One of the indicators that take into account only leaching is the Groundwater Ubiquity Score or GUS (Gustafson, 1989). The GUS is an experimentally calculated value that relates pesticide half-life and Koc (from laboratory data). The GUS may be used to rank pesticides for their potential to move toward groundwater. It can be calculated as
GUS = log10 (half-life) x [4 - log10 (Koc)].
Although the GUS index is interesting in terms of its simplicity to use – only few data are required and these data are widely available in environmental fate databases, it has a low validation status and ignores the contribution of the weather, soils and the subsoil to the risk of leaching.