J. Schnoebelen, 1Kent
D. Becher, 1Matthew
W. Bobier and
1U.S. Geological Survey, Iowa City, IA
2Iowa Department of Natural Resources, Des Moines, IA
The full report is available in pdf. Links to the pdf.
Water-quality data from 17
surface-water monitoring sites were compiled for 1970 through 1995 and
analyzed to determine historical waterquality conditions and possible trends
in the Eastern Iowa Basins study unit as part of the U.S. Geological Survey's
National Water-Quality Assessment Program. The Eastern Iowa Basins encompasses
the Wapsipinicon, Cedar, Iowa, and Skunk River Basins and covers about 19,500
square miles. Seven of the monitoring sites were sampled by the Iowa
Department of Natural Resources, three sites by the Minnesota Pollution
Control Agency, three sites by the University of Iowa Institute for Hydraulic
Research, and four sites by the U.S. Geological Survey. Water-quality analyses
typically consisted of nitrate, ammonia, total nitrogen, and total phosphorus,
with limited analyses available for organic nitrogen, dissolved phosphorus,
dissolved orthophosphate, and water-soluble pesticides. Long-term historical
nutrient and pesticide data were not available for the Wapsipinicon River
Median concentrations for total nitrogen ranged
from 4.6 to 9.4 milligrams per liter, and maximum concentrations of total
nitrogen ranged from 4.6 to 31 milligrams per liter. The majority of nitrogen
transported in surface waters of the Eastern Iowa Basins study unit is in the
form of nitrate (nitrogen). Median concentrations of total phosphorus ranged
from less than 0.10 to 0.66 milligram per liter, and maximum concentrations of
total phosphorus ranged from less than 0.10 to 5.4 milligrams per liter.
Nitrate varied seasonally. Median concentrations of
nitrate were largest during the spring and the winter (6.0 to 7.0 milligrams
per liter) compared to the summer and fall (2.0 to 4.0 milligrams per liter).
Concentrations of nitrate greater than 10 milligrams per liter typically
occurred during spring runoff. Median ammonia concentrations generally were
highest during the winter (approximately 0.3-0.5 milligram per liter) compared
to the spring and summer when ammonia concentrations were often close to the
detection limit (0.01 milligram per liter). In general, the median
concentrations of total phosphorus varied less than 0.1 milligram per liter
The statistical analysis of the nutrient data
typically indicated a strong positive correlation of nitrate with streamflow.
Total phosphorus concentrations with streamflow showed greater variability
than nitrate, perhaps reflecting the greater potential of transport of
phosphorus on sediment rather than in the dissolved phase as with nitrate.
Ammonia and ammonia plus organic nitrogen showed no correlation with
streamflow or a weak positive correlation. Seasonal variations and the
relations of nutrients and pesticides to streamflow generally corresponded
with nonpoint‑source loadings, although possible point sources for
nutrients were indicated by the data at selected monitoring sites. Statistical
trend tests for concentrations and loads were computed for nitrate, ammonia,
and total phosphorus. Trend analysis indicated decreases for ammonia and total
phosphorus concentrations at several sites and increases for nitrate
concentrations at other sites in the study unit.
Purpose and Scope
Description of the Eastern Iowa Basins
Population, Land Use, and Water Use
Methods of Investigation
Sources of Nutrient, Pesticide, and Streamflow Data
Iowa Department of Natural Resources and Minnesota Pollution Control Agency
University of Iowa Institute of Hydraulic Research
U.S. Geological Survey
Data Compilation and Statistical Methods
Spatial and Temporal Records
Selected Nutrients and Pesticides in Streams
Relations Between Concentrations and Streamflow
Time Trends in Concentrations and Loads
Seasonal and Annual Variability
Implications for Data Collection and Analysis.
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