Groundwater Ubiquity Score (GUS)

In the environment, a number of things can influence a pesticide’s potential to move toward and contaminate groundwater. This includes characteristics of both the soil and the pesticide, as well as water movement in soil, and soil surface disturbances (crop agriculture, insect or animal activity, landscaping, etc.).

Characteristics that determine soil sensitivity include: sorption (driven by clay and organic matter content), permeability, and water table features. For example:

• Soils that have slow permeability, high clay and/or organic matter, and a low water table may be less sensitive.
• Soils that have fast permeability, low clay and/or organic matter, and a high water table may be more sensitive.

Physical and chemical properties of the pesticide can also influence its potential for movement through soil. Some of these factors include pesticide persistence (half-life) in different settings, solubility in water, sorption potential (Koc) (binding affinity), and its ability to become a vapor (vapor pressure), among other things.

• Pesticides that have a short half-life, low water solubility, and high sorption potential may have less potential to move through soil.
• Pesticides that have a long half-life, high water solubility, and low sorption potential may have greater potential to move through soil.

The GUS or Groundwater Ubiquity Score 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. GUS = log10 (half-life) x [4 - log10 (Koc)].

(Citation: Gustafson, D.I. 1989. Groundwater ubiquity score: A simple method for assessing pesticide leachability. Environmental Toxicology and Chemistry 8:339-357)

Source: Wauchope, R.D., Buttler, T.M., Hornsby, A.G., Augustijn-Beckers, P.W.M., Burt, J.P. (1992). The SCS/ARS/CES Pesticide Properties Database for Environmental Decision-Making. In: Ware, G.W. (eds) Reviews of Environmental Contamination and Toxicology. "Reviews of Environmental Contamination and Toxicology", vol 123. Springer, New York, NY. https://doi.org/10.1007/978-1-4612-2862-2_1

If you have questions about this, or any pesticide-related topic, please call NPIC at 800-858-7378 (8:00am - 12:00pm PST), or email at npic@ace.orst.edu.