SUMMARY OF MAJOR FINDINGS
|The Eastern Iowa Basins Study Unit encompasses the Wapsipinicon, the
Cedar, the Iowa, and the Skunk River Basins and covers about 19,500 mi2
in eastern Iowa and southern Minnesota. In 1990, about 40 percent of the
more than 1 million people in the Study Unit were concentrated in cities
with populations of greater than 20,000 people. Cedar Rapids is the only
city with a population greater than 100,000. Ground water is the major
source for municipal, industrial, and domestic supplies. During the study,
Iowa City was the largest municipal user of surface water. Over 90 percent
of the land in the Study Unit is used for agricultural purposes. Forested
areas account for only 4 percent of the land. Data from Eros Data Center,
Stream And River
Highlights Nitrogen and phosphorus concentrations in
streams in the Eastern Iowa Basins Study Unit rank as some of the highest
in the Corn Belt (see map, p. 8)
as well as the Nation and were higher than the drinking-water standard
in many samples. These conditions reflect intensive use of the land for
growing crops and dense populations of livestock in some basins.
- Nitrate concentrations in 22 percent of the stream
samples exceeded the U.S. Environmental Protection Agency (USEPA) drinking-water
standard of 10 mg/L (milligrams per liter). The standard was most frequently
exceeded during June, soon after spring fertilizer application. Although
many of the streams sampled are not currently used for drinking-water
supplies, the Cedar and Iowa Rivers are the direct or indirect source
for Cedar Rapids and Iowa City—two of the largest cities in the study
- The highest nitrate concentrations occurred in medium-sized
streams draining basins with the most intensive row-crop agriculture
and in a stream draining a basin with both intensive row-crop agriculture
and dense concentrations of large-scale animal feeding operations. Nitrate
concentrations in these streams exceeded 10 mg/L in almost 50 percent
of the samples. Conversely, nitrate concentrations were lowest in basins
that had greater percentages of pasture, grassland, and forest.
- Total phosphorus concentrations frequently (75 percent
of the samples) exceeded the 0.1-mg/L USEPA-recommended goal to minimize
algal growth in rivers. Total phosphorus concentrations were greatest
in streams and rivers that drain basins with more highly erodible soils
and in large river basins that contain the largest cities and towns
in the Study Unit.
- The large amounts of nitrogen and phosphorus that
are transported to the Mississippi River from the Study Unit represent
an economic loss to farmers and a potential environmental threat to
downstream waters. The estimated annual loss of 17 to 41 lb/acre (pounds
per acre) of nitrogen and 1.2 to 1.5 lb/acre of phosphorus represents
a potential loss in crop yield or the cost of additional fertilizer
needed to compensate for that flushed from the fields. Nutrients transported
to the Mississippi River likely reach the Gulf of Mexico where they
contribute to eutrophication and hypoxia.
- Although the use of herbicides and insecticides
in the Study Unit is among the most intense nationwide, herbicide concentrations
in streams were not among the highest 25 percent nationally, and insecticide
concentrations were in the lowest 25 percent nationally. Breakdown compounds
(degradates), whose widespread occurrence has only recently been discovered
and about which little is known of the human and environmental effects,
generally accounted for the majority of the pesticide compounds present
in rivers and streams.
The most commonly used herbicides were the most frequently detected
and were generally present in the greatest concentrations. Atrazine
and metolachlor were detected in all stream samples. Concentrations
generally ranged from 0.1 to 1.0 µg/L (microgram per liter). Atrazine
concentrations exceeded the USEPA 3.0-µg/L drinking-water standard
in about 10 percent of the samples; exceedances occurred mainly during
- Acetochlor, a conditionally registered herbicide
that is intended to replace several other commonly used herbicides,
was frequently detected, but concentrations were less than 0.1 µg/L
in 75 percent of the samples. Mean annual acetochlor concentrations
did not exceed the 2.0-µg/L USEPA registration requirement at
any site, but concentrations did exceed that level in about 3 percent
of the individual samples. The maximum concentration measured during
the study (10.6 µg/L) exceeded the level that would require biweekly
sampling by water-supply systems.
- Alachlor, metolachlor, and acetochlor degradates
are present in relatively high concentrations throughout the year, indicating
that they are more persistent than their parent compounds.
- Carbofuran and chlorpyrifos, insecticides that have
been identified as posing a high risk to aquatic insects and mussels,
were present in as much as 60 percent of the monthly samples during
the summer when these insecticides are normally applied.
- Riparian buffer zones influence the quality of water
in streams and rivers. Biological communities respond to tree density
in riparian buffer zones. Invertebrate taxa indicative of good stream
quality increased with increased numbers of trees. In contrast, streams
that were not shaded by trees contained large algal growths considered
indicative of eutrophication.
Major influences on streams and rivers
- Agricultural storm runoff
- Animal feeding operations
- Tile-line drainage Urban areas
Compared to surface water, ground water in the Eastern
Iowa Basins had substantially lower nutrient concentrations and less frequent
detections. Land use, however, had a substantial effect on the quality
of water in alluvial aquifers. Pesticide degradates were some of the most
commonly detected constituents in these aquifers. Nitrate and methyl tert-butyl
ether (MTBE) exceeded the USEPA standard or advisory in some of the samples.
In contrast, bedrock aquifers, which are generally protected by clay and
shale layers, typically had low nitrate concentrations and low frequency
of pesticide detections.
- Nitrate concentrations generally decreased with depth
in the alluvial aquifers. Biological denitrification may result in decreased
nitrate concentration with depth, but it is also possible that the deeper
water infiltrated during past years when less fertilizer was used for
- Nutrients move from ground water to streams by natural
drainage and tile lines. Nitrate concentrations in 24 of 25 medium-sized
streams exceeded 10 mg/L during the sampling period in May 1998 when
streamflow originated primarily from ground-water discharge. Nitrate
concentrations consistently exceeded 10 mg/L in water from a selected
tile line draining to the Iowa River.
- Pesticides were detected in alluvial aquifers underlying
both agricultural and urban areas, but shallow ground water in agricultural
areas contained greater concentrations than urban areas. A greater variety
of pesticide compounds was detected in urban areas than agricultural
areas, reflecting a more diverse usage.
- Pesticides most commonly detected in the alluvial
aquifers underlying urban areas were atrazine, prometon, and metolachlor.
Pesticide concentrations did not exceed established drinking-water standards.
- With the exception of atrazine and metolachlor and
prometon in urban areas, pesticides were infrequently detected in alluvial
aquifers. Pesticide degradates generally were more commonly detected
in the alluvial aquifers than their parent compounds. The greater presence
of degradates indicates that many pesticides break down in the soil
and that the resulting pesticide degradates are transported to the shallow
- MTBE, a common gasoline additive used to increase
the octane content or ensure cleaner burning, was detected in 23 percent
of samples from alluvial aquifers in urban areas. Concentrations exceeded
the USEPA drinking-water advisory in samples from 6 percent of the wells.
Major influences on ground water
- Lawn, garden, and agricultural fertilizers
- Agricultural and urban pesticides
- Leaking underground fuel-storage tanks
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U.S. Geological Survey Circular
Kalkhoff, S.J., Barnes, K.K., Becher, K.D., Savoca, M.E., Schnoebelen, D.J., Sadorf, E.M., Porter, S.D., and Sullivan, D.J., 2000, Water Quality in the Eastern Iowa Basins, Iowa and Minnesota, 199698: U.S. Geological Survey Circular 1210, 37 p., on-line at https://pubs.water.usgs.gov/circ1210/