Evaluating the effectiveness of landfill liners

A new method for evaluating the effectiveness of landfill liner systems has been developed that is based on contaminant transport.  The current approach used by designers is to assume that leakage rate, or advective transport, is the only mode of mass transport through a composite liner.  This is not necessarily true because diffusive mass transport occurs through defects and volatile organic compounds (VOCs) can diffuse through intact composite liners.  The newly developed method accounts for advective and diffusive transport through intact and defective composite liner systems and does not involve the use of empirical equations for estimating leakage rates.  The processes modeled include advection and diffusion through defects in composite liners and diffusion of VOCs through intact composite liners.  This new method has been used to compare three alternative liner designs:  (1) a U.S. Environmental Protect Agency (EPA) Subtitle D liner, (2) a State of Wisconsin liner, and (3) a composite liner having a geosynthetic clay liner.  The composite liner having a geosynthetic clay liner performed best in terms of leakage rate.  However, composite liners having thicker soil liners performed best in terms of contaminant transport through the base of the liner system.  The thicker composite liners had longer breakthrough times and less contaminant flux from the base.  Results from this analysis can be linked to a hydrogeologic transport model and use the predict the impact of contaminant transport through the liner system on groundwater quality at some compliance point.