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A. Long, C. Eastoe (UA-GEO)
We are investigating recharge processes
in semi-arid basins, with emphasis on questions concerning
the age and origin of groundwater, mapping groundwater
flow paths, and the location of zones where recharge
is rapid. We approach this research problem by applying
a variety of isotope measurement techniques (stable
O, H, S, C; 3H,
14C) available
in our laboratory to ground and surface water samples.
Our principal focus has been on the Tucson Basin, where
our study has demonstrated the advantage of concurrent
application of different isotope systems. We expect
that a similar approach in other basins, e.g. the Hueco
Bolson (2.9), will produce equally valuable results.
We believe that detailed knowledge of recharge zones
and flow paths is essential to basin modeling studies.
Activities and Results
In the current reporting period
our primary emphasis has been on processing our data
for the central part of Tucson Basin and have begun
a manuscript on that area. We have also mapped
d18O(sulfate)
in groundwaters and evaporites of the basin, finding
that this parameter distinguishes sulfate originating
by oxidation of sulfide in crystalline rock from dust
and evaporite sulfate in the basin sediments. And finally
we have extended sampling to fill a small number of
gaps in our data set.
We obtained
d18O(sulfate)
values for about 60 water and gypsum specimens from
Tucson Basin. Dissolved sulfate and gypsum from sediment
in the central basin have higher
d18O(sulfate)
values (>5‰), in contrast with near-zero values
for sulfate in water drawn from fractures in crystalline
rock near the Rincon Mountains. Comparing with acid
mine drainage in the Patagonia area, we interpret the
low
d18O values to indicate oxidation of
sulfide. We will apply this information as we attempt
to identify recharge from the mountain block to shallow
sediment cover in Rincon Valley in future studies.
Figure 5 illustrates the groundwater
domains in Tucson basin overlaid on a satellite photo
of Tucson and vicinity. Principal washes are blue, urban
areas are outlined in red and the study area is within
the green rectangle. Few or no data exist for areas
without color. The domains can be related to major washes
by location and by values of
d18O
and
d34S,
except for that shown in light brown, in which may represent
water of mountain origin upwelling through gypsum with
d34S = 12‰ in the SE corner of
the basin.
Plans
The isotope study of the central
part of the basin is complete. What remains is writing
journal articles, information dissemination through
public speaking engagements, and the resolution of some
problems regarding the peripheral parts of the basin.
Our results to date illustrate the importance of recharge
through alluvial channels. What remains unclear is the
nature of the connection between the mountain block
and basin sediments. We plan to obtain more samples
near the edge of the basin to explore this connection:
Rincon Valley. The local
well-owners' association has offered to cooperate in
a thorough study of the area. Rincon Valley differs
from the central part of Tucson basin in having only
a thin sedimentary cover over crystalline rock of the
upper detachment plate. Data in hand suggest that much
of the groundwater in the area is supplied through fractures
in the crystalline rock beneath the sediments. Rincon
Valley is thus the only extensive area of the Tucson
basin where we can study recharge from the mountain
block into basin sediment, a process that is almost
certainly also happening beneath the very deep sediment
in the basin center.
Tucson Mountains. We currently
have relatively few samples from the area, including
the foothills on the eastern side. Available data suggest
that hot water with d18O
lower than expected from low-elevation rain is emerging
from bedrock at several locations; we do not yet have
an explanation for the origin of these waters, or an
adequate knowledge of their extent.
Catalina Foothills. We have
little data between Rillito Creek and the Santa Catalina
Mountains. We are aware of only a few wells in the area,
but we will seek extra samples (probably 5) and obtain
a full suite of measurements.
Oracle Junction to Oracle.
No isotope study has been attempted in this area, in
which large suburban developments have been proposed.
Water will most likely be obtained locally for new subdivisions
(as in Catalina and Oro Valley to the south where pre-1950's
water is being pumped in certain wellfields). Local
government should be aware of the potential limitations
of water supply before issuing permits. We envision
sampling existing wells from Oracle to Oracle Junction
(20 to 30 sites) to determine water origins and ages
using our full suite of isotopic measurements.
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