This is important because landfill liners are selected to be compatible with the wastes that will be placed in a landfill. However, as this statement says, the mixing of wastes in a landfill will produced unanticipated chemical combinations with unpredictable results. A landfill liner selected to withstand attack from chemicals X, Y and Z may not withstand attack from chemicals X and Z in combination, or Y and Z in combination. The more chemicals involved, the greater the number of possible combinations, the more complex the interactions will be, and the less predictable the results become. The chemical interactions within a landfill do not cease when the dumping stops.
Leakage is inevitable.
There is no such thing as a secure landfill.