The study of interactions within Earth’s critical
zone, or surficial environment, provides an
understanding of the integral links between
the hydrosphere, lithosphere, atmosphere,
and biosphere. Central to the study of the
critical zone are chemical weathering, surface,
and near-surface hydrology. Geochemical
tracers provide insight into the interactions
between processes in the critical zone. In the
series of studies described herein,
chlorofluorocarbons (CFCs), tritium, major ion
chemistry, as well as oxygen, hydrogen, and
helium isotopes were used to understand
groundwater residence times, groundwater
movement, and the origin of solutes in the
near surface zone. The two main field sites,
Sagehen basin and the Mission Tunnel, differ
greatly in their geologic settings and provide
an interesting contrast and broad test for the
applicability of using environmental tracers in
near surface systems.

Sagehen basin is located in the Sierra
Nevada of California. The chemical and
isotope composition of groundwater springs
in Sagehen basin were used to decipher an
archive of internal and external changes. A
positive correlation was found between
groundwater residence times and the
chemistry of the waters. This correlation is
strong evidence for the chemical evolution of
groundwater and suggests internal changes
in the system are recorded by shallow
groundwater and is accessible with springs.
The oxygen isotope content of the spring
waters also correlates with groundwater
residence times. This correlation is related to
air temperature and atmospheric circulation
pattern changes, suggesting groundwater
systems also archive external changes.

The Mission Tunnel of Santa Barbara,
California, provides a very different
environment for studying groundwater
flowpaths and solute chemistry. By using
multiple geochemical tracers, different age
components of the groundwater system were
separated. Due to the highly fractured nature
of the aquifers, no correlation was found
between the minimum flowpath lengths and
groundwater age components. Ion chemistry
of the groundwaters suggest that flow is
primarily in the vertical direction. In addition,
time series analysis of long-term records of
precipitation and groundwater seepage into
the tunnel suggest that there is a 3 month lag
between precipitation and seepage events
and that antecedent conditions of the
groundwater system strongly control the
response to precipitation loading.