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EIWA Abstracts
Occurrence,
Distribution, and Transport of Nutrients in Eastern Iowa Rivers,
1996-98
Kent D. Becher, Hydrologist, U.S. Geological Survey, Iowa City,
Iowa
Proceedings from Agriculture and the Environment: State and Federal
Initiatives conference at Iowa State University, March 5-7, 2001
Eleven streams in the Wapsipinicon, Cedar, Iowa, and Skunk River
basins in eastern Iowa were sampled monthly from March 1996 through
September 1998 to assess the occurrence, distribution, and transport
of nitrogen, phosphorus, and suspended sediment as part of the
U.S. Geological Survey's National Water-Quality Assessment Program.
At least one form of dissolved nitrogen and phosphorus was detected
in every sample collected. Nitrate accounted for 92 percent of
the dissolved nitrogen. About 22 percent of the samples contained
nitrate that exceeded the U.S. Environmental Protection Agency's
Maximum Contaminant Level of 10 milligrams per liter (mg/L) as
nitrogen in drinking water. The median concentration of total
nitrogen for surface water in the study area was 7.2 mg/L. Dissolved
phosphorus was predominately in the form of orthophosphate. The
median total phosphorus concentration for the study area was 0.22
mg/L. About 75 percent of the total phosphorus concentrations
exceeded the U.S. Environmental Protection Agency recommended
total phosphorus concentration of 0.10 mg/L or less to minimize
algal growth in streams not discharging directly into lakes or
impoundments. Median suspended-sediment concentration for the
study area was 82 mg/L.
Nitrogen, phosphorus, and suspended-sediment concentrations varied
annually and seasonally. Nitrogen, phosphorus, and suspended-sediment
concentrations increased each year of the study due to increased
precipitation and runoff. Concentrations were typically higher
in the spring after fertilizer application and runoff. In winter,
nitrogen concentrations typically increased when there was little
instream assimilation by aquatic plants and algae. Nitrogen and
phosphorus concentrations decreased in late summer when there
was less runoff and instream assimilation of nitrogen and phosphorus
by aquatic plants and algae was high. Suspended-sediment concentrations
were highest in early summer during runoff and lowest in January
when there was ice cover with very little overland flow contributing
to rivers and streams.
Streams draining small- to medium-sized watersheds that have uniform
land use and geology had significantly (p < 0.05) higher total
dissolved-nitrogen concentrations (median, 8.2 mg/L) than did
samples from large rivers (median, 6.2 mg/L) that drain basins
with mixed land use and geology. Samples from large rivers typically
had significantly (p< 0.05) higher total phosphorus and suspended-sediment
concentrations than did samples from small rivers and streams.
Concentrations varied between drainage basins due to land use
and physiography. Basins that are located in areas with a higher
percentage of row-crop agriculture typically had samples with
higher nitrogen concentrations than did basins with less row-crop
agriculture. In addition, basins that drain the Southern Iowa
Drift Plain and the Des Moines Lobe typically had samples with
higher total phosphorus and suspended-sediment concentrations
than did basins that drain other landform regions.
Total nitrogen and total phosphorus loads were typically proportional
to the size of the drainage basin. Total nitrogen loads increased
each year from 1996 through 1998 in conjunction with increased
concentrations and runoff. However, total phosphorus loads in
the Skunk River Basin decreased in 1997 due to less runoff. Total
phosphorus loads followed the same pattern as total nitrogen loads
with increases in 1998. Total nitrogen and total phosphorus loads
varied seasonally and the highest loads typically occurred in
early spring and summer after fertilizer application and runoff.
Loads were lowest in January and September when there was little
runoff to transport nitrogen and phosphorus to the rivers and
streams.
Total nitrogen loads contributed to the Mississippi River from
the Eastern Iowa Basins during 1996, 1997, and 1998 were 97,000,
120,000, and 230,000 metric tons respectively. Total phosphorus
loads contributed to the Mississippi River from the Eastern Iowa
Basins during 1996, 1997, and 1998 were 6,900, 4,600, and 8,800
metric tons, respectively. The highest nitrogen and phosphorus
yields typically occurred in streams draining small watersheds
that were dominated by a single land use and geology. Sampling
sites located in drainage basins with higher row-crop percentage
typically had higher nitrogen and phosphorus yields. Sites that
were located in the Des Moines Lobe and the Southern Iowa Drift
Plain typically had higher phosphorus yields probably due to more
erodible soils and steeper slopes.
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