In a study using GeoChip 2.0 to examine three atrazinecontaminated aquifers and a background site, Liebich et al. (2009) detected more genes in the background site compared to the contaminated sites. The aquifer with the highest level of contamination had the highest number of genes, most involved in contaminant degradation, compared to the other contaminated samples. Atrazine-degradation genes were detected in all contaminated samples and verified by PCR. These results indicated that even small amounts of contaminant were enough to select for specific degrading populations.