Potential solution to pollution of groundwater by diffusion of volatile organic compounds through the primary HDPE geomembrane in composite lining systems of landfills

Abstract

Waste production is not a modern concept. It has always been a byproduct of human beings use of the earth s natural resources for survival. The safe and reliable long-term disposal of solid waste residues is an important component of integrated waste management. Solid waste residues are waste components that are not recycled, that remain after processing at a material recovery facility, or that remain after the recovery of energy. Historically, solid waste was placed in depressions in the soil of the earth s crust through a process called landfilling. In South Africa, most waste produced by households and industries is disposed of on landfill sites. By law, all landfills and waste containment structures are required to have an engineered containment barrier installed that adheres to the minimum standards described in the waste classification and management regulations of the South African National Environmental Management: Waste Act (Act 59 of 2008). When waste materials in a landfill or any other waste body is exposed to the chemicals and heat generated over time, they produce harmful fluids in the form of leachate or landfill gas that migrate from the landfill towards the liner or capping, and include organic contaminants. These organic contaminants include a group commonly referred to as volatile organic compounds (VOCs) that have been known to migrate to and pollute the underlying groundwater (Prosser & Janechek, 1995). The High Density Polyethylene (HDPE) Geomembrane is often believed to be the primary barrier to contaminant transport, but the clay component in the composite liner usually controls the rate of transport of VOCs since researchers like Edil (2003) have shown that VOCs diffuse through geomembranes at appreciable rates. Therefore, the effectiveness of modern landfill liner systems in minimising the migration of VOCs merits scrutiny. The aim of this study was to obtain reliable data on the reduction in diffusion of VOCs through the HDPE geomembranes (GM) component in the composite liner systems of landfills by extracting air through the leakage detection layer or drainage layer of the composite liner. The objective was to undertake tests in three phases: Phase 1 aimed to prove that the chosen VOCs diffuse from source to receptor through a GM layer and to compare this to the results obtained from the literature. Phase 2 aimed to prove that, even if the separation between the source and receptor consisted of two GMs separated by an air-filled pervious zone, diffusion of the VOCs would still occur from the source to the receptor volumes. Phase 3 aimed to prove that, by introducing airflow into the pervious zone between the two GMs, the concentration of VOCs in the receptor volume (due to diffusion through the GM) could be reduced significantly. The testing in this phase also aimed to determine if the rate of air removal would play a role in the diffusive process and the resultant VOC concentrations in the receptor. Laboratory tests were carried out at the South Campus of the University of Pretoria in South Africa. The tests undertaken were based on the methods used by Prof Kerry Rowe at the Geo-Engineering Centre at the Queens University in Kingston, Canada as demonstrated during a visit to their facility. To undertake these tests it would be required to calculate the Sorption (Sgf) and Diffusion (Dg) coefficients for the compound and GM in question. Sorption/Immersion, Diffusion and Weight Gain tests were done to determine the sorption coefficient for the GM and permeant in question. The Diffusion coefficient (Dg) was inferred using the variation in source and receptor concentrations with time (Fick s second law) at the given boundary conditions. This was done using POLLUTEv7®, which solves the one-dimensional contaminant migration equation subject to boundary conditions at the top and bottom of the GM being modelled. The data entry into POLLUTEv7® includes information such as thickness and density on the layers to be modelled as well as the boundary conditions to be used for modelling. Phase 1 testing had challenges and limitations but it met its objective of proving that the VOCs in question diffuse from the source, through the 2 mm GM, into the receptor that represents groundwater, at rates that were comparable to those found in literature. Phase 2 tests took longer to reach equilibrium since the sorption and diffusion process had to take place over two GMs and the 0.8 cm air-filled pervious zone. The temperature under which phase 2 tests were undertaken was higher than that of phase 1 and, as indicated by literature, diffusion occurs faster at elevated temperatures. Undertaking the tests at different temperatures was not on purpose but rather a factor of laboratory conditions and setup. Data on the diffusion across two GMs separated by air, was not readily available to compare the difference that the increased temperature had on the system, but Phase 2 testing successfully met its aim of proving that the diffusion of BTEX and Chloroform takes place from source to receptor across a divide consisting of two 1 mm GMs separated by an air filled pervious zone. Phase 3 testing showed that, even though the optimal rate of airflow would require additional testing, introducing a flow of air through a pervious zone adjacent to the GM layer in a landfill liner would significantly reduce the concentrations of VOCs in the groundwater beneath landfills and waste containment facilities.