The Arizona Geological Survey (AZGS) performed a study to measure the tritium content of waters form various sources in the Safford Basin, part the upper Gila River watershed. The concentration of the naturally occurring radioactive isotope of hydrogen, known as tritium, was measured to determine the "age" of groundwater from different sources. The project area is shown on Figure 1. Water balance calculations and aquifer tests suggest upwelling of groundwater from the deep basin-fIll aquifer into the shallow alluvial aquifer of the Gila floodplain (Hanson and Brown, 1972; Brown, 1989). Based on the widespread occurrence of evaporites in the Safford basin fill (Harris, 1997) and the isotopic compositions of evaporites, deep well water, and river water (Harris, 1999a), natural water sources such as artesian leakage and springs may be an important source ofTDS in the Gila River. If this is the case, attempts to mitigate water quality problems in the Gila River by focusing on specific human activities, such as agriculture, may be misdirected. The purpose of the pilot study is to assess the utility of the tritium technique as applied to determining the degree of mixing between "old", deep groundwater in contact with highly soluble salts in the basin-fill sediments, and "young", shallow groundwater that constitutes a mixture of subflow from tributaries, infiltration of Gila River water, and possible infiltration of irrigation water. This study will help water management agencies make informed decisions regarding issues of water quality in the Gila River and how to apply appropriate mitigation measures regarding salinity. (56 pages)