STABLE ISOTOPES AS A SEDIMENT FINGERPRINTING TECHNIQUE.
Thanos N Papanicolaou1 and James F Fox2
1Associate Professor, IIHR-Hydroscience and Engineering, University of Iowa,
Iowa City, IA 52242, USA, e-mail: apapanic@engineering.uiowa.edu
2Graduate Research Assistant, IIHR-Hydroscience and Engineering, University of
Iowa, Iowa City, IA 52242, USA, e-mail: apapanic@engineering.uiowa.edu
The United States Geological Survey (USGS) and the National Institutes for Water
Resources (NIWR) have highlighted non-point source sediment-bound pollutants,
their source and fate, as Research Priority Area A in the request for proposals.
One goal in the second decade of the National Water-Quality Assessment (NAWQA)
program is to improve explanation and understanding of the sources of
contaminants and their transport through the hydrologic system.  Basin and
subbasin scale data in Iowa State, such as those collected for the mainstem
Mississippi River (e.g., Mississippi River at Clinton) and Lower Cedar River
Watershed in Eastern Iowa as part of NAWQA, demonstrate that soil erosion is a
substantial problem.  During intense runoff, large quantities of soil originated
from multiple land uses (e.g. agriculture, deforestation, cattle grazing, urban)
are transported to streams causing increased turbidity and siltation and thus
degrading the aquatic habitat and aesthetic quality of the streams.
There is, therefore, a clear need to develop an effective tracing technique to
identify the source of sediment within a basin that is essential for developing
appropriate strategies to control sediment mobilization and associated offside
effects, such as the siltation of river channels and reservoir sedimentation.
The research that is proposed here explores the potential of geochemical
techniques such as carbon and nitrogen stable isotopes (i.e., isotopes that do
not decay with geological time) for identifying sediment sources in a case study
for the Lower Cedar River Watershed.  The multiple land uses that exist within
the Lower Cedar River Watershed include the production of row crops, such as
corn, the production of cover crops, such as alfalfa and small grains, deciduous
forest cover, and industrial facilities and urban housing.  Erosion prone areas
of the watershed are comprised of nearly 60 percent agricultural/urban riparian
systems and 40 percent riparian.  These vegetative differences allow the use of
C/N atomic ratios and C and N isotopic signatures for source identification.
Carbon isotopes can be used to distinguish between organic matter derived from
different plants, particularly between plants that use C3 (trees, shrubs and
some grasses) vs. C4 (bamboo, corn, and many grasses) photosynthetic pathways
because the two pathways impart very distinct isotopic signatures on the plant
tissue (Stevenson 1997).  The nitrogen isotopic composition of sediments will
reflect the isotopic composition of their source sediment.  Importantly,
introduction of fertilizers and microbial activity associated with agriculture
can either lower or increase the nitrogen composition of a soil, depending on
the type of fertilizer used.  Therefore cultivated soils are often isotopically
distinct with respect to nitrogen from their uncultivated equivalent (e.g.
Broadbent et al., 1980).  The C/N ratios will reflect the presence of an active
biological milieu altering the elemental composition of soils.
Keywords: Stable isotopes, geochemical tracers, sediment sources, Palouse
watershed