This study assesses water quality in the aquifers of southern Sanpete and central Sevier Valleys to determine likely sources of nitrate pollution and determine the relative age of high-nitrate water. Samples having high nitrate concentration were also analyzed for other environmental tracers. One spring and two shallow wells contain water characterized as modern, seven wells contain water greater than 500 years old but that also has a modern-age component, and 13 wells and one spring have mixed ground water recharged during the 20th century. Because environmental tracer data show all samples as having a historical recharge component, the major sources of nitrate contamination are probably human related rather than from a natural geologic source. 132 pages + 6 plates