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B. Ekwurzel, S. Einloth (UA-HWR)
This project is the first application
of 32Si to determine long-term recharge rates and the
first rigorous study of 32Si
activities in soil. 32Si has a 140-year half-life and
theoretically can be a useful tracer for processes on
the 100 to 1000 year time-scale, a time length for which
there are few other available tracers. Studies have
found that 32Si is not an ideal groundwater tracer because
most of the tracer is lost during transit through the
vadose zone before reaching the groundwater table. It
is this "loss" in the vadose zone that is
exploited for recharge research in semi-arid regions.
The research plan for this work includes three phases:
development of analysis capabilities and determination
of feasibility; application to well-defined systems;
and extension to other regions of interest for recharge
purposes. The major advancements made this year were
the development of laboratory facilities with the capability
to quantitatively separate silica from soil and to analyze
for 32Si by liquid scintillation counting of the daughter
product 32P, and the collection of precipitation and
soil samples from key SAHRA field locations. We have
collected samples from the three southwestern deserts
(Sonoran, Mojave, and Chihuahuan), and from three different
segments of semi-arid regions: valley floor, stream
and wash channels, and the mountain-front. The understanding
gained about the basic recharge processes in these types
of environments can potentially be generalized and applied
basin-wide and between basins. This can be achieved
by a categorization of desert areas and through correlation
with other geochemical and geophysical properties measured.
Activities and Results
The past reporting period encompassed
the laboratory development and heavy field sampling
stages of the research. The net result is that water
sample lab procedures have been completed and will be
published in the next reporting year. Soil sample handling
procedures are still being refined, but we expected
them to be finalized by September 2002. The majority
of the samples that will be analyzed and interpreted
were collected in this report year.
Precipitation: To understand the 32Si
input to semi-arid systems we need to collect sufficient
precipitation and surface water samples. Several large-volume
rain gauges have been collecting precipitation at the
two locations: the San Pedro River Basin and the Mojave
Desert. In addition we have been collecting event-based
precipitation samples within the Tucson Basin to test
for any trends in 32Si
deposition. The Tucson Basin was chosen for the logistical
needs necessary for capturing large volume event-based
precipitation samples.
Soil Sample Collection: A second major component
is to understand the influences of recharge location,
vegetation impact, and soil properties on long-term recharge
in semi-arid regions. Below is a brief synopsis of field
activities:
- San Pedro River Basin - Eleven
vadose zone boreholes were drilled over a total of
170 m and 615 32Si
samples were collected. Concurrently the USGS and
SAHRA TA1 researchers collected geophysical, sedimentary,
and geochemical (3H,
18O, D,
Cl) data.
- Mojave Desert - Mojave vadose
zone borehole cores are archived and the USGS has
agreed to share core samples (J. Izbicki, USGS, pers.
comm.). This field sampling campaign is similar to
the one described above for the San Pedro River Basin.
- Socorro, NM Cooperative Trench
Study - Soil samples were collected from four vertical
sections. Two profiles were beneath established vegetation
and two profiles lacking surface vegetation. Again,
32Si interpretations
will benefit from the extensive data (e.g. total Cl,
root density, soil properties) collected by New Mexico
Tech TA2 researchers.
Lab Development: Three separate
facilities were established: the Soil-Leaching Laboratory,
the Scintillation Preparation Laboratory, and the Scintillation
Counting Facility. Due to the nature of the laboratory-building
phase of the research, the achievements were predominantly
in the area of process improvements leading to a published
method. We have established two notable improvements
to the methods:
1) The focus of the current project
is on the vadose zone, which required rigorous study
of soil leaching parameters and development of procedures
to generate reproducible results.
2) For water analysis ~ 50-500 L of sample are required,
therefore a small filter device was designed to extract
the silica from the water source onto a polypropylene
filter impregnated with iron hydroxide.
Plans
The next steps for this research
include analyzing the soil and precipitation samples;
evaluating the data along the vertical transects and
comparing the data with geochemical and geophysical
properties in order to assess infiltration depths and
recharge rates; and, if necessary based on the field
results, conducting column studies. The most immediate
task is sample analysis. The first set of samples analyzed
will be the precipitation (snow and rain) and the soil
cuttings from the San Pedro region. The precipitation
and the first set of San Pedro soil samples (half of
the wells) have a projected completion date of December
2002. The transect evaluation and modeling of the first
results will occur between approximately November 2002
and April 2003. A manuscript on the new analysis process
is to be written by December 2002.
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