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1
Effect of Geochemical Processes on Selenium in
Soil and Water, Central South Dakota Md. Salah
Uddin Sharif, Phil Gerla, Scott Korom
(Department of Geology and Geological
Engineering, University of North Dakota) and John
Finley, (Grand Forks Human Nutrition Center, USDA
Agricultural Research Service)

• Introduction

Soils and Sampling
Soil Water/Groundwater
Soil Geochemistry
Selenium, an essential element in humans, can be
fatally toxic to humans, livestock, and
waterfowl, depending on the amount and chemical
form present in the environment. Most early
reports focused on Se toxicity problems, although
more recent research indicates the necessity of
Se in both animal and human nutrition and
benefits to human health (e.g. Clark et al. 1996,
Finley 2000, and Davies et al. 1999). Some of
the highest concentrations of Se worldwide occur
in the central Great Plains of the United States.
The sedimentary rocks that make up the Cretaceous
formations, most notably the Pierre Shale,
contain the greatest concentrations. These shales
were deposited under typically anoxic conditions
in a shallow, warm sea that covered the present
area of the Great Plains during the Mesozoic
Era. The selenium that occurs in the shales has
been mobilized, transported, and redistributed
through the on-going processes of soil formation.
The movement of soil water, groundwater, and
surface runoff further modifies the original
distribution. Plants growing on these soils
absorb Se, which can then be transported into
animal tissues through the food chain. This
report describes the results of soil and water
sampling at the Hovland Ranch in South Dakota.
Earlier work showed that areas of the ranch have
high concentrations of Se in soils, crops, and
native range vegetation. The objectives of the
work reported here is to show (1) the
correspondence of Se and other geochemical
parameters in soils and waters at the ranch, and
(2) reveal the spatial distribution of Se and
associated elements to better understand the
geochemical cycle of Se in the natural range and
cropland environment at the ranch.
A rough correlation can be seen between Se and
Na, Mg2, and SO42-, but not Ca2 (not shown)
suggesting that more soluble sulfate minerals may
act as a temporary sink for Se.
west transect
Soil water chemistry also shows a correlation
between EC and TOC, although there does not
appear to be much similarity between soil water
and saturated paste/hot water extractable Se in
soils adjacent to the sampler.
Acid-extracted Se shows some correlation with
total organic carbon (TOC), but not with Se
released by the saturated paste extraction.
east transects
Groundwater sampled from seeps and shallow wells
shows a weak correlation between EC and sulfate,
but no clear trend to organic carbon.
Abundant Astragalus racemosus (Pursch) along
transect C revealed significant uptake of Se
(unpub, J. Caton, NDSU). Previous work by South
Dakota State University also showed elevated
concentrations of Se in wheat (ppb) at the ranch
Nimbro Series (Nc) very deep, well-drained soils
formed in alluvium on bottomlands and low stream
terraces. The surface layer is gray silty clay
loam 8 thick. The substratum is grayish brown to
light brownish gray stratified silty clay loam
and clay loam. Opal Series (Od) moderately deep,
well-drained soils in residuum weathered from
clay shales on uplands. The surface layer is dark
grayish brown clay 2 thick. The subsoil is dark
grayish brown, very firm clay 24 thick. Pierre
Series (Pe) moderately deep, well-drained soils
formed in residuum weathered from clay shale on
uplands. The surface layer is dark grayish brown
and brownish gray clay 7 thick. The subsoil is
light brownish gray clay 20 thick. Promise
Series (Pr) very deep, well-drained soils formed
in clayey sediments on uplands, colluvial fans,
and terraces. The surface layer is dark gray
clay, 8 thick. The subsoil is grayish brown and
light brownish gray very firm clay 17
thick. Samsil Series (SbF) shallow, well-drained
soils formed in residuum weathered from shale.
The surface layer is light brownish gray clay 2
thick. The layer below is 5 of light gray
clay. The substratum in all but the Nimbro
Series consists of 10-20 of light gray or light
brownish gray clay that overlies shale. The
Pierre-Samsil (Pk) and Opal-Promise (Oe)
associations are also shown on the maps.
Conclusions
Se concentration in soils and waters vary
greatly, suggesting that near-surface geochemical
processes strongly modify the original
distribution of Se in unweathered Pierre
Shale. Hot water and saturated paste
extractions generate about 50 times less Se than
that recovered by soil digestion, and show
generally poor correlation. This indicates that
only a small fraction of the total Se is mobile
and labile. Se concentrations roughly increase
with both increasing saturated paste EC and total
organic carbon. Some correlation is observed
between Se and Mg2, Na, and SO42- suggesting
that non-gypsum sulfate minerals may act as a
temporary sink for Se. The greatest
concentrations of Se occur in areas of
groundwater seeps. Leaching on upper slopes and
evaporative precipitation at discharge points is
likely an important process that redistributes Se
at the ranch and in the region. Future work at
the ranch will focus on coupling soil
geochemistry with plant and animal uptake of Se.
Soil Geochemistry
The following Eh-pH diagram shows that selenate
(SeO42-) (6) constitutes the most typical form
of Se in most soils. Selenite (SeO32-) (4)
occurs under mildly oxidizing conditions, but was
detected in only a few soil samples. Seo can
occur in reducing environments. Microbes are
known to catalyze reactions and generate organic
Se.
Acknowledgments
We thank Mr. Allen Hovland for not only his
permission to work at the ranch, but for his
hospitality, time, and unfailing interest in the
effort. Results are based upon work supported by
the U.S. Department of Agriculture, under
agreement 59-5450-1-9034. Any opinions,
conclusions, or recommendations expressed in this
poster or accompanying abstract are those of the
authors and do not necessarily reflect the view
of the U.S. Department of Agriculture.
Hot water and saturated paste extraction compared
to soil digestion suggests that less than a few
percent of the total Se is mobile and labile.
Seeps, swales, and water courses tend to have
elevated concentrations of mobile Se.