Water-Resources Investigations Report 01–4175

NATIONAL WATER–QUALITY ASSESSMENT PROGRAM

Water–Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus,

Suspended Sediment, and Organic Carbon In Surface Water, 1996–98 

U.S. Department of the Interior

U.S. Geological Survey
Technical Assistance

James Giglierano, Iowa Geological Survey Bureau

Robert Libra, Iowa Geological Survey Bureau

Clint Beckert, U.S. Army Corps of Engineers

Dana Kolpin, U.S. Geological Survey

Craig Harvey, U.S. Geological Survey

Data Collection, Site Construction, Data Entry or Verification

Jim Cerveny, U.S. Geological Survey

Eric Sadorf, U.S. Geological Survey

Debra Sneck-Fahrer, U.S. Geological Survey

Matt Bobier, U.S. Geological Survey

Jennifer Tobias, U.S. Geological Survey

Joel Galloway, U.S. Geological Survey

Jon Nania, U.S. Geological Survey

Jim Klaus, U.S. Geological Survey

Denise Montgomery, U.S. Geological Survey

Jake Freier, U.S. Geological Survey

Erik Smith, U.S. Geological Survey

Bob Einhellig, U.S. Geological Survey

Jeff Harms, U.S. Geological Survey

Dave Conell, U.S. Geological Survey

Joe Gorman, U.S. Geological Survey

Al Conkling, U.S. Geological Survey

Jim Sondag, U.S. Geological Survey

Pat Lustgraaf, U.S. Geological Survey

Mike Turco, U.S. Geological Survey

Pat Sweeney, U.S. Geological Survey

Daron Tanko, U.S. Geological Survey

Editorial Review

Lanna Combs, U.S. Geological Survey

Technical Review

Mary Skopec, Iowa Geological Survey Bureau

Greg Nalley, U.S. Geological Survey

Russell Dash, U.S. Geological Survey

Patrick Edelmann, U.S. Geological Survey

Robert Buchmiller, U.S. Geological Survey

John O. Helgesen, U.S. Geological Survey

ACKNOWLEDGMENTS
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01–4175

Water-Quality Assessment of the Eastern 

Iowa Basins—Nitrogen, Phosphorus, 

Suspended Sediment, and Organic Carbon 

in Surface Water, 1996–98

By Kent D. Becher, Stephen J. Kalkhoff, Douglas J. Schnoebelen, 

Kimberlee K. Barnes, and Von E. Miller

Iowa City, Iowa

2001

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

P.O. Box 1230

Iowa City, IA 52244
The U.S. Geological Survey (USGS) is committed

to serve the Nation with accurate and timely scientific

information that helps enhance and protect the overall

quality of life, and facilitates effective management of

water, biological, energy, and mineral resources. Information on the quality of the Nation’s water resources

is of critical interest to the USGS because it is so integrally linked to the long-term availability of water that

is clean and safe for drinking and recreation and that is

suitable for industry, irrigation, and habitat for fish and

wildlife. Escalating population growth and increasing

demands for the multiple water uses make water availability, now measured in terms of quantity and quality,

even more critical to the long-term sustainability of

our communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support

national, regional, and local information needs and

decisions related to water-quality management and

policy. Shaped by and coordinated with ongoing

efforts of other Federal, State, and local agencies, the

NAWQA Program is designed to answer: What is the

condition of our Nation’s streams and ground water?

How are the conditions changing over time? How do

natural features and human activities affect the quality

of streams and ground water, and where are those

effects most pronounced? By combining information

on water chemistry, physical characteristics, stream

habitat, and aquatic life, the NAWQA Program aims to

provide science-based insights for current and emerging water issues. NAWQA results can contribute to

informed decisions that result in practical and effective

water-resource management and strategies that protect

and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than

50 of the Nation’s most important river basins and

aquifers, referred to as Study Units. Collectively,

these Study Units account for more than 60 percent

of the overall water use and population served by

public water supply, and are representative of the

Nation’s major hydrologic landscapes, priority

ecological resources, and agricultural, urban, and

natural sources of contamination.

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The assessments thereby build local knowledge

about water-quality issues and trends in a particular

stream or aquifer while providing an understanding

of how and why water quality varies regionally and

nationally. The consistent, multi-scale approach helps

to determine if certain types of water-quality issues

are isolated or pervasive, and allows direct comparisons of how human activities and natural processes

affect water quality and ecological health in the

Nation’s diverse geographic and environmental

settings. Comprehensive assessments on pesticides,

nutrients, volatile organic compounds, trace metals,

and aquatic ecology are developed at the national

scale through comparative analysis of the StudyUnit findings.

The USGS places high value on the communication and dissemination of credible, timely, and relevant

science so that the most recent and available knowledge about water resources can be applied in management and policy decisions. We hope this NAWQA

publication will provide you the needed insights and

information to meet your needs, and thereby foster

increased awareness and involvement in the protection

and restoration of our Nation’s waters.

The NAWQA Program recognizes that a national

assessment by a single program cannot address all

water-resource issues of interest. External coordination at all levels is critical for a fully integrated

understanding of watersheds and for cost-effective

management, regulation, and conservation of our

Nation’s water resources. The Program, therefore,

depends extensively on the advice, cooperation,

and information from other Federal, State, interstate,

Tribal, and local agencies, non-government organizations, industry, academia, and other stakeholder

groups. The assistance and suggestions of all are

greatly appreciated.

Robert M. Hirsch

Associate Director for Water

FOR E WOR D III

FOR E WOR D
CONTENTS V

CONTENTS

Foreword ................................................................................................................................................................................ III

Glossary ................................................................................................................................................................................. IX

Abstract.................................................................................................................................................................................. 1

Introduction............................................................................................................................................................................ 2

Importance of Nitrogen, Phosphorus, Suspended Sediment, and Organic Carbon ..................................................... 2

Purpose and Scope....................................................................................................................................................... 4

Description of Study Unit...................................................................................................................................................... 5

Hydrology .................................................................................................................................................................... 5

Landforms.................................................................................................................................................................... 5

Climate......................................................................................................................................................................... 7

Land Use...................................................................................................................................................................... 7

Methods and Data Analysis ................................................................................................................................................... 9

Sample Collection and Chemical Analysis.................................................................................................................. 11

Quality Assurance and Quality Control....................................................................................................................... 11

Statistics....................................................................................................................................................................... 13

Relation of Constituent Concentration and Streamflow .............................................................................................. 13

Hydrograph Separation ................................................................................................................................................ 14

Load Calculations ........................................................................................................................................................ 14

Concentrations of Nitrogen, Phosphorus, Suspended Sediment, and Organic Carbon ......................................................... 15

Overall Occurrence of Concentrations ........................................................................................................................ 15

Nitrogen ............................................................................................................................................................. 15

Phosphorus and Sediment.................................................................................................................................. 17

Organic Carbon.................................................................................................................................................. 17

Relations Between Constituent Concentrations and Streamflow ................................................................................ 17

Annual Variations .............................................................................................................................................. 17

Seasonal Variations............................................................................................................................................ 20

Nonpoint and Point Sources .............................................................................................................................. 22

Spatial Variability ........................................................................................................................................................ 22

Nitrogen ............................................................................................................................................................. 22

Phosphorus and Sediment.................................................................................................................................. 26

Dissolved Organic Carbon................................................................................................................................. 29

Transport of Nitrogen, Phosphorus, and Suspended Sediment.............................................................................................. 31

Loads............................................................................................................................................................................ 31

Yields ........................................................................................................................................................................... 35

Synoptic Studies .................................................................................................................................................................... 37

Variability Among Basic-Fixed and Synoptic Sites .................................................................................................... 38

Spatial Variability ........................................................................................................................................................ 40

Variability Among Base-Flow Conditions................................................................................................................... 41

Summary................................................................................................................................................................................ 43

References.............................................................................................................................................................................. 44

Appendix................................................................................................................................................................................ 49

FIGURES

1. Map showing landform regions and surface-water-quality sampling sites in the Eastern Iowa 

Basins study unit ......................................................................................................................................................... 3

2. Schematic showing physiography of landform regions in the Eastern Iowa Basins study unit.................................. 6

3. Map showing land cover and location of permitted livestock facilities in relation to surfacewater-quality sampling sites in the Eastern Iowa Basins study unit ........................................................................... 8

4. Graphs of precipitation and discharge at selected sites in the Eastern Iowa Basins study unit, 

1996–98, and map showing site locations .................................................................................................................. 18
VI CONTENTS

5. Boxplots showing yearly concentrations of selected constituents in surface-water samples in the 

Eastern Iowa Basins, 1996–98.................................................................................................................................... 19

6. Boxplots showing monthly variability of selected constituents in surface-water samples in the 

Eastern Iowa Basins study unit, March 1996–September 1998 ................................................................................. 21

7. Graph showing relations between nitrite plus nitrate and total phosphorus concentrations and 

streamflow in the Iowa River at Wapello, Iowa (site 11), 1970–98............................................................................ 23

8. Boxplots showing total nitrogen concentrations for indicator and integrator sites in surface-water 

samples from the Eastern Iowa Basins study unit, March 1996–September 1998..................................................... 24

9. Graph showing relation between median total dissolved nitrogen concentrations and percentage of 

drainage basin in row crops in the Eastern Iowa Basins study unit............................................................................ 25

10. Boxplots showing total phosphorus concentrations for indicator and integrator sites in surface-water 

samples from the Eastern Iowa Basins study unit, March 1996–September 1998..................................................... 27

11. Graph showing relation between total phosphorus concentrations and suspended-sediment concentrations 

in surface-water samples from integrator sites in the Eastern Iowa Basins, March 1996–September 1998 .............. 28

12. Boxplots showing suspended-sediment concentrations for indicator and integrator sites in surface-water 

samples from the Eastern Iowa Basins study unit, March 1996–September 1998..................................................... 29

13. Boxplots showing dissolved organic-carbon concentrations for indicator and integrator sites in surfacewater samples from the Eastern Iowa Basins study unit, March 1996–September 1998........................................... 30

14. Graphs showing estimated monthly loads of total nitrogen at selected sampling sites in the Eastern 

Iowa Basins study unit, 1996–98................................................................................................................................ 33

15. Graphs showing estimated monthly loads for total phosphorus at selected sampling sites in the Eastern 

Iowa Basins study unit, 1996–98................................................................................................................................ 34

16. Maps showing 3-year average estimated yields of total nitrogen and total phosphorus in the Eastern 

Iowa Basins study unit, 1996–98................................................................................................................................ 36

17. Graphs showing base flow and total flow at three surface-water-quality sampling sites during low and 

high base-flow conditions in the Eastern Iowa Basins study unit, July and August 1997 and April and 

May 1998.................................................................................................................................................................... 38

18. Graphs showing concentrations of selected nitrogen, phosphorus, and organic-carbon compounds in 

samples from 6 basic-fixed sites and 19 synoptic sites during low and high base flow............................................. 40

19. Maps showing spatial variability of nitrate and total phosphorus concentrations during low and high 

base-flow conditions in the Eastern Iowa Basins study unit....................................................................................... 42

TABLES

1. Description of surface-water-quality monitoring sites in the Eastern Iowa Basins study unit ............................... 10

2. Summary of field-blank and replicate-sample quality-assurance data for nitrogen, 

phosphorus, suspended sediment, and organic carbon in the Eastern Iowa Basins study 

unit, March 1996–September 1998 ......................................................................................................................... 12

3. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon 

concentrations in the Eastern Iowa Basins study unit, March 1996–September 1998 ........................................... 16

4. Estimated total nitrogen and total phosphorus loads and yields for streams in the Eastern Iowa 

Basins study unit, 1996–98 ..................................................................................................................................... 32

5. Estimated suspended-sediment loads and yields for selected rivers in the Eastern Iowa Basins 

study unit, 1996–98................................................................................................................................................. 35

6. Statistical summary of selected physical properties and chemical constituents in samples from 

25 synoptic sites in the Eastern Iowa Basins study unit, August 1997 (low base flow) and 

May 1998 (high base flow) ..................................................................................................................................... 39

7. Median concentrations of selected nitrogen constituents in samples from 25 synoptic sites in 

relation to landform region in the Eastern Iowa Basins study unit, August 1997 (low base flow) 

and May 1998 (high base flow)............................................................................................................................... 41

APPENDIX TABLE

Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon 

concentrations in the Eastern Iowa Basins study unit, March 1996–September 1998 ........................................... 51
CONTENTS VII

CONVERSION FACTORS, VERTICAL DATUM, and ABBREVIATIONS

Temperature in degrees Celsius (° C) may be converted to degrees Fahrenheit (° F) as follows:

° F = 1.8 (° C) + 32

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).

Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. For example, the 

water year ending September 30, 1996, is called water year 1996.

Multiply By To obtain

Length

millimeter (mm) 0.3937 inch

centimeter (cm) 0.03937 inch

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

Area

square meter (m2

) 10.76 square foot

square hectometer (hm2

) 2.471 acre

square kilometer (km2

) 0.3861 square mile

Volume

cubic hectometer (hm3

) 810.7 acre-foot

liter (L) 33.82 ounce

milliliter (mL) 0.0338 ounce

Flow

cubic meter per second (m3

/s) 35.31 cubic foot per second

cubic meter per second per square kilometer [(m3

/s)/km2] 91.46 cubic foot per second per square mile

Mass

kilogram (kg) 2.205 pound

metric ton 1.102 ton, short

kilogram per day (kg/d) 2.205 pound per day

kilogram per hectare (kg/ha) 0.8924 pound per acre

kilogram per square kilometer (kg/km2) 5.711 pound per square mile

kilogram per square kilometer per year [(kg/km2

)/yr] 5.711 pound per square mile per year

gram per kilogram (g/kg) 0.0160 ounce per pound

Concentration

milligram per liter (mg/L) 1.0 parts per million
VIII CONTENTS

Abbreviation Description

DOC dissolved organic carbon

EIWA Eastern Iowa Basins Assessment

GIS geographic information system

LOWESS locally weighted scatterplot smoothing

MCL maximum contaminant level

MRL minimum reporting limit

NAWQA National Water-Quality Assessment

NWQL National Water-Quality Laboratory

RPD relative percentage difference

SAS Statistical Analysis System

SOC suspended organic carbon

STATSGO State Soil Geographic data base

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

N nitrogen

P phosphorus

° C degrees Celsius

° F degrees Fahrenheit

cm centimeter

m meter

km kilometer

hm hectometer

mg/L milligram per liter

kg/d kilogram per day

kg/ha kilogram per hectare

yr year

< less than
GLOSSARY IX

GLOSSARY

Ammonia. A compound of nitrogen and hydrogen 

(NH3) that is a common by-product of animal waste. 

Ammonia readily converts to nitrate in soils and 

streams.

Analyte. A specific compound or element of interest undergoing chemical analysis.

Atmospheric deposition. The transfer of substances from the 

air to the surface of the Earth, either in wet form (rain, 

fog, snow, dew, frost, hail) or in dry form (gases, aerosols, particles).

Base flow. Sustained, low flow in a stream; ground-water 

discharge is the source of base flow in most places.

Basic-fixed site. Site on a stream at which streamflow is 

measured and samples are collected for temperature, 

specific conductance, pH, dissolved oxygen, nutrients, 

major ions, pesticides, suspended sediment, and 

organic carbon to assess the broad-scale spatial and 

temporal character and transport of inorganic and 

organic constituents of stream water in relation to 

hydrologic conditions and environmental setting.

Constituent. A chemical or biological substance in water, 

sediment, or biota that can be measured by an analytical method. 

Detection limit. The concentration below which a particular 

analytical method cannot determine, with a high 

degree of certainty, a concentration of an analyte.

Drainage area. The area, measured in a horizontal plane, 

which is enclosed by a drainage divide.

Drainage basin. The portion of the surface of the Earth that 

contributes water to a stream through overland runoff, 

including tributaries and impoundments.

Drinking-water regulation or guideline. A threshold 

concentration in a public drinking-water supply, 

designed to protect human health. As defined here, 

regulations are U.S. Environmental Protection Agency 

regulations that specify the maximum contamination 

levels for public water systems required to protect the 

public welfare; guidelines have no regulatory status 

and are issued in an advisory capacity.

Environmental sample. A water sample collected from a 

stream or river for the purpose of chemical, physical, 

or biological characterization of the sampled resource.

Equal-width increment (EWI) sample. A composite 

sample across a section of stream with equal spacing 

between verticals and equal transit rates within each 

vertical that yields a representative sample of stream 

conditions. 

Erosion. The process whereby materials of the Earth’s crust 

are loosened, dissolved, or worn away and simultaneously moved from one place to another. 

Eutrophication. The process by which water becomes 

enriched with nutrients, most commonly phosphorus 

and nitrogen.

Field equipment blank. A solution of water that 

contains analytes of interest below detection limits 

and is subjected to all aspects of sample collection, 

processing, preservation, transportation, and laboratory 

handling as an environmental sample but is collected 

at the sampling site immediately before the environmental sample.

Hypoxia. Seasonal depletion of dissolved oxygen (less 

than 2.0 milligrams per liter) within a water body, 

which is related to eutrophication of the water body. 

Most aquatic species cannot survive at such low 

oxygen levels.

Indicator site. Stream sampling site located at an outlet of a 

drainage basin with relatively homogeneous land use 

and physiographic conditions; indicator sites have 

drainage areas ranging from 320 to 1,080 square kilometers within the Eastern Iowa Basins.

Integrator site. Stream sampling site located at an outlet 

of a drainage basin that contains multiple environmental settings. Integrator sites are located on major 

rivers with drainage areas ranging form 6,050 to 

32,400 square kilometers within the Eastern Iowa 

Basins.

Intensive-fixed site. A basic-fixed site with increased 

sampling frequency during selected seasonal periods 

and analysis of dissolved pesticides for 1 year.

Karst. A type of topography that results from dissolution 

and collapse of carbonate rocks such as limestone and 

dolomite and characterized by closed depressions or 

sinkholes, caves, and underground drainage.

Load. General term that refers to a material or constituent 

in solution, in suspension, or in transport; usually 

expressed in terms of mass or volume.

Loess. Homogeneous, fine-grained sediment made up 

primarily of silt and clay and deposited over a wide 

area (typically by wind).

Maximum contaminant level (MCL). Maximum permissible 

level of a contaminant in water that is delivered to any 

user of a public water system. MCLs are enforceable 

regulations established by the U.S. Environmental 

Protection Agency.

Mean. The average of a set of observations, unless otherwise specified.

Mean discharge. The arithmetic average of individual daily 

average discharges during a specific period, usually 

daily, monthly, or annually.

Median. The middle or central value in a distribution of 

data ranked in order of magnitude. The median also

is known as the 50th percentile.
X GLOSSARY

Minimum reporting limit (MRL). The smallest measured 

concentration of a constituent that may be reliably 

reported by an analytical method. The minimum 

reporting level is generally higher than the detection 

limit because of unpredictable matrix effects for 

different waters.

Nitrate. An ion consisting of nitrogen and oxygen (NO3

–

). 

Nitrate is a nutrient and is very soluble in water.

Nonpoint source. A pollution source that cannot be 

defined as originating from discrete points, such as 

pipe discharge. Areas of fertilizer and pesticide applications, atmospheric deposition, manure, and natural 

inputs from plants are types of nonpoint-source 

pollution.

Nutrient. Chemical element that is essential to plant and 

animal nutrition. Nitrogen and phosphorus are nutrients that are important to aquatic life, but in high 

concentrations, they can be contaminants in water. 

These nutrients occur in a variety of forms. Both are 

affected by chemical and biological processes that can 

change their form and can transfer them to or from 

water, soil, biological organisms, and the atmosphere.

Overland flow. The part of surface runoff flowing over land 

surfaces toward stream channels.

Phosphorus. A nutrient essential for growth that can play 

a key role in simulating aquatic growth in lakes and 

streams.

Point source. A source of chemical, physical, biological 

input to a water body at a discrete location such as a 

discharge pipe, drainage ditch, tunnel, well, concentrated livestock operation, or floating craft.

Quality assurance. Evaluation of quality-control data to 

allow quantitative determination of the quality of 

chemical data collected during a study. Techniques 

used to collect, process, and analyze water samples 

are evaluated.

Reference site. Site at the headwater of a drainage basin 

that has been minimally affected by humans.

Sediment. Particles, derived from rocks or biological 

materials, that have been transported by a fluid or 

other natural process, suspended or settled in water.

Split sample. A sample prepared by dividing it into two or 

more equal volumes, where each volume is considered 

a separate sample but representative of the entire 

sample.

Study unit. A major hydrologic system of the United States 

in which NAWQA studies are focused. NAWQA study 

units are geographically defined by a combination of 

ground- and surface-water features and usually encompass more than 10,000 square kilometers.

Synoptic sites. Sites sampled during short time periods 

(1 week or less) to evaluate the spatial distribution 

of water quality during specified hydrologic 

conditions.

Tile drain (line). A buried perforated pipe designed to 

remove excess water from soils.

Yield. The mass of material or constituent transported 

by a river in a specified period of time divided by 

the drainage area of the river basin.
Abstract 1

Water-Quality Assessment of the Eastern Iowa 

Basins—Nitrogen, Phosphorus, Suspended 

Sediment, and Organic Carbon in Surface Water, 

1996–98

By Kent D. Becher, Stephen J. Kalkhoff, Douglas J. Schnoebelen, Kimberlee K. Barnes, 

and Von E. Miller

Abstract

Twelve sites on streams and rivers in the 

Eastern Iowa Basins study unit were sampled 

monthly and during selected storm events from 

March 1996 through September 1998 to assess 

the occurrence, distribution, and transport of 

nitrogen, phosphorus, suspended sediment, and 

organic carbon as part of the U.S. Geological 

Survey’s National Water-Quality Assessment 

Program. One site was dropped from monthly 

sampling after 1996. Dissolved nitrogen and 

phosphorus were detected in every water sample 

collected. Nitrate accounted for 92 percent of the 

total dissolved nitrogen. About 22 percent of the 

samples had nitrate concentrations that exceeded 

the U.S. Environmental Protection Agency’s maximum contaminant level of 10 milligrams per liter 

as nitrogen for drinking-water regulations. The 

median concentration of total dissolved nitrogen 

for surface water in the study unit was 7.2 milligrams per liter. The median total phosphorus concentration for the study unit was 0.22 milligram 

per liter. About 75 percent of the total phosphorus 

concentrations exceeded the U.S. Environmental 

Protection Agency recommended total phosphorus concentration of 0.10 milligram per liter or 

less to minimize algal growth. Median suspended 

sediment and dissolved organic-carbon concentrations for the study unit were 82 and 3.5 milligrams per liter, respectively.

Median concentrations of nitrogen, phosphorus, and suspended sediment varied annually 

and seasonally. Nitrogen, phosphorus, and 

suspended-sediment concentrations increased 

each year of the study due to increased precipitation and runoff. Median concentrations of dissolved organic carbon were constant from 1996 

to 1998. Nitrogen concentrations were typically 

higher in the spring after fertilizer application and 

runoff. During winter, nitrogen concentrations 

typically increased when there was little in-stream 

processing by biota. Nitrogen and phosphorus 

concentrations decreased in late summer when 

there was less runoff and in-stream processing 

of nitrogen and phosphorus was high. Dissolved 

organic carbon was highest in February and 

March when decaying vegetation and manure 

were transported during snowmelt. Suspendedsediment concentrations were highest in early 

summer (May–June) during runoff and lowest in 

January when there was ice cover with very little 

overland flow contributing to rivers and streams. 

Based on historical and study-unit data, eastern 

Iowa streams and rivers are impacted by both 

nonpoint and point-source pollution.

Indicator sites that have homogeneous land 

use, and geology had samples with significantly 

higher concentrations of total dissolved nitrogen 

(median, 8.2 milligrams per liter) than did samples from integrator sites (median, 6.2 milligrams 

per liter) that were more heterogeneous in land 

use and geology. Samples from integrator sites 
2 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

typically had significantly higher total phosphorus 

and suspended-sediment concentrations than did 

samples from indicator sites. Typically, there was 

very little difference in median dissolved organiccarbon concentrations in samples from indicator 

and integrator sites. 

Concentrations of nitrogen and phosphorus 

varied across the study unit 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. Basins that drain the Southern Iowa 

Drift Plain and the Des Moines Lobe typically 

had samples with higher total phosphorus and 

suspended-sediment concentrations.

Total nitrogen loads increased each year 

from 1996 through 1998 in conjunction with 

increased concentrations and runoff. Total phosphorus loads in the Skunk River Basin decreased 

in 1997 due to less runoff and decreased sediment 

transport, but increased in 1998 due to higher runoff and increased sediment transport. Total nitrogen and total phosphorus loads varied seasonally. 

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 typically very little 

runoff to transport nitrogen and phosphorus 

in the soil to the rivers and streams.

Total nitrogen loads contributed to the 

Mississippi River from the Eastern Iowa Basins 

during 1996, 1997, and 1998 were 97,600, 

120,000, and 234,000 metric tons, respectively. 

Total phosphorus loads contributed to the 

Mississippi River from the Eastern Iowa Basins 

during 1996, 1997, and 1998 were 6,860, 4,550, 

and 8,830 metric tons, respectively. Suspendedsediment loads contributed to the Mississippi 

River from the Eastern Iowa Basins during 1996, 

1997, and 1998 were 7,480,000, 4,450,000, and 

8,690,000 metric tons, respectively. The highest 

total nitrogen and total phosphorus yields typically occurred in samples from indicator sites. 

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 physiographic features 

(for example, erodible soils, steeper slopes).

Synoptic samples collected during low 

and high base flow had nitrogen, phosphorus, 

and organic-carbon concentrations that varied 

spatially and seasonally. Comparisons of waterquality data from six basic-fixed sampling sites 

and 19 other synoptic sites suggest that the waterquality data from basic-fixed sampling sites were 

representative of the entire study unit during periods of low and high base flow when most streamflow originates from ground water.

INTRODUCTION 

The Eastern Iowa Basins (EIWA) study 

unit (fig. 1) is 1 of 15 U.S. Geological Survey 

(USGS) National Water-Quality Assessment Program 

(NAWQA) study units activated in 1994 (fig. 1). Information on water-quality conditions in the EIWA is 

necessary for planning and management. Part of the 

assessment of the study unit addressed surface-water 

conditions. Specific surface-water-quality issues of 

primary concern in the EIWA include degradation of 

aquatic ecosystems, soil erosion (sediment), and elevated concentrations of nitrogen, phosphorus, organic 

carbon, and synthetic organic compounds (including 

pesticides) (Iowa Department of Natural Resources, 

1994; Kalkhoff, 1994; Hallberg and others, 1996; 

Goolsby and others, 1997). 

Importance of Nitrogen, Phosphorus, 

Suspended Sediment, and Organic 

Carbon

Nitrogen and phosphorus compounds can 

occur naturally at small concentrations in stream water 

(Hem, 1985). In addition, nitrogen and phosphorus can 

be introduced into the environment from sources such 

as chemical fertilizer, animal manure, wastewater 

effluent, atmospheric deposition, soil mineralization, 

and legume fixation.

Nitrogen compounds are a water-quality concern primarily because they contribute to aquatic plant 

growth, eutrophication, and toxicity. Algae generally 

prefer ammonia over nitrate for growth (Brezonik, 

1973, p. 11), but both the reduced species of nitrogen 
INTRODUCTION 3

MINNESOTA

IOWA

93°

43°

42°

41°

91°

92°

Marshalltown

Waterloo

Marion

Cedar

Rapids

Coralville

Iowa City

Mason

City

Albert

Lea Austin

Cedar

Falls

Ames

Landform regions (modified 

 from Prior, 1991)

Des Moines Lobe

Iowan Karst

Iowan Surface

Southern Iowa Drift Plain

Major city

Major lakes

State boundary

County boundary

Study unit boundary

Major river

Basic-fixed site and map reference number

Synoptic site and map reference number or letter

8

8

s r

r

EXPLANATION

q

n

g

p f

o

e

k

h

b

c

d

m

i

a

j l

12

11

2

10

6

5

4

3 1

9

7

Base from U.S. Geological Survey digital data, 1:250,000, 1967

Albers Equal-Area Conic projection

Standard parallels 41°30' and 44°30', central meridian -92°

IOWA

MINNESOTA

Eastern

Iowa

Basins

Mississippi River

Index Map

0 20 40 60 MILES

0 20 40 60 KILOMETERS

Figure 1. Landform regions and surface-water-quality sampling sites in the Eastern Iowa Basins 

study unit.
4 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

(ammonia and organic nitrogen) and the oxidized 

species (nitrite and nitrate) can be used as nutrients 

for algal growth.

Considerable concern in recent years has been 

expressed over health effects of nitrate in drinking 

water (Neill, 1989). Nitrite and nitrate have been 

linked to infantile methemoglobinemia (whereby the 

blood loses its ability to transport oxygen) and are suspected of causing the formation of carcinogenic nitrosamines and nitrosamides (Neill, 1989). In addition, 

an association between nitrate in drinking water 

and bladder cancer in women has been identified 

(Weyer and others, 2001). The maximum contaminant 

level (MCL) for drinking water established by the 

U.S. Environmental Protection Agency (USEPA) is 

10 mg/L of nitrate as nitrogen (U.S. Environmental 

Protection Agency, 1996).

Phosphorus compounds are a water-quality concern because, like nitrogen, phosphorus is an essential 

nutrient for plant growth. To prevent the excessive 

growth of aquatic plants in water bodies, the USEPA 

recommends that total phosphorus concentrations not 

exceed 0.10 mg/L as phosphorus (U.S. Environmental 

Protection Agency, 1986). About 95 percent of the 

phosphorus transported by rivers is adsorbed to sediment (Maybeck, 1982). However, algae and other 

aquatic plants most readily use the soluble or dissolved 

compounds of phosphorus. Sources of phosphorus 

include wastewater-treatment-plant effluent, detergents, manure, fertilizers, and sediment from surfacewater runoff. 

The Midwestern United States has been identified as a major contributor of nitrogen and phosphorus 

to the Mississippi River (Goolsby and others, 1997). In 

nutrient-poor freshwater, inorganic phosphate is often 

the factor limiting the growth of aquatic plants and 

algae. However, nitrate as nitrogen tends to become 

the limiting factor when phosphorus is plentiful 

(Allen, 1995, p. 89). Recent studies in the Gulf of 

Mexico have indicated areas with small dissolvedoxygen concentrations (less than 2.0 mg/L), a condition known as hypoxia. The occurrence of hypoxia in 

the Gulf of Mexico has been linked to nitrogen and 

phosphorus loads discharged from the Mississippi 

River (Turner and Rabalais, 1994; Goolsby and others, 

1999). Increases in nitrogen and phosphorus loads to 

the Gulf of Mexico can promote the excessive growth 

of algae which eventually die and decompose, depleting the water column of dissolved oxygen, which can 

kill or otherwise adversely affect fish and other aquatic 

life.

Sediment is a water-quality concern because it 

can fill reservoirs, prevent sunlight from penetrating 

the water to reach aquatic plants, and adsorb and transport many toxic compounds. Suspended sediment 

originates from erosion within streams and on uplands. 

Phosphorus is one of the constituents that tends to 

adsorb to sediment.

The organic layer in soils is a major source of 

soluble organic compounds (Drever, 1988). Organic 

carbon is a water-quality concern because it plays a 

major role in chemical processes and the transport of 

trace metals. Dissolved organic carbon (DOC) and 

suspended organic carbon (SOC) are normally present 

in surface water from decaying vegetation, living 

organisms suspended in the water column such as 

algae and bacteria, sewage, and manure.

Purpose and Scope

Water-quality data presented in this report 

were collected at 12 surface-water-quality sites 

from March 1996 through September 1998 as 

part of the EIWA NAWQA. The objectives of this 

report are to: (1) summarize nitrogen, phosphorus, 

suspended-sediment, and organic-carbon data 

in drainage basins both spatially and temporally 

during 1996–98; (2) compare nitrogen, phosphorus, 

suspended-sediment, and organic-carbon concentrations among drainage basins and relate differences 

to land use, basin physical characteristics, streamflow, 

and human factors; and (3) estimate loads and yields 

of total nitrogen, total phosphorus, and suspended 

sediment transported by rivers and streams. Twelve 

sites were sampled during the first year of the study 

(1996) and 11 were sampled during the last 2 years 

of the study (1997–98). In addition, results from 

synoptic studies conducted in 25 small watersheds 

(310–1,480 km2

) during high and low base-flow 

conditions are discussed.

Schnoebelen and others (1999) summarized historical surface-water nitrogen and phosphorus data in 

the EIWA study unit collected by various agencies 

from 1970 through 1995. The present EIWA study 

updates nitrogen and phosphorus findings within the 

study unit for the sampling period (1996–98) and 

expands information to drainage basins where limited 

water-quality data were available. In addition, EIWA 

water-quality data will be useful for national comparisons with other NAWQA study units. 
DESCRIPTION OF STUDY UNIT 5

DESCRIPTION OF STUDY UNIT

Hydrology

The EIWA study unit consists of four major 

drainage basins and covers about 49,700 km2

(Kalkhoff, 1994). The major drainage basins are 

the Wapsipinicon River Basin (6,100 km2

), the 

Cedar River Basin (20,200 km2), the Iowa River 

Basin (12,200 km2), and the Skunk River Basin 

(11,200 km2) (fig. 1). The four major rivers in the 

study unit have their headwaters in the northwestern 

part of the study unit and flow to the southeast, discharging to the Mississippi River. The Wapsipinicon 

River originates in southeastern Minnesota and is 

about 362 km long, with an average basin width of 

about 16 km. The Cedar River originates in southern 

Minnesota and forms the largest basin in the study 

unit. The Cedar River Basin ranges from 32 to 96 km 

in width. The Cedar River joins the Iowa River about 

48 km upstream from the confluence of the Iowa and 

Mississippi Rivers. The Iowa River originates in northcentral Iowa and averages 32 km in width. Together, 

the Cedar and Iowa River Basins cover 32,380 km2

and drain about 64 percent of the study unit. The 

Skunk River originates in central Iowa and averages 

39 km in width.

Overland flow and ground-water discharge are 

the major sources of streamflow. Flooding occurs as a 

result of rapid spring melting of the snowpack, often 

combined with rainfall, or thunderstorm activity. 

Droughts can result from the shift of the normal seasonal atmospheric storm track by high-pressure conditions, a block or decrease in moist airflows, or lack of 

thunderstorm development.

Interflow is that part of subsurface flow that 

moves at shallow depths and reaches the surface 

stream channel in a relatively short period of time and, 

therefore, commonly is considered part of overland 

flow. During a storm period, interflow slowly 

increases up to the end of the storm period, followed 

by a gradual recession (Viessman and others, 1989). 

Tile drains (lines) beneath fields can enhance the 

subsurface-drainage component of flow to streams. 

Yearly total streamflow from the study unit averages 

11,350 hm3 (Kalkhoff, 1994). The mean annual 

streamflow from the Wapsipinicon River Basin, 

the Cedar-Iowa River Basins, and the Skunk River 

Basin averages about 1,360, 7,770, and 2,220 hm3

,

respectively.

Landforms

The EIWA study unit includes three major 

landform regions (Des Moines Lobe, Iowan Surface, 

Southern Iowa Drift Plain) (Prior, 1991) and one subregion (Iowan Karst, a subregion of the Iowan Surface) 

that are based on distinct spatial differences in topography, geology, soils, and vegetation (figs. 1 and 2). 

These regions are broadly coincident with ecoregions 

and subecoregions of Iowa (Griffith and others, 1994). 

The Des Moines Lobe, in the western part of the 

study unit, is one of the youngest landforms in Iowa 

and is characterized by low local relief (15–30 m) on 

the land surface. The Des Moines Lobe was formed by 

the last glaciation in Iowa approximately 12,000 to 

14,000 years ago (Wisconsinan age) and has been 

altered only slightly since that time (Prior, 1991). The 

topography consists of predominantly flat and slightly 

rolling land broken by arcuate bands of “knob and 

kettle” terrain (Buchmiller and others, 1985). Historically, ponds and wetlands and poor drainage characterized the Des Moines Lobe. Extensive ditching and 

tiling of fields during 1900–20 augmented the natural 

surface drainage in this area. The potential natural vegetation is bluestem prairie (Griffith and others, 1994); 

corn and soybean crop production presently (2001) 

dominates. Stream development is poor with many 

small, low-gradient streams that drain into a few larger 

rivers. Surficial material consists of glacial till that has 

an average thickness of about 30 m, and alluvium in 

association with the larger streams. Surficial loess is 

absent (fig. 2). 

The Iowan Surface is characterized by gently 

rolling topography with long slopes and low relief 

(15–30 m). Drainage is well developed, although 

streams generally have slight gradients (fig. 2). Surficial material consists of pre-Illinoian-age (500,000 to 

700,000 years old) glacial till covered by a thin veneer 

of windblown loess that transitions from little to none 

in the north to thicker deposits on the ridges in the 

south with alluvium near the streams (Prior, 1991). 

Potential natural vegetation includes bluestem prairie 

and oak-hickory forest; corn and soybean crop production presently (2001) dominates the land surface 

(Prior, 1991; Griffith and others, 1994).

The Iowan Karst is a subregion of the Iowan 

Surface where dissolution of soluble limestone and 

dolomite bedrock under a thin or nonexistent cover of 

glacial drift has caused localized collapse of the land 

surface resulting in karst topography with numerous 

sinkholes. The surface drainage is well developed, 
6 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

Leached glacial till

Leached glacial till

Calcareous glacial till

Des Moines Lobe

Iowan Surface

Southern Iowa

Drift Plain

EXPLANATION

SLOPE—Numbers indicate ranges

 in slope, in percent

9-30

Loess

Paleosol

Glacial till

Alluvium

NOT TO SCALE

NOT TO SCALE

NOT TO SCALE

Calcareous glacial till

Sand

wedges

Glacial

outwash

Figure 2. Physiography of landform regions in the Eastern Iowa Basins study unit (modified from 

Oschwald and others, 1965).
DESCRIPTION OF STUDY UNIT 7

although local direct infiltration to bedrock is common. This area is used extensively for agriculture, and 

some fields are drained through agricultural drainage 

wells (gravity based). Field tile lines are connected 

to these drainage wells and may become conduits for 

surface runoff to enter the underlying bedrock. Floyd 

County, Iowa, located in the Cedar River Basin, has 

the most registered agricultural drainage wells in the 

EIWA study unit (Libra and others, 1996). Sinkholes, 

a natural feature of the Iowan Karst, can affect groundwater quality in a manner similar to agricultural drainage wells.

The Southern Iowa Drift Plain is characterized by steeply rolling terrain with moderate relief 

(30–91 m) separated by flat, tabular divides (fig. 2). 

Surficial material consists of pre-Illinoian glacial 

deposits mantled by loess. Soils on the lower slopes 

commonly are derived from till, whereas soils on the 

higher slopes and upland flats are derived from loess. 

Alluvium is found in association with streams that 

form a well-developed drainage pattern. Potential 

natural vegetation includes bluestem prairie and oakhickory forests (Griffith and others, 1994); the land 

presently (2001) is used for agriculture.

Climate

The climate in the study unit is continental, with 

large differences in seasonal temperatures that result in 

distinct winter and summer seasons. Primary climatic 

effects in the EIWA study unit are warm, moist air 

from the Gulf of Mexico and surges of cold, dry air 

from Canada, which predominate in the summer and 

winter, respectively (U.S. Department of Commerce, 

1959). Mean monthly temperatures range from –16°C 

for the lows in January to 28°C for the highs in July 

(Wendland and others, 1992). The growing season 

(generally April–September) lasts about 127 days and 

is characterized by mean temperatures of 19° C in the 

southern part of the EIWA study unit to 16°C in the 

northern part.

Precipitation occurs mostly as rain associated 

with thunderstorms that occur from April through 

September. About 71 percent of the annual rainfall 

occurs during this period (Harry Hillaker, Iowa 

Department of Agriculture and Land Stewardship, oral 

commun., 2000). Peak precipitation occurs in June and 

diminishes sharply during the autumn. Precipitation 

during the cooler months of the year generally is of 

long duration and of moderate or low intensity, 

whereas precipitation during the late spring and summer tends to be of shorter duration and higher intensity. Snow during the colder winter months can remain 

on the land surface from December to March. The 

mean annual precipitation ranges from 76 cm in the 

north to 94 cm in the southeastern part of the study 

unit (Wendland and others, 1992).

Land Use

Land use is based primarily on agriculture 

and agriculture-related industry, accounting for 

92.9 percent of the land use in the study unit (fig. 3). 

Other land uses are forests (4.0 percent), urban 

(1.8 percent), and other (1.3 percent). The principal 

crops are corn, soybeans, oats, hay, and pasture on 

unirrigated land. Iowa ranked first in the Nation in 

the production of corn and soybeans in 1995 and 

1996 (Sands and Holden, 1996, 1997). 

In addition to row-crop agriculture, Iowa 

produces large quantities of hogs, cattle, sheep, and 

poultry. Iowa hog production ranked first in the Nation 

in 1995 (14.4 million head) and 1996 (12.2 million 

head) (Sands and Holden, 1996, 1997). Numerous 

confined-feeding hog facilities began operation in 

the 1990’s in the study unit. Figure 3 shows the location of regulated livestock facilities in the EIWA study 

unit, and the majority of these facilities are for hog 

production. Confined-feeding facilities must obtain 

permits from the Iowa Department of Natural 

Resources before construction if the facility is 

designed for an animal weight capacity higher than 

181,440 kg bovine or 90,720 kg for other animal species (Iowa Department of Natural Resources, 1998). 

The numbers of hog facilities have doubled since 1993 

in the upstream parts of the Iowa River and Skunk 

River Basins (fig. 3). The potential negative effects of 

these hog-production facilities on water quality are not 

known and are of concern.

The use of chemical fertilizers to increase 

crop production increased in the United States from 

1970 to 1994 (Goolsby and Battaglin, 1995). Fertilizer 

use in eastern Iowa has paralleled the national trend. 

In recent years, fertilizer (nitrogen) application rates 

on corn in Iowa decreased from 1985 (162 kg/ha) 

to 1994 (135 kg/ha), but application rates then 

increased from 134 kg/ha in 1995 to 148 kg/ha in 
8 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98































 







  



 

  

  

 

  

   

 

  

     

 

  

  



 

  





  

 



  

 

     

  







  

 

  

  

  

 



  

 



 

  

  

  

 



  

     

  

 



 

       

 





 



  

  

  

  







 



  



  

 



  

  

  

  

  

  

 



  

  

  

  



    









 

  

 

 



  

  

 







 

  

  

  

     

  

     

 



  













  

  







  

 

  

  

  

  

  

 

  

  

 





 

  

  







  



 

  

  

 









 







  



 



 







 



  





  



 

  

   

   



  

  

   





 

  

 

   

 

  

  

  

    



  

   

 



  

  

  

 



 

  

 





 



 

  



 

  

 

  

  

  



 

  

  

  

 









 



    







  

 









 





  

     

 



  







  

     

















































 

 







  





 

     

 



 

        

 

               

 

 









     

  



 

 





    





  

   

  

    

   





    

 

 



 

  





 



 

  

  



    

 



 





 





  







   

Iowa River

12

11

10

9

8

7

6

5

4

3

2

1

o 91

o 92

o 93

o 43

o 42

o 41

Coralville

Reservoir

Ames Marshalltown

Iowa City

Coralville

Cedar

Rapids

Marion

Waterloo

Cedar

Falls

Mason

City

Austin

Clear

Lake

Rice

Lake

Bear

Lake

Albert Lea

Lake

Geneva

Lake

Albert

Lea

IOWA

MINNESOTA 

DES

MOINES

VAN

BUREN

LEE

DES MOINES

HENRY

LOUISA

MUSCATINE

SCOTT

CLINTON

JOHNSON

CEDAR

JONES

DELAWARE

LINN

MARION

MAHASKA

KEOKUK

WAPELLO JEFFERSON

WASHINGTON

IOWA

JASPER POWESHIEK POLK

BUTLER

FAYETTE

BREMER

DODGE

MOWER

FREEBORN

HOWARD

CHICKASAW

FLOYD

MITCHELL

WORTH

WINNEBAGO

HANCOCK

CERRO

GORDO

WRIGHT

FRANKLIN

BUCHANAN

BLACK HAWK

BENTON

TAMA

GRUNDY HARDIN

HAMILTON

MARSHALL BOONE

STORY

s

r

q

p

o

n

m

l

k

j

i

h

g

f

e

d

c

b

a

b

8

Livestock-facility data from

Iowa Department of Natural

Resources (1998) and David

Johnson, Minnesota Pollution

Control Agency, written

commun., 1997)

EXPLANATION

Livestock facilities

 Permit issued before 1993

 Permit issued during 1993–98

Land cover (based on satellite imagery from

U.S. Geological Survey, EROS Data Center, 1994)

Row crops

Urban

Forest

Grassland, pasture, hay

Water

Wetlands

State boundary

County boundary

  Study unit boundary

Synoptic site and map reference number or letter

 

Basic-fixed site and map reference number

Base from U.S. Geological Survey digital data, 1:250,000, 1967

Albers Equal-area Conic projection

Standard parallels 41 30 and 44 30, central meridian -92

0

0 20 40 60 KILOMETERS

20 40 60 MILES

o o

Figure 3. Land cover and location of permitted livestock facilities in relation to surface-water-quality 

sampling sites in the Eastern Iowa Basins study unit.
METHODS AND DATA ANALYSIS 9

1996 (G.R. Hallberg, University of Iowa Hygienic 

Laboratory, written commun., 1997). Application rates 

in 1997 and 1998 were 136 kg/ha and 148 kg/ha, 

respectively (U.S. Department of Agriculture, 1998, 

1999).

METHODS AND DATA ANALYSIS

Twelve basic-fixed surface-water-quality sites 

were selected on rivers and streams following the 

NAWQA sampling design protocol (Hirsch and others, 

1988; Kalkhoff, 1994) to serve as indicator or integrator sites (table 1, fig. 1). Indicator sites represent 

smaller drainage basins (320 to 1,080 km2

) with 

relatively homogeneous land use and geology. Integrator sites represent larger drainage basins (6,050 to 

32,400 km2

) and are affected by combinations of land 

use and geology. The basic-fixed sites were monitored 

continuously for stream discharge and sampled 

monthly to assess the broad-scale spatial and temporal 

character of water-quality constituents. In addition, 

three basic-fixed sites, the Iowa River near Rowan 

(site 3, fig. 1), Wolf Creek near Dysart (site 9, fig. 1), 

and the Iowa River at Wapello (site 11, fig. 1), were 

sampled weekly in the spring and early summer of 

1997 and biweekly at the end of the summer of 1997 

(intensive sites, table 1) to determine short-term fluctuations and variability of selected constituents.

The indicator sites (table 1, fig. 1) were 

selected to represent typical drainage basins in 

the three major landforms—Iowa River near Rowan 

(site 3, fig. 1)–Des Moines Lobe; Wolf Creek near 

Dysart (site 9, fig. 1)–Iowan Surface; Flood Creek near 

Powersville (site 7, fig. 1)–Iowan Karst (a subregion of 

the Iowan Surface); and Old Mans Creek near Iowa 

City (site 6, fig. 1)–Southern Iowa Drift Plain. The 

primary difference between indicator sites is physiographic (soils, topography, and geology) as the land 

use within these areas is consistently about 95 percent 

agriculture (table 1). In addition, the Wapsipinicon 

River near Tripoli (site 1, fig. 1) was selected as a reference site because the drainage basin retains a higher 

percentage of its stream-corridor wetlands compared 

to other basins in the study unit (table 1). The South 

Fork Iowa River near New Providence (site 4, fig. 1) 

was selected as an indicator site to assess the effects 

of large-scale, hog-confinement facilities on streamwater quality.

Integrator sites (table 1) were sampled to assess 

broad-scale spatial and temporal differences in waterquality characteristics in basins that contain a mixture 

of land use and geologic features. Water-quality sampling of the Cedar River near Conesville (site 10, 

fig. 1) was moved about 8 km upstream because of 

bridge construction in October 1996. Water-quality 

data collected from this upstream location were considered to be equivalent to data collected from the 

downstream location because there are no major tributaries or urban areas draining into the river within the 

8-km reach. Water-quality sampling of the Cedar 

River at Gilbertville (site 8, fig. 1) was discontinued 

in March 1997 after one full year of sampling because 

the site did not have a continuous gaging station. 

Eleven basic-fixed sites were sampled 

monthly beginning in March 1996 and ending in 

September 1998. Additional samples were collected 

near peak river stage after significant rainfall events. 

The intensive sites were sampled weekly beginning 

in April 1997 (fertilizer application period and early 

growing season) and biweekly from July 1997 through 

November 1997 (after fertilizer application and middle 

to late growing season). 

Samples also were collected at 25 synoptic 

sites during low base-flow and high base-flow conditions in August 1997 and May 1998 (fig. 1, table 1) to 

better define spatial variability. The synoptic sampling 

included six indicator basic-fixed sites. The low baseflow samples were collected as part of the NAWQA 

Midwest Regional synoptic study (Sorenson and 

others, 1999) conducted by personnel from the Upper 

Mississippi River Basin, EIWA, and the Lower Illinois 

River Basin NAWQA study units. Candidate sampling 

sites were selected for drainage basin areas generally 

larger than 260 km2 and smaller than 2,600 km2. All 

basins represented by the synoptic samples had at least 

85 percent agricultural land use (table 1); agricultural 

land use among all surface-water-quality sampling 

sites in the study averaged more than 90 percent. 

Drainage basin selection criteria also included moderately well drained soils (more than 50 percent of the 

basin area in the State Soil Geographic data base 

(STATSGO) soil hydrologic groups A or B) to poorly 

drained soils (more than 50 percent of the basin area in 

STATSGO soil hydrologic groups C or D) (Sorenson 

and others, 1999).
10 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment,

and Organic Carbon in Surface Water, 1996–98

Table 1. Description of surface-water-quality monitoring sites in the Eastern Iowa Basins study unit[<, less than]

Map reference

number or letter

(fig. 1)

Site number Site name

Drainage

area

(square

kilometers)

Site type1

Land use in contribut

(percen

Agriculture Urban Fore

1 05420680 Wapsipinicon River near Tripoli, IA 900 bfs, ind, syn 88.5 1.8 5.2 05422000 Wapsipinicon River near De Witt, IA 6,050 bfs, int 87.4 2.1 7.3 05449500 Iowa River near Rowan, IA 1,080 bfs, intens, ind, syn 94.6 1.8 1.4 05451210 South Fork Iowa River near New Providence, IA 580 bfs, ind, syn 95.1 1.5 2.5 05453100 Iowa River at Marengo, IA 7,240 bfs, int 91.0 2.5 3.6 05455100 Old Mans Creek near Iowa City, IA 520 bfs, ind, syn 91.9 2.6 4.7 05461390 Flood Creek near Powersville, IA 320 bfs, ind, syn 95.3 1.1 2.8 05464020 Cedar River at Gilbertville, IA2 13,600 bfs, int 90.7 3.0 3.9 05464220 Wolf Creek near Dysart, IA 770 bfs, intens, ind, syn 95.6 1.8 1.

10 05465000 Cedar River near Conesville, IA 20,200 bfs, int 89.5 3.3 4.11 05465500 Iowa River at Wapello, IA 32,400 bfs, intens, int 89.0 3.1 4.12 05474000 Skunk River at Augusta, IA 11,200 bfs, int 87.1 2.9 7.

a 05420720 East Fork Wapsipinicon River near Tripoli, IA 370 syn 90.4 1.3 5.

b 05420900 Little Wapsipinicon River at Littleton, IA 380 syn 89.0 2.3 7.

c 05421700 Buffalo Creek near Stone City, IA 600 syn 90.1 2.2 6.

d 05421870 Mud Creek near Donahue, IA 310 syn 94.7 2.2 2.

e 05449200 East Branch Iowa River at Belmond, IA 500 syn 95.4 1.8 1.

f 05452020 Salt Creek at Belle Plaine, IA 560 syn 93.0 2.4 3.

g 05455500 English River near Kalona, IA 1,500 syn 91.2 2.5 4.h 05456510 Turtle Creek at Austin, MN 400 syn 90.7 1.9 3.

i 05457950 Little Cedar River near Floyd, IA 610 syn 93.4 1.3 3.j 05458870 Maynes Creek near Kelsey, IA 350 syn 95.0 1.1 3.

k 05459300 Winnebago River near Fertile, IA 760 syn 91.4 2.0 2.

l 05462770 Beaver Creek near Parkersburg, IA 370 syn 95.5 1.4 2.

m 05463510 Black Hawk Creek at Waterloo, IA 850 syn 95.4 2.0 1.

n 05465310 Long Creek near Columbus Junction, IA 400 syn 90.8 2.3 6.o 05469980 South Skunk River near Story City, IA 570 syn 94.0 3.1 1.p 05471120 East Branch Indian Creek near Iowa Center, IA 330 syn 93.6 2.2 3.q 05473060 Crooked Creek at Coppock, IA 740 syn 90.5 2.4 6.

r 05473400 Cedar Creek near Oakland Mills, IA 1,400 syn 85.4 2.2 10.

s 05473550 Big Creek near Lowell, IA 420 syn 87.2 3.3 8.

1Site type: bfs, basic-fixed site, sampled monthly; ind, indicator site representing drainage basins of 320 to 1,080 square kilo

meters; int, integrator site representing 

32,400 square kilometers; intens, intensive site sampled weekly in spring and early summer 1997 and biweekly at end of summer 1997; syn, synoptic site. 2Cedar River at Gilbertville was discontinued as a basic-fixed site in March 1997.

3Land-use data from Hitt, 1994.
METHODS AND DATA ANALYSIS 11

Sample Collection and Chemical Analysis

A complete discussion of the collection and 

processing of surface-water samples is described 

in Shelton (1994). All surface-water samples were 

obtained by collecting depth-integrated subsamples 

at equally spaced vertical sections across the stream 

(Ward and Harr, 1990) to integrate water-quality differences within the stream. Samples for analysis of 

organic carbon were collected using a single-point 

depth-integrated sampler. At each surface-waterquality sampling site, a minimum of 10 equally 

spaced, depth-integrated water samples were collected 

using cable-mounted or hand-held samplers (Shelton, 

1994). During ice conditions, samples were collected 

from at least three equal-width vertical sections. Laboratory analysis for nutrients, suspended sediment, and 

organic carbon was done using methods described by 

Fishman (1993) and Guy (1969).

Streamflow discharge was obtained from 

instantaneous discharge measurements or from discharge records obtained from continuous-recording 

gaging stations located at each of the sites. Measurements of specific conductance, pH, water temperature, 

and dissolved oxygen were made at equal-width increments across the stream cross section. The median 

value for each was recorded as the field-determined 

value.

Quality Assurance and Quality Control

The NAWQA quality-control design is 

described in detail by Mueller and others (1997). 

Equipment-blank samples of deionized water certified 

to be free of organic compounds and inorganic compounds were passed through all sampling equipment 

during February or March for each year of the sampling to verify initial cleanliness. About 13 percent of 

the total samples collected for the EIWA study were 

used for quality assurance (field-blank and replicate 

samples).

A field-blank sample is a specific type of blank 

sample used to demonstrate that: (1) equipment has 

been adequately cleaned to remove contamination 

introduced by previous use; (2) sample collection and 

processing have not resulted in contamination; and 

(3) sample handling, transport, and laboratory analysis 

have not introduced contamination (Mueller, 1998). 

The same type of deionized water that was used for 

equipment-blank samples was used for field-blank 

samples collected by passing the deionized water 

through all pumps, filter plates, and filters to verify 

cleanliness of sampling equipment. 

The objective of collecting replicate samples 

was to estimate the precision of concentration values 

from sample processing and analysis. Laboratory analyses of organic constituents generally are more variable than analyses of inorganic constituents. Each 

replicate sample is an aliquot of the environmental 

water sample processed through the cone splitter, 

passed through the same sample equipment, and processed in the same manner. 

Table 2 summarizes quality-assurance data for 

field-blank and replicate samples. Field-blank sample 

concentrations for nitrogen and phosphorus constituents were typically equivalent to the minimum reporting limit (MRL) or within a few hundredths of a 

milligram per liter of the MRL. This indicates that 

there was very little, if any, cross contamination of 

samples from sampling equipment. Low levels of 

DOC were detected in all 15 blank samples. Low-level 

contamination from DOC may have originated from 

methanol that was used in the sampling and decontamination for pesticides, from ineffective decontamination of sampling equipment between samples, or from 

laboratory grade blank water that contained some 

organic compounds. SOC was detected in 71 percent 

of samples, but the concentrations were close to the 

MRL.

Replicate samples were compared to the environmental samples by calculating relative percentage 

differences (RPD). In general, replicate concentrations 

for all constituents were generally within 10 percent 

of the environmental sample. RPD between replicate 

concentrations was calculated by using the following 

equation: 

, (1)

where 

S1 is the concentration from the environmental 

sample; and

S2 is the concentration from the replicate 

sample.

RPD S1 S2 – S1 S2 = ⁄ ( ) + ⁄ 2 × 10
12 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98 Table 2. Summary of field-blank and replicate-sample quality-assurance data for nitrogen, phosphorus, suspended sediment, and org

Iowa Basins study unit, March 1996–September 1998 [MRL, minimum reporting limit; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; RPD, relative percentage difference; <, less than; --, no data] Constituent MRL Number of blank

samples

Maximum

concentration

(mg/L)

Median

concentration

(mg/L)

Number

of blank

samples with

concentrations

greater than

MRL

Number

of replicate

samples

Ammonia (as N), dissolved 10.015 11 0.03 <0.02 4 31 

Ammonia (as N) plus organic nitrogen, total .1 11 <.10 <.10 0 31 Ammonia (as N) plus organic nitrogen, dissolved .1 11 <.10 <.10 0 31 Nitrite (as N), dissolved .01 11 .02 <.01 2 31 Nitrite plus nitrate (as N), dissolved .05 11 .08 .06 6 31 Phosphorus (as P), total .01 11 .03 <.01 1 31 Orthophosphate (as P), dissolved .01 11 .02 <.01 3 31 Phosphorus (as P), dissolved .01 11 .02 <.01 1 31 Suspended sediment .1 -- -- -- -- 21 Organic carbon, dissolved .1 15 2.1 .4 15 23 Organic carbon, suspended .1 14 .30 .1 10 23 

1The MRL for dissolved ammonia was increased to 0.02 mg/L in November 1997.
METHODS AND DATA ANALYSIS 13

The median RPD for nitrogen, phosphorus, suspended sediment, DOC, and SOC ranged from 

1.3 to 8.2 percent. The number of replicate samples 

for each constituent with RPDs greater than 10 percent 

is listed in table 2. When comparing the small concentrations and the slight differences between some replicate samples, the RPDs can be relatively large, which 

is the case for the majority of the RPDs that are greater 

than 10 percent.

Statistics

Water-quality data were analyzed by a variety 

of graphical and statistical methods. Quality-assurance 

data such as replicate samples were removed from the 

data set before statistical analysis to avoid biasing the 

data. If a constituent was reported as a less than value, 

the value was set to one-half of the detection limit for 

that constituent. For example, if a dissolved nitrate 

concentration was reported as less than 0.1 mg/L, 

it would be set to 0.05 mg/L for statistical calculations. 

The Statistical Analysis System (SAS) software package was used to calculate summary statistics such as 

mean, median, minimum and maximum concentrations, and quartiles (Statistical Analysis System, 

1990). In addition, boxplots were used to show 

the central tendency and variability of data. In a 

boxplot diagram, a box is drawn from the 25th to 

the 75th percentile concentrations, and the median is 

drawn as a horizontal line in the box. Vertical lines are 

extended beyond the box to data values less than or 

equal to 1.5 times the interquartile ranges outside the 

quartile. In addition, data values are marked as outliers 

with asterisks and circles for values beyond the vertical lines.

Nonparametric statistics were used because 

the water-quality data were not normally distributed. 

Statistical analysis for Spearman’s rank correlation, 

the Wilcoxon rank-sum test, the Kruskal-Wallis test, 

and analysis of variance test (on the ranks) were 

used for comparison of water-quality data (probability 

less than or equal to 0.05 was used to indicate statistical significance) among sites (Statistical Analysis 

System, 1990). The null hypothesis of no relation 

between variables was rejected if the probability 

of obtaining the correlation by chance was less than 

or equal to 0.05. In addition, only samples that were 

collected during the first week of the month from each 

site were used in the nonparametric analyses. To avoid 

bias and to have similar sample size for comparison, 

samples that were collected at specific sites weekly 

and biweekly were removed from data sets when running the Wilcoxon rank-sum, Krustal-Wallis test, and 

analysis of variance test.

Relation of Constituent Concentration 

and Streamflow

Schnoebelen and others (1999) plotted historical concentration and streamflow for many of the 

EIWA sampling sites by using data collected from 

1970 through 1995. Data from the present study 

(1996–98) appends historical concentration and 

streamflow data compiled for the Iowa River at 

Wapello (site 11, fig. 1) illustrated in Schnoebelen 

and others (1999). That site was a sampling site 

common to both studies. 

To improve visualization of nonlinear relations in the concentration and streamflow data, 

LOWESS (locally weighted scatterplot smoothing) 

lines (Cleveland, 1979; Helsel and Hirsch, 1992) 

were calculated and plotted for nitrite plus nitrate 

and total phosphorus. The LOWESS lines illustrate 

relations between concentrations and streamflow 

that are difficult to discern in a typical scatterplot. 

The LOWESS line is computed by fitting a weighted 

least-squares equation to the concentration and streamflow data (Helsel and Hirsch, 1992, p. 288–291). 

The “smoothing” used to calculate the LOWESS 

line is a particularly useful technique because no 

assumptions about linearity of the data are required. 

The smoothing algorithm uses nearby data points to 

calculate a “smoothed value” for every data point. 

Each nearby data point is weighted so that the more 

distant points affect the smoothed value less than 

points that are closer. A line then is drawn through 

the smoothed values. The number of nearby points 

used to calculate a smoothed value is controlled by 

the smoothness factor. A smoothness factor of 0.5 

was used for all LOWESS trend lines in this report. 

This means that the closest 50 percent of all the 

data points were used to calculate each smoothed 

value.
14 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

Hydrograph Separation

Sources of streamflow at the time of sampling 

were estimated by separating streamflow hydrographs 

into their base-flow and surface-water-runoff components using the USGS program HYSEP (Sloto and 

Crouse, 1996) and the Base-Flow Indicator (BFI) program (Wahl and Wahl, 1995). The data were used for 

comparisons of base flow for the low base-flow and 

high base-flow synoptic studies.

Load Calculations

The 1996–98 annual loads for total nitrogen 

(dissolved nitrite plus nitrate plus total ammonia 

plus organic nitrogen) and total phosphorus were 

estimated for each sampling site by using a minimum variance unbiased estimator (Cohn and others, 

1989). The Fortran program ESTIMATOR (Cohn 

and others, 1989) uses daily mean discharge and periodic nitrogen and phosphorus concentrations to estimate the annual and monthly loads for a site. The 

following equation was applied to the estimation 

of loads for each site: 

(2)

where

N = estimated load, in kilograms per day;

B0, B1, B2, B3, B4, B5, B6 = regression 

coefficients;

Q = daily mean stream discharge, in cubic 

feet per second; 

T = time, in decimal years; and

ε = error term.

The same regressors for load calculations 

were used for each site for consistency between 

sites and with other national studies (Donald Goolsby, 

U.S. Geological Survey, oral commun., 1998) within 

the Mississippi River Basin. The squares of the multiple correlation coefficient (R2) and the plots of residuals were used to determine which regression equation 

had the best fit for the load model. The nearer that R2

is to 1, the better the fitted equation explains the variation in the data. Water-quality data from January 1996 

through December 1998 were used in the model calibration. The data set consisted of at least one waterquality sample per month at each site, with the exception of January and February 1996 and October 

through December 1998 when no water-quality data 

were collected for any of the sites. Water-sample collection occurred on a monthly basis including highflow event samples that covered a wide range of 

streamflow and seasonal concentrations. In addition, 

the 3 years of the study included a below-normal year, 

a normal year, and an above-normal year for runoff. 

Thus, the sampling covered a wide range of streamflow, runoff, and seasonal conditions that adequately 

represented the study unit. 

To run ESTIMATOR, daily mean discharges 

(for each day of the month) for the complete water 

year must be entered into the model. The South Fork 

Iowa River near New Providence (site 4), Flood Creek 

near Powersville (site 7), and Wolf Creek near Dysart 

(site 9) were established in late October of 1995 and 

thus did not have daily mean discharge records for the 

early part of October 1995. The Wapsipinicon River 

near Tripoli (site 1) was not established until the middle of April 1996 and did not have daily mean discharge from October 1995 to the middle of April 1996. 

Because 1997 was a normal runoff year for most of the 

study unit, 1997 daily mean discharge data were used 

for South Fork Iowa River near New Providence, 

Flood Creek near Powersville, Wolf Creek near 

Dysart, and Wapsipinicon River near Tripoli where 

these sites had missing daily mean discharge data 

in water year 1996. Water year 1996 was a belownormal year for runoff in the study unit, so the 1996 

loads for the four sites (South Fork Iowa River near 

New Providence, Flood Creek near Powersville, 

Wolf Creek near Dysart, and Wapsipinicon River 

near Tripoli) probably are biased high during the 

winter months and for total load estimates for water 

year 1996. However, when running the model, the 

time period that had water-quality data (March 1996 

through September 1998) was used for calibration. 

The use of the 1997 daily mean discharge data for 

these four sites probably had only a minimal effect 

on the overall constituent loads in the drainage basins 

because the data represented a low-flow period.

1n (N) B0 B1 1n(Q) B2 1n(Q)2 = ++ + B3T

B4T2 B5 sin B ( ) 2πT 6 ++ + + cos( ) 2πT ε
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 15

The 1996–98 annual loads for suspended 

sediment were estimated for the Wapsipinicon River 

near Dewitt (site 2), Iowa River at Wapello (site 11), 

and the Skunk River at Augusta (site 12) using the 

program SEDCALC (Koltun and others, 1994). USGS 

and U.S. Army Corps of Engineers observers at these 

three sites collected daily suspended-sediment samples. SEDCALC load estimates (calendar year) for 

the Iowa River at Wapello (site 11) and Skunk River at 

Augusta (site 12) were obtained from May and others 

(1996, 1997, 1998) and Nalley and others (1999). 

Suspended-sediment data for the Wapsipinicon 

River near Dewitt (site 2) were obtained from 

the U.S. Army Corps of Engineers and calculated 

using the SEDCALC program.

The determination of daily sediment concentrations and daily loads was accomplished by first plotting the instantaneous sediment concentrations and 

discharge daily unit values. A curve was developed 

between points, mainly by straight-line interpolation, 

with additional points added to better define breaks 

in the concentration curve. SEDCALC calculates an 

equal-interval sediment discharge using the following 

equation:

(3)

where

Qs = instantaneous suspended-sediment 

discharge, in tons per day;

Qw = instantaneous stream discharge, in cubic 

feet per second;

C = instantaneous suspended-sediment concentration, in milligrams per liter; and

k = a units conversion constant (0.0027).

After equal-interval sediment discharges have 

been computed, daily sediment discharges are determined by numerical integration. The integration is 

accomplished by summing the equal-interval instantaneous sediment discharges for the calendar day and 

dividing by the number of intervals per day. For this 

report, tons per year were converted to metric tons 

per year.

CONCENTRATIONS OF NITROGEN, 

PHOSPHORUS, SUSPENDED SEDIMENT, 

AND ORGANIC CARBON

Overall Occurrence of Concentrations

Four hundred and fifty-five samples were 

collected at 12 basic-fixed sites in the EIWA study 

unit from March 1996 through September 1998. 

Table 3 summarizes the statistical data for each 

constituent collected. Summary statistics for each 

basic-fixed site are given in the Appendix at the end 

of the report. Water-quality data collected in the EIWA 

study unit are published in data reports by Akers and 

others (1999, 2000). Nitrogen, phosphorus, and

suspended-sediment concentrations are affected 

by many environmental and human factors such 

as climate, in-stream processes, soils, and proximity 

to sources, land use, and magnitude of runoff events. 

DOC concentrations in rivers vary with the size of the 

river, climate, and the nature of vegetation in the river 

basin (Thurman, 1985).

At least one form of dissolved nitrogen and 

phosphorus was detected in every sample. The 

frequency of detection of individual dissolved nitrogen 

compounds ranged from 65 to 98 percent. Dissolved 

phosphorus was detected in 90 percent of the samples 

collected. DOC and suspended sediment were detected 

in 100 percent of the samples collected. SOC was 

detected in 99.8 percent of the samples collected. The 

high percentage of detection for nitrogen and phosphorus in EIWA rivers and streams is not unexpected 

because there are many potential sources of these 

constituents.

Nitrogen

Nitrogen in the EIWA streams and rivers is 

present mainly as dissolved nitrate. About 92 percent 

of the median total dissolved nitrogen concentration 

was in the form of nitrate. Dissolved ammonia concentrations ranged from less than 0.015 to 1.2 mg/L, 

with a median concentration of 0.03 mg/L. Dissolved 

nitrite concentrations ranged from less than 0.01 to 

0.25 mg/L, with a median concentration of 0.04 mg/L. 

Nitrite plus nitrate concentrations ranged from less 

than 0.05 to 22 mg/L, with a median concentration 

Qs = QwCk
16 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment,

and Organic Carbon in Surface Water, 1996–98 Table 3. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon concentrations in the Eastern Iowa Bas

March 1996–September 1998 [mg/L; milligrams per liter; N, nitrogen; P, phosphorus; <, less than] Constituent Number of samples Number of detections Concentrations (mg/L) Minimum 25th percentile Median 75

Ammonia (as N), dissolved 455 294 <0.0154 <0.02 0.03 Ammonia and organic nitrogen (as N), total 455 450 <.1 .6 1.1 Ammonia and organic nitrogen, dissolved 455 425 <.1 .3 .4 Organic nitrogen (as N),2 dissolved 455 425 <.1 .2 .36 Nitrite (as N), dissolved 455 436 <.01 .04 .04 Nitrite plus nitrate (as N), dissolved 455 448 <.05 3.6 6.6 Nitrate (as N),1 dissolved 455 448 <.05 3.6 6.6 Nitrogen (as N), total,3 dissolved 455 455 .2 4.4 7.2 Phosphorus (as P), dissolved 455 409 <.01 .04 .08 Orthophosphate (as P), dissolved 455 409 <.01 .03 .08 Phosphorus (as P), total 455 455 .01 .1 .22 Suspended sediment 454 454 .3 33 82 Organic carbon, dissolved 446 446 .8 2.8 3.5 Organic carbon, suspended 449 448 <.1 .8 1.6 

1Dissolved nitrite plus nitrate minus dissolved nitrite.2Dissolved ammonia and organic nitrogen minus dissolved ammonia.3Dissolved nitrite plus nitrate plus dissolved ammonia and organic nitrogen.4Dissolved ammonia minimum reporting level was increased to 0.02 mg/L in November 1997.
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 17

of 6.6 mg/L. About 22 percent (101 out of 455) of the 

nitrate samples collected exceeded the USEPA MCL 

of 10 mg/L for drinking water. Dissolved ammonia 

and organic-nitrogen concentrations ranged from 

less than 0.10 to 3.1 mg/L, with a median concentration of 0.4 mg/L. Dissolved organic-nitrogen concentrations ranged from less than 0.10 to 1.9 mg/L, with 

a median concentration of 0.36 mg/L. Total ammonia 

and organic-nitrogen concentrations ranged from less 

than 0.1 to 7.5 mg/L, with a median concentration 

of 1.1 mg/L. Total dissolved nitrogen concentrations 

ranged from 0.2 to 22 mg/L, with a median concentration of 7.2 mg/L.

Phosphorus and Sediment

Dissolved phosphorus concentrations ranged 

from less than 0.01 to 1.4 mg/L, with a median concentration of 0.08 mg/L. Dissolved orthophosphate 

concentrations ranged from less than 0.01 to 1.3 mg/L, 

with a median concentration of 0.08 mg/L. Total phosphorus concentrations ranged from 0.01 to 3.4 mg/L, 

with a median concentration of 0.22 mg/L. About 

75 percent of the total phosphorus concentrations 

(341 out of 455 samples) exceeded the USEPA 

recommended concentration of 0.1 mg/L to prevent 

excessive growth of aquatic plants in water bodies 

(U.S. Environmental Protection Agency, 1986). 

Suspended-sediment concentrations ranged from less 

than 0.3 to 3,500 mg/L, with a median concentration 

of 82 mg/L.

Organic Carbon

Excluding water draining swamps and 

wetlands, the normal range in DOC concentrations 

for natural water is from about 2 to 15 mg/L, with a 

mean of about 4 to 6 mg/L (Degens, 1982). DOC concentrations in water draining swamps and wetlands 

range from about 5 to 60 mg/L, with a mean of 

25 mg/L (Thurman, 1985). DOC concentrations in 

EIWA samples ranged from 0.8 to 44 mg/L, with a 

median concentration of 3.5 mg/L. The highest concentration of 44 mg/L occurred at Wolf Creek near 

Dysart (site 9) in May of 1997 during an increase 

in flow. SOC concentrations ranged from less than 

0.1 to 17 mg/L, with a median concentration of 

1.6 mg/L.

Relations Between Constituent 

Concentrations and Streamflow

Streamflow in the EIWA varied during the study 

with generally below-normal flows in 1996, normal 

flows in 1997 (with the exception of the Southern Iowa 

Drift Plain), and above-normal flows in 1998. The 

streamflows varied across the study unit with varying 

rainfall, as figure 4 shows. Figure 4 shows the historical monthly fiftieth percentile of long-term discharge 

and the 1996–98 streamflows from selected sites 

across the study unit. Iowa River near Rowan (site 3), 

which is located in the northwestern part of the study 

unit, and the Iowa River at Wapello (site 11), which 

is located in the southeastern part of the study unit, 

showed a similar pattern of rainfall and streamflow. 

The southern part of the study unit (Southern Iowa 

Drift Plain) was extremely dry in 1997 as figure 4 indicates. The timing of rainfall-runoff events and location 

of runoff events affects the concentrations and transport of constituents. In 1997, the southern part of the 

study unit received less runoff than the rest of the 

study unit, which is reflected in the constituent concentrations that are discussed later in this section. 

Annual Variations

Concentrations of nitrogen and phosphorus 

compounds, suspended sediment, and organic carbon 

varied annually. Median concentrations of all nitrogen 

and phosphorus compounds, with the exception of 

dissolved ammonia, increased from 1996 to 1998. 

Figure 5 shows boxplots of yearly concentrations for 

total dissolved nitrogen, total phosphorus, suspended 

sediment, and DOC. The increases in concentrations 

follow the trend for overall increased streamflow from 

1996 to 1998 for the study unit. Median total dissolved 

nitrogen concentrations increased from 4.7 mg/L in 

1996 to 6.7 mg/L in 1997 and to 9.4 mg/L in 1998. 

Median total phosphorus concentrations increased 

from 0.18 mg/L in 1996 to 0.21 mg/L in 1997 and to 

0.26 mg/L in 1998. Median suspended-sediment concentrations increased each year (62 mg/L in 1996, 

71 mg/L in 1997, and 125 mg/L in 1998). Median 

DOC concentrations remained the same from 1996 to 

1998 with a concentration of 3.5 mg/L. SOC did not 

follow the same trends as the other constituents. The 

median SOC concentration decreased from 1.90 mg/L 

in 1996 to 1.4 mg/L in 1997 and then increased to 

2.0 mg/L in 1998.
18 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base from U.S. Geological Survey digital data, 1:250,000, 1967

Albers Equal-Area Conic projection

Standard parallels 41°30' and 44°30', central meridian -92°

Precipitation data from

National Oceanic and Atmospheric Administration

93°

43°

42°

41°

92°

91°

EXPLANATION

Basic-fixed site and map reference number

Site sampled in August 1997

 and May 1998 and map reference number or letter

Blue shading in stream hydrographs is

 middle 50th percentile of long-term discharge

South Skunk River

North Skunk River

English River

Iowa River Iowa River

West Fork Cedar River

Cedar River

Skunk River

Wapsipinicon R

Cedar River

Shell Rock R MINNESOTA

IOWA

12

11

10a

2

6

7

5

9

4

3

7

1

8

u

v

e

r

i

h

d

g

q

b

a

m

n

k

e

o

f

s

t

j

p

0

15

5

10

MONTHLY 

PRECIPITATION,

IN INCHES

1997 1998 0

2,000

500

1,000

1,500

MEAN MONTHLY 

DISCHARGE, IN CUBIC

FEET PER SECOND

Mason City, IA

Iowa River near Rowan, IA

1996

0

15

5

10

MONTHLY 

PRECIPITATION, 

IN INCHES

1997 1998 0

20,000

5,000

10,000

15,000

MEAN MONTHLY 

DICHARGE, IN CUBIC 

FEET PER SECOND

Skunk River at Augusta, IA

Burlington, IA

1996

0

15

5

10

MONTHLY 

PRECIPITATION, 

IN INCHES

1997 1998 0

40,000

10,000

20,000

30,000

MEAN MONTHLY 

DICHARGE, IN CUBIC 

FEET PER SECOND

Iowa River at Wapello, IA

Wapello, IA

1996

w

Figure 4. Graphs of precipitation and discharge at selected sites in the Eastern Iowa Basins study unit, 1996–98, and map 

showing site locations.
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 19

EXPLANATION

25th percentile

75th percentile

interquartile range outside the quartile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

and more than 1.5 times the

Outlier data value less than or equal to 

interquartile range outside the quartile

Outlier data value more than 3 times the

(128) Number of observations

(128) (202) (125)

(128) (202) (125)

(128) (193) (125)

(128) (201) (125)

1996 1997 1998

0.1

1

0.1

0.01

1

10

10

0

10

20

30

100

1

10

100

1,000

10,000

TOTAL DISSOLVED NITROGEN

TOTAL PHOSPHORUS

SUSPENDED SEDIMENT

DISSOLVED ORGANIC CARBON

CONCENTRATION, IN MILLIGRAMS PER LITER

Median

Figure 5. Yearly concentrations of selected constituents in surface-water samples in the Eastern Iowa 

Basins, 1996–98.
20 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

Seasonal Variations

Concentrations of nitrogen, phosphorus, 

suspended sediment, and organic carbon varied 

seasonally in surface-water samples from the EIWA. 

Boxplots of monthly concentrations for total dissolved 

nitrogen, total phosphorus, suspended sediment, and 

DOC are presented in figure 6. Median total dissolved 

nitrogen concentrations were highest during May 

(9.7 mg/L) and June (11.5 mg/L). The maximum 

total dissolved nitrogen concentration occurred in 

June (22 mg/L) in a sample from the South Fork 

Iowa River near New Providence (site 4). The median 

total dissolved nitrogen concentrations decreased from 

August (6.0 mg/L) to October (0.7 mg/L). The median 

total dissolved nitrogen concentration increased again 

in November (6.8 mg/L), December (7.6 mg/L), and 

January (7.8 mg/L), followed by a gradual decrease 

in February (6.7 mg/L) and March (5.4 mg/L).

Higher median concentrations of total dissolved 

nitrogen in the spring and early summer (April–June) 

probably result from the transport of nitrate accumulated in the soil after the application of chemical 

fertilizers and manure during the spring and previous 

autumn. The excess nitrate is transported to rivers and 

streams from spring snowmelt and rainfall events. In 

the late summer and early autumn (August–October), 

total dissolved nitrogen concentrations decrease 

because crops and other vegetation have utilized much 

of the fertilizer and because there is less runoff from 

rainfall to transport the nitrogen and phosphorus. During stable hydrologic conditions in the late summer, 

relatively low dissolved nitrogen and phosphorus concentrations have been measured in Iowa streams that 

contained substantial amounts of algae, as indicated by 

phytoplankton chlorophyll-a concentrations (Sorenson 

and others, 1999). Isenhart and Crumpton (1989) also 

reported significant losses of nitrate in a central Iowa 

stream from algal nitrogen assimilation during lowflow conditions during August.

During late autumn, fertilizers were applied to 

about 47 percent of the land used for row-crop production in the EIWA (Iowa Agricultural Statistics, 1996, 

1997, 1998a). The percentage of row-crop acreage 

that received autumn fertilizer application increased 

from 31 percent in 1996 to 60 percent in 1998 (Iowa 

Agricultural Statistics, 1996, 1997, 1998a). The timing 

of chemical fertilizer and manure application with runoff events is important to the transport of nitrogen and 

phosphorus within the study unit. The total dissolved 

nitrogen concentrations increased during late autumn 

because crops and vegetation are not growing or using 

the nitrogen at that time, the rate of uptake by algal 

and aquatic plants decreases with the lower water temperature and shorter days, and ground water containing nitrogen is a significant component of streamflow 

during base-flow conditions. Dissolved nitrate concentrations in the study unit commonly exceeded the 

USEPA MCL of 10 mg/L for nitrate as nitrogen during 

high flows after spring fertilizer application and during 

low flows in the winter. The MCL refers to drinkingwater supplies; and the Iowa Department of Natural 

Resources and the USEPA are currently developing 

nutrient criteria for Iowa streams (U.S. Environmental 

Protection Agency, 1998).

Median total phosphorus concentrations varied 

on a seasonal basis from 0.09 to 0.36 mg/L (fig. 6). 

Median total phosphorus concentrations were slightly 

higher during February (0.33 mg/L) and March 

(0.36 mg/L) than the rest of the year. In addition, 

the maximum concentration of total phosphorus 

was detected in March (3.4 mg/L) in a sample from 

Old Mans Creek near Iowa City (site 6). The lowest 

median concentration of total phosphorus was detected 

in December (0.09 mg/L). The higher total phosphorus 

concentrations during February and March may be 

related to increased runoff from snowmelt which 

transports sediment and phosphorus. In addition, 

manure that was applied to the frozen fields during 

the winter may be a source of the increased phosphorus during this time period. 

Suspended-sediment concentrations followed 

the trend of precipitation and streamflow. During high 

runoff, suspended-sediment concentrations increased 

across the study unit. Median suspended-sediment 

concentrations were highest in May (132 mg/L), June 

(204 mg/L), and July (133 mg/L) when the most 

intense storm events usually occur. The maximum 

concentration of suspended sediment occurred in 

May (3,500 mg/L) in a sample from the Skunk River 

at Augusta (site 12). Median suspended-sediment concentrations were the lowest during January (17 mg/L) 

when the rivers and streams are ice covered and 

streamflow is mostly from ground water. Median 

suspended-sediment concentrations increased in 

March (106 mg/L) when there was increased precipitation and very little vegetation in the fields to hold sediment in place. 

DOC had a similar seasonal pattern as total 

phosphorus. Median DOC concentrations varied 

on a seasonal basis from 2.7 to 5.0 mg/L (fig. 6). 
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 21

0.1

1

0.1

0.01

1

10

10

0

10

20

30

100

10

1

100

1,000

5,000

TOTAL DISSOLVED NITROGEN

TOTAL PHOSPHORUS

SUSPENDED SEDIMENT

DISSOLVED ORGANIC CARBON

CONCENTRATION, IN MILLIGRAMS PER LITER

(23) (46) (23) (60) (25) (42) (39) (67) (35) (23) (43) (28)

(23) (46) (23) (61) (25) (42) (39) (67) (35) (23) (43) (28)

(23) (46) (23) (61) (25) (42) (39) (67) (35) (23) (43) (28)

(23) (46) (23) (53) (24) (42) (39) (67) (35) (23) (43) (28)

EXPLANATION

25th percentile

Median

75th percentile

interquartile range outside the quartile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

and more than 1.5 times the

Outlier data value less than or equal to 

interquartile range outside the quartile

Outlier data value more than 3 times the

(23) Number of observations

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

Figure 6. Monthly variability of selected constituents in surface-water samples in the Eastern Iowa Basins study 

unit, March 1996–September 1998.
22 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

Median DOC concentrations were highest during 

February (4.6 mg/L) and March (5.0 mg/L). The 

lowest median concentration of DOC was detected 

in December (2.7 mg/L). The higher median DOC 

concentrations in February and March may be 

due to decaying vegetation held in place over the 

winter and then released during snowmelt. Decaying 

crop residue and DOC originating from manure 

also may be a source of the higher median 

concentrations. 

Nonpoint and Point Sources

Schnoebelen and others (1999) reported that, 

in general, nitrogen and phosphorus concentrations 

in surface-water samples from the EIWA study unit 

during 1970–95 were not linearly related to streamflow. Figure 7 shows the same relation in samples 

from the Iowa River at Wapello (site 11), which drains 

about 64 percent of the EIWA study unit. The concentrations of nutrients associated with nonpoint sources 

generally increased as streamflow increased because 

of runoff and(or) agricultural tile-drain flow, reflecting 

a positive relation between concentration and streamflow (fig. 7A). The increase in concentration occurred 

only up to a certain level beyond which concentrations 

then decreased with increasing streamflow. The 

decrease in concentration at the higher rates of streamflow is probably due to dilution of the nitrate once it 

has reached a maximum rate of release. These nonpoint relations are complex and are affected by antecedent soil conditions, timing of fertilizer application, 

land cover, and the location, duration, and intensity of 

precipitation.

The opposite effect typically is observed for 

point-source constituents. Concentrations of constituents associated with point-source locations tend to 

decrease in the stream due to dilution as streamflow 

increases, reflecting in a negative relation between 

concentration and streamflow. Rivers and stream 

point-source inputs often have higher concentrations 

at the lowest streamflows because point-source 

contaminants discharged to the stream can remain 

constant and are not diluted during periods of low 

streamflow. Schnoebelen and others (1999) reported 

a permitted point-source location upstream from the 

Iowa River at Wapello site. Figure 7B represents a 

typical LOWESS line of a point source of phosphorus 

during low-flow conditions.

Spatial Variability

Nitrogen

Median total dissolved nitrogen concentrations 

in samples from the indicator sites ranged from 6.3 

to 10 mg/L (fig. 8). Median total dissolved nitrogen 

concentrations in samples from integrator sites ranged 

from 4.2 to 6.9 mg/L (fig. 8). The median total 

dissolved nitrogen concentration in samples from 

all of the integrator sites was 6.2 mg/L compared 

to 8.2 mg/L in samples from all the indicator sites. 

The lowest median concentration for total dissolved 

nitrogen of 4.2 mg/L was from the Cedar River at 

Gilbertville (site 8), which had only 1 year (1996) of 

data collection. Samples from the Cedar River near 

Conesville (site 10) had the lowest median concentration of 5.5 mg/L for the 3-year sampling period. Samples from the Cedar River near Conesville (site 10) 

had a lower median concentration of total dissolved 

nitrogen than the rest of the sites, perhaps due to 

its morphologic features. The Cedar River is wide 

and shallow compared to the Iowa River. Therefore, 

there is more surface area for light penetration to promote algal growth and more contact with bed material 

for denitrification in the Cedar River compared to the 

Iowa River. These differences support more in-stream 

processing of nutrients in the Cedar River.

Total dissolved nitrogen concentrations in 

samples from indicator sites South Fork Iowa River 

near New Providence (site 4), Flood Creek near 

Powersville (site 7), and Wolf Creek near Dysart 

(site 9) were significantly higher (p < 0.05, KruskalWallis test) than in samples from the rest of the indicator and integrator sites, with the exception of the Iowa 

River near Rowan (site 3). The lower total dissolved 

nitrogen concentrations in samples from integrator 

sites relate to many factors, including the amount 

and type of riparian zones, the effects of in-stream 

processing by plants, and the amount of groundwater discharge.

There may be several reasons for the higher 

total dissolved nitrogen concentrations in samples 

from South Fork Iowa River near New Providence 

(site 4), Flood Creek near Powersville (site 7), and 

Wolf Creek near Dysart (site 9). All three of these 

sites have at least 80 percent of their total drainage 

basin areas used for row-crop agriculture. There 
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 23

STREAMFLOW, IN CUBIC METERS PER SECOND

TOTAL PHOSPHORUS, IN MILLIGRAMS PER LITER NITRITE PLUS NITRATE, IN MILLIGRAMS PER LITER

0 500 1,000 1,500 2,000 2,500

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

5

10

15

20

LOWESS line

Historical data, 1970–95 (Schnoebelen 

 and others, 1999)

Study data, 1996–98

A. Nitrite plus nitrate as nitrogen

B. Total phosphorus

Figure 7. Relations between nitrite plus nitrate and total phosphorus concentrations and streamflow in the Iowa River at 

Wapello, Iowa (site 11), 1970–98.
24 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

Median

(36) (54) (52) (38) (39) (34) (35) (34) (34) (34) (53)

EXPLANATION

25th percentile

75th percentile

Number of observations

Outlier data value more than 3 times the

 interquartile range outside the quartile

Outlier data value less than or equal to 

 3 and more than 1.5 times the 

 interquartile range outside the quartile

Data value less than or equal to 1.5 times the

 interquartile range outside the quartile

(36)

TOTAL DISSOLVED NITROGEN CONCENTRATION, 

IN MILLIGRAMS PER LITER AS N

0

10

20

30

Reference site

Des Moines Lobe

Des Moines Lobe (animal

feeding operations)

Southern Iowa Drift Plain

Iowan KarstIowan Surface

Wapsipinicon R., De Witt

Iowa R., Marengo

Cedar R., Conesville

Iowa R., Wapello

Skunk R., Augusta

LANDFORMS SITES

Indicator sites

(headwaters streams or

tributaries with one predominant

landform or land use)

Integrator sites

(large rivers with mixed

landforms and land uses)

Site 1

Site 2

Site 5

Site 10

Site 11

Site 12

Site 3

Site 4

Site 6

Site 7

Site 9

Figure 8. Total nitrogen concentrations for indicator and integrator sites in surface-water samples from the 

Eastern Iowa Basins study unit, March 1996–September 1998.
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 25

Predicted 

Measured—Number is map reference number for basic-fixed sampling site

1 2

3

4

6 5

7

9

10

11

12

12

Median total dissolved nitrogen concentration = 0.110 (percentage of row crop) –0.405

Correlation coefficient (R ) = 0.59 2

PERCENTAGE OF DRAINAGE BASIN IN ROW CROPS

MEDIAN CONCENTRATION, IN MILLIGRAMS PER LITER

50

0

5

10

15

60 70 80 90

Cedar River at Gilbertville (site 8) not included in data set

is a correlation between total dissolved nitrogen concentrations and row-crop agricultural practices as indicated in figure 9. However, the correlation coefficient 

(R2) of 0.59 suggests other factors also affect the water 

quality at these sites.

Another explanation for higher total dissolved 

nitrogen concentrations in samples from the South 

Fork Iowa River near New Providence (site 4) may 

be that the sampling site is located in an area of 

intense hog production (site 4, fig. 3). Since 1987, 

approximately 23 large-scale hog facilities have been 

permitted in this drainage basin (Iowa Department of 

Natural Resources, 1998). Also, the South Fork Iowa 

River has significantly higher (p < 0.05, KruskalWallis test) total nitrogen concentrations than the Iowa 

River near Rowan, which is located in the same physiographic region (Des Moines Lobe) and has similar 

row-crop percentages. By comparison with Iowa River 

near Rowan, the combination of row-crop agriculture 

and large-scale animal feeding operations might 

explain the water-quality variations within the South 

Fork Iowa River Basin.

Figure 9. Relation between median total dissolved nitrogen concentrations and percentage of drainage basin in 

row crops in the Eastern Iowa Basins study unit.
26 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

The higher total dissolved nitrogen concentrations observed in samples from Flood Creek near 

Powersville (site 7, fig. 2) when compared to other 

sites may be due to differences in geology at the sites. 

Flood Creek is located in an area of karst landform 

where soil thickness is minimal compared to other 

sites. Karst landforms commonly can provide a rapid 

pathway for migration of water to the stream. 

Wolf Creek near Dysart (site 9, fig. 2) had the 

second highest row-crop percentage and the highest 

median total dissolved nitrogen concentration. Concentrations of total dissolved nitrogen at Wolf Creek 

typically are higher than at most sites throughout 

the year. Even when concentrations decreased during 

late summer and autumn at other sites, Wolf Creek 

typically had high total dissolved nitrogen concentrations throughout the year. Initially, it was thought 

that Wolf Creek may have had some point-source 

discharge upstream from the sampling location, but a 

synoptic sampling of Wolf Creek and its tributaries for 

nitrate upstream from the sampling location proved to 

be inconclusive. The reason for the year-round high 

concentrations of total dissolved nitrogen is unknown.

Phosphorus and Sediment

Median total phosphorus concentrations in 

samples from indicator sites ranged from 0.07 to 

0.2 mg/L (fig. 10). Median total phosphorus concentrations in samples from integrator sites ranged 

from 0.23 to 0.35 mg/L (fig. 10). The median total 

phosphorus concentration in samples from all of the 

integrator sites was 0.29 compared to 0.12 mg/L 

for samples from indicator sites. Samples from the 

Iowa River at Marengo (site 5) had the highest median 

total phosphorus concentration, 0.35 mg/L. Samples 

from the South Fork Iowa River near New Providence 

(site 4) had the lowest median total phosphorus 

concentration, 0.07 mg/L, but as the boxplot indicates in figure 10, South Fork Iowa River near New 

Providence had a wide range of total phosphorus 

concentrations.

Median total phosphorus concentrations were 

higher than 0.20 mg/L for samples from all integrator 

sites and typically were 0.20 mg/L or less for samples 

from all indicator sites. Nine of the 12 sampling sites 

had median total phosphorus concentrations exceeding 

the USEPA recommended level of 0.1 mg/L to prevent 

excessive growth of aquatic plants in water bodies 

(U.S. Environmental Protection Agency, 1986). Total 

phosphorus concentrations were significantly higher 

(p < 0.05, Kruskal-Wallis test) in samples from integrator sites than in samples from indicator sites. Total 

phosphorus concentrations in samples from the Iowa 

River at Marengo (site 5) and the Skunk River at 

Augusta (site 12) were significantly higher (p < 0.05) 

than in samples from the Wapsipinicon River near 

De Witt (site 2) and in samples from all of the indicator sites. The differences probably are related to physiographic features which include slope, soil erodibility, 

and point sources. The total phosphorus concentrations in samples from indicator sites at Iowa River 

near Rowan (site 3), Old Mans Creek near Iowa City 

(site 6), and Wolf Creek near Dysart (site 7) were significantly higher (p < 0.05) than samples from indicator sites Wapsipinicon River near Tripoli (site 1) and 

Flood Creek near Powersville (site 7).

Phosphorus commonly is strongly adsorbed to 

sediment. The higher median phosphorus concentrations in samples from integrator sites may be related 

to increased sediment transport. However, the correlation coefficient (R2

) was only 0.62 when total phosphorus concentrations were plotted against suspendedsediment concentrations in samples from integrator 

sites, which indicates that other factors also influence 

total phosphorus concentrations (fig. 11). In addition, 

the integrator sites represented drainage basins that 

typically had point-source inputs that may contribute 

additional phosphorus and which may explain 

some of the differences in total phosphorus concentration between samples from indicator and integrator 

sites.

Median suspended-sediment concentrations 

in samples from indicator sites ranged from 20 to 

100 mg/L (fig. 12). Median suspended-sediment 

concentrations in samples from integrator sites ranged 

from 43 to 211 mg/L (fig. 12). Samples from the Iowa 

River at Marengo (site 5) had the highest median 

suspended-sediment concentration, 211 mg/L. The 

lowest median concentration of suspended sediment 

for the 1996–98 sampling period was 20 mg/L, in 

samples from the Wapsipinicon River at Tripoli 

(site 1). 

Samples from the Iowa River at Marengo 

(site 5) and Skunk River at Augusta (site 12) were 

significantly higher (p < 0.05) in suspended-sediment 

concentrations than samples from the remaining sites, 

with the exception of the Iowa River at Wapello 

(site 11). About 75 percent of the Skunk River at 
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 27

(36)

(36) (54) (52) (38) (39) (34) (35) (34) (34) (34) (53)

EXPLANATION

25th percentile

Median

75th percentile

Number of observations

Outlier data value more than 3 times the

 interquartile range outside the quartile

Outlier data value less than or equal to 

 3 and more than 1.5 times the 

 interquartile range outside the quartile

Data value less than or equal to 1.5 times the

 interquartile range outside the quartile

TOTAL PHOSPHORUS CONCENTRATION, 

IN MILLIGRAMS PER LITER AS P

0.01

10

5

2

1

0.5

0.2

0.1

0.05

0.02

Reference site

Des Moines Lobe

Southern Iowa Drift Plain

Iowan Karst

Iowan Surface

Wapsipinicon R., De Witt

Iowa R., Marengo

Cedar R., Conesville

Iowa R., Wapello

Skunk R., Augusta

LANDFORMS SITES

Indicator sites

(headwaters streams or

tributaries with one predominant

landform or land use)

Integrator sites

(large rivers with mixed

landforms and land uses)

Site 1

Site 3

Site 4

Site 6

Site 7

Site 9

Site 2

Site 5

Site 10

Site 11

Site 12

Des Moines Lobe (animal

feeding operations)

Figure 10. Total phosphorus concentrations for indicator and integrator sites in surface-water samples from the 

Eastern Iowa Basins study unit, March 1996–September 1998.
28 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98 TOTAL PHOSPHORUS CONCENTRATION, IN MILLIGRAMS PER LITER

SUSPENDED-SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

Predicted point

Measured point

Total phosphorus concentration = 0.0005(sediment concentration) + 0.2317

Correlation coefficient (R )= 0.62 2

0

0

0.5

1.0

1.5

2.0

2.5

3.0

1,000 2,000 3,000 4,000

Augusta (site 12) drainage basin is located in the 

Southern Iowa Drift Plain, which has steeper slopes 

than the rest of the study unit and contains loess 

deposits that are easily eroded (Schwarz and 

Alexander, 1995). The Iowa River at Marengo 

(site 5) drainage basin is about 44 percent within 

the Des Moines Lobe, 37 percent within the Southern 

Iowa Drift Plain, and 18 percent within the Iowan 

Surface. Channelization upstream from Iowa River at 

Marengo (site 5) has caused increased channel aggradation (Eash, 1995). The channel aggradation may 

lead to more bank erosion, thus increasing sediment 

transport and increasing total phosphorus concentrations adsorbed to the sediment.

Suspended-sediment concentrations in samples 

from the Iowa River near Rowan (site 3) were significantly higher (p < 0.05) than concentrations in samples 

from all of the indicator sites and one integrator site, 

Cedar River near Conesville (site 10). It is unclear 

why samples from Iowa River near Rowan (site 3) had 

higher suspended-sediment concentrations than the 

rest of the indicator sites. The physical characteristics 

of the Iowa River near Rowan drainage basin do not 

suggest any characteristics that would cause higher 

suspended-sediment concentrations.

Suspended-sediment concentrations in samples 

from the Wapsipinicon River near Tripoli (site 1) 

were significantly lower (p < 0.05) than the rest of 

the sites in the EIWA study unit. The drainage basin 

represented by the Wapsipinicon River near Tripoli 

(site 1) has a lower percentage of row-crop agriculture 

(64 percent) and fewer channeled segments when 

compared to the rest of the study unit and contains a 

higher percentage of wetland and undisturbed flood 

plain than the rest of the drainage basins represented 

by the sampling sites in the study unit.

Figure 11. Relation between total phosphorus concentrations and suspended-sediment concentrations in surfacewater samples from integrator sites in the Eastern Iowa Basins, March 1996–September 1998.
CONCENTRATIONS OF NITROGEN, PHOSPHORUS, SUSPENDED SEDIMENT, AND ORGANIC CARBON 29

(36) (54) (52) (37) (39) (34) (35) (34) (34) (34) (53)

(36)

EXPLANATION

25th percentile

75th percentile

Median

Number of observations

Outlier data value more than 3 times the

 interquartile range outside the quartile

Outlier data value less than or equal to 

 3 and more than 1.5 times the 

 interquartile range outside the quartile

Data value less than or equal to 1.5 times the

 interquartile range outside the quartile

SUSPENDED-SEDIMENT CONCENTRATION, 

IN MILLIGRAMS PER LITER

0

2

5

10

20

50

100

200

500

1,000

2,000

5,000

10,000

Reference site

Des Moines Lobe

Southern Iowa Drift Plain

Iowan Karst

Iowan Surface

Wapsipinicon R., De Witt

Iowa R., Marengo

Cedar R., Conesville

Iowa R., Wapello

Skunk R., Augusta

LANDFORMS SITES

Indicator sites

(headwaters streams or

tributaries with one predominant

landform or land use)

Integrator sites

(large rivers with mixed

landforms and land uses)

Site 1

Site 3

Site 4

Site 6

Site 7

Site 9

Site 2

Site 5

Site 10

Site 11

Site 12

Des Moines Lobe (animal

feeding operations)

Dissolved Organic Carbon

Median DOC concentration in samples from 

indicator sites ranged from 1.5 to 4.2 mg/L (fig. 13). 

Median DOC concentrations in samples from integrator sites ranged from 3.0 to 4.2 mg/L (fig. 13). The 

median DOC concentration in samples from all of the 

integrator sites was 3.6 mg/L compared to 3.5 mg/L in 

samples from indicator sites. The lowest median DOC 

concentration was 1.5 mg/L in samples from Flood 

Creek near Powersville (site 7). The highest DOC 

median concentration of 4.2 mg/L was detected in 

samples from the Iowa River near Rowan (site 3) 

and the Skunk River at Augusta (site 12).

DOC concentrations were significantly 

lower (p < 0.05) in samples from Flood Creek near 

Powersville (site 7) than in samples from all other 

basic-fixed sampling sites. This is probably due to the 

karst landform characterized by very thin soils. DOC 

concentrations were significantly higher (p < 0.05) in 

Figure 12. Suspended-sediment concentrations for indicator and integrator sites in surface-water 

samples from the Eastern Iowa Basins study unit, March 1996–September 1998.
30 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

(36)

EXPLANATION

25th percentile

Median

75th percentile

Number of observations

Outlier data value more than 3 times the

 interquartile range outside the quartile

Outlier data value less than or equal to 

 3 and more than 1.5 times the 

 interquartile range outside the quartile

Data value less than or equal to 1.5 times the

 interquartile range outside the quartile

DISSOLVED ORGANIC-CARBON CONCENTRATION, 

IN MILLIGRAMS PER LITER

0

10

20

30

Reference site

Des Moines Lobe

Southern Iowa Drift Plain

Iowan Karst

Iowan Surface

Wapsipinicon R., Dewitt

Iowa R., Marengo

Cedar R., Conesville

Iowa R., Wapello

Skunk R., Augusta

LANDFORMS SITES

Indicator sites

(headwaters streams or

tributaries with one predominant

landform or land use)

Integrator sites

(large rivers with mixed

landforms and land uses)

Site 1

Site 3

Site 4

Site 6

Site 7

Site 9

Site 2

Site 5

Site 10

Site 11

Site 12

Des Moines Lobe (animal

feeding operations)

(36) (52) (50) (38) (37) (34) (35) (34) (34) (34) (52)

samples from the Iowa River near Rowan (site 3), the 

South Fork Iowa River near New Providence (site 4), 

and the Skunk River at Augusta (site 12) than in samples from most other sites. On the basis of STATSGO 

data (Schwarz and Alexander, 1995), the drainage 

basin represented by the Iowa River near Rowan 

(site 3) had soil samples with higher average soil 

organic-carbon concentrations (35 g/kg) than the 

rest of the drainage basins represented by sampling 

sites in the EIWA study unit. However, the drainage 

basin represented by the South Fork Iowa River near 

New Providence (site 4) had soil samples with the 

lowest average organic-carbon concentration of 

(4.2 g/kg), but still had surface-water samples with 

high DOC concentrations. The drainage basin represented by the Skunk River at Augusta (site 12) had 

soils with average organic-carbon concentrations of 

about 10 g/kg (Schwarz and Alexander, 1995).

Figure 13. Dissolved organic-carbon concentrations for indicator and integrator sites in surface-water 

samples from the Eastern Iowa Basins study unit, March 1996–September 1998.
TRANSPORT OF NITROGEN, PHOSPHORUS, AND SUSPENDED SEDIMENT 31

TRANSPORT OF NITROGEN, 

PHOSPHORUS, AND SUSPENDED 

SEDIMENT

Loads

Load is the mass of a constituent passing 

through a stream transect per given unit of time. Loads 

for all of the nitrogen and phosphorus compounds 

were estimated for 1996–98 for 11 of the 12 (table 4) 

basic-fixed sites using a minimum variance unbiased 

estimator (ESTIMATOR; Cohn and others, 1989). For 

this report, only total nitrogen and total phosphorus are 

reported because they represent total loads for the 

main nutrient compounds of concern. 

Annual and monthly loads were calculated for 

11 basic-fixed sites. Cedar River at Gilbertville (site 8) 

had only 1 year of concentration data and no discharge 

data; therefore, loads were not calculated for this site. 

Values of the squares of multiple correlation coefficient (R2

) ranged from 0.91 to 0.97 for the total nitrogen model and ranged from 0.82 to 0.95 for the total 

phosphorus model. The equations with lower R2 values may still provide satisfactory estimates of loads, 

even with a regression that does not have an ideal fit 

(Cohn and others, 1992). The calculated loads are 

strictly estimates used to characterize differences 

between drainage basins in the EIWA study unit and 

loads that are transported to the Mississippi River.

Iowa River at Wapello (site 11), which represents drainage from 64 percent of the study unit, had 

the highest estimated total nitrogen load for each year 

(1996, 57,600 metric tons; 1997, 75,100 metric tons; 

1998, 154,000 metric tons) and the highest estimated 

total phosphorus load in 1997 (3,120 metric tons). 

Skunk River at Augusta (site 12) had the highest estimated total phosphorus loads in 1996 and 1998 (3,960 

and 4,140 metric tons, respectively). The highest total 

nitrogen load at an indicator site was in 1998 at Wolf 

Creek near Dysart (site 9) (4,780 metric tons). Old 

Mans Creek near Iowa City (site 6) had the highest 

total phosphorus load at an indicator site in 1996 with 

a load of 390 metric tons. During 1996, a large spring 

runoff event occurred that transported large loads of 

total phosphorus from this watershed.

Total nitrogen loads typically increased 

every year for each site during the study period 

with only a few exceptions (Old Mans Creek and 

Skunk River in 1997), which were probably related 

to less runoff. The Iowa River at Wapello (site 11) 

includes flow and nutrient loads from both the 

Cedar and Iowa River Basins because the rivers join 

about 16 km upstream from Wapello. The Cedar 

River contributes the higher percentage of the total 

nitrogen load between the two rivers, possibly due to 

its larger drainage area. The Iowa River upstream from 

the confluence with the Cedar River contributed an 

estimated 44 percent (25,700 metric tons) in 1996, 

30 percent (22,900 metric tons) in 1997, and 37 percent (57,000 metric tons) in 1998 of the total nitrogen 

load estimated at the Iowa River at Wapello. Total 

phosphorus loads did not increase each year for every 

site listed in table 4 and were more variable with runoff events that moved large sediment loads.

Estimated total nitrogen and total phosphorus 

loads varied seasonally both at indicator sites and integrator sites (figs. 14 and 15). Estimated loads for total 

nitrogen and total phosphorus at the 11 basic-fixed 

sites listed in table 4 followed a similar seasonal trend 

as did concentrations. The highest loads typically 

occurred in early spring and summer after fertilizer 

applications and runoff. Estimated loads began to 

decrease in the late summer and autumn as the available nitrogen and phosphorus were processed by 

plants as less runoff was available to transport the 

nitrogen and phosphorus. The loads then increased 

again in late autumn and early winter after autumn 

application of fertilizer and lack of processing due 

to vegetation die-off and dormancy. Total estimated 

monthly loads were lowest in January and September 

when there is typically very little runoff to transport 

nitrogen and phosphorus in the soil to streams and 

rivers. Loads increased in February when snowmelt 

transported nitrogen and phosphorus to rivers and 

streams. Some of the sites had higher total estimated 

loads in the autumn of 1998 than in the spring of 1998. 

The autumn of 1998 was relatively wet and had higher 

nitrogen and phosphorus concentrations than in 1996 

and 1997.

The three major basins draining eastern Iowa 

and southeastern Minnesota (Wapsipinicon River, 

Iowa River [includes the Cedar River], and Skunk 

River) contributed an estimated total of 97,600 metric 

tons of nitrogen and 6,860 metric tons of phosphorus 

to the Mississippi River in 1996. Estimated loads of 

total nitrogen and total phosphorus transported into 

the Mississippi River in 1996 represent about 19 percent of the estimated sources of nitrogen and 9 percent 

of the estimated sources of phosphorus (Becher and 

others, 2000). However, these estimations of sources 
32 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

did not include estimation of a mass balance of inputs 

and outputs within the basins. Soil mineralization and 

nitrogen fixation by legumes can be a significant 

source of nitrogen (Burkhart and James, 1999). In 

1997, an estimated 120,000 metric tons of nitrogen 

and 4,550 metric tons of phosphorus were transported 

to the Mississippi River from EIWA. The lower load 

for phosphorus in 1997 compared to 1996 probably 

is related primarily to less intense rains and lack of 

rain in the Skunk River Basin, which resulted in lower 

sediment concentrations to transport the phosphorus 

that adsorbs to the sediment particles. In 1998, an 

estimated 234,000 metric tons of nitrogen and 

8,830 metric tons of phosphorus were transported 

to the Mississippi River. The higher loads of nitrogen 

and phosphorus in 1998 compared to previous years 

(1996–97) can be attributed to higher concentrations 

and higher runoff. 

Table 4. Estimated total nitrogen and total phosphorus loads and yields for streams in the Eastern Iowa Basins study unit, 

1996–98

[kg/km2, kilograms per square kilometer]

Site name

(map reference

number)

Year

Total nitrogen Total phosphorus

Load

(metric tons)

Yield

(kg/km2)

Load

(metric tons)

Yield

(kg/km2)

Wapsipinicon River near Tripoli (1) 1996 2,040 2,260 66 73

1997 2,830 3,150 63 70

1998 3,570 3,960 85 95

Wapsipinicon River near De Witt (2) 1996 9,990 1,650 540 89

1997 21,700 3,580 440 73

1998 31,100 5,120 760 130

Iowa River near Rowan (3) 1996 1,490 1,380 44 41

1997 2,210 2,050 96 88

1998 3,150 2,920 140 130

South Fork Iowa River near New Providence (4) 1996 1,040 1,890 39 68

1997 2,000 3,440 100 180

1998 3,790 6,540 240 420

Iowa River at Marengo (5) 1996 11,900 1,650 760 100

1997 19,300 2,670 720 100

1998 35,500 4,900 1,140 160

Old Mans Creek near Iowa City (6) 1996 1,600 3,080 390 750

1997 920 1,780 150 280

1998 3,170 6,100 170 320

Flood Creek near Powersville (7) 1996 240 760 2.2 7

1997 610 1,910 20 62

1998 1,510 4,730 12 36

Wolf Creek near Dysart (9) 1996 1,050 1,360 26 33

1997 1,920 2,490 47 62

1998 4,780 6,200 180 230

Cedar River near Conesville (10) 1996 31,900 1,580 1,560 77

1997 52,200 2,580 1,860 92

1998 97,000 4,800 2,470 120

Iowa River at Wapello (11) 1996 57,600 1,650 2,360 73

1997 75,100 2,320 3,120 96

1998 154,000 4,740 3,930 120

Skunk River at Augusta (12) 1996 30,000 2,680 3,960 350

1997 23,500 2,100 990 89

1998 49,400 4,410 4,140 370
TRANSPORT OF NITROGEN, PHOSPHORUS, AND SUSPENDED SEDIMENT 33

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

0.1

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

10,000

20,000

50,000

100,000

0.1

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

10,000

20,000

50,000

100,000

Iowa River near Rowan (site 3, fig.1)

Iowa River at Wapello (site 11, fig.1)

MONTHLY LOADS, IN METRIC TONS

1996 1997 1998

1996 1997 1998

Figure 14. Estimated monthly loads of total nitrogen at selected sampling sites in the Eastern Iowa Basins study unit, 

1996–98.
34 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

0.1

0.05

0.02

0.01

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

0.1

0.05

0.02

0.01

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

Iowa River near Rowan (site 3, fig.1)

Iowa River at Wapello (site 11, fig.1)

1996 1997

1998

1996 1997

1998

MONTHLY LOADS, IN METRIC TONS

Figure 15. Estimated monthly loads for total phosphorus at selected sampling sites in the Eastern Iowa Basins study 

unit, 1996–98.
TRANSPORT OF NITROGEN, PHOSPHORUS, AND SUSPENDED SEDIMENT 35

Goolsby and others (1999) reported that 

the average estimated total nitrogen load was 

74,200 metric tons for the Iowa River at Wapello 

(site 11) from 1980 to 1996 and was 22,450 metric 

tons for the Skunk River at Augusta (site 12). The historical average total nitrogen load for these two sites 

was less than the total nitrogen loads estimated in this 

report for 1996 through 1998, with the exception of 

the Iowa River at Wapello (site 11), which was less 

than (57,600 metric tons) the historical average in 

1996. In addition, Goolsby and others (1999) reported 

that the average total phosphorus load for 1980–96 

was 3,076 metric tons for the Iowa River at Wapello 

(site 11) and 1,338 metric tons for Skunk River at 

Augusta (site 12). Estimated total phosphorus loads 

for the Iowa River at Wapello (site 11) were less than 

the 1980–96 average in 1996 and greater than the 

1980–96 average in 1997 and 1998. Estimated total 

phosphorus loads for the Skunk River at Augusta 

(site 12) were greater than the 1980–96 average in 

1996 and 1998 but less than the 1980–96 average in 

1997.

Suspended-sediment loads were calculated for 

the three sites (Wapsipinicon River near De Witt, Iowa 

River at Wapello, and Skunk River at Augusta) that 

were near the mouths of the main river systems in the 

EIWA. These three sites had daily suspended-sediment 

samples collected by observers at each site. Table 5 

lists the estimated suspended-sediment loads for these 

three sites. Sediment transport is highly variable and is 

highly dependent on timing of runoff events. In 1996, 

sediment transport was high during the late winter and 

spring during runoff events when there was very little 

vegetation to hold sediment in place. Sediment transport was generally lower throughout the EIWA in 1997 

when there were few major runoff events during the 

late winter and early spring. In 1998, sediment transport increased due to runoff events, especially in the 

Skunk River Basin. The Skunk River Basin drains 

the Southern Iowa Drift Plain, which has easily 

erodible loess deposits. The Skunk River at Augusta 

(site 12, fig. 1) had the highest load of the 3 years 

with 5,300,000 metric tons transported in 1998. 

The total suspended-sediment load transported from 

these three sites to the Mississippi River from 1996 

to 1998 was 7,480,000, 4,450,000, and 8,690,000, 

respectively.

Yields

Yield is the mass of a constituent that has been 

transported from a unit of area per specific amount of 

time (kg/km2

)/yr. Table 4 lists the estimated yields for 

11 basic-fixed sites for 1996–98. Indicator sites typically had higher yields for total nitrogen than did integrator sites. The South Fork Iowa River near New 

Providence (site 4) had the highest estimated total 

nitrogen yield (6,540 kg/km2

 in 1998). Flood Creek 

near Powersville (site 7) had the lowest estimated total 

nitrogen yield (760 kg/km2 in 1996) during a dry year 

when there were many days with no flow. Total nitrogen yields typically increased every year for each site 

during the study period with only a few exceptions 

(Old Mans Creek and Skunk River in 1997), which 

were probably related to less runoff. This can be attributed to the overall increase in runoff and higher concentrations each year of the study. The average 

estimated total nitrogen yields for the Cedar River 

and the Iowa River during the study period were about 

the same [2,990 and 2,900 (kg/km2)/yr, respectively]. 

Figure 16 shows the 3-year average nitrogen and phosphorus yields for each site in the study unit.

Old Mans Creek near Iowa City (site 6) 

had the highest estimated total phosphorus yield of 

750 kg/km2, in 1996. Flood Creek near Powersville 

(site 7) had the lowest estimated total phosphorus yield 

Table 5. Estimated suspended-sediment loads and yields 

for selected rivers in the Eastern Iowa Basins study unit, 

1996–98

[km2, square kilometer]

Site name

(map reference

number)

Year

Suspended sediment

Load

(metric

tons)

Yield

(metric

tons/km2)

Wapsipinicon River near

De Witt (2)

1996 580,000 95.4

1997 450,000 74.0

1998 490,000 80.6

Iowa River at Wapello (11) 1996 3,000,000 92.6

1997 1,800,000 55.6

1998 2,900,000 89.5

Skunk River at Augusta (12) 1996 3,900,000 348

1997 2,200,000 196

1998 5,300,000 473
36 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

12

11

2

6 10

5

4

3 1

9

7

12

11

2

6 10

5

5

4

3 1

9

7

Total nitrogen yield, kilograms per 

 square kilometer per year

Number is map reference number

 used in tables

5

Number is map reference number

 used in tables

Total phosphorus yield, kilograms

 per square kilometer per year

2,120 to 2,470

2,471 to 2,990

2,991 to 3,120

3,121 to 3,650

3,651 to 3,960

37

38 to 97

98 to 120

121 to 270

447

93°

43°

42°

41°

91°

92°

93°

43°

42°

41°

91°

92°

0 20 40 60 MILES

0 20 40 60 KILOMETERS

0 20 40 60 MILES

0 20 40 60 KILOMETERS

EXPLANATION

EXPLANATION

Figure 16. Three-year average estimated yields of total nitrogen and total phosphorus in 

the Eastern Iowa Basins study unit, 1996–98.
SYNOPTIC STUDIES 37

of 7 kg/km2 in 1996. Again, this low yield can be 

attributed to low flow in Flood Creek during 1996. 

The 11 basic-fixed sites in table 4 typically had 

increases in total phosphorus yields for each year of 

the study with a few exceptions. In 1997, Old Mans 

Creek near Iowa City (site 6) and the Skunk River at 

Augusta (site 12) had lower total phosphorus yields 

than in 1996 and 1998. This was due to less runoff 

and less intense rain events in the associated drainage 

basins. However, with the exception of 1997, the 

drainage basins represented by Old Mans Creek 

near Iowa City (site 6) and Skunk River at Augusta 

(site 12) had higher estimated total phosphorus 

yields than other drainage basins within the study unit. 

Samples from these two sites had higher suspendedsediment concentrations during runoff events than did 

samples from the other sites, which may explain the 

higher total phosphorus yields at these two sites. In 

addition, Old Mans Creek near Iowa City (site 6) 

and Skunk River at Augusta (site 12) mainly drain 

the Southern Iowa Drift Plain, which consists of 

loess deposits that erode relatively easily, which 

can increase sediment and phosphorus loads to 

rivers and streams.

The South Fork Iowa River near New 

Providence (site 4) had large increases in estimated 

total nitrogen and total phosphorus yields each year 

of the study. This site had the highest yields for total 

nitrogen and total phosphorus in 1998, which may 

reflect the large number of hog confinement operations in this drainage basin, which in turn may be 

affecting the water quality. Other drainage basins that 

have a high number of hog confinement operations, 

such as the Iowa River near Rowan (site 3), showed 

similar increasing total nitrogen and total phosphorus 

yields, but not as substantial as the South Fork Iowa 

River near New Providence (site 4). 

Goolsby and others (1999) reported that the 

average estimated total nitrogen yield for 1980–96 

at the Iowa River at Wapello sampling site (site 11) 

was 2,290 (kg/km2

)/yr and at the Skunk River at 

Augusta (site 12) was 2,020 (kg/km2

)/yr. Estimated 

total nitrogen yields at the Iowa River at Wapello 

(site 11) and Skunk River at Augusta (site 12) were 

greater than the 1980–96 average from 1996–98, with 

the exception of the Iowa River at Wapello (site 11, 

fig. 1), which was less than (1,650 kg/km2

) the historical nitrogen yield (table 4). The average total phosphorus yield for 1980–96 at the Iowa River at Wapello 

sampling site (site 11) was 94.9 (kg/km2

)/yr and 

120.5 (kg/km2

)/yr for the Skunk River at Augusta 

(site 12) sampling site. Estimated total phosphorus 

yields for Iowa River at Wapello (site 11) were less 

than the 1980–96 average in 1996 but were higher 

than average in 1997 and 1998. Estimated total phosphorus yields for the Skunk River at Augusta (site 12) 

were higher than the 1980–86 average in 1996 and 

1998 but were lower than the 1980–86 average in 

1997 [89 (kg/km2

)/yr].

Skunk River at Augusta (site 12, fig. 1) had 

the highest estimated suspended-sediment yield of 

473 metric tons/km2

 in 1998. Iowa River at Wapello 

(site 11, fig. 1) had the lowest estimated suspendedsediment yield of 55.6 metric tons/km2 in 1997. 

The average suspended-sediment yields for the 

Wapsipinicon River near De Witt (site 2, fig. 1), Iowa 

River at Wapello (site 11, fig. 1), and the Skunk River 

at Augusta (site 12) were 83, 79, and 339 (metric 

tons/km2

)/yr, respectively. Again, the variation in 

suspended-sediment yields in the EIWA is due to runoff events and the landform type that the watershed 

drains.

SYNOPTIC STUDIES

Two synoptic studies were conducted to better 

understand water quality and variability during low 

base-flow and high base-flow conditions in mediumsized (320–1,300 km2) drainage basins of the EIWA 

study unit. Hydrographs of stream discharge and 

ground-water inflow at surface-water-quality sampling 

sites distributed from north to south across the 

study unit (fig. 17) show that ground water contributed 

most of the streamflow during the synoptic sampling 

periods (August 1997 and May 1998). The median 

stream discharge per unit drainage area was more 

than five times higher during the high base-flow 

sampling in May 1998 [0.046 (m3/s)/km2

] than 

during the low base-flow sampling in August 1997 

[0.008 (m3/s)/km2

]. Ground water may have been contributed by direct infiltration from adjacent aquifers or 

from tile-line flow. Streamflow during the low baseflow period in August 1997 was characterized by 

decreasing stream discharge. During the month before 

sampling, rainfall produced little runoff. Although 

streamflow was decreasing and consisted primarily of 

ground-water inflow in the weeks before sampling in 

May 1998, several storms in April produced substantial runoff events before sampling.
38 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

EXPLANATION

Base flow

Total flow

July Aug

1997

Apr May

1998

Iowa River near Rowan (site 3, fig. 1)

Wolf Creek near Dysart (site 9, fig. 1)

Cedar Creek near Oakland Mills (site r, fig. 1)

DISCHARGE, IN CUBIC METERS PER SECOND

0

50

10

20

30

40

0

50

10

20

30

40

0

200

50

100

150

With the exception of stream discharge and 

dissolved-oxygen concentrations, overall stream 

conditions at the time of sampling were very similar 

(table 6). Specific conductance, pH, water temperature, and suspended sediment were not significantly 

different (p > 0.05) during the two synoptic sampling 

periods. However, samples from the streams did contain higher dissolved-oxygen concentrations and were 

more highly oxygen saturated during the August 1997 

low base-flow sampling. Stable flow conditions and 

possibly lower turbidity in late summer than in spring 

may have been conducive to the growth of oxygenproducing plants and algae in the streams. 

Variability Among Basic-Fixed and 

Synoptic Sites

Six basic-fixed/synoptic sites on mediumsized drainage basins (320–1,300 km2) were selected 

initially to represent the range of environmental settings found in the EIWA study unit. The settings 

generally vary due to differences in landforms and 

agricultural land-use practices. The drainage basins of 

the six basic-fixed/synoptic sites (sites 1, 3, 4, 6, 7, and 

9) account for about 5 percent of the drainage area of 

the EIWA study unit. The representativeness of the 

entire study unit by these basic-fixed/synoptic sites 

is unknown. Comparison of water-quality data among 

the six basic-fixed/synoptic sites and the 19 synoptic 

sites would answer this question, at least for the conditions during the synoptic sampling periods. The synoptic drainage basins represent about 30 percent of the 

EIWA study unit. 

Comparison of water-quality data suggests 

that the water quality from the six basic-fixed/synoptic 

sites is representative of the entire study unit during 

periods of low and high base flow when most streamflow originates from ground water. None of the concentrations of nitrogen and phosphorus compounds 

analyzed were significantly different (p < 0.05, 

Kruskal-Wallis test) in samples from the 19 synoptic 

sites than in samples from basic-fixed/synoptic 

sites. Selected nitrogen, phosphorus, and organiccarbon compounds are shown in relation to the type of 

site in figure 18. Although not significantly different, 

concentrations in samples from the 19 synoptic sites 

were generally more variable than in samples from the 

basic-fixed/synoptic sites. Increased variability is at 

least partially due to the larger number of synoptic 

sites.

Only water temperature was significantly different between the basic-fixed/synoptic and synoptic 

sites. The median water temperature was significantly 

less (p < 0.05, Kruskal-Wallis test) in samples from the 

basic-fixed/synoptic sites (18.2°C) than in samples 

from the 19 synoptic sites (21.0°C) during low base 

flow. The reason for this difference is unknown.

Figure 17. Base flow and total flow at three surface-waterquality sampling sites during low and high base-flow conditions in the Eastern Iowa Basins study unit, July and 

August 1997 and April and May 1998.
SYNOPTIC STUDIES 39

Table 6. Statistical summary of selected physical properties and chemical constituents in samples from 25 synoptic sites in 

the Eastern Iowa Basins study unit, August 1997 (low base flow) and May 1998 (high base flow)

[<, less than detection limit indicated]

Constituent Base-flow

type

Property or constituent value Coefficient

Minimum Median Maximum of variation

Physical properties

Stream discharge, in cubic meters per second Low 0.11 0.79 2.1 0.61

High 49. 119 147 .59

Specific conductance, in microsiemens per centimeter

at 25 degrees Celsius

Low 396 573 1,220 .27

High 436 590 792 .15

pH, in standard units Low 7.5 8.0 8.4 .03

High 7.7 8.0 8.2 .02

Water temperature, in degrees Celsius Low 15.5 20.5 25.5 .11

High 15.0 20.5 24.0 .11

Dissolved oxygen, in milligrams per liter Low 4.9 9.4 13.8 .22

High 6.8 8.1 9.2 .08

Dissolved oxygen, percent saturation Low 56 106 168 .24

High 79 91 106 .08

Suspended sediment, in milligrams per liter Low 6. 40 330 1.2

High 12 86 171 .58

Constituent concentrations,

in milligrams per liter

Ammonia (as N), dissolved Low <.015 <.015 1.3 3.4

High <.02 .04 .08 .56

Ammonia ammonia plus organic nitrogen (as N), total Low .20 .39 2.1 .81

High .17 .40 .78 .47

Organic nitrogen, dissolved Low .12 .39 1.0 .54

High .15 .38 .71 .41

Nitrite plus nitrate (as N), dissolved Low .05 1.5 8.3 1.0

High 8.5 12 14 .15

Nitrogen (as N), total, dissolved Low .46 2.3 8.5 .74

High 8.9 12 15 .15

Phosphorus (as P), dissolved Low <.01 .04 .31 1.1

High <.01 .06 .13 .70

Orthophosphate (as P), dissolved Low <.10 .04 .31 1.4

High <.10 .03 .09 1.1

Phosphorus (as P), total Low <.01 .12 .16 1.1

High .02 .10 .32 .63

Organic carbon, dissolved Low 1.6 3.7 7.5 .42

High 1.3 2.3 5.1 .34

Organic carbon, suspended Low .30 .80 4.5 .88

High .60 1.2 2.7 .49
40 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

(6)

(6) (19)

0

20

5

10

15

CONCENTRATION, IN MILLIGRAMS PER LITER

Total dissolved nitrogen

Total dissolved phosphorus

0

0.5

1.0

BFS SYN

0

10

2

4

6

8

Dissoved organic carbon

EXPLANATION

Low base flow

High base flow

25th percentile

75th percentile

 interquartile range outside the quartile

Data value less than or equal to 1.5 times the

 interquartile range outside the quartile 

 and more than 1.5 times the

Outlier data value less than or equal to 

 interquartile range outside the quartile

Outlier data value more than 3 times the

Number of observations

Median

Spatial Variability

Although water quality did not vary among the 

basic-fixed/synoptic and synoptic sites, spatial variability was evident during both low and high base-flow 

conditions. Much of the spatial variability was related 

to landform type and geologic and hydrologic differences among drainage basins. Nitrogen and phosphorus 

concentrations were significantly different (p < 0.05) in 

samples from sites in at least one landform region in 

the study unit (table 7) during low and high base-flow 

conditions. Orthophosphorus was the only constituent 

that had similar concentrations in samples from all 

landform regions for both base-flow periods. 

Ammonia, organic-nitrogen, phosphorus, and 

organic-carbon compounds commonly transported 

on and with suspended sediment were generally 

present in higher concentrations in drainage basins 

on the Des Moines Lobe and the Southern Iowa Drift 

Plain than in basins draining the Iowan Surface and 

Iowan Karst. Even though median concentrations of 

suspended sediment were low (less than 110 mg/L) 

(table 7) in samples from the Des Moines Lobe and 

Southern Iowa Drift Plain, suspended-sediment and 

DOC concentrations were generally higher than 

those in stream samples from the Iowan Surface and 

Iowan Karst. The presence of higher concentrations of 

organic compounds (organic enrichment) suggest one 

or more of the following: (1) biological production by 

algae, zooplankton, and bacteria; (2) input of organic 

compounds from soils with higher organic content; 

and (3) input of organic compounds from animalfeeding operations. Determining the source of organic 

enrichment is beyond the scope of this report, but a 

combination of the three sources may be possible. 

In contrast, nitrite plus nitrate concentrations 

varied among landform regions only during low baseflow conditions. Nitrite plus nitrate concentrations 

were high throughout the study unit during the high 

base-flow sampling in May 1998 (fig. 19). Nitrate plus 

nitrite concentrations were higher than 10 mg/L 

at all but three sites. The highest nitrite plus nitrate 

concentrations were in samples from streams draining 

parts of the Iowan Surface and Iowan Karst in the 

northern part of the study unit. During low base-flow 

sampling (August 1997), algal populations, as indicated by chlorophyll-a concentrations, were inversely 

related to dissolved nitrate concentrations. This relation suggests that when conditions are conducive for 

growth, algal production may be a factor in reducing 

nitrogen in streams. 

Figure 18. Concentrations of selected 

nitrogen, phosphorus, and organic-carbon 

compounds in samples from 6 basic-fixed sites 

(BFS) and 19 synoptic (SYN) sites during low 

and high base flow.
SYNOPTIC STUDIES 41

Variability Among Base-Flow Conditions

Three of the 10 selected nitrogen and phosphorus compounds had significant concentration differences between the low base flow in August 1997 and 

high base flow in May of 1998. Concentrations of dissolved ammonia and nitrite plus nitrate were significantly higher (p < 0.05, Kruskal-Wallis test) during 

high base flow than during low base flow (table 6). 

The higher concentrations may be due to the availability of nitrogen for transport to streams in spring. More 

than 90 percent of the corn acres had been fertilized 

(Iowa Agricultural Statistics, 1998b) in the autumn 

of 1997 and the spring of 1998, with a statewide 

average application of 14,800 kg/km2

 of nitrogen 

(U.S. Department of Agriculture, 1999) before sampling took place. Manure, another source of nitrogen 

and phosphorus, commonly is applied in the autumn 

after harvest and in early spring before planting. The 

availability of nitrogen and phosphorus in a drainage 

basin decreases during the summer because of uptake 

by plants and possible denitrification in underlying 

aquifers. Ground-water inflow from tile lines, which 

have been shown to transport substantial amounts of 

nitrogen (Soenksen, 1996; Cambardella and others, 

1999), normally decreases during late summer due 

Table 7. Median concentrations of selected nitrogen constituents in samples from 25 synoptic sites in relation to landform 

region in the Eastern Iowa Basins study unit, August 1997 (low base flow) and May 1998 (high base flow)

[Concentrations are in milligrams per liter except chlorophyll-a; N, nitrogen; P, phosphorus; <, less than]

Constituent

Base-flow type

(bold type indicates 

concentrations in one 

or more landforms are 

significantly different)

Median concentrations in samples

from each landform region

(defined by Prior, 1991)

Des Moines

Lobe

(7 sites)

Iowan Karst

(2 sites)

Iowan Surface

(9 sites)

Southern Iowa

Drift Plains

(7 sites)

Ammonia (as N), dissolved Low <0.015 <0.015 <0.015 0.08

High .05 .03 .04 .05

Ammonia plus organic nitrogen (as N), total Low 1.1 .3 .5 1.2

High 1.1 .3 .6 .7

Organic nitrogen, dissolved Low .4 .3 .2 .4

High .6 .2 .3 .3

Nitrite plus nitrate (as N), dissolved Low 1.5 5.2 3.4 .2

High 13 12 11 12

Nitrogen, total dissolved Low 1.8 5.4 3.6 1.2

High 14 12 11 12

Phosphorus (as P), dissolved Low .04 .03 .04 .08

High .06 .03 .04 .08

Orthophosphate (as P), dissolved Low .03 .02 .03 .06

High .02 .07 .01 .06

Phosphorus (as P), total Low .18 .02 .05 .2

High .10 .09 .06 .1

Sediment, suspended Low 69 14.5 21.5 72

High 109 22 71.0 61

Organic carbon, dissolved Low 4.1 2.0 2.4 5.2

High 3.0 1.6 2.1 2.3

Organic carbon, suspended Low .6 .6 .5 2.5

High 1.4 .6 1.2 1.2

Chlorophyll-a (micrograms per liter) Low 72 15 13 50

High 29 3.7 8.0 12
42 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

3

9

6

7

r

k

1

h

o

4

f

d

j

n

b

i

g

a

e

q

c

m

s

l

p

High base flow

(May 1998)

3

9

6

7

r

k

1

h

o

4

f

d

j

n

b

i

g

a

e

q

c

m

s

l

p

High base flow

(May 1998)

3

9

6

7

r

k

1

h

o

4

f

d

j

n

b

i

g

a

e

q

c

m

s

l

p

Nitrite plus nitrate

Total phosphorus

Low base flow

(August 1997)

3

9

6

7

r

k

1

h

o

4

f

d

j

n

b

i

g

a

e

q

c

m

s

l

p

Low base flow

(August 1997)

Total phosphorus concentration

 in milligrams per liter

0 - 0.049

0.05 - 0.099

0.1 - 0.8

Nitrite plus nitrate concentration

 in milligrams per liter

0 - 2.99

3 - 9.99

10 - 15

Landform region

Iowan Karst

Des Moines Lobe Iowan Surface

Southern Iowa Drift Plain

Figure 19. Spatial variability of nitrate and total phosphorus concentrations during low and high baseflow conditions in the Eastern Iowa Basins study unit.
SUMMARY 43

to reduced precipitation and increased evapotransporation. The pattern follows the seasonal trend described 

in the “Seasonal Variations” section of this report. 

Although concentrations of many nitrogen and 

phosphorus compounds were similar, the relative variability was higher during low base flow in late summer 

than during high base flow in late spring. Higher variability may be due to the effects of differences in geologic and hydrologic factors among landform regions.

SUMMARY

Twelve streams and rivers in the Eastern 

Iowa Basins (EIWA) study unit were sampled monthly 

from March 1996 through September 1998 for nitrogen, phosphorus, suspended sediment, and organic 

carbon. One sampling location was discontinued 

after 1996.

At least one compound of dissolved nitrogen 

and phosphorus was detected in every sample. About 

92 percent of nitrogen was in the form of nitrate. 

About 22 percent of the nitrate concentrations 

exceeded the U.S. Environmental Protection Agency 

maximum contaminant level of 10 mg/L as N for 

drinking water. Total nitrogen concentrations ranged 

from 0.20 to 22 mg/L, with a median concentration 

of 7.2 mg/L. Dissolved phosphorus was predominately 

in the form of orthophosphate. Total phosphorus concentrations ranged from 0.01 to 3.4 mg/L, with a 

median concentration of 0.22 mg/L. About 75 percent 

of the total phosphorus concentrations exceeded the 

U.S. Environmental Protection Agency recommended 

total phosphorus concentration of 0.1 mg/L to minimize algal growth. Suspended-sediment concentrations ranged from less than 0.1 to 3,500 mg/L, with a 

median concentration of 82 mg/L. Dissolved organiccarbon (DOC) concentrations ranged from 0.80 to 

44 mg/L, with a median concentration of 3.5 mg/L.

Concentrations of nitrogen and phosphorus 

varied both annually and seasonally and were related 

to precipitation, runoff, and fertilizer application. 

Precipitation and runoff varied during the 3-year 

study. In 1996, the study unit received below-normal 

precipitation and runoff. In 1997, precipitation and 

runoff were normal except for the southern part of 

the study unit, which received less precipitation 

and runoff. In 1998, precipitation and runoff 

were above normal. All nitrogen and phosphorus 

compounds, with the exception of dissolved 

ammonia, showed increases in median concentrations from 1996 to 98. Median concentrations of 

suspended-sediment increased from 1996 to 1998. 

Median concentrations of DOC remained the same 

from 1996 to 1998.

Concentrations of nitrogen, phosphorus, 

and suspended sediment varied seasonally, and concentrations were higher in the spring after fertilizer 

application and runoff events. In winter, nitrogen 

and phosphorus concentrations increased when there 

was little in-stream processing by biota. Nitrogen and 

phosphorus concentrations decreased in late summer 

when there was less runoff and in-stream processing 

of nitrogen and phosphorus was high. Suspendedsediment concentrations were highest in early summer 

during runoff events and lowest in January when there 

was ice cover with very little overland flow. DOC concentrations were highest in February and March when 

decaying vegetation and manure were transported to 

streams during snowmelt events.

Historical and study-unit data indicate there 

are trends that show both nonpoint- and point-source 

pollution in eastern Iowa streams and rivers. Typically, 

for nonpoint sources, the chemical concentrations 

increase as runoff increases. A decrease in concentration at the highest rate of streamflow indicates that 

after extended periods of runoff, nitrate available for 

transport to streams changes little, and further rainfall 

and runoff dilute the nitrate concentrations. Compound concentrations for point sources typically are 

high during low flow and decrease as flow increases 

due to dilution.

Samples from indicator sites that have homogeneous land use and geology had significantly higher 

(p < 0.05) concentrations of total dissolved nitrogen 

(median, 8.2 mg/L) than did samples from integrator 

sites (median, 6.2 mg/L) that are more heterogeneous 

in land use and geology. Samples from integrator sites 

had significantly higher (p < 0.05) total phosphorus 

concentrations (median, 0.29 mg/L) than did samples 

from indicator sites (median, 0.12 mg/L). Integrator 

sites had significantly (p < 0.05) higher suspendedsediment concentrations (median, 114 mg/L) than did 

indicator sites (median, 55 mg/L). There was little difference in median concentrations of DOC in samples 

from indicator and integrator sites (3.5 and 3.6 mg/L, 

respectively).
44 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

Statistical analysis of chemical concentrations 

indicated that there were differences in water quality 

for nitrogen, phosphorus, and suspended sediment 

among sites, which are related to land use and differences in landform regions. Drainage basins with large 

percentages of row-crop agriculture and animalfeeding operations typically had higher nitrogen 

concentrations than did drainage basins with lower 

percentages of row-crop agriculture and animalfeeding operations. Basins that drain the Des Moines 

Lobe and Southern Iowa Drift Plain typically had 

higher total phosphorus and suspended-sediment 

concentrations than did other basins in this study.

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. Total phosphorus loads decreased in 

1997 due to less runoff in the southern part of the 

study unit, which contributes large sediment concentrations. Total nitrogen and total phosphorus loads 

vary seasonally. The highest loads typically occurred 

in early spring and summer after fertilizer application 

and runoff. The loads were typically lowest in January 

and September when there was very little runoff to 

transport nitrogen and phosphorus adsorbed to the soil 

to the rivers and streams.

Total nitrogen loads contributed to the 

Mississippi River from the EIWA from 1996, 1997, 

and 1998 were 97,600, 120,000, and 234,000 metric 

tons, respectively. Total phosphorus loads contributed 

to the Mississippi River from the EIWA during 1996, 

1997, and 1998 were 6,860, 4,550, and 8,830 metric 

tons, respectively. Suspended-sediment loads contributed to the Mississippi River from the EIWA during 

1996, 1997, and 1998 were 7,480,000, 4,450,000, and 

8,690,00 metric tons, respectively. The highest total 

nitrogen and total phosphorus yields typically 

occurred at indicator sites that were dominated by 

row-crop agriculture land use and within a single 

landform region. Sampling sites located in drainage 

basins with a higher row-crop percentage and that 

contained more animal-feeding operations typically 

had higher yields of total nitrogen. In addition, landform regions are a determining factor in yields. Sites 

that were on the Des Moines Lobe and the Southern 

Iowa Drift Plain typically had higher total phosphorus 

yields than sites on the Iowan Surface and Iowan 

Karst.

Synoptic sampling during low base-flow 

and high base-flow conditions in 25 selected sites 

indicated that there were no water-quality differences 

among samples collected from six indicator basicfixed sites and 19 other synoptic-sampling sites; thus 

the six basic-fixed sites selected as indicator sites were 

representative of the entire study unit. There were 

spatial differences in compound concentrations in 

samples from the 25 synoptic-sampling sites. Ammonia, organic nitrogen, and phosphorus—chemicals 

commonly transported by and with suspended 

sediment—were present in higher concentrations in 

basins draining the Des Moines Lobe and Southern 

Iowa Drift Plain than in basins draining the Iowa 

Surface and Iowa Karst. Concentrations of nitrogen 

and phosphorus were generally higher during high 

base-flow conditions than during low base-flow 

conditions.

REFERENCES

Akers, K.K.B., Schnoebelen, D.J., Savoca, M.E., Roberts, 

L.R., and Becher, K.D., 1999, Water-quality assessment of the Eastern Iowa Basins—Hydrologic and 

ecological data, water years 1995–96: U.S. Geological 

Survey Open-File Report 99–66, 153 p.

Akers, K.K.B., Montgomery, D.L., Christiansen, D.E., 

Schnoebelen, D.J., Savoca, M.E., and Becher, K.D., 

2000, Water-quality assessment of the Eastern Iowa 

Basins—Hydrologic and biologic data, October 1996 

through September 1998: U.S. Geological Survey 

Open-File Report 00–67, 153 p. 

Allen, J.D., 1995, Stream ecology—Structure and function 

of running waters: New York, Chapman and Hall 

Publishers, 388 p.

Becher, K.D., Schnoebelen, D.J., and Akers, K.K.B, 

2000, Nutrients discharged into the Mississippi River 

from eastern Iowa streams, 1996–97: Journal of the 

American Water Resources Association, v. 36, no. 1, 

p. 161–173.

Brezonik, P.L., 1973, Nitrogen sources and cycling 

in natural waters: U.S. Environmental Protection 

Agency, Office of Research and Development, 

EPA–660/3–73–002, 167 p.

Buchmiller, Robert, Gaillot, Gary, and Soenksen, P.J., 1985, 

Water resources of north-central Iowa: State of Iowa, 

Iowa Geological Survey Water Atlas 7, 93 p.
REFERENCES 45

Burkhart, M.R., and James, D.E., 1999, Agriculturalnitrogen contributions to hypoxia in the Gulf of 

Mexico: Journal of Environmental Quality, v. 28, 

no. 3, May–June, p. 850–859.

Cambardella, C.A., Moorman, T.B., Jaynes, D.B., 

Hatfield, J.L., Parkin, T.B., Simpkins, W.W., and 

Karlan, D.L., 1999, Water quality in the Walnut 

Creek watershed—Nitrate-nitrogen in soils, subsurface 

drainage water, and shallow groundwater: Journal of 

Environmental Quality, v. 28, no. 1, p. 25–34.

Cleveland, W.S., 1979, Robust locally weighted regression 

and smoothing scatterplots: Journal of American 

Statistics Association, v. 74, p. 829–836.

Cohn, T.A., Caulder, D.L., Gilroy, R.J., Zynjuk, L.D., 

and Summers, R.M., 1992, The validity of a simple 

statistical model for estimating fluvial compound 

loads—An empirical study involving nutrient loads 

entering Chesapeake Bay: Water Resources Research, 

v. 28, no. 9, p. 2353–2363.

Cohn, T.A., DeLong, L.L., Gilroy, E.J., Hirsch, R.M., and 

Wells, D.K., 1989, Estimating compound loads: Water 

Resources Research, v. 25, no. 5, p. 937–942.

Degens, E.T., 1982, Transport of carbon and minerals 

in major world rivers, SCOPE/UNEP: Sonderband, 

Mitteilungen aus dem Geologisch-Paläontologischen 

Institut der Universität Hamburg, Heft 52, 765 p.

Drever, J.I., 1988, The geochemistry of natural waters 

(2d ed.): Englewood Cliffs, N.J., Prentice-Hall, 

437 p.

Eash, D.A., 1995, Flood-plain and channel aggradation 

at selected bridge sites in the Iowa and Skunk River 

Basins, Iowa: U.S. Geological Survey WaterResources Investigations Report 95–4290, 44 p.

Fishman, M.J., 1993, Methods of analysis by the 

U.S. Geological Survey National Water-Quality 

Laboratory—Determination of inorganic and organic 

compounds in water and fluvial sediments: U.S. 

Geological Survey Open-File Report 93–125, 217 p.

Goolsby, D.A., and Battaglin, W.A., 1995, Spatial data in 

geographic information system format on agricultural 

chemical use, land use, and cropping practices in 

the United States: U.S. Geological Survey WaterResources Investigations Report 94–4176, 87 p.

Goolsby, D.A., Battaglin, W.A., and Hooper R.P., 1997, 

Sources and transport of nitrogen in the Mississippi 

River Basin, conference proceedings, from the Corn 

Belt to the Gulf—Agriculture and hypoxia in the 

Mississippi River watershed: St. Louis, Mo., 

American Farm Bureau Federation Workshop, 

7 p.

Goolsby, D.A., Battaglin, W.A., Lawrence, G.B., Artz, 

R.S., Aulenbach, B.T., Hooper, R.P., Keeney, D.R., 

and Stensland, G.J., 1999, Flux and sources of nutrients in the Mississippi-Atchafalaya River Basin, 

Topic 3 Report of the integrated assessment on 

hypoxia in the Gulf of Mexico: National Oceanic and 

Atmospheric Administration Coastal Ocean Program 

Decision Analysis Series 17, NOAA Coastal Ocean 

Program, Silver Spring, Md., 130 p.

Griffith, G.E., Omernik, J.M., Wilton, T.F., and Pierson, 

S.M., 1994, Ecoregions and subregions of Iowa—A 

framework for water-quality assessment and management: Journal of the Iowa Academy of Science, 

p. 5–13.

Guy, H.P., 1969, Laboratory theory and methods for sediment analysis: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 5, chap. C1, 

58 p.

Hallberg, G.R., Riley, D.G., Kantamneni, J.R., Weyer, P.J., 

and Kelley, R.D., 1996, Assessment of Iowa safe 

drinking water act monitoring data, 1988–1995: Iowa 

City, University of Iowa Hygienic Laboratory Research 

Report 97–1, 132 p.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods 

in water resources: New York, Elsevier Science 

Publishers, 522 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p.

Hirsch, R.M., Alley, W.M., and Wilber, W.G., 1988, 

Concepts for a National Water-Quality Assessment 

Program: U.S. Geological Survey Circular 1021, 42 p.

Hitt, K.J., 1994, Refining 1970’s land-use data with 1990 

population data to indicate new residential development: U.S. Geological Survey Water-Resources Investigations Report 94–4250, 15 p.

Iowa Agricultural Statistics, 1996, Iowa crops and weather: 

Des Moines, Iowa, November 18, 1996, v. 96–34, 4 p. 

———1997, Iowa crops and weather: Des Moines, Iowa, 

November 24, 1997, v. 97, no. 36, 4 p.

———1998a, Iowa crops and weather: Des Moines, Iowa, 

November 29, 1998, v. 98, no. 38, 4 p.

———1998b, Iowa crops and weather: Des Moines, Iowa, 

May 17, 1996, v. 98, no. 10, 4 p.

Iowa Department of Natural Resources, 1994, Water quality 

in Iowa during 1992 and 1993: Iowa Environmental 

Protection Division, Water Resources Section 305(b) 

Report, 226 p.
46 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98

———1998, Confinement feeding operations: accessed 

January 12, 1998, at URL http://www.state.ia.us/

epd/wastewtr/feedlot/feedlt/htm

Isenhart, T.M., and Crumpton, W.G., 1989, Transformation 

and loss of nitrate in an agricultural stream: Journal of 

Freshwater Ecology, v. 5, no. 2, p. 123–129.

Kalkhoff, S.J., 1994, National Water-Quality Assessment 

Program—Eastern Iowa Basins: U.S. Geological 

Survey Fact Sheet FS 94–031, 2 p.

Koltun, G.F., Gray, John R., and McElhone, T.J., 1994, 

User’s manual for SEDCALC, a computer program 

for calculation of suspended-sediment discharge: U.S. 

Geological Survey Open-File Report 94–459, 68 p.

Libra, R.D., Quade, D.J., and Rieck-Hinz, A., 1996, Agricultural drainage wells and ground-water quality, hogs, 

bogs, and logs, in Quaternary deposits and environmental geology of the Des Moines Lobe: Geological 

Survey Bureau Guidebook Series 12, North-Central 

Section Annual Meeting of the Geological Survey of 

America, Field Trip 5, p. 155–170.

May, J.E., Gorman, J.G., Goodrich, R.D., Bobier, M.W., 

and Miller, V.E., 1996, Water resources data, Iowa, 

water year 1996: U.S. Geological Survey Water-Data 

Report IA–96–1, 578 p.

May, J.E., Gorman, J.G., Goodrich, R.D., and Miller, V.E., 

1997, Water resources data, Iowa, water year 1997: 

U.S. Geological Survey Water-Data Report IA–97–1, 

v. 1, 395 p.

May, J.E., Gorman, J.G., Goodrich, R.D., Miller, V.E., 

Turco, M.J., and Linhart, S.M., 1998, Water resources 

data, Iowa, water year 1998: U.S. Geological Survey 

Water-Data Report IA–98–1, v. 1, 374 p.

Maybeck, Michel, 1982, Carbon nitrogen, and phosphorus 

transport by world rivers: American Journal of 

Science, v. 282, p. 401–450.

Mueller, D.K., 1998, Quality of nutrient data from streams 

and ground water sampled during 1993–95—National 

Water-Quality Assessment Program: U.S. Geological 

Survey Open-File Report 98–276, 25 p.

Mueller, D.K., Martin, J.D., and Lopes T.J., 1997, Qualitycontrol design for surface-water sampling in the 

National Water-Quality Assessment Program: U.S. 

Geological Survey Open-File Report 97–223, 17 p.

Nalley, G.M., Gorman, J.G., Goodrich, R.D., Miller, V.E., 

Turco, M.J., and Linhart, S.M., 1999: Water resources 

data, Iowa, water year 1999: U.S. Geological Survey 

Water-Data Report IA–99–1, v. 1, 363 p.

Neill, M., 1989, Nitrate concentrations in river waters in the 

southeast of Ireland and their relationship with agricultural practice: Water Research, v. 23, p. 1339–1355.

Oschwald, W.R., Riecken, F.F., Dideriksen, W.H., and 

Schaller, F.W., 1965, Principal soils of Iowa: Ames, 

Iowa State University Cooperative Extension Service 

Special Report 42, 76 p.

Prior, J.C., 1991, Landforms of Iowa: Iowa City, University 

of Iowa Press, p. 30–75.

Sands, J.K., and Holden, H.H., 1996, Iowa agricultural 

statistics: U.S. Department of Agriculture and Iowa 

State University, 131 p.

———1997, Iowa agricultural statistics: U.S. Department 

of Agriculture and Iowa State University, 131 p.

Schnoebelen, D.J., Becher, K.D., Bobier, M.W., and 

Wilton, Tom, 1999, Selected nutrients and pesticides 

in streams of the Eastern Iowa Basins, 1970–95, U.S. 

Geological Survey Water-Resources Investigations 

Report 99–4028, 105 p.

Schwarz, G.E., and Alexander, R.B., 1995, State Soil 

Geographic (STATSGO) database for the conterminous United States: U.S. Geological Survey OpenFile Report 95–489.

Shelton, L.R., 1994, Field guide for collection and 

processing stream-water samples for the National 

Water-Quality Assessment Program: U.S. Geological 

Survey Open-File Report 94–455, 42 p.

Sloto, R.A., and Crouse, M.Y., 1996, HYSEP—A computer 

program for streamflow hydrograph separation and 

analysis: U.S. Geological Survey Water-Resources 

Investigations Report 96–1040, 46 p.

Soenksen, P.J., 1996, Transport of agricultural chemicals 

in surface flow, tile flow, and streamflow of the 

Walnut Creek watershed near Ames, Iowa, 

April 1991–September 1993: U.S. Geological Survey 

Water-Resources Investigations Report 96–4017, 41 p.

Sorenson, S.K., Porter, S.D., Akers, K.K.B., Harris, M.A., 

Kalkhoff, S.J., Lee, K.E., Roberts, L.R., and Terrio, 

P.J., 1999, Water-quality and habitat conditions in 

Upper Midwest streams relative to riparian vegetation 

and soil characteristics, August 1997—Study design, 

methods, and data: U.S. Geological Survey Open-File 

Report 99–202, 53 p.

Statistical Analysis System, 1990, Statistical Analysis 

System user guide, version 6: Cary, North Carolina, 

SAS Institute Inc., 840 p.

Thurman, E.M., 1985, Organic geochemistry of natural 

waters: Dordrecht, Netherlands, Martinus Nijhoff/

Dr. W. Junk, 497 p.

Turner, R.E., and Rabalais, N.N., 1994, Coastal eutrophication near the Mississippi River Delta: Nature, v. 364, 

p. 619–621.
REFERENCES 47

U.S. Department of Agriculture, 1998, Agricultural 

chemical usage: accessed July 23, 1999, at URL 

http://usda.mannlib.cornell.edu (accessed 11/01).

———1999, Agricultural chemical usage: accessed 

July 23, 1999, at URL http://usda.mannlib.cornell.edu 

(accessed 11/01).

U.S. Department of Commerce, 1959, Climates of the 

States—Climate Iowa: U.S. Government Printing 

Office, Climatology of the United States, no. 6013, 

2 p.

U.S. Environmental Protection Agency, 1986, Quality 

criteria for water 1986: Washington, D.C., 

Report 440/5–86–001, 453 p.

———1996, Drinking water regulations and health advisories: Washington, D.C., Report 822–R–96–001, 16 p.

———1998, National strategy for development of 

regional nutrient criteria: Washington, D.C., 

Report 822–R–98–002, 47 p.

Viessman, Warren, Jr., Levis, G.L., and Knapp, J.W., 1989, 

Introduction to hydrology: New York, Harper and Row 

Publishers, 780 p.

Wahl, K.L., and Wahl, T.L., 1995, Determining the 

flow of Comal Springs at New Braunfels, Texas: 

San Antonio, Texas, Water ’95, American Society 

of Civil Engineers, August 16–17, 1995, p. 77–86.

Ward, J.R., and Harr, C.A., 1990, Methods for collection 

and processing of surface-water and bed-material 

samples for physical and chemical analysis: U.S. 

Geological Survey Open-File Report 90–140, 71 p.

Wendland, W.M., Kunkel, K.E., Conner, G., Decker, 

W.L., Hillaker, H., Naber-Knox, P., Nurmberger, V., 

Rogers, J., Scheeringa, K., and Zaldo, J., 1992, Mean 

1961–1990 temperature and precipitation over the 

Upper Midwest: Illinois State Water Survey, Research 

Report 92–10, 27 p.

Weyer, P.J., Cerhan, J.R., Kross, B.R., Hallberg, G.R., 

Kantamneni, J., Bruer, George, Jones, M.P., Zheng, 

Wei, and Lynch, C.F., 2001, Municipal drinking water 

nitrate level and cancer risk in older women—The 

Iowa Women’s health study: Epidemiology, v. 2, no. 3, 

p. 327–338.
APPENDIX
A

PPENDIX51

Appendix table. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon concentrations in the Eastern IMarch 1996–September 1998[MCL, Maximum Contamination Level; RAL, recommended action level; N, nitrogen; P, phosphorus; na, not applicable; <, less than]

Constituent

Number of

samples

Number ofdetections

Number of detections

greater thanMCL or RAL

Concentrations,

in milligrams per lite

Minimum

25th

percentile

Median

p

Site 1—Wapsipinicon River near Tripoli (indicator site, fig. 1)

Ammonia (as N), dissolved 36 21 na <0.02 <0.02 0.03 Ammonia plus organic nitrogen (as N), total 36 36 na .2 .5 .6 Ammonia plus organic nitrogen (as N), dissolved 36 36 na <.1 .3 .4 Organic nitrogen,2 dissolved 36 35 na .2 .2 .3 Nitrite (as N), dissolved 36 34 na <.01 .02 .04 Nitrite plus nitrate (as N), dissolved 36 35 na .11 2.8 5.6 Nitrate (as N),1 dissolved 36 35 4 <.02 2.8 5.5 Nitrogen (as N), total,3 dissolved 36 36 na .4 3.1 6.3 Phosphorus (as P), dissolved 36 30 na <.01 .02 .03 Orthophosphate (as P), dissolved 36 32 na <.01 .02 .03 Phosphorus (as P), total 36 36 15 .01 .05 .09 Suspended sediment 36 36 na 2.0 8.5 20 Organic carbon, dissolved 36 36 na 2.1 2.7 3.2 Organic carbon, suspended 35 35 na .3 .4 .9 

Site 2—Wapsipinicon River near De Witt (integrator site, fig. 1)

Ammonia (as N), dissolved 35 18 na <.02 <.02 .02 Ammonia plus organic nitrogen (as N), total 35 35 na .3 1.0 1.7 Ammonia plus organic nitrogen (as N), dissolved 35 33 na <.1 .3 .4 Organic nitrogen,2 dissolved 35 33 na .2 .2 .3 Nitrite (as N), dissolved 35 34 na <.01 .02 .03 Nitrite plus nitrate (as N), dissolved 35 35 na .11 1.4 6.2 Nitrate (as N),1 dissolved 35 35 8 .09 1.4 6.1 Nitrogen (as N), total,3 dissolved 35 35 na .3 1.7 6.4 Dissolved phosphorus (as P) 35 27 <.01 <.01 .05 .09 Orthophosphate (as P), dissolved 35 29 na <.01 .01 .06 Phosphorus (as P), total 35 35 32 .04 .16 .23 Suspended sediment 35 35 na 7.0 51 92 Organic carbon, dissolved 35 35 na 2.1 2.8 3.3 Organic carbon, suspended 35 34 na .2 1.4 2.8 
52 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98 Site 3—Iowa River near Rowan (indicator site, fig. 1) Ammonia (as N), dissolved 52 35 na <0.02 <0.02 0.04 

Ammonia plus organic nitrogen (as N), total 52 52 na .2 .7 1.0 Ammonia plus organic nitrogen (as N), dissolved 52 51 na <.1 .4 .4 Organic nitrogen,2 dissolved 52 51 na .2 .3 .4 

Nitrite (as N), dissolved 52 51 na <.01 .03 .04 Nitrite plus nitrate (as N), dissolved 52 52 na 1.2 3.0 6.3 Nitrate (as N),1 dissolved 52 52 9 1.2 3.0 6.3 

Nitrogen (as N), total3 dissolved 52 52 na 1.5 3.7 6.8 

Phosphorus (as P), dissolved 52 51 na <.01 .06 .09 Orthophosphate (as P), dissolved 52 50 na <.01 .06 .09 Phosphorus (as P), total 52 52 41 .05 .11 .2 Suspended sediment 52 52 na 13 60 100 Organic carbon, dissolved 51 51 na 2.7 3.6 4.2 Organic carbon, suspended 50 50 na .3 .7 1.4 

Site 4—South Fork Iowa River near New Providence (indicator site, fig. 1)

Ammonia (as N), dissolved 38 25 na <.02 <.02 .04 Ammonia plus organic nitrogen (as N), total 38 38 na .4 .5 .6 Ammonia plus organic nitrogen (as N), dissolved 38 38 na .2 .3 .4 Organic nitrogen,2 dissolved 38 38 na .2 .3 .4 

Nitrite (as N), dissolved 38 35 na <.01 .02 .04 Nitrite plus nitrate (as N), dissolved 38 38 na .02 4.2 9.5 Nitrate (as N),1 dissolved 38 37 17 .02 4.1 9.5 

Nitrogen (as N), total3 dissolved 38 38 na .4 5.0 10 

Phosphorus (as P), dissolved 38 30 na <.01 .02 .04 Orthophosphate (as P), dissolved 38 32 na <.01 .01 .06 Phosphorus (as P), total 38 38 16 .02 .05 .07 Suspended sediment 37 37 na 13 24 47 Organic carbon, dissolved 38 38 na 2.5 3.1 4.0 Organic carbon, suspended 38 38 na .4 .6 .9 

Appendix table. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon concentrations in the Eastern I

March 1996–September 1998—Continued

[MCL, Maximum Contamination Level; RAL, recommended action level; N, nitrogen; P, phosphorus; na, not applicable; <, less than]

Constituent

Number of

samples

Number ofdetections

Number of detections

greater thanMCL or RAL

Concentrations,

in milligrams per lite

Minimum

25th

percentile

Median
APPENDIX 53

Site 5—Iowa River at Marengo (integrator site, fig. 1)

Ammonia (as N), dissolved 34 21 na <0.02 <0.02 0.03 Ammonia plus organic nitrogen (as N), total 34 34 na .3 1.1 1.4 Ammonia plus organic nitrogen (as N), dissolved 34 33 na <.1 .3 .4 Organic nitrogen,2 dissolved 34 33 na .1 .3 .4 

Nitrite (as N), dissolved 34 33 na <.01 .02 .03 Nitrite plus nitrate (as N), dissolved 34 33 na <.05 3.9 6.5 Nitrate (as N),1 dissolved 34 32 4 <.05 3.9 6.5 

Nitrogen (as N), total3 dissolved 34 34 na .4 4.5 6.9 

Phosphorus (as P), dissolved 34 31 na <.01 .08 .13 Orthophosphate (as P), dissolved 34 30 na <.01 .06 .12 Phosphorus (as P), total 34 34 34 .13 .27 .35 Suspended sediment 34 34 na 11 81 211 Organic carbon, dissolved 34 34 na 2.2 2.9 3.2 Organic carbon, suspended 34 34 na .1 1.3 3.0 

Site 6—Old Mans Creek near Iowa City (indicator site, fig. 1)

Ammonia (as N), dissolved 39 35 na <.02 .03 .08 Ammonia plus organic nitrogen (as N), total 39 38 na <.1 .5 .7 Ammonia plus organic nitrogen (as N), dissolved 39 39 na .2 .3 .4 Organic nitrogen,2 dissolved 39 39 na .1 .3 .4 

Nitrite (as N), dissolved 39 37 na <.01 .03 .05 Nitrite plus nitrate (as N), dissolved 39 37 na <.05 2.20 4.90 Nitrate (as N),1 dissolved 39 37 10 .04 2.2 4.9 

Nitrogen (as N), total3 dissolved 39 39 na .4 2.7 6.4 

Phosphorus (as P), dissolved 39 39 na .01 .05 .08 Orthophosphate (as P), dissolved 39 38 na <.01 .04 .06 Phosphorus (as P), total 39 39 25 .03 .09 .17 Suspended sediment 39 39 na 7.0 16 69 Organic carbon, dissolved 39 39 na 1.8 2.9 3.6 Organic carbon, suspended 38 38 na .2 .8 1.1 

Appendix table. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon concentrations in the Eastern I

March 1996–September 1998—Continued

[MCL, Maximum Contamination Level; RAL, recommended action level; N, nitrogen; P, phosphorus; na, not applicable; <, less than]

Constituent

Number of

samples

Number ofdetections

Number of detections

greater thanMCL or RAL

Concentrations,

in milligrams per lite

Minimum

25th

percentile

Median

p
54 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98 Site 7—Flood Creek near Powersville (indicator site, fig. 1) Ammonia (as N), dissolved 34 22 na <0.02 <0.02 0.03 

Ammonia plus organic nitrogen (as N), total 34 30 na <.1 .2 .4 Ammonia plus organic nitrogen (as N), dissolved 34 24 na <.1 <.1 .2 Organic nitrogen,2 dissolved 34 24 na 1.1 1.3 1.5 

Nitrite (as N), dissolved 34 33 na <.01 .03 .04 Nitrite plus nitrate (as N), dissolved 34 34 na 3.3 6.3 8.4 Nitrate (as N),1 dissolved 34 34 10 3.2 6.2 8.3 

Nitrogen (as N), total3 dissolved 34 34 na 3.8 6.8 8.6 

Phosphorus (as P), dissolved 34 33 na <.01 .06 .07 Orthophosphate (as P), dissolved 34 33 na <.01 .06 .08 Phosphorus (as P), total 34 34 15 .03 .05 .09 Suspended sediment 34 34 na .3 11 28 Organic carbon, dissolved 34 34 na 1.1 1.3 1.5 Organic carbon, suspended 34 34 na .1 .3 .6 

Site 8—Cedar River at Gilbertville (integrator site, fig. 1)

Ammonia (as N), dissolved 12 9 na <.02 <.02 .04 Ammonia plus organic nitrogen (as N), total 12 12 na .6 1.0 1.3 Ammonia plus organic nitrogen (as N), dissolved 12 11 na <.1 .3 .5 Organic nitrogen,2 dissolved 12 11 na .2 .3 .3 

Nitrite (as N), dissolved 12 12 na .03 .03 .04 Nitrite plus nitrate (as N), dissolved 12 12 na 1.8 3.2 3.8 Nitrate (as N),1 dissolved 12 12 1 1.7 3.2 3.7 

Nitrogen (as N), total3 dissolved 12 12 na 2.1 3.4 4.2 

Phosphorus (as P), dissolved 12 8 na <.01 <.01 .07 Orthophosphate (as P), dissolved 12 9 na <.01 <.01 .08 Phosphorus (as P), total 12 12 11 .06 .18 .22 Suspended sediment 12 12 na 5.0 23 43 Organic carbon, dissolved 12 12 na 2.5 2.7 3.0 Organic carbon, suspended 12 12 na .2 .8 2.0 

Appendix table. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon concentrations in the Eastern I

March 1996–September 1998—Continued

[MCL, Maximum Contamination Level; RAL, recommended action level; N, nitrogen; P, phosphorus; na, not applicable; <, less than]

Constituent

Number of

samples

Number ofdetections

Number of detections

greater thanMCL or RAL

Concentrations,

in milligrams per lite

Minimum

25th

percentile

Median
APPENDIX 55

Site 9—Wolf Creek near Dysart (indicator site, fig. 1)

Ammonia (as N), dissolved 54 31 na <0.02 <0.02 0.02 Ammonia plus organic nitrogen (as N), total 54 54 na .2 .4 .6 Ammonia plus organic nitrogen (as N), dissolved 54 42 na <.1 .2 .3 Organic nitrogen,2 dissolved 54 42 na .1 .2 .3 

Nitrite (as N), dissolved 54 53 na <.01 .03 .04 Nitrite plus nitrate (as N), dissolved 54 54 na 3.2 6.9 10 Nitrate (as N),1 dissolved 54 54 27 3.1 6.9 10 

Nitrogen (as N), total3 dissolved 54 54 na 3.5 7.6 10 

Phosphorus (as P), dissolved 54 53 na <.01 .04 .08 Orthophosphate (as P), dissolved 54 54 na <.01 .04 .07 Phosphorus (as P), total 54 54 33 .03 .08 .13 Suspended sediment 54 54 na 13 35 81 Organic carbon, dissolved 53 53 na 1.5 2.0 2.4 Organic carbon, suspended 53 53 na .3 .7 1.2 

Site 10—Cedar River near Conesville (integrator site, fig. 1)

Ammonia (as N), dissolved 34 21 na <.02 <.02 .03 Ammonia plus organic nitrogen (as N), total 34 34 na .3 1.0 1.6 Ammonia plus organic nitrogen (as N), dissolved 34 34 na .2 .3 .4 Organic nitrogen,2 dissolved 34 34 na .2 .3 .4 

Nitrite (as N), dissolved 34 34 na .01 .02 .03 Nitrite plus nitrate (as N), dissolved 34 34 na .13 2.7 4.9 Nitrate (as N),1 dissolved 34 34 2 .11 2.6 4.9 

Nitrogen (as N), total3 dissolved 34 34 na .5 3.1 5.5 

Phosphorus (as P), dissolved 34 27 na <.01 .01 .12 Orthophosphate (as P), dissolved 34 28 na <.01 .02 .11 Phosphorus (as P), total 34 34 32 .06 .22 .26 Suspended sediment 34 34 na 12 40 78 Organic carbon, dissolved 34 34 na 2.2 3.1 3.6 Organic carbon, suspended 33 33 na .3 1.4 4.5 

Appendix table. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon concentrations in the Eastern I

March 1996–September 1998—Continued

[MCL, Maximum Contamination Level; RAL, recommended action level; N, nitrogen; P, phosphorus; na, not applicable; <, less than]

Constituent

Number of

samples

Number ofdetections

Number of detections

greater thanMCL or RAL

Concentrations,

in milligrams per lite

Minimum

25th

percentile

Median

p
56 Water-Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended Sediment, 

and Organic Carbon in Surface Water, 1996–98 Site 11—Iowa River at Wapello (integrator site, fig. 1) Ammonia (as N), dissolved 53 33 na <0.02 <0.02 0.03 

Ammonia plus organic nitrogen (as N), total 53 53 na .4 1.1 1.5 Ammonia plus organic nitrogen (as N), dissolved 53 51 na <.1 .3 .3 Organic nitrogen,2 dissolved 53 51 na .2 .3 .3 

Nitrite (as N), dissolved 53 48 na <.01 .02 .03 Nitrite plus nitrate (as N), dissolved 53 52 na .02 3.6 5.4 Nitrate (as N),1 dissolved 53 52 3 <.10 3.6 5.9 

Nitrogen (as N), total3 dissolved 53 53 na .3 4.2 5.9 

Phosphorus (as P), dissolved 53 46 na <.01 .01 .12 Orthophosphate (as P), dissolved 53 43 na <.01 .02 .08 Phosphorus (as P), total 53 53 52 .07 .24 .29 Suspended sediment 53 53 na 13 84 130 Organic carbon, dissolved 52 52 na 2.5 3.2 3.7 Organic carbon, suspended 53 53 na .3 1.4 4.5 

Site 12—Skunk River at Augusta (integrator site, fig. 1)

Ammonia (as N), dissolved 34 23 na <.02 <.02 .03 Ammonia plus organic nitrogen (as N), total 34 34 na .5 .9 1.3 Ammonia plus organic nitrogen (as N), dissolved 34 34 na .2 .3 .4 Organic nitrogen,2 dissolved 34 34 na .2 .3 .4 

Nitrite (as N), dissolved 34 32 na <.01 .02 .03 Nitrite plus nitrate (as N), dissolved 34 32 na .02 3.7 6.0 Nitrate (as N),1 dissolved 34 34 6 .04 3.6 5.9 

Nitrogen (as N), total3 dissolved 34 34 na .2 4.7 6.8 

Phosphorus (as P), dissolved 34 33 na <.01 .11 .14 Orthophosphate (as P), dissolved 34 32 na <.01 .10 .14 Phosphorus (as P), total 34 34 34 .12 .22 .33 Suspended sediment 34 34 na 17 99 180 Organic carbon, dissolved 34 34 na .8 3.2 4.2 Organic carbon, suspended 34 34 na .3 1.7 3.9 

1Dissolved nitrite plus nitrate minus dissolved nitrite.2Dissolved ammonia plus organic nitrogen minus dissolved ammonia.3Dissolved nitrite plus nitrate plus dissolved ammonia plus organic nitrogen.

Appendix table. Statistical summary of nitrogen, phosphorus, suspended-sediment, and organic-carbon concentrations in the Eastern I

March 1996–September 1998—Continued

[MCL, Maximum Contamination Level; RAL, recommended action level; N, nitrogen; P, phosphorus; na, not applicable; <, less than]

Constituent

Number of

samples

Number ofdetections

Number of detections

greater thanMCL or RAL

Concentrations,

in milligrams per lite

Minimum

25th

percentile

Median
USGS/WRIR 01–4175

 Sediment, and Organic Carbon in Surface Water, 1996–98Becher and others—Water–Quality Assessment of the Eastern Iowa Basins—Nitrogen, Phosphorus, Suspended 

Printed on recycled paper