urban areas of South Africa

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Phophi, Thilivhali Samuel
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University of the Free State
Petroleum liquids are a basic building block of our modern lives. Uses include fuels, lubricants, and the raw material for manufactured products. The by-product of these uses has been the inadvertent release of petroleum liquids. A result of our utilization of petroleum liquids is a legacy of soil and groundwater impacted by petroleum liquids. The aim of this research is to provide an overview of LNAPLs in South African urban areas, transport mechanism of the LNAPLs in the subsurface, framework for detecting and evaluating LNAPLs under South African conditions. LNAPL is a convenient label for petroleum liquids in soils and groundwater. The acronym stands for Light Non Aqueous Phase Liquid. “Light” highlights the fact that petroleum liquids (i.e., gasoline) are less dense than water; “Non Aqueous” highlights the fact that petroleum is not completely miscible in water. An LNAPL contamination assessment was conducted at a service station after a complaint was raised by a resident who found free product (petrol) in her borehole. A multitude of private boreholes were found during the hydrocensus survey. A petroleum contamination assessment was done through soil vapour survey (SVS), hand auger holes and rotary percussion drilling. No significant petroleum vapours were detected due to clay soil which has low transmission of vapours. Hand auger holes were restricted to shallow depth due to the lack of penetration though the clay soils. Percussion drilling was needed to gather groundwater information (i.e., quality and quantity). Free product (petrol) was found within the percussion and some of the private boreholes. Groundwater samples were collected and analyzed for BTEX (Benzene, Toluene, Ethyl benzene and Xylenes) compounds. No detectable levels of BTEX were found in the soil samples. Risk assessment was done suing the RBCA approach and BP RISC software. BTEX concentrations of groundwater samples have triggered the Tier 1 risk based screening level for the risk values of carcinogenic and non-carcinogenic through groundwater ingestion, indoor and outdoor vapour inhalation exposure pathways. BP RISC was used to conduct Tier 2 evaluation and carcinogenic risk does exist in the receptor borehole through groundwater ingestion risk pathway. AQUA-WIN finite element model was used to determine the abstraction rate that could be used to conduct a pump-and-treat system. Free product could be recovered within two years after commencing with pump and treat system. Due to the lack of South African regulations with regard to petroleum contamination, the pump and treat system was stopped as soon as the product was recovered and monitoring of the dissolved phase hydrocarbons was continued. The establishment of South African guidelines and risk assessment protocols for petroleum hydrocarbons is outlined and strongly recommended for implementation.
Dissertation (M.Sc. (Geohydrology))--University of the Free State, 2005, Petroleum products -- Environmental aspects, Petroleum pollution of soils -- South Africa, Urban pollution -- South Africa, Hydrocarbons -- Environmental aspects, Light nonaqueous phase liquids -- Environmental aspects