Agriculture’s use of inorganic fertilizer and animal manure is the most dominant
and widespread nitrate source in the Southwest, although urban areas, primarily unsewered areas, can also contribute significant nitrate to groundwater. The major regions with high groundwater nitrate pollution are therefore not surprisingly the major agricultural regions: Imperial, Central, Salinas, and other coastal valleys in California; the Snake River Plain in Idaho; the Wasatch Front in north- central Utah; the Rio Grande Valley
in New Mexico; and the Gila and Salt River valleys in Central Arizona.