A methodology to assess the movement of hydrocarbons in the subsurface and associated remediation thereof

Abstract

Petroleum serves as a great source of energy, however, with such principle importance it poses a problem as a global contaminant. Hydrocarbon contamination is a huge threat to groundwater as it contains toxic substances that are insoluble in water and is referred to as the free phase. These toxins are carcinogenic and mutagenic, and have a major impact on human health and ecosystem stability. When spilled, hydrocarbons will move downward through the unsaturated zone under the influence of gravity and capillary forces, trapping small amounts in the pore spaces. Some of the components within the free phase can dissolve and move as an aqueous plume by diffusion and advection within the groundwater. There is a long term effect on ecosystems, as the insoluble free phase mass slowly decays into the aquifer making it more difficult to model and control. The net result is that some hydrocarbon fractions are transported faster than others and a contamination plume of varying intensity may spread over a large area. The ultimate aim of this study was to develop a methodology to assess the movement and remediation of hydrocarbons in the subsurface with the use of a numerical model, to improve the management of areas contaminated by these compounds. This includes the migration and delineation of the free phase and dissolved plume. Additionally, it was necessary to simulate a number of remediation options to elevate the risk associated with the contaminant. According to the Manual for site assessment at DNAPL contaminated sites in South Africa, MODFLOW and UTCHEM can be used as a facilitator in managing and containing hydrocarbon contamination. The software package UTCHEM was therefore used to model the migration of the non-aqueous and aqueous hydrocarbon phases and surfactant and co-solvent enhanced remediation, while MODFLOW was used to facilitate the migration and extraction of the dissolved plume. The methodology was demonstrated by means of a case study of an industrial site located within the Durban harbour. After simulating the movement and remediation of tetrachloroethene (PCE) and benzene results reflect the complexity of the problem. UTCHEM could not accurately model the migration as the generation of a working model was far too simple due to software restrictions however it was possible to simulate the change in mobility with a surfactant SDS, polymer xanthan gum, and ethanol that demonstrate trends related to literature. The proposed methodology includes a site investigation, collection of historical data, delineating and characterisation of the NAPL using non-invasive methods, chemical borehole logging, a chemical risk assessment, the modelling of the dissolved contaminated plume using MODFLOW to determine the migration and extraction of the plume, and the remediation of the free and dissolved phase using a simplified UTCHEM model in order to determine the best option for a specific site if necessary.