Description:
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)