ABSTRACT
Abraham Thomas1 and John Tellam2 1Department of Earth Sciences, University of the Western Cape, P Bag X17, Bellville 7535, Cape Town, South Africa
2Hydrogeology Research Group, Earth Sciences, School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom
ABSTRACT: An ArcView GIS based “Petrol Station BTEX Pollution Model” has been developed to assess the reactive dissolved phase migration from ground level to the water table for cases where spills of (multi-compound) non-aqueous phase liquids (NAPLs) have occurred. It uses inputs from an urban recharge GIS model. The model calculation is undertaken in seven steps, viz. 1) input of data; 2) estimation of volumetric water content in the unsaturated zone; 3) calculation of soil/water, air/water, and NAPL/water partitioning coefficients; 4) multiphase partitioning of spilled NAPLs into the four phases; 5) calculation of initial leachate concentrations for each NAPL component; 6) calculation of retardation factors; 7) calculation of final concentrations and fluxes reaching the water table taking into account degradation. The main input data required are: locations, volumes, and areas of spills; soil texture, hydraulic properties, and organic carbon content; recharge rates; and water table depths. The model has been trialled on the Birmingham (UK) aquifer for the case of petrol stations. In this aquifer, the risks to groundwater appear to be minimal for small-scale spills, except where the unsaturated zone thickness is limited close to the main river. The program could be used for vulnerability mapping, determining threats from existing NAPL storage sites, and in planning locations for future sites.
