Colloids in landfill leachate

Heavy metal contents of landfill leachate sometimes are unexpectedly high. This may be related to the presence of colloidal matter with high metal contents that, due to lack of strict protocols on sampling and solid separation, some times are found in the sampled leachate.
Leachate has been sampled by special techniques at four Danish full scale landfills and the colloidal content (down to about 1 nm) has been characterized. The four leachates had low contents of heavy metals except high concentrations of zinc in one leachate sample. A substantial part of the heavy metals were associated with the colloidal matter suggesting that the presence of colloidal matter in leachate samples may cause high metal concentrations. Sampling protocols should precisely specify the filtration procedures for leachate sampling.