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