DNAPLs in South African fractured aquifers : occurrence, fate and management

[摘要] English: The potential for the widespread contamination of groundwater by Dense Non-aqueous Liquids (DNAPLs) in South Africa is substantial, because of the extensive production, transport, utilisation, and disposal of large volumes of DNAPL chemicals. There are a great number of potential sites where DNAPLs may have been released to the subsurface in varying quantities. A basic understanding of the nature and occurrence of groundwater in South Africa aquifer systems is a prerequisite for assessment, monitoring and management of DNAPL contaminated sites. The physical properties of an aquifer that have the greatest impact on the fate and transport of DNAPL contaminants, are the flow rate and flow mechanism present, and the hydraulic conductivity. The major South African aquifer systems have been classified in relation to the dominant flow mechanisms and flow characteristics. The majority of the utilised South African aquifers can be classified as intergranular fractured aquifers. From the results of this study, which included laboratory experiments and the controlled injection of a surrogate DNAPL in the field, it is clear that preferential pathways in fractured rock will determine the flow path of any DNAPL phase contamination. Aqueous plumes of DNAPL contaminants will also be influenced by these pathways (dissolving and or diffusing from the NAPL into the water in fractures and matrix) which can result in spatially variable aqueous plumes in these aquifer systems. The local variations in fracture strike and dip play a far more important role in DNAPL flow than the regional fracture dip or groundwater flow directions. Natural attenuation processes are important consideration under South African conditions. Relatively high organic carbon in the shallow zones assists in retardation of the organic contaminants, while the large unsaturated zone, arid climate and high temperatures leads to significant loss of contaminant mass through volatilization. Although the National Water and Environmental Acts of South Africa are very clear on prevention of pollution to, and management of water resources, no guidelines exist on how to deal with DNAPL contaminated sites. Recommendations have been made relating to the regulations that are required for: · Site assessment · Sampling and monitoring · Implementation of monitored natural attenuation.

[发布机构] University of the Free State