FACTORS AFFECTING N15 FOR TERRESTRIAL AND INSTREAM 
SEDIMENTS

Ranjani B. Theregowda and Thanos A. Papanicolaou
Hydroscience and Engineering, 100 C. Maxwell Stanley Hydraulics Laboratory, The 
University of Iowa, Iowa City, IA 52242

Determining reasons for N15 variability in the sediment transport process is being 
conversed in this paper due to the identification of nitrogen isotope as a significant 
signature abetting in linking sediments to their terrestrial source which aids in estimating 
the delivered sediment impact on aquatic systems.  This study is being performed at the 
Clear Creek Watershed, located at Upper South Amana.  Clear Creek is a tributary of the 
Iowa River, IA.  The watershed is mainly divided into 60% agricultural covered by row 
crops and 20% being covered by pasture/hay, remnant 20% represents roads, floodplain, 
and NRCS CRP as land use.  It has been a targeted watershed by the State’s Geological 
Survey, IDNR, NRCS and other non-profit organizations (e.g. Clear Watershed Board).  
Since 1940s, agricultural activities (e.g., tilling) and natural events (e.g., floods) have 
contributed significantly to the increased influx of fine sediments and associated 
pollutants into the Creek.  The experiment performed to attain the objective involves 
characterization of the terrestrial (source) soil and then simulating natural conditions 
using flumes at the laboratory to study the significance of instream factors towards spatial 
variability of the sediments.  Primary factors mediating variability at the terrestrial source 
can be distinguished as biogeochemical characteristics: physical/geological (e.g., 
moisture content, temperature, light intensity, slope variability, vegetation, soil type), 
chemical (e.g., pH, CEC, concentration & form of nitrogen available) and biological 
(e.g., photosynthetic pathway of vegetation, litter and concentration & type of biota).  In 
the stream though chemical aspects have the same weight as in the terrestrial 
environment, all physical factors may not be significant contributors, and conversely 
biological properties (e.g., phyto & zoo-plankton) may show consumption at diverse 
levels.  Reference and instream sediments collected are subjected to isotopic and 
biogeochemical tests.  During the experiment, 5 different terrestrial soils will be 
considered corresponding to 5 different land coverage.  In order to asses the effects of 
instream processes on N15 variability, stream water will be used during the experiments.  
Hence, the biological factors causing N15 variability will be isolated.  The 5 flumes will 
be placed in a temperature controlled environment that can match the existing 
temperature conditions and ranges found in the region.  Water flow recirculation will be 
maintained throughout the tests to simulate flow rates found in CC in order to prevent 
changes of the biological conditions in the water column.  pH will be also kept the same 
with the one found in the stream to minimize variability in sediment agglomeration.  The 
results of this study will be compared with findings reported in the Upper Mississippi 
watershed and other regions of the world.

Keywords: N15 variability, Clear Creek Watershed, biogeochemical, terrestrial, instream.