The U.S. Environmental Protection Agency requested that the Navajo Nation conduct an assessment of aquifer sensitivity on Navajo Nation lands and an assessment of ground-water vulnerability to pesticide contamination on the Navajo Indian Irrigation Project. Navajo Nation lands include about 17,000 square miles in northeastern Arizona, northwestern New Mexico, and southeastern Utah. The Navajo Indian Irrigation Project in northwestern New Mexico is the largest area of agriculture on the Navajo Nation. The Navajo Indian Irrigation Project began operation in 1976; presently (2001) about 62,000 acres are available for irrigated agriculture. Numerous pesticides have been used on the Navajo Indian Irrigation Project during its operation.

Aquifer sensitivity is defined by the U.S. Environmental Protection Agency as 'The relative ease with which a contaminant [pesticide] applied on or near a land surface can migrate to the aquifer of interest. Aquifer sensitivity is a function of the intrinsic characteristics of the geologic material in question, any underlying saturated materials, and the overlying unsaturated zone. Sensitivity is not dependent on agronomic practices or pesticide characteristics.' Ground-water vulnerability is defined by the U.S. Environmental Protection Agency as 'The relative ease with which a contaminant [pesticide] applied on or near a land surface can migrate to the aquifer of interest under a given set of agronomic management practices, pesticide characteristics, and aquifer sensitivity conditions.'

The results of the aquifer sensitivity assessment on Navajo Nation and adjacent lands indicated relative sensitivity within the boundaries of the study area. About 22 percent of the study area was not an area of recharge to bedrock aquifers or an area of unconsolidated deposits and was thus assessed to have an insignificant potential for contamination. About 72 percent of the Navajo Nation study area was assessed to be in the categories of most potential or intermediate potential for contamination. About 6 percent of the study area was assessed to have the least potential for contamination, mostly in areas where the slope of the land surface is more than 12 percent. Nearly all fields on the Navajo Indian Irrigation Project were assessed to have the most potential for contamination.

The assessment of ground-water vulnerability to pesticide contamination on the Navajo Indian Irrigation Project was based on pesticide application to various crops on part of the Navajo Indian Irrigation Project during 1997-99. The assessment indicated that ground water underlying fields of beans, wheat, barley, and alfalfa was most vulnerable to pesticide contamination; ground water underlying fields of corn and potatoes was intermediately vulnerable to pesticide contamination; and ground water underlying fields of hay was least vulnerable to pesticide contamination.