Tucson Mountains in the east, Brawley watershed in the west, Tortolita Mountains in the north, and S

1
Identification of Sources of Recharge in Avra
Valley Alluvial Basin Dody A1., Eastoe C2., Long
A1,2. 1Hydrology and Water Resources, The
University of Arizona 2Geosciences Department,
The University of Arizona
Results and Discussion ?18O vs. ?D plot a.  Show
Tucson rain data (weighted mean, 1991-2000 -7.5
, -51 -- source W.E. Wright, pers. comm.),
plus winter average for Santa Catalina Mountains
(-10.1 , -67 source, Kalin 1994). b. Most
waters show evaporation before recharge. Upper
end cluster of data is below agricultural land,
and doesnt correspond to rain data, therefore is
thought to be recharged irrigation water. c.  The
rest of the data form a trend between mean
low-elevation rain and mean high-elevation rain,
with some evaporation. d. Two wells (10-150
(3,4)) north of the Santa Cruz River located on
both sides of I-10, had depleted and not
evaporated water. This contradicts the assumption
that the source is CAP recharge water. These two
wells had ?13C values of about -10 , compared
with 3 in the CAP water.
Objective Identification of groundwater recharge
sources into the aquifer, and determination of
groundwater ages. Methodology Radioactive and
stable environmental isotopes of C, H, and O,
were chosen as tracers. Isotope values of water
samples from wells were overlaid on a base map
using GIS, to show the spatial relationship with
the two streams.
Introduction Tucson Mountains in the east,
Brawley watershed in the west, Tortolita
Mountains in the north, and Sierrita Mountains in
the south bound the study area of Avra Valley.
Avra Valley is about 40 miles long and varies in
width from 7 to 16 miles (520 mile2). The average
altitude of the surrounding mountains is about
4,500 ft. Most of the area is agricultural fields
irrigated either with groundwater or CAP water.
Since 1940 the observed water level decline in
the aquifer has been higher than 150 ft.
Map of ?18O a.  Most of the valley has water with
?18O between -8.3 and -7.0, corresponding to
low-elevation rain there is little
high-elevation area around the valley itself. b.
The main exception is the Santa Cruz flood plain
around Marana, which yields groundwater with ?18O
between -9 and -10. This is water supplied from
the Catalina and Rincon Mountains by way of the
Rillito, Canada del Oro and Santa Cruz rivers.
It is not water from the upper Santa Cruz basin,
or from the Tortolita Mountains, which in both
cases has ?18O between -7 and -8. The low ?18O
water has not recharged the flood plain
downstream of Marana town. c.  Other exceptions
include wells of Avra Water Coop, -9 to -8 and
Diablo Water, -9.
Physiographic Setting Avra Valley is a part of
the Basin and Range physiographic province (Hess,
1992). It is located in an arid/semiarid climate
zone with annual precipitation of about 10 inches
(Hanson et al., 1990). The streams that drain the
valley are the Santa Cruz River in the north and
Brawley Wash along the valley axis. Flow is
ephemeral occurring mainly after winter storms or
summer monsoonal thunderstorms. The Santa Cruz
River has a base flow discharged from Tucson
sewage treatment plant. The subsurface lithology
was divided into two formations, both consisting
on alluvial material. The transmisivity is highly
variable, low in the central basin (2600 ft2/day)
and high along the Santa Cruz River (13,000
ft2/day) (Hess, 1992). The direction of the
subsurface flow is due north in the south-central
part of the basin and due northwest in the north
part of the basin. The Tucson Mountains bound the
alluvial basin on the east side, and consist
igneous and sedimentary crystalline rocks.
Map of Temperature Well AVW6, which is located in
the central valley, with measured temperature of
40.50C, suggests seepage of water arriving from
deeper formations.
Map of 14C a.  High pMC is mainly limited to the
Santa Cruz flood plain near Marana. Values of pMC
decrease north and south of the Santa Cruz
River. b. Lowest pMC value is 22, corresponding
with high temperature and low ?18O in Avra Water
Coop.s area. This seems to be an area of
upwelling of deep, old water. c.  Several
occurrences of low pMC water in Tucson Mountains
at Becker well, and at SASI, where low pMC warm
water is mixed with tritiated water in a shallow
aquifer.
Bibliography 1. Hess G.S., 1992. Chemical and
Isotope evidence for irrigation return flow in
Avra Valley, Arizona. M.Sc, Department of
Geosciences, The University of Arizona, 37 pp. 2.
Hanson R.T., S.R. Anderson, and D.R. Pool, 1990.
Simulation of groundwater flow and potential land
subsidence, Avra Valley, Arizona, USGS, Water-
Resources Invet. Report 90-4178, 41 pp. 3. Kalin
R.M., 1994. The hydrogeochemical evolution of the
groundwater of the Tucson Basin with application
to 3 dimensional groundwater flow modelling. Ph.D
dissertation, Department of Geosciences, The
University of Arizona, 510 pp.
CONCLUSIONS 1. Rapid recharge (last 45 years)
limited to Santa Cruz flood plain. Water
sources are Santa Cruz and Brawley washes. 2. No
evidence of widespread recharge in the last 45
years in Central-Southern Avra Valley,
including recharge from the flanks of the
Tucson Mountains. 3. There is a source of old
water, some of it warm, beneath parts of
the Tucson Mountains. 4. There is no evidence for
CAP water in any of the samples.
Map of tritium a.  Water with detectable tritium
(above 9 TU) is mostly along the Santa Cruz flood
plain around Marana, including water with -8 to
-7 (?18O) downstream of Marana. Therefore there
has been rapid recharge of water from both
Brawley Wash and the Santa Cruz River in the
flood plain. b. Exceptions include shallow
aquifers in the Tucson Mountains, and a well in
Brawley Wash flood plain near Manville Road.
Acknowledgment This material is based upon work
support in part by SAHRA (Sustainability of
semi-Arid Hydrology and Riparian Areas) under the
STC Program of the National Science Foundation,
Agreement No. EAR-9876800.