Science for Solutions 

NOAA COASTAL OCEAN PROGRAM 

Decision Analysis Series No. 17 

Flux and Sources of Nutrients in the 

Mississippi-Atchafalaya River Basin 

Topic 3 Report for the Integrated Assessment 

on Hypoxia in the Gulf of Mexico 

Donald A. Goolsby, William A. Battaglin, Gregory B. Lawrence, 

Richard S. Artz, Brent T. Aulenbach, Richard P. Hooper, 

Dennis R. Keeney, and Gary J. Stensland 

May 1999 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Ocean Service 

Coastal Ocean Program 
GULF OF MEXICO HYPOXIA ASSESSMENT 

This report is the third in a series of six reports developed 

as the scientific basis for an integrated assessment of the 

causes and consequences of hypoxia in the Gulf of Mexico, as 

requested by the White House Office of science and Technology Policy and as required by Section 604a of P.L. 105-

383. For more information on the assessment and the assessment process, please contact the National Centers for 

Coastal Ocean Science at (30I)7 13-3060. 

DECISION ANALYSIS SERIES 

The Decision Analysis Series has been established by 

NOAA's Coastal Ocean Program (COP) to present documents for coastal decision makers which contain analytical 

treatments of major issues or topics. The issues, topics, and 

principal investigators have been selected through an extensive peer review process. To learn more about the COP or 

the Decision Analysis Series, please write: 

NOAA 

National Centers for Coastal Ocean Science 

Coastal Ocean Program 

13 15 East-West Highway, Room 9700 

Silver Spring, Maryland 209 I0 

phone: 30 1-713-3338 

fax: 30 1 -7 1 3-4044 

web: http://www.cop.noaa.gov 

e-mail: coastalocean@cop.noaa.gov 

Total nitrogen 

yields, in 

kgJkm2Jyr 

/ 90 to320 

Cover image: Spatial distribution of 

total nitrogen yields (lkm2lyr) in nine large basins during 1980-96. 

A gaging station 
Science for Solutions 

NOAA COASTAL OCEAN PROGRAM 

Decision Analysis Series No. 17 

Flux and Sources of Nutrients in the 

Mississippi-Atchafalaya River Basin 

Topic 3 Report for the Integrated Assessment 

on Hypoxia in the Gulf of Mexico 

Donald A. Goolsby, U.S. Geological Survey 

William A. Battaglin, U.S. Geological Survey 

Gregory B. Lawrence, U.S. Geological Survey 

Richard S. Artz, National Oceanic and Atmospheric Administration 

Brent T. Aulenbach, U.S. Geological Survey 

Richard P. Hooper, U.S. Geological Survey 

Dennis R. Keeney, Leopold Center for SustainableAgriculture 

Gary J. Stensland, Illinois State Water Survey 

May 1999 

U.S. DEPARTMENT OF COMMERCE 

William M. Daley, Secretary 

National Oceanic and Atmospheric Administration 

D.James Baker, Under Secretary for Oceans and Atmosphere 

National Ocean Service 

Nancy Foster, Assistant Administrator 

National Centers for Coastal Ocean Science 

Donald Scavia, Director 

Center for Sponsored Coastal Ocean Research-Coastal Ocean Program 

David Johnson, Director 
This publication should be cited as: 

Goolsby, Donald A., William A. Battaglin, Gregory B. Lawrence, Richard S. Artz, Brent T. 

Aulenbach, Richard P. Hooper, Dennis R. Keeney, and Gary J. Stensland. 1999. Flux and 

Sources of Nutrients in the Mississippi-Atchafalaya River Basin: Topic 3 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision 

Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD. 130 pp. 

Corresponding author information: 

Donald A. Goolsby 

U.S. Geological Survey 

P.O. Box 25046, MS 406 

Denver, Colorado 80225-0046 

Ths publication does not constitute an endorsement of any commercial product or 

intend to be an opinion beyond scientific or other results obtained by the National 

Oceanic and Atmospheric Administration (NOAA). No reference shall be made to 

NOAA, or to this publication furnished by NOAA, in any advertising or sales promotion that would indicate or imply that NOAA recommends or endorses any proprietary product mentioned herein, or that has as its purpose an interest to cause 

duectly or inlrectly the advertised product to be used or purchased because of hs 

publication. 
Contents 

LIST OF FIGURESAND TABLES................................................................................................................ vi 

CONVERSIONFACTORSAND ABBREVIATIONS................................................................................. X 

ACKNOWLEDGMENTS................................................................................................................................. X 

FOREWORD................................................................................................................................................... xi 

... EXECUTIVESUMMARY............................................................................................................................. xlll 

INTRODUCTION ..........................................................................................................I 

Purpose and Scope of This Report.....................................................................................................I 

The Mississippi-Atchafalaya River Basin ............................................................................................ I 

Hydrology................................................................................................................................................ 2 

Physical. Land Use. and Cultural Features ........................................................................................ 4 

METHODS...............................................................................................................................................8 

Sources and Description of Data ........................................................................................................9 

Selection of Basins for Flux Estimation............................................................................................ I0 

NUTRIENTCONCENTRATIONS................................................................................................... 16 

Current Nitrogen Concentrations................................................................................................. 16 

Historical Nitrogen Concentrations................................................................................................22 

Current Phosphorus Concentrations ..............................................................................................24 

Current and Historical Dissolved Silica Concentrations............................................................27 

NUTRIENTFLUX AND SOURCES ..............................................................................29 

Flux Estimation Methods ....................................................................................................................29 

4.1 .I Model structure ......................................................................................................................30 

Flux of Nutrients to the Gulf of Mexico .........................................................................................I 

4.2.1 Climate effects on nutrient flux...........................................................................................36 

Sources of Nitrogen ............................................................................................................................37 

Sources of Phosphorus .......................................................................................................................45 

Sources of Silica ...................................................................................................................................48 

State-Level Nitrogen Flux Estimates ................................................................................................54 

iii 
iv Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

NUTRIENTINPUTSAND OUTPUTS ..........................................................................55 

Atmospheric Inputs to the Mississippi-Atchafalaya River Basin ................................................ 55 

5.1 . I Methods ................................................................................................................................... 55 

5.1.2 Available data ........................................................................................................................... 56 

5.1.3 Estimation methods................................................................................................................ 56 

5.1.4 Results-Wet deposition ...................................................................................................... 58 

5.1.5 Relations between wet and dry deposition ...................................................................... I 

5.1.6 Discussion ................................................................................................................................. 63 

5.1.7 Budget implications................................................................................................................. 65 

Agricultural Inputs and Outputs ....................................................................................................... 66 

5.2.1 Fertilizer67 

5.2.2 Legume fixation ....................................................................................................................... 68 

5.2.3 Animal manure ........................................................................................................................ 72 

5.2.4 Soil mineralization................................................................................................................... 74 

5.2.5 Nutrient removal in crops .................................................................................................... 76 

5.2.6 Volatilization losses ................................................................................................................ I 

5.2.7 Denitrification losses .............................................................................................................. 83 

5.2.8 Immobilization ......................................................................................................................... 83 

5.2.9 Erosion...................................................................................................................................... 85 

Municipal and Industrial Inputs .......................................................................................................... 85 

5.3.1 Methods ................................................................................................................................... 86 

5.3.2 Results ................................................................................................................................... 86 

Atmospheric Inputs Directly to the Gulf of Mexico .................................................................... 87 

5.4.1 Dry deposition......................................................................................................................... 87 

5.4.1. I Sumatra. Florida. CASTNet site .........................................................................87 

5.4.1.2 Tampa Bay National Estuary Program data ..................................................... 87 

5.4.1.3 Long Key Program ................................................................................................. 88 

5.4.1.4 IMPROVE data........................................................................................................ 88 

5.4.1.5 Parungo air chemistry studies over the Gulf of Mexico ...............................88 

5.4.2 Wet deposition ....................................................................................................................... 88 

5.4.2. I National Atmospheric Deposition Program .................................................... 88 

5.4.2.2 Florida atmospheric deposition study ...............................................................89 

5.4.2.3 Gulf of Mexico cruise data ................................................................................... 90 

5.4.2.4 Xalapa. Veracruz. Mexico. precipitation chemistry estimates..................... 90 

5.4.3 Wet plus dry deposition ....................................................................................................... 90 

5.4.4 Model estimates ...................................................................................................................... I 

5.4.5 Comparisons with Chesapeake Bay and the open Atlantic Ocean.............................91 

5.4.6 Meteorological considerations ............................................................................................. I 

LINKING NUTRIENT FLUX TO NUTRIENT INPUTS AND HUMANACTIVITIES .........93 

Graphic Approach ....................................................................................................................................... 93 

Nitrogen Mass Balance ............................................................................................................................. 108 

Regression Models .................................................................................................................................... 108 

6.3. 1 Nitrogen yield models ................................................................................................................ 108 

6.3.2 Total phosphorus yield model ..................................................................................................113 
Contents v 

7. RESEARCH NEEDS..................................................................................................1 15 

7.1 Nitrogen Dynamics in Soils ............................................................................................................ 115 

7.2 Small Watersheds.............................................................................................................................. 115 

7.3 In-stream Processes................................................................................... ...................................... 116 

7.4 SPARROW Model............................................................................................................................. 116 

7.5 Nitrogen Mass Balance..................................................................................................................... 116 

7.6 Stable Isotopes ................................................................................................................................ 116 

7.7 Atmospheric Deposition in the MARB......................................................................................... 117 

7.8 Atmospheric Deposition in the Gulf of Mexico.........................................................................117 

8. CONCLUSIONSAND RECOMMENDATIONS............................................................1 18 

8.1 Conclusions ................................................................................................................................ 118 

8.1.I Flux and sources .................................................................................................................. 118 

8.1.1 Relative importance of human activities in contributing to nutrient flux ...............118 

8.1.I Climatic effects on nutrient flux ....................................................................................... 119 

8.2 Recommendations............................................................................................................................. 120 

8.2.1 Nutrient monitoring program........................................................................................... 120 

8.2.2 Effluent monitoring program............................................................................................. 120 

8.2.3 Monitoring of atmospheric deposition ........................................................................... 120 

8.2.4 Interdisciplinary forum........................................................................................................ 120 

8.2.5 Long-term research............................................................................................................. 120 
List of Figures andTables 

FIGURES 

FIGURE1.l. Location map showing drainage and land use in the MississippiAtchafalaya River Basin ......................................................................................................... 3 

FIGURE 1.2. Daily mean streamflow for the Mississippi River at Vicksburg, Mississippi ................. 4 

FIGURE 1.3. Harvested cropland in the Mississippi River Basin as a percentage of 

area of hydrologic accounting units ..................................................................................... 5 

FIGURE 1.4. Drained agricultural land in the Mississippi River Basin as a percentage 

of the area of hydrologic accounting units ......................................................................... 6 

FIGURE1.5. Density of population with hydrologic accounting units in the 

Mississippi River Basin .......................................................................................................... 7 

FIGURE 2.1. Location of nine large basins used for nutrient flux and yield estimates ..................... 11 

FIGURE 2.2. Location of 42 interior basins used for nutrient flux and yield estimates .................... 13 

FIGURE 3.1. Boxplots showing the Istribution of (A) nitrate plus nitrite, (B) total organic 

nitrogen, and (C) ammonia nitrogen concentrations in the 42 interior basins............19 

FIGURE3.2. Relationship between mean nitrate concentrations and percent of the ..... . . Mlsslsslpp~fiver Basm m row crops ................................................................................. 20 

FIGURE 3.3. Daily streamflow and daily nitrate concentrations in the Raccoon fiver 

at Des Moines, Iowa, during 1983-89 ............................................................................... 21 

FIGURE 3.4. Long-term patterns in nitrate concentrations in (A) the Mississippi fiver at 

St. Francisville, Louisiana, and (B) the Ohio Rwer at Grand Chain, Ihois...............25 

FIGURE3.5. Boxplots showing the Qstribution of (A) orthophosphate and (B) total 

phosphorus concentrations in 42 interior basins ............................................................. 26 

FIGURE 3.6. (A) Boxplots showing the Istribution of s&ca concentrations in the 42 

interior basins, and (B) long-term pattern in sllica concentrations in the 

Mississippi River at St. Francisvdle, Louisiana, 1955-96 ................................................ 28 

FIGURE 4.1. Hydrograph of the daily flux of nitrate in the Mississippi fiver at St. . . . . Franc~sv~Ue, Loumana, 1980-98.. ...................................................................................... .34 

FIGURE 4.2. Annual flux of nitrate and organic nitrogen and mean annual streamflow 

from the Mississippi fiver Basin to the Gulf of Mexico ................................................34 
List of Figures and Tables vii 

FIGURE 4.3. 

FIGURE 4.4. 

FIGURE 4.5. 

FIGURE 4.6. 

FIGURE 4.7. 

FIGURE 4.8. 

FIGURE 4.9. 

FIGURE 4.11. 

FIGURE 5.1. 

FIGURE 5.3. 

FIGURE 5.4. 

Annual flux of total phosphorus and mean annual streamflow from the 

Mississippi fiver Basin to the Gulf of Mexico .......................................................................35 

Annual flux of sdica and mean annual streamflow from the Mississippi 

River Basin to the Gulf of Mexico ..........................................................................................36 

Spatial distribution of the average nutrient and chloride yields in nine large 

basins during 1980-96: (A) nitrate-nitrogen, (B) total nitrogen, (C) total phosphorus, (D)orthophosphate phosphorus, (E) sihca as Si, and (F) chloride ......................41 

Temporal patterns in nitrate yields in the Mtddle Mississippi, Ohio, 

Lower Mtssouri, and entire Mississippi River Basin, 1970-96 .............................................42 

(A) Spatial distribution of nitrate yields in the 42 interior basins, and (B) 

boxplots showing distribution of nitrate yields in the 42 interior basins ...........................43 

(A) Spatial distribution of total nitrogen yields in the 42 interior basins, 

and (B) boxplots showing distribution of total nitrogen yields in the . . 42 intenor basins ......................................................................................................................... 44 

(A) Spatial distribution of total phosphorus yields in the 42 interior 

basins, and (B) boxplots showing distribution of total phosphorus yields 

in the 42 interior basins .............................................................................................................. 49 

(A) Spatial distribution of orthophosphate yields in the 42 interior 

basins, and (B) boxplots showing chstribution of orthophosphate yields . . in the 42 intenor basins .............................................................................................................. 50 

(A) Spatial distribution of sihca (Si) yields in the 42 interior basins, and 

(B) boxplots showing distribution of Si yields in the 42 interior basins .............................53 

Wet deposition of NO,-, averaged for 1990-96 data from the National 

Atmospheric Deposition Program (NADP, http://nadp.sws.uiuc.edu/) in 

each the 133 accounting units that make up the Mtssissippi River Basin ..........................57 

Wet deposition of NH4+, averaged for 1990-96 data from the National 

Atmospheric Deposition Program (NADP, http://nadp.sws.uiuc.edu/) in 

the 133 accounting units that make up the Mississippi fiver Basin ...................................58 

Wet deposition of NO3- and NH4+ in 42 sub-basins of the Mississippi 

River Basin, averaged for 1990-96 data from the National Atmospheric 

Deposition Program (NADP, http://nadp.sws.uiuc.edu/) ................................................. 59 

Dry deposition data from CASTNet (Clean Air Status and Trends 

Network) as a function of wet deposition data from the National 

Atmospheric Deposition Program (NADP, htt~://nad~.sws.uiuc.edu/) 

at 14 sites in the Mississippi River Basin where dry and wet deposition 

measurement stations are co-located ..................................................................................... 61 

Distribution of the ratio of dry deposition data from CASTNet (Clean 

Air Status and Trends Network) to wet deposition data from the National 

Atmospheric Deposition Program (NADP, htt~://nad~.sws.uiuc.edu/)at 

14 sites in the Mtssissippi Rwer Basin where dry and wet deposition 

measurement stations are co-located ....................................................................................... 62 
viii Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Chemical species of wet deposition data from the National Atmospheric 

Deposition Program (NADP, htm://nad~.sws.uiuc.edu/)and dry deposition 

data from CASTNet (Clean Air Status and Trends Network) measured at 

4 sites on a west-to-east transect across the Mississippi River Basin for 

December 1992 through November 1993........................................................................ 63 

Annual nitrogen inputs to the Mississippi-Atchafalaya Rwer Basin, 1951-96 ...........67 

Nitrogen inputs in hydrologic accounting units in 1992 from (A) ferthzer, 

(B) legumes, (C) livestock manure, (D) soil mineralization, (E) industrial . . point sources, and (F) munlclpal point sources ............................................................... 69 

Annual nitrogen outputs from the Mississippi-Atchafalaya kver 

Basin, 1951-96 ...................................................................................................................... 77 

Nitrogen outputs in hydrologic units in 1992 from (A) harvested crops, 

(B) ferthzer volathzation, (C) manure volatilization, (D) plant senescence, 

(E) soil denitrification, and (F) soil imrnobiltzation ........................................................ 78 

Scatter plots showing relationshps between total nitrogen yield and 

nitrogen inputs from (A) ferthzer, (B) mineralized soil, (C) legumes, 

(D) manure, (E) atmospheric deposition of nitrate (wet plus dry), and . . (F) mumclpal and industrial point sources ................................................................. 94-95 

(A) Nitrogen yields (river outputs from basins) and point-source nitrogen 

inputs in the 42 interior basins and the entire Mississippi-Atchafalaya River 

Basin (MARB). (B) Inputs of newly fixed nitrogen to the 42 interior basins 

and the MhRB from ferthzer, N fixation by legumes, and atmospheric 

deposition (wet and dry) of nitrate. (C) Inputs of recycled nitrogen derived 

from manure and potential mineralization of soil organic nitrogen and plant 

residue, and atmospheric wet deposition of ammonium................................................ 96 

Bar graphs showing (A) yields of orthophosphate, suspended and total 

phosphorus and (B) phosphorus inputs from point sources in 42 interior 

basins and the entire Mississippi-Atchafalaya River Basin ............................................ 99 

Graphs showing (A) the annual nitrogen inputs and outputs for the 

Mississippi-Atchafalaya River Basin from all major sources, and annual 

nitrogen residual (inputs minus outputs) from nitrogen mass balance for 

1951-96. (B) Cumulative nitrogen residual for 1955-96 .............................................. 102 

Graphs showing the relationshp between the annual nitrogen residual from 

mass balance and annual flux of nitrate to the Gulf of Mexico for 1951-96 ............104 

Graphs showing the relationshp between the annual nitrogen residual 

from mass balance and the average annual streamflow from the 

Mississippi-Atchafalaya bver Basin to the Gulf of Mexico ........................................ 105 

Bar graphs showing the average nitrogen mass balance for the 

Mississippi-Atchafalaya River Basin for 1980-96 ......................................................... 107 
List of Figures and Tables ix 

TABLES 

TABLE2.I. 

TABLE 3.4. 

Analytical methods used to determine nutrient concentrations in 

water samples used in hs assessment................................................................................. 9 

Sites used to estimate nutrient flux from nine large basins and from 

the entire Mississippi-Atchafalaya River Basin................................................................ 12 

Sites used to estimate nutrient flux from 42 interior basins during 1980-96 ..............14 

Mean concentrations of nitrogen, phosphorus, sllica, and chloride at large 

river sites in the Mississippi-Atchafalaya River Basin during 1980-96 ........................ 17 

Mean concentrations of nitrogen and phosphorus compounds, sllica, and 

chloride in the 42 interior basins during 1980-96 ........................................................... 18 

Historical and current (1980-96) nitrate concentrations in the small ... streams in the Mississippi hver Basin............................................................................... 23 

Historical nitrate concentrations for sites on the Arkansas, Mississippi, 

and Missouri hvers .............................................................................................................. 23 

Mean annual flux of nutrients from the Mississippi-Atchafalaya River 

Basin to the Gulf of Mexico, 1980-96 ............................................................................... 32 

Average annual flux and yields of nitrogen from large watersheds 

in the Mississippi-Atchafalaya River Basin during 1980-96, estimated 

from regression models ....................................................................................................... 38 

Average annual flux and yields of nitrate and total nitrogen from the 42 

interior basins during 1980-96, estimated with regression models...............................39 

Average annual flux and yields of phosphorus from large watersheds in 

the Mississippi-Atchafalaya River Basin, during 1980-96, estimated from 

regression models ................................................................................................................. 46 

Average annual flux and yields of orthophosphate and total phosphorus from 

the 42 interior basins during 1980-96, estimated with regression models ...................47 

Average annual flux and yields of sllica (Si) and chloride from large 

watersheds in the Mississippi-Atchafalaya River Basin during 1980-96, 

estimated from regression models ..................................................................................... 51 

Average flux and yields of siltca and chloride from the 42 interior basins 

during 1980-96, estimated with regression models.......................................................... 52 

Approximate percentage of total nitrogen flux to the Gulf of Mexico 

contributed by selected states in the Mississippi-Atchafalaya River Basin.................. 54 

Wet, dry, and total nitrogen deposition values for the overall Mississippi 

River Basin and 42 interior basins averaged for 1990-96 .............................................. 60 

Estimated annual inputs of nitrogen as N in the Mississippi-Atchafalaya 

River Basin, 9 large sub-basins, and 42 interior basins ...................................................70 
x Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE5.3. Estimated annual inputs of phosphorus as P in the Mississippi-Atchafalaya 

River basin, 9 large sub-basins, and 42 interior basins .................................................... 71 

TABLE5.4. Estimated rates of nitrogen fixation by legumes in kg N/km2/yr ................................ 72 

Estimates of the nitrogen and phosphorus voided in animal manure in 

kdograms per day.................................................................................................................. 73 

Estimated rates of nitrogen and phosphorus removal in harvested crops in 

kg N or P per common yield unit ...................................................................................... 76 

Estimated annual outputs of nitrogen as N in the Mississippi-Atchafalaya 

fiver Basin, 9 large sub-basins, and 42 interior basins ................................................... 79 

Estimated annual outputs of phosphorus as P in the MississippiAtchafalaya fiver Basin, 9 large sub-basins, and 42 interior basins.............................. 80 

Estimated rates of nitrogen (ammonia) vola&zation from fertilizer, 

as a percent ............................................................................................................................ 81 

Estimated rates of nitrogen (ammonia) volaidzation from senescing 

plant leaves in kg N per square kdometer ......................................................................... 83 

Estimated average soil organic content and denitrification rate for states 

in the Mississippi-Atchafalaya fiver Basin ...................................................................... 84 

Estimated point-source phosphorus and nitrogen hscharges for the 

Mississippi-Atchafalaya Rwer Basin .................................................................................. 86 

U.S. Navy Clunatic Atlas of the World (1995) grid point data for latitude 

28.0 N, longitude 92.0 W .................................................................................................... 92 

Nitrogen mass-balance data for the Mississippi-Atchafalaya River Basin 

for 1980-96 (except as noted for atmospheric deposition and point sources) .........106 

Regression model results for total nitrogen and nitrate-N yields from the 

42 interior basins................................................................................................................. 109 

Estimated contributions of nitrogen input sources to the total nitrogen 

yield of the Mississippi-Atchafalaya River Basin and total nitrogen flux 

to the Gulf of Mexico ........................................................................................................ 111 

Estimated contributions of nitrogen input sources to the nitrate-nitrogen 

yield of the Mississippi-Atchafalaya River Basin and nitrate-nitrogen flux 

to the Gulf of Mexico ........................................................................................................ 112 

Regression model results for total phosphorus yields from 42 interior basins .........113 

Estimated contributions of phosphorus input sources to the total 

phosphorus yield of the Mississippi-Atchafalaya River Basin and total 

phosphorus flux to the Gulf of Mexico .......................................................................... 114 
Conversion Factors and Abbreviations 

MULTIPLY TO OBTAIN 

meter (m) foot 

kilometer (km) mile 

square kilometer (km? square mde 

cubic kdometer (km3) cubic mde 

centimeter (cm inch 

liter (L) quart 

milliliter (mL) fluid ounce 

kdogram (kg) pound, avoirdupois 

metric ton (t) pound, avoirdupois 

cubic meter per second (m"/s) cubic feet per second 

kilogram per square kilometer (kg/km2) pounds per acre 

CONCENTRATION UNIT APPROXIMATELY EQUALS 

dgram per liter (mg/L) part per don 

The following equations were used to compute flux of chemicals: 

concentration (mg/L) x flow (mvs) x 8.64 x lo-' = metric tons per day 

concentration (mg/L) x flow (m3/s) x 8.64 x 10-5= kdogram per day 
Acknowlednrnents 

Because of this report's comprehensive nature and broad scope, its preparation required the collaboration and assistance of many people. Scientists who contributed sigmficantly to this effort but are 

not listed as authors include: Robert Hoeft, University of Illinois; Michael Burkart, U.S. Department 

of Agriculture/Agricultural Research Service (USDA/ARS), Ames, Iowa; Gyles Randall, University 

of Minnesota, Southern Experiment Station; and Keith A. Kelling, University of Wisconsin. 

We also wish to acknowledge the efforts of U.S. Environmental Protection Agency (USEPA) scientists and managers in EPA Regions 111, IV, V, VI, VII, and VIII, and the National Health and Environmental Effects Research Laboratory in Gulf Breeze, Florida, who assisted in the compilation of 

point-source discharge data. Ths effort was coordinated by Larinda Norton at the Gulf of Mexico 

Program Office at Stennis Space Center, Mississippi, and Mary Belefsh at EPA headquarters in 

Washmgton, D.C. 

Other scientists who assisted with data compilation and management include: Yvonne H. Baevsky, 

Laurie L. Boyer, Gail Theh, Larry Puckett, and Kathy Fitzgerald, U.S. Geological Survey (USGS); 

Gary Lear, USEPA; David James, USDA/ARS; and John Mdler, U.S. Army Corps of Engineers. 

Manuscript preparation was completed with sipficant assistance from Laurie L. Boyer, USGS. We 

thank John Flager of the USGS for hs review of the report and for hls advice and many helpful 

suggestions for preparation of the manuscript; &chard Alexander and Greg Clark of the USGS for 

their technical review of the manuscript; and eight anonymous reviewers selected by the Committee 

on Environment and Natural Resources (CENR) Editorial Board and George Hallberg of the Cadmus Group for their technical review of the manuscript. 

We wish to acknowledge Gary Stensland of the Illinois State Water Survey in Champaign, Illinois, 

for his assistance in preparing the section of this report on atmospheric deposition of nitrogen compounds. For his contribution he is included as a co-author of this report, but we wish to note that 

his involvement was hted to section 5.1, Atmospheric Inputs to the Mississippi-Atchafalaya River 

Basin. 

Funding to support the preparation of this report was provided by the following USGS Programs: 

National Stream Quality Accounting Network, Toxic Substances Hydrology, and National Trends 

Network. 

xii 
Foreword 

Nutrient overenrichment from anthropogenic sources is one of the major stresses on coastal ecosystems. Generally, excess nutrients increase algal production and the availabhty of organic carbon 

with an ecosystem-a process known as eutrophication. Scientific investigations in the northern 

Gulf of Mexico have documented a large area of the Louisiana continental shelf with seasonally depleted oxygen levels (< 2 mg/l). Most aquatic species cannot survive at such low oxygen levels. The 

oxygen depletion, referred to as hypoxia, forms in the middle of the most important commercial and 

recreational fisheries in the conterminous United States and could threaten the economy of this region of the Gulf. 

As part of a process of considering options for respondmg to hypoxia, the U.S. Environmental Protection Agency (EPA) formed the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 

during the fall of 1997, and asked the Whte House Office of Science and Technology Policy to 

conduct a scientific assessment of the causes and consequences of Gulf hypoxia through its Committee on Environment and Natural Resources (CENR). A Hypoxia Workmg Group was assembled 

from federal agency representatives, and the group developed a plan to conduct the scientific assessment. 

The National Oceanic and Atmospheric Ahstration (NOAA) has led the CENR assessment, 

although oversight is spread among several federal agencies. The objectives are to provide scientific 

information that can be used to evaluate management strateges, and to identify gaps in our understanding of this complex problem. WMe the assessment focuses on hypoxia in the Gulf of Mexico, 

it also addresses the effects of changes in nutrient concentrations and loads and nutrient ratios on 

water quality condttions within the Mississippi-Atchafalaya River system. 

As a foundation for the assessment, six interrelated reports were developed by six teams with experts from within and outside of government. Each of the reports underwent extensive peer review 

by independent experts. To facilitate tls comprehensive review, an editorial board was selected 

based on nominations from the task force and other organizations. Board members were Dr. Donald Boesch, University of Maryland; Dr. Jerry Hatfield, U.S. Department of Agriculture; Dr. George 

Hallberg, Cadrnus Group; Dr. Fred Bryan, Louisiana State University; Dr. Sandra Batie, Michigan 

State University; and Dr. Rodney Foil, Mississippi State University. The six reports are entitled: 

Topic 1: Characterization of Hypoxia. Describes the seasonal, interannual, and long-term 

variations of hypoxia in the northern Gulf of Mexico and its relationship to nutrient loadings. 

Lead: Nany N.Rabalais, Louisiana UniversitiesMarine Consortittm. 

Topic 2: Ecological and Economic Consequences of Hypoxia. Evaluates the ecological 

and economic consequences of nutrient loading, including impacts on the regional economy. Coleads: Robert J. Dia3 Virginia Institttte ofMarine Science, andAndrew Solow, WoodsHole OceanographicInstitution, CenterforMarine Poky. 

xiii 
xiv Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Topic 3: Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin. 

Identifies the sources of nutrients within the Mississippi-Atchafalaya system and Gulf of Mexico. Lead Donald A. Goolsb, US. GeologicalSz/my. 

Topic 4: Effects of Reducing Nutrient Loads to Surface Waters Within the Mississippi 

River Basin and Gulf of Mexico. Estimates the effects of nutrient-source reductions on water 

quality. Co-leads:Patrick L Brexonik, Universi~ofMinnesota, and VictorJ. Bierman,Jr., hno-Tech, Inc. 

Topic 5: Reducing Nutrient Loads, Especially Nitrate-Nitrogen, to Surface Water, 

Ground Water, and the Gulf of Mexico. Identifies and evaluates methods for reducing nutrient loads. Lead: WiIliamJ. Mitsch, Ohio State Univers9. 

Topic 6: Evaluation of the Economic Costs and Benefits of Methods for Reducing Nutrient Loads to the Gulf of Mexico. Evaluates the social and economic costs and benefits of 

the methods identified in Topic 5 for reducing nutrient loads. Lead: Otto C. Doerin& Purdue UniversiQ. 

These six individual reports provide a foundation for the final integrated assessment, which the task 

force wd use to evaluate alternative solutions and management strategies called for in Public Law 

105-383. 

As a contribution to the Decision Analysis Series, this report provides a critical synthesis of the best 

available scientific information regarding the ecological and economic consequences of hypoxia in 

the Gulf of Mexico. As with all of its products, the Coastal Ocean Program is very interested in ascertaining the uulity of the Decision Analysis Series, particularly with regard to its application to the 

management decision process. Therefore, we encourage you to write, fax, call, or e-mail us with your 

comments. Our address and telephone and fax numbers are on the inside front cover of this report. 

David Johnson, Director 

Coastal Ocean Program 

Donald Scavia, Chief Scientist 

National Ocean Service 
Executive Summary 

Ths report addresses the following two questions: 

What are the loads (flux) of nutrients transported from the Mississippi-Atchafalaya River 

Basin to the Gulf of Mexico, and where do they come from within the basin? 

What is the relative importance of specific human activities, such as agriculture, point-source 

discharges, and atmospheric deposition in contributing to these loads? 

These questions were addressed by first estimating the flux of nutrients from the MississippiAtchafalaya fiver Basin and about 50 interior basins in the Mississippi fiver system using measured 

historical streamflow and water quality data. Annual nutrient inputs and outputs to each basin were 

estimated using data from the National Agricultural Statistics Service, National Atmospheric Deposition Program, and point-source data provided by the USEPA. Next, a nitrogen mass balance was 

developed using agricultural statistics, estimates of nutrient cychng in agricultural systems, and a 

geographic information system. Finally, multiple regression models were developed to estimate the 

relative contributions of the major input sources to the flux of nitrogen and phosphorus to the Gulf 

of Mexico. 

MAJORFINDINGS 

The major findmgs from this assessment are summarized below. 

Flux and Sources of Nutrients 

The current (1980-96) mean annual flux of total nitrogen from the Mississippi-Atchafalaya 

Rver Basin to the Gulf of Mexico is about 1.6 don metric tons, and the average total nitrogen yield for the entire basin is 489 kg/km2/yr. The nitrogen is about 61% nitrate, 37% dssolved and particulate organic N, and 2% ammonium. 

Nitrate concentrations in the Mississippi fiver and some of its tributaries in the upper Midwest 

have increased two- to five-fold in the last century. 

Nitrate flux from the Mississippi Basin to the Gulf of Mexico has averaged nearly 1 don metric tons per year since 1980 and is about three times larger than it was 30 years ago. Most of the 

increase in nitrate flux to the Gulf occurred between 1970 and 1983. 

Streamflow in the Mississippi River also increased about 30% during 1970-83 as a result of increased precipitation. The increase in nitrate flux to the Gulf is attributed to both an increase in 

nitrate concentration and an increase in streamflow. 
xvi Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Since about 1980, the annual nitrogen flux has become hghly variable due, in part, to variable 

amounts of precipitation and increased annual nitrogen inputs to the basin. Episokc events, 

such as the 1993 flood, can nearly double the annual nitrate flux to the Gulf as a result of increased leachmg of nitrate from the basin's soil/ground-water system. High annual nitrate fluxes 

associated with flood events can be expected to occur in the future. 

The 1980-96 average annual flux of phosphorus to the Gulf was about 136,000 metric tons. On 

average about 69% of the phosphorus is in particulate and/or organic material, and 31% is 

transported as &ssolved orthophosphate. There has been no statistically significant increase or 

decrease in the annual flux of phosphorus since records began in the early 1970s. 

The average annual flux of dissolved silica to the Gulf for 1980-96 was 2.1 don metric tons 

(as Si). Dissolved silica concentrations in the Lower Mississippi kver decreased from 4-5 mg/L 

in the 1950s to about 3 mg/L in the mid-1970s. However, there has been no statistically significant long-term decrease in the flux of dissolved silica to the Gulf. Ths apparent contradiction 

results, in part, from an increase in streamflow, whch could dilute silica concentrations without 

altering the flux. Removal of Qssolved siltca by increased &atom production in the Mississippi 

River as a result of increased nitrogen concentrations is another possible reason for the decrease. 

The principal source areas for the nitrogen that discharges to the Gulf are watersheds draining 

intensively cultivated regions in southern Minnesota, Iowa, Illinois, Indiana, and Oho. These regions contribute several times more nitrogen per unit area than other areas. 

Streams draining Iowa and Illinois contribute as much as 35% of the total nitrogen flux of the 

Mississippi fiver during years of average rainfall, and much more during years of high rainfall. 

However, these two states comprise only about 9% of the area of the Mississippi-Atchafalaya 

Basin. During the flood year of 1993, Iowa, with only 4.5% of the basin area, contributed about 

35% of the nitrate discharged to the Gulf of Mexico. Because these amounts assume no instream losses of nitrogen in the Mississippi fiver, they represent maximum contributions. Some 

in-stream nitrogen losses probably occur, but they are believed to be relatively small in large rivers, such as the Mississippi. 

The soils, unsaturated zones, and ground-water systems underlying cropland in the basin serve 

as storage reservoirs that can accumulate and store nitrogen. Accumulation of nitrate can be significant during years with low crop yields and dry climatic periods when leachmg by precipitation is minimal. During periods of hlgh precipitation, large amounts of the accumulated nitrate 

can be leached from these reservoirs into agricultural drains and streams, and eventually discharged to the Gulf of Mexico. 

Drainage of agricultural land by tile drains and other means contributes to the high nitrate concentrations and flux in the Mississippi River. Tile drains short-circuit the flow of ground water 

by draining the top of the ground-water system into tile lines and Itches, and eventually to the 

Mississippi River. Tile drainage water can have very high nitrate concentrations. 

Relative Importance of Human Activities in 

Contributing to Nutrient Flux 

* Nonpoint sources contribute about 90% of the nitrogen and phosphorus discharpg to the Gulf 

of Mexico. Agricultural activities are the largest contributors of both nitrogen and phosphorus. 

Multiple-regression models showed fertilizer plus mineralized soil organic nitrogen to be the 

largest nitrogen source, contributing about 50% of the annual total nitrogen flux to the Gulf. 
Executive Summary xvii 

However, nitrogen inputs from fertlllzer and mineralized soil are so strongly correlated that the 

relative importance of each of these sources could not be determined. Nitrogen sources, such as 

atmospheric deposition, ground-water dscharge, and soil erosion, which are associated with basin runoff, are estimated to contribute 24% of the total nitrogen flux to the Gulf. Animal manure is estimated to contribute about 15% of the nitrogen flux, and municipal and industrial 

point sources contribute about 11'10. Legumes do not appear to be significant contributors of nitrogen to the Gulf. 

About 31% of the phosphorus flux to the Gulf is estimated to come from fertlllzer, 18% is from 

manure, and 10% is from municipal and industrial point sources. About 41% of the annual 

phosphorus flux comes from sources that are not quantified but are associated with basin runoff. The most important of these is believed to be phosphorus in sediment associated with soil 

erosion. 

Of all of the major agricultural nitrogen inputs to cropland, only fer&zer and legume inputs 

have increased sipficantly since the 1950s. Fertilizer nitrogen input has increased seven-fold, 

and fixation of nitrogen by legumes has increased by about 50%. The nitrogen input to the basin 

from animal manure has actually decreased slightly over the last 40 years, although the spatial 

pattern of the manure input has changed from a highly dispersed to a hghly concentrated distribution. The amount of nitrogen removed from the basin in harvested crops has more than doubled since the 1950s,parallehg the increase in fer&zer use. 

In contrast to results reported for Chesapeake Bay and elsewhere in the eastern United States, 

atmospheric deposition appears to be a significant but relatively small contributor to the total nitrogen flux to the Gulf of Mexico. Atmospheric inputs (wet and dry) of nitrate are very significant in watersheds in much of the upper Oho fiver Basin, and atmospheric deposition of 

ammonia, presumably from manure, is high in Iowa and parts of Minnesota and Illinois. However, these inputs are small relative to other nitrogen inputs to most of the MississippiAtchafalaya fiver Basin. 

Atmospheric deposition of nitrogen on a 30,000-km2area (twice the size of the hypoxic zone) of 

the Gulf of Mexico is estimated to be less than 1% of the nitrogen input to the Gulf from the 

Mississippi-Atchafalaya River Basin. 

In the future, the flux of nitrate to the Gulf wdl most hkely respond quickly and dramatically to 

variations in precipitation and runoff. Because of the readily avadable pool of nitrate in the 

soil/ground-water system and the extensive drainage network, nitrate fluxes wdl be high in wet 

years and low in dry years. However, the nitrogen balance of the soil/ground-water system will 

adjust relatively slowly to increases or decreases in nitrogen inputs and outputs. As a result, the 

flux of nitrate to the Gulf will most likely change slowly in response to changes in nitrogen inputs. The response time of the basin to changes in inputs and outputs is unknown, but may be 

several years, or longer. This implies that it could take several years, or longer, for the effects of 

significant reductions (20%) in nitrogen inputs to produce a noticeable reduction in nitrogen 

flux to the Gulf of Mexico. In the short term, precipitation and runoff will control nitrate flux. 
CHAPTERI 

Introduction 

The CENR Topic 3 team was asked to determine the flux and sources of nutrients transported from 

the Mississippi Rtver Basin to the Gulf of Mexico. Nutrients of concern for this assessment are nitrogen, phosphorus, and silica. The first part of this task was to identify where the nutrients come 

from in the basin and which parts of the basin contribute the most sipficant flux of nutrients to 

the surface-water system. The second part was to estimate the relative importance of specific human 

activities, such as agriculture, point-source Ischarges, and atmospheric emissions, in contributing 

nutrient flux to the Mississippi fiver and Gulf of Mexico. This report presents the results of the assessment. It is based entirely on an analysis of existing information. 

The Mississippi River Basin is the largest river basin in North America and the third largest river basin in the world (van der Leeden et al. 1990). Only the Amazon River Basin in South America and 

the Congo Rtver Basin in Africa are larger. About 70 don people live within the Mississippi Rtver 

Basin, whose drainage area includes all or parts of 30 states. 

The basin is one of the most productive farming regions in the world, producing the majority of the 

corn, soybeans, wheat, cattle, and hogs grown in the United States. Because of hs intensive agnculture, the majority of all fertilizers and pesticides used in the United States are applied to cropland 

wih the basin. About 58% of the basin is cropland (Figure 1.1). Other significant land uses and 

their percentages of the basin include woodland (18%), range and barren land (21°/0) ,wetlands and 

water (2.4O/), and urban land (0.6Yo). Runoff from these diverse land uses discharges into streams 

and reservoirs, carrying with it suspended sediment, naturally occurring chemicals weathered from 

the soil, and such contaminants as nutrients and pesticides from urban and agricultural activities in 

the basin. The water, along with much of its dissolved and suspended contents, eventually flows into 

the Mississippi Rtver and ultimately discharges into the Gulf of Mexico. Naturally occurring chernicals and man-made contaminants also leach to ground-water systems. This water eventually is discharged to streams and rivers that flow into the Mississippi River. The ground water and its 

associated chemical load can take from a few days to decades, or longer, to reach a point of &scharge on a river gXrinter et al. 1998). 

The Gulf of Mexico has one of the most productive fisheries in the world. The combined economic 

value of the farm industry of the Mississippi Rwer Basin and the fishing industry of the Gulf is estimated to be more than $100 blllion (Malakoff 1998). The land-use and cultural changes that have 

occurred in the basin this century have had measurable and sometimes deleterious effects on the 
2 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

quality of water in the Mississippi &very its tributaries, and the Gulf of Mexico. In particular, increases in the flux (loads) of nutrients transported from the basin in recent decades are believed to 

contribute to eutrophication, algal blooms, and hypoxia in the Gulf of Mexico (Rabalais et al. 1996). 

Further, it is hkely that changes in land use and increases in nutrient flux will continue as the population of the basin grows and as crop production increases to meet the growing national and global 

demands for food. 

The Mssissippi-Atchafalaya River Basin (MARB) drains an area of nearly 3,208,700 square kdometers 

(kmz), or about 41% of the conterminous United States (Figure 1.1). It extends from the Appalachan 

Mountains in western Pennsylvania and New York to the Rocky Mountains in western Montana and 

from southern Canada to the Gulf of Mexico. The mainstem of the Mississippi River originates in northern Mmnesota and flows southward more than 3,700 km to the Gulf. At St. Louis, Missouri, its flow 

nearly doubles from about 3,530 m3/s to 6,790 m3/s, due to inflow from the Missouri and Illinois 

Rivers. About 320 km downstream from St. Louis the Mississippi's flow more than doubles again to 

about 15,340 m3/s due to inflow from the Ohio fiver. From its confluence with the Ohio River to 

Vicksburg, Mississippi-a distance of about 860 river h-the average flow of the river increases only 

about 1496, to 17,400 m3/s, with most of this increase coming from the Arkansas and White Rivers. 

About 225 km downstream from Vicksburg nearly 25% of the Mrssissippi's flow, on average, is diverted 

to the west through the Old River outflow channel. The diverted flow combines with the Red and 

Ouachita Rivers to form the Xtchafalaya River, whch is comprised predominately of Mississippi River 

water. The Xtchafalaya then flows southward about 200 km to the Gulf. 

The combined long-term average annual discharge of the Mississippi and Atchafalaya Rivers to the Gulf 

of Mexico is about 19,920 m3/s based on streamflow records (1950-96) for the Mississippi at Tarbert's 

Landing, Mississippi (near St. Francisville), and the Atchafalaya fiver at Sirnrnsport, Louisiana (streamflow data provided by the U.S. Army Corps of Engineers). Streamflow varies considerably over long periods of time. For example, the mean annual flow of the Mssissippi increased about 30% between 1955 

and 1996 (see section 4.2.1). 

Figure 1.2 is a hydrograph of daily streamflow for the Mississippi fiver at Vicksburg, Mssissippi, for the 

period 1980-97. The seasonal pattern of streamflow shown in this figure is typical of other large unregulated rivers in the Mississippi River Basin (MRB). Streamflow is lowest in the fall, with the lowest flows 

occurring in September and October. Streamflow typically begins to increase in mid-winter and usually 

reaches a peak in April or May (Baldwin and Lall 1999).The majority of the annual transport of water, 

sediment, nutrients, and other chemicals from the Mississippi and its tributaries occurs between December and June of each year. 

The hydrology of the Mississippi River system has been greatly altered by locks, dams, and reservoirs 

since the early 1900s (Meade 1995). The Mississippi has a series of 29 lock-and-dam structures between 

St. Louis, Missouri, and St. Paul, Mnnesota, whch have been constructed to maintain water sufficiently 

deep for navigation by boats and barges. Sdarly, the Ohio River currently (1999) has 20 lock-and-dam 

structures for navigation between its mouth at Cairo, Illinois, and Pittsburgh, Pennsylvania. Also, two 

large tributaries to the Ohio River-the Tennessee and Cumberland River-have large reservoirs just 

above their confluence with the Ohio. The Missouri River has a series of large reservoirs in Montana and 

North and South Dakota, most of whch were constructed and filled in the 1950s and early 1960s. The 

storage capacity of these reservoirs is equal to several years of discharge of the Missouri River and has a 

sipficant effect 
Chapter I: Introduction 3 
4 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

on the transport of water, sediment, and nutrients. In addition to storing water, the reservoirs on the 

Missouri River and the pools formed behind the lock-and-dam structures on the Mississippi and 

Ohio Rivers also trap organic and inorganic material, including sediment (Meade 1995), nutrients, 

and organic carbon, altering the flow of these materials to the Gulf of Mexico. For example, since 

construction of the Missouri Rwer reservoirs, sediment discharge from the MRB to the Gulf has 

decreased by more than 50% (Meade and Parker 1985). 

FIGURE 1.2. Daily mean streamflow for the Mississippi River at Vicksburg, Mississippi. (Data from U.S.Army Corps of Engineers, Vicksburg, MS). 

1.4 PHYSICAL, LANDUSE, AND CULTURAL FEATURES 

The climate, land use, soils, and population vary widely across the MRB. The annual runoff ranges 

from less than 5 centimeters (cm) per year in the arid western part of the basin to more than 60 cm 

per year in the humid eastern part. The central part of the basin is used primarily for cropland (Figure 1.3) and produces most of the corn, soybeans, wheat, and sorghum grown in the United States. 

In some parts of the MRB-particularly in Iowa, Illinois, and Indiana-more than 50% of all land in 

some hydrologic accounting units is used for growing crops (indicated by the red areas in Flgure 

1.3). Most of the fertilizers and pesticides used in the United States are applied to cropland in this 

part of the basin. Large numbers of livestock (cattle and hogs) and poultry also are produced in the 

central part of the basin. 
Chapter I: Introduction 5 

FIGURE 1.3. Harvested cropland in the Mississippi River Basin as a percentageof area of 

hydrologic accounting units. 

During 1870-1920 and 1945-60, the central part of the MRB was subjected to extensive agricultural 

drainage to lower the water table to make farming more economical and efficient (Zucker and 

Brown 1998; Pavelis et al. 1987).This practice essentially drains the top of the ground water system 

into tile lines and ditches that flow into streams, rivers, and eventually the Mississippi River. More 

than 50 million acres of mostly cropland in the MRB have been drained by tile lines, ditches, and 

other means. Figure 1.4 shows the percentage of land in each hydrologic accounting unit in the 

MARB that has been drained. 

Woodland is more common in the eastern, noah central, and south central parts of the MRB (Figure 1.I). Rangeland and barren land are common in the western part of the basin, while wetlands are 

most common in the extreme northern and extreme southern pans. Most of the MRB's 70 million 

people live in the eastern half, particularly in the Ohio Basin, as illustrated by the red areas in Figure 

1.1 and the red and pink areas in Figure 1.5. 
6 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

FIGURE 1.4. Drained agricultural land in the Mississippi River Basin as a percentageof the 

area of hydrologic accounting units. 
Chapter I: Introduction 7 

FIGURE 1.5. Density of population within hydrologic accounting units in the Mississippi 

River Basin. 
CHAPTER2 

Methods 

This section provides an overview of the methods used to assess the flux and sources of nutrients in 

the MARB. More detailed descriptions of the methods are presented in later sections of the report. 

The first step in the assessment was to develop annual and long-term average estimates of nutrient 

fluxes (mass transport per unit time) from the MARB to the Gulf of Mexico and to determine where 

the most significant nutrient fluxes were coming from wih the MARB. The five nutrient compounds considered most critical to the hypoxia issue were nitrate, total nitrogen, total phosphorus, 

orthophosphate, and silica. Flux estimates were developed for each of these compounds using available data on nutrient concentrations and stream discharge. Estimates of chloride fluxes were also 

developed. Chloride, a nonreactive solute, was used to develop ratios and test mass balances. Multiple-regression analysis (described in detail later in this report) was used to develop statistical models 

to estimate nutrient fluxes from the entire MARB and about 50 sub-basins that had streamflow data 

and sufficient historical data on nutrient concentrations. Predictor variables used in regression models were daily streamflow, time, and mathematical terms to handle seasonal variations in nutrient 

flux. Daily fluxes estimated from the models were summed over time to provide seasonal, annual, 

and long-term average fluxes from the selected basins and to the Gulf of Mexico. Nutrient yields 

(mass per unit area per unit time) were calculated by dividing the estimated annual fluxes by basin 

areas. This normalized the nutrient fluxes and provided a means to compare nutrient contributions 

among basins of all sizes. 

The second step in this assessment was to estimate the annual inputs of nitrogen and phosphorus to 

the MRB .from all major known sources. The inputs considered included agriculture (feralizer, manure, legumes, and soil mineralization), atmospheric deposition of nitrate (wet and dry) and ammonium, and municipal and ,industrialpoint sources. Inputs of newly formed nitrogen from feralizer, 

legumes, and atmospheric.deposition of nitrate (Jordan and Weller 1996; Howarth et al. 1996) were 

differentiated from nitrogen that was already in the system (recycled nitrogen). These recycled nitrogen inputs, which include manure, mineralization of soil organic matter and plant residue, and atmospheric deposition of'ammonium, can be the immediate source of some of the nitrate that 

leaches into streams and ground water, just as can the newly formed nitrogen. A geographic information system (ARC/INFO) was used to develop estimates of annual nutrient inputs and outputs 

for selected basins and to display the spatial distribution of these inputs and outputs. Finally, a nitrogen mass balance was developed for the basin to estimate the total inputs of nitrogen (new and recycled) and total outputs of nitrogen from the MARB on an annual basis. The mass balance also 

provided a means to estimate the amount of nitrogen that might be available for leaching to streams 

and ground water. 
Chapter 2: Methods 9 

2.1 SOURCESAND DESCRIPTIONOF DATA 

The nutrient concentration data used in hs analysis to develop flux estimates were obtained from 

the U.S. Geological Survey (USGS) National Water Information System (NWIS) database. These 

data were collected as part of various USGS programs in the basin during 1974-97. The principal 

source of data on nutrient concentrations used in this assessment is the USGS National Stream 

Quality Accounting Network (NASQAN) (Ficke and Hawkinson 1975;Alexander et al. 1996). This 

program conducted extensive sampling for nutrients in the 1970s and early 1980s. The sampling was 

continued at a reduced scale and frequency through the early 1990s, at which time the program was 

redesigned to focus on large rivers, such at the Mississippi (Hooper et al. 1997). Data on nitrate, 

ammonia, total organic nitrogen, ortho and total phosphorus, and silica were collected routinely 

since the start of the NASQAN program. Additional nutrient data were obtained from the USGS 

National Water Information System (NWIS). These data were collected as part of various USGS 

programs beginning in the early 1900s. Data from reports published in the early 1900s were also 

used. Table 2.1 describes the analytical methods used to obtain these data. Nitrate was analyzed by 

the phenoldisulfonic acid method (Rainwater and Thatcher 1960) before the early 1970s, and by 

cadmium reduction afterward. The two methods are reported to have comparable accuracy (Friedman and Fishman 1989). The possible implications of this change in methods for analyzing longterm trends are discussed in section 3.2. 

TABLE 2.1. Analytical methods used to determine nutrient concentrations in water samples used in this assessment. 

Nutrient Period Analytical Method References 

Nitrate Pre-1970 Colorimetric phenoldisulfonic acid Rainwater and Thatcher 1960 

1970-96 Colorimetric cadmium reduction Fishman and Friedman 1989 

Organic nitrogen 1975-96 Colorimetric kjeldahl Fishman and Friedman 1989 

Orthophosphate 1975-96 Colorimetric phosphomolybdate Fishman and Friedman 1989 

Total phosphorus 1975-96 Digestion, colorimetric Fishman and Friedman 1989 

phosphomolybdate 

Silica Pre- 1980 Colorimetric molybdate Fishman and Friedman 1989 

1980-96 Colorimetric molybdate and Fishman and Friedman 1989 

inductivelycoupled plasma 

A large body of additional data on nutrient concentrations is available from numerous state, local, 

and federal agencies in the MRB. However, these data were not used because of the short time 

frame for this analysis and the effort that would have been required to obtain the data and ensure 

that sample collection and analytical methods were comparable with USGS methods. 

Daily streamflow data used in this report were obtained from several sources. Streamflow data for 

the Lower Mississippi, Atchafalaya, and Red Rivers were obtained from the U.S. Army Corps of 

Engmeers. The Tennessee Valley Authority provided streamflow data for the Tennessee River. The 

remaining streamflow data were obtained from the USGS/NWIS. 
10 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Data on nitrogen and phosphorus inputs and outputs in agriculture were obtained from numerous 

sources. Data on crop production, livestock, and poultry were obtained from the U.S. Department 

of Commerce's (USDC's) Census of Agriculture and the U.S. Department of AgncultureYs(USDAYs) 

National Agricultural Statistics Service (NASS). These data were used to develop estimates of nitrogen and phosphorus inputs from legume crops and animal manure, and nitrogen and phosphorus 

outputs in harvested crops. Data on nitrogen and phosphorus inputs from fertilizer were obtained 

from NASS, the Tennessee Valley Authority, the Fertilizer Institute, and published reports. Nitrogen 

inputs and outputs from soil mineralization and immobdization, denitrification, and vola&zation 

were obtained through the assistance of soil scientists in the USDA and the academic sector, and 

from the literature. Methods used to obtain agricultural inputs and outputs are described in detail 

later in this report. 

Recent (1996) data on nitrogen and phosphorus from municipal and industrial point sources for 

more than 11,000 facilities in the MARB were developed by the U.S. Environmental Protection 

Agency (USEPA). The methods used to develop these estimates are described in detail later in this 

report. Historical data on point-source discharges were obtained from published reports. 

Nitrogen inputs to the MARB from atmospheric wet deposition were estimated from data on nitrate 

and ammonium in rainfall obtained from the National Atmospheric Deposition Program (NADP). 

Nitrogen in dry deposition was estimated with statistical models using data from the CASTNet 

(Clean Air Status and Trends Network), AIRMoN (Atmospheric Integrated Research Monitoring 

Network), and NADP programs. Atmospheric deposition of nitrogen on the Gulf of Mexico was 

estimated based on a literature review and very limted deposition data. 

Estimates of nutrient flux in the MARB were made at three scales: the entire MississippiAtchafalaya River Basin, 9 large basins that in aggregate comprise the MARB, and 42 smaller basins, 

referred to as interior basins. The largest scale, the entire MARB, shown in Figures 1.1 and 2.1, provided long-term (42 years) estimates of nutrient flux to the Gulf of Mexico from the entire basin. 

Historical data available at two sampling stations listed in Table 2.2-the Mississippi River at St. 

Francisvdle, Louisiana, and the Atchafalaya River at Simmsport, Louisiana-provided estimates of 

nutrient flux at this scale. The St. Francisville site, located about 150 miles upstream from New Orleans, Louisiana, provided estimates of nutrient flux to the Gulf via the Mississippi Rwer channel 

and the nutrient flux diverted into the Atchafalaya River. Data on nitrate and silica concentrations 

have been collected at this site numerous times each year since 1955. The Atchafalaya site provided 

estimates of nutrient flux to the Gulf from the Red and Ouachita Rivers and the Mississippi River 

diversion via the Old River Outflow Channel. 
Chapter 2: Methods I I 

USGS gaging station XX) KILOMETERS 

LLIL I i -1 

FIGURE 2.1. Location of nine large basins used for nutrient flux and yield estimates. NOTE: 

See Table 2.2 for descriptions. 

The nine large basins were selected to provide estimates of nutrient flux at a large-basin scale and to 

develop solute balances for nutrient flux in the MARB. These basins are shown in Figure 2.1 and are 

described in Table 2.2. They cover nearly the entire MARB, except for a small area in southern Louisiana, and provide data on the relative contributions of nutrient flux to the Gulf of Mexico from 

nine large areas of the MARB. Because of the cumulative nature of streamflow in these basins, it was 

necessary to calculate the flux from some of the basins as the difference in flux measured at upstream and downstream sampling stations. The sum of the nutrient fluxes measured for these 9 basins is nearly equal to the total flux from the MARB to the Gulf of Mexico. Only a small area (< 2% 

of the MARB) in southern Louisiana below St. Francisville on the Mississippi River and below Melville on the Atchafalaya River is not included. 

The 42 interior basins were selected to provide detailed information on nutrient fluxes and nutrient 

yields in various parts of the MARB and to help identify areas having abnormally high inputs. These 

42 basins are shown in Figure 2.2. Table 2.3 presents a listing of the basins and their drainage areas, 

number of years of nutrient data, average discharges, average runoff, population density, and percent 

cropland. 
I2 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE 2.2. Sites used to estimate nutrient flux from nine large basins and from the 

entire Mississippi-Atchafalaya River Basin. NOTE: Locations are shown in Figure 2.1. 

Large 

Basin 

Basin Name Area 

(km2) 

Average 

Discharge 

Sampling Stations Used for Flux 

Calculations and USGS Station 

Map 

ID 

1 980-96 ID Number 

(in m3/s) 

Outflow to Gulf of Mexico from Entire Mississippi-Atchafalaya River Basin 

1-8 Mississippi River 15,390 Mississippi River at St. Francisville, LA, streamflow from Tarbert's Landing, MS 

Atchafalaya River 6,600 Atchafalaya River at Melville, LA, and streamflow from Simmsport, LA 

1-9 Entire MississippiAtchafalaya Basin 

2 1,990 Mississippi River at St. Francisville, LA, plus 

Atchafalaya River at Melville, LA 

Large Basins 

I Upper Ohio Ohio River at Cannelton Dam, KY (03303280) 

2 Lower Ohio Ohio River at Grand Chain, IL (036 12500) and 

Ohio River at Cannelton Dam, KY 

I + 2 Entire Ohio Basin Ohio River at Grand Chain, IL (036 12500) 

3 Upper Missouri Missouri River at Omaha, NE (06610000) 

4 Lower Missouri Missouri River at Hermann, MO (06934500) and 

Missouri River at Omaha, NE 

3 + 4 Entire Missouri Ba- Missouri River at Hermann, MO (06934500) 

sin 

5 Upper Mississippi Mississippi River at Clinton, IA (05420500) 

None Mississippi River Mississippi River below Grafton. IL (05587455) 

above Missouri 

River Basin 

6 Middle Mississippi Mississippi River at Thebes, IL (07022000) 

Mississippi River at Clinton, IA; Missouri River at 

Hermann, MO 

7 Arkansas Arkansas River at Little Rock, AR (07263620) 

8 Lower Mississippi Mississippi River at St. Francisville, LA (073734201 

and streamflow from Tarbert's Landing, LA; Old 

River outflow at Knox Landing (Mississippi diversion); Arkansas R. at Little Rock, AR; Ohio River 

at Grand Chain, IL; Mississippi River at Thebes, IL 

9 Red and Ouachita 242,700 Atchafalaya River at Melville. LA (07381495), and 

streamflow from Atchafalaya at Simmsport, LA; 

Old River outflow at Knox Landing, LA 
Chapter 2: Methods 13 

500 KILOMETERS 'r 

Small basin sites \ / O , I I I 

1 -ALLEGHENY R, NEW KENSINGTON, PA 23 -ILLINOIS R, VALLEY CITY, IL 

2 -MONONGAHELA R, BRADDOCK, PA 24 -KASKASKIA R, VENEDY STATION, IL 

3 -MUSKINGUM R, MCCONNELSVILLE. OH 25 -MILK R, NASHUA, MT 

4 -KANAWHA R, WINFIELD, WV 26 -MISSOURI R, CULBERTSON, MT 

5 -SCIOTO R, HIGBY, OH 27 -BIGHORN R, BIGHORN, MT 

6 -G MIAMI R, NEW BALTIMORE, OH 28 -YELLOWSTONE R, SIDNEY, MT 

7 -KENTUCKY R, LOCK 2, LOCKPORT. KY 29 -CHEYENNE R, CHERRY CREEK, SD 

8 -WABASH R, NEW HARMONY, IN 30 -JAMES R, SCOTLAND, SD 

9 -CUMBERLAND R, GRAND RIVERS, KY 31 -PLATTE R, LOUISVILLE, NE 

10 -TENNESSEE R, HIGHWAY 60, PADUCAH, K 32 -KANSAS R, DESOTO, KS 

I1 -MISSISSIPPI R, ROYALTON, MN 

12 -MINNESOTA R, JORDAN, MN 

13-ST CRO1X R, ST CROIX FALLS, Wl 

14 -CHIPPEWA R, DURAND. WI 

15 -WISCONSIN R, MUSCODA, WI 

16 -ROCK R, JOSLIN, 1L 

17 -CEDAR R, CEDAR FALLS, 1A 

18 -IOWA R, WAPELLO, IA 

19-SKUNK R, AUGUSTA, IA 

20 -RACCOON R, VAN METER, 1A 

21 -DES MOINES R, KEOSAUQUA, IA 

22 -ILLINOIS R, MARSEILLES, IL 

FIGURE 2.2. Location of 42 interior 

NOTE: See Table 2.3 for descriptions. 

33 -GRAND R, SUMNER, MO 

34 -OSAGE R, ST THOMAS, MO 

35 -ST FRANCIS BAY, RIVERFRONT, AR 

36 -WHITE R, CLARENDON, AR 

37 -ARKANSAS R, TULSA, OK 

38 -CANADIAN R, CALVIN, OK 

39 -YAZOO R, REDWOOD. MS 

40 -BIG BLACK R, BOVINA, MS 

41 -RED R, ALEXANDRIA, LA 

42 -OUACHITA R, COLUMBIA, LA 

USGS GAGING STATION 

basins used for nutrient flux and yield estimates. 
14 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE 2.3. Sites used to estimate nutrient flux from 42 interior basins during 1980-96. 

NOTE: Basin locations are shown in Figure 2.2. 

Basin 

ID 

No. 

Name and Location of Sampling 

Station Used for Flux Estimation 

Area 

(km2) 

Years of 

Nutrient 

Data 

Average 

Discharge 

(m31s) 

Annual 

Runoff 

(cmlyr) 

People 

per 

km2 

Percent 

Cropland 

Allegheny River at New Kensington, PA 

Monongahela River at Braddock, PA 

Muskingham River at McConnelsville, OH 

Kanawha River at Winfield, WV 

Scioto River at Higby, OH 

Great Miami at New Baltimore, OH 

Kentucky River at Lockport, KY 

Wabash River at New Harmony, IN 

Cumberland River near Grand Rivers, KY 

Tennessee River near Paducah, KY 

Mississippi River near Royalton, MN 

Minnesota River at Jordan, MN 

St. Croix River at St. Croix Falls, WI 

Chippewa River at Durand, WI 

Wisconsin River at Muscoda, WI 

Rock River near Joslin, IL 

Cedar River at Cedar Falls, IA 

(daily streamflow measured at Waterloo, IA) 

Iowa River at Wapello, IA 

Skunk River at Augusta, IA 

Raccoon River at Van MeterIDes Moines, IA 

Des Moines at St. Francisville, MO 

(daily streamflow measured at Keosaqua, IA) 

Illinois River at Marseilles, IL 

Lower Illinois River Basin 

Illinois River at Valley City, IL (basins 22 & 23) 

Kaskaskia River near Venedy Station, IL 

Milk River near Nashua, MT 

Missouri River near Culbertson, MT 

Bighorn River near Bighorn, MT 

Yellowstone River near Sydney, MT 

Cheyenne River at Cherry Creek, SD 

JamesRiver near Scotland, SD 

Platte River near Louisville, NE 

Kansas River at Desoto, KS 

Grand River near-Sumner, MO 

Osage River below St Thomas, MO 

St. Francis Bay at Riverfront, AR 

White River at Clarendon, AR 

Arkansas River at Tulsa, OK 

Canadian River at Calvin, OK 

Yazoo River at Redwood, MS 

Big Black River near Bovina, MS 

Red River at Alexandria, LA 

Ouachita River near Columbia, LA 

*Sueamflow was measuredonly when water samples were collected. 
Cha~ter2: Methods 15 

There were insufficient historical data on nutrient concentrations to estimate nutrient flux at this 

scale for the entire MARB. However, these 42 basins cover the full range of land uses and population density in the MARB and provide a good spatial representation of the entire basin. Also, the 

aggregate drainage areas of these 42 basins account for about 70% of the drainage area of the 

MARB. Data were available at many of these sites to estimate nutrient flux for the 17-year period 

from 1980 to 1996. The actual number of years for which nutrient data were available and flux estimates were made for each of these basins is shown in Table 2.3. Nutrient yields from these 42 basins were used in conjunction with data on nutrient inputs to the basins from point and nonpoint 

sources to examine the relative importance of specific human activities in contributing nutrients to 

streams in the MARB. 
CHAPTER3 

Nutrient Concentrations 

Data on the concentrations of nutrients are needed to estimate nutrient flux to the Gulf of Mexico. 

However, nutrient concentrations also are of importance in addressing the overall water quality and 

health of streams in the MARB. High concentrations of nitrogen and phosphorus in streams can 

cause eutrophication and can present problems for public drinking-water supplies. These nutrients 

are derived from both natural sources and sources associated with human activities, such as waste 

disposal and agriculture. Human activities can lead to significant increases in nitrogen and phosphorus concentrations in streams, which, in turn, can lead to increased nutrient flux in streams and to 

the Gulf. This section briefly summarizes data on current and historical nutrient concentrations in 

the basin and discusses their temporal and spatial patterns. A more detailed discussion of the water 

quality in the MARB is presented in a companion CENR hypoxia assessment report (Brezonik et al. 

1999). 

3.1 CURRENT NITROGEN CONCENTRATIONS 

The mean concentrations of nitrogen, phosphorus, silica, and chloride in water discharging from the 

large basins selected for this study (Figure 2.1) appear in Table 3.1. Mean concentrations for the 42 

interior basins (Figure 2.2) are summarized in Table 3.2. Nitrogen is the nutrient believed to be most 

responsible for producing the increased growths of algae in the Gulf that lead to seasonal oxygen 

depletion and hypoxia (Rabalais et al. 1996). 

The two principal forms in which nitrogen occurs in streams of the MARB are nitrate (N03-) and 

organic nitrogen (dissolved and particulate). Significant amounts of ammonia (mostly NH4+) also 

may occur in some stream reaches, particularly downstream from sources of municipal and animal 

wastes. However, ammonia is quickly transformed to nitrate, and concentrations are generally much 

less than 0.1 mg/L in the lower reaches of the Mississippi River (Antweiler et al. 1995). Trace 

amounts of nitrite (N02-) nitrogen also occur briefly during the oxidation of ammonia to nitrate, 

which is the end product of the aerobic biochemical oxidation of organic and ammonia nitrogen in 

soil and water. Nitrate is the most soluble and mobile form of nitrogen and is easily leached from 

soils by precipitation into ground water, tile drains, and streams. 

The distribution of nitrate plus nitrite-nitrogen, hereafter referred to as nitrate, at the sites representing the 42 interior basins is shown in boxplot A of Fgure 3.1. As the figure shows, there are two 

distinct groups of basins: 12 have median nitrate concentrations ranging from about 2.5 mg/L to 

more than 6 mg/L, while the remaining 30 have medians of less than 1.5 mg/L. Boxplot A of Fgure 3.1 also shows that the maximum nitrate concentration at several of these sites occasionally exceeds the drinking-water standard of 10mg/L. 
TABLE3.1. Mean concentrations (mglL) of nitrogen, phosphorus, silica (Si), and chloride at large river sites in 

- - -

Atchafalaya River Basin during 1980-96. 

Ohio R. at Cannelton Dam, KY 65 1.240 0.089 

Ohio R. at Grand Chain. IL 181 1.020 0.053 

Mississippi R, at Clinton, IA 157 1.720 0.079 

Mississippi R at Grafton, IL I I 1 2.780 0.123 

Missouri R. at Omaha, NE 76 1.020 0.101 

Missouri R. at Hermann, MO 227 1.230 0.052 

Mississippi R. at Thebes, IL 225 2.410 0.096 

Arkansas R. at Little Rock, AR 129 0.280 0.077 

Mississippi R. at St. Francisville, LA 1 96 1.4 10 0.050 

Atchafalaya R at Melville, LA 192 1 .040 0.052 

Chapter 3: Nutrient Concentrations 19 

95th percentile 

75th percentile 

median -------------------

25th percentile 

5th percentile 

0 3 6 9 12 15 18 TI 24 27 30 33 36 39 42 

BASIN IDENTIFICATION NUMBER 

BASIN IDENTIFICATION NUMBER 

BASIN IDENTIFICATION NUMBER 

FIGURE 3.1. Boxplots showing the distribution of (A) nitrate plus nitrite, (B) total organic nitrogen, and (C) ammonia nitrogen concentrationsin the 42 interior basins. 
20 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Comparison of the nitrate concentrations with land use and population data in Table 2.3 shows that the 

high nitrate concentrations are associated with basins having either a high percentage of land in row 

crops (corn, soybeans, or sorghum) or a high population density (people per kmz), or both. The generalized relation between mean nitrate concentrations and percent cropland is shown in Figure 3.2. Basin 20 

(Raccoon River, IA) had the highest mean nitrate concentration (6.7 mg/L) (median: 6.4 mg/L) and the 

highest percent cropland (74%). Basins 5 and 6 in Ohio and basin 22 in Illinois have population densities 

ranging from 100 to more than 300 people per km2 and more than 45% of the basins in row crops (Table 2.3). The range of row cropland expressed as a percentage of the basin area for the 12 basins with 

highest median nitrate concentrations was 44-74%. The percentage of row crop land in the remaining 30 

basins, except basin 24, was < 0.1-35%. The mean nitrate concentration in basin 24 (Kaskaskia River, 

IL) was much lower (0.83 mg/L) (median: 0.59 mg/L) than would be expected based on the relation to 

percent cropland given in Figure 3.2. This can probably be attributed, at least in part, to two large reservoirs on the Kaskaskia River in which nitrate could be assimilated by algae and subsequently be stored in 

lake sediments as particulate organic N. Another possible nitrate removal mechanism is denitrificationin 

anoxic bed sediments of the reservoir, which would be promoted by the longer residence time of water 

in the reservoir (Howarth 1996). Basin 35 (St. Francis Bay, AR) also does not fit the relation in the figure, possibly because most of the row cropland in this basin is soybeans, which require very little nitrogen from external sources. 

Boxplots B and C of Figure 3.1 show the distribution of total organic nitrogen (dissolved and particulate) and ammonia nitrogen. Median values for total organic nitrogen (TON) range from < 0.5 mg/L to 

about 1.5 mg/L. The highest concentrations of TON are associated with high population density, intense cropland activity, and/or high suspended-sediment concentrations. Median concentrations of ammonia were < 0.2 mg/L in all 42 basins, except the upper Illinois River (basin 22). This basin is 

dominated by municipal wastes from the Chicago metropolitan area and had a median ammonia concentration of about 0.5 mg/L. Ammonia concentrations in basin 23 (lower Illinois River Basin), which receives the inflow from basin 22, are similar to those in other basins and provide evidence for the rapid 

conversion of ammonia to nitrate. 

0 10 20 30 40 50 60 70 80 90 100 

PERCENT OF BASIN IN ROW CROPS 

FIGURE 3.2. Relationship between mean nitrate concentrations and percent of the 

Mississippi River Basin in row crops. NOTE: Data are presented in tables 2.3 and 3.3. 
Chapter 3: Nutrient Concentrations 2 1 

Nitrate concentrations in basins where the supply of nitrate in soils is abundant can vary seasonally 

over a large range in response to climatic and hydrologic condtions. Concentrations tend to be 

highest in the late winter and spring when streamflow is highest, and lowest in the late summer and 

fall when streamflow is low. This is illustrated in Figure 3.3, which is a plot of daily nitrate concentrations and streamflow in the Raccoon River at Des Moines, Iowa, for the period 1983-89, based 

on nitrate data collected by the Des Moines Water Treatment Plant.. 

The direct relationship between nitrate concentrations and streamflow in the Midwest has been reported by other investigators (Keeney and DeLuca 1993; Lucey and Goolsby 1993; Fenelon 1998). 

It indicates that most of the nitrate in these streams is from nonpoint sources. If the nitrate were 

predominantly from point sources, concentrations would decrease as streamflow increases due to 

dilution. Instead, nitrate concentrations in streams increase in response to rainfall or snowmelt that 

leaches nitrate that has accumulated in the soil. There is scientific evidence that nitrate levels can 

build up in soils during dry years from mineralization processes and reduced uptake by crops, and 

can be flushed out in larger than normal amounts in succeeding wet years (Randall et al. 1995). Nitrate can enter the streams though agricultural drains, ground-water discharge, and direct runoff. Nitrate concentrations generally decrease in the summer and fall as streamflow and agricultural 

drainage decrease (Figure 3.3). Assimilation of nitrate by agricultural crops on the land and aquatic 

plants in streams also helps decrease the nitrate concentrations in streams during the summer. Instream denitrification rates would also increase during the summer due to increased temperatures 

and longer residence times of water in the streams. 

FIGURE 3.3. Daily streamflow and daily nitrate concentrations in the Raccoon River at 

Des Moines, Iowa, during 1983-89. (Niuate data from Des Moines Water Treatment Plant.) 
22 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Agricultural drainage plays a major role in transporting nitrate from cropland to streams in the 

MARB. More than 50 don acres, mostly cropland, have been drained in Illinois, Indiana, Iowa, 

Ohio, Minnesota, Missouri, and Wisconsin (Pavelis et al. 1987). Ths practice 0short circuits0 the 

flow of water by draining the top of the saturated zone into tile drains, ditches, and streams, and 

eventually the Mississippi River. Drainage practices can result in the leaching of large amounts of 

nitrate from the soil and unsaturated zones into the drains and ditches. Nitrate concentrations in 

agricultural drains can be very high-20-40 mg/L nitrogen or more (Fenelon 1998; Gentry et al. 

1998; Zucker and Brown 1998; David et al. 1997; Randall et al. 1997). 

3.2 HISTORICALNITROGEN CONCENTRATIONS 

Historical data on nitrogen concentrations in the MARB are available from numerous publications. 

Some of the earliest data on nitrogen concentrations were published in a report by the University of 

Ihois (Palmer, no date). Nitrogen data were also published by USGS in a report containing testimony from a lawsuit heard by the U.S. Supreme Court in 1905 on pollution of the Illinois and Mississippi Rivers by Chicago sewage (Leighton 1907). These reports also contain data on hundreds of 

analyses for nitrate, nitrite, ammonia, and organic nitrogen on samples from the Illinois River Basin 

and Mississippi River in the vicinity of St. Louis, Missouri, during 1897-1902. 

Historical nutrient data are also available from a USGS study in whch water samples were collected 

daily during 1906-07 from 62 major rivers in the eastern half of the United States. Sampling sites 

included several sites on the Mississippi kver from Minneapolis, Minnesota, to New Orleans, Louisiana, and sites on rivers in Arkansas, Illinois, Indiana, Iowa, Minnesota, Ohio, and Pennsylvania. 

The daily samples were composited at about 10-day intervals and analyzed for numerous solutes, 

including nitrate. Results of these analyses have been published in at least two USGS reports (Dole 

1909 and Clarke 1924). A search of the USGS NWIS database provided additional data on nitrate 

concentrations for several rivers in Iowa, including the Cedar, Raccoon, and Des Moines Rivers for 

1944-51, and for sites on the Ohio and Mississippi Rivers for 1954 to the present. 

While the historical nitrate concentrations probably do not represent natural background conditions, 

they do provide a baseline from which changes that have occurred in the past 90-100 years can be 

determined. Table 3.3 summarizes historical nitrate concentrations in a few interior basins in the 

MARB from the late 1890s to about 1965. Mean concentrations for samples collected from these 

same streams during 1980-96 near where the historical samples were obtained are shown for ,comparison. 

Table 3.4 contains similar data for sites on the Arkansas, Mississippi, and Missouri Rivers. These 

data clearly show that the concentration of nitrate in the Mississippi River and its tributaries has increased sigmficantly in the last 100 years. No attempt was made to determine streamflow conditions 

for the historical data. Nevertheless, the results shown in Tables 3.3 and 3.4 suggest that average nitrate concentrations in the small rivers and the Mississippi and Missouri Rivers have increase by factors of two to more than five. The only exception is the Arkansas River, in which nitrate 

concentrations appear to have decreased; this exception may be the result of recently constructed 

impoundments on the river that would create an environment more favorable to the growth of algae 

and conversion of nitrate to organic matter. Impoundments can also increase the rate of denitrification due to increased retention time and increased contact between water and benthic deposits 

(Howarth et al. 1996). 
Chapter 3: Nutrient Concentrations 23 

TABLE 3.3. Historical and current (1 980-96) nitrate concentrations in the small streams 

in the Mississippi River Basin. (From Leighton 1907, Dole 1909, and Clarke 1924.) 

Mean Concentrations 

Year(s) N mglL 

Allegheny River 35 

76 

Muskingum River 27 

84 

Wabash River 3 1 

44 

Minnesota River 30 

1 22 

Rock River 36 

152 

Cedar .River 37 

1 75 

83 

Raccoon River 55 

48 

Des Moines River 37 

28 

88 

Upper Illinois River Weekly 

36 

175 

Lower Illinois River Weekly 

36 

I87 

TABLE 3.4. Historical nitrate concentrations for sites on the Arkansas, Mississippi, and 

Missouri Rivers. 

Location Mean Concentrations 

Year($) N mglL 

Arkansas River at Little Rock, AR 190647 22 0.45 

1980-96 129 0.28 

Mississippi River at Minneapolis1Ninninger, MN 190647 35 0.32 

1980-96 67 2.40 

Wabash River at VincenneslNew Harmony, IN 190647 3 1 1.44 

1980-96 44 2.55 

Mississippi River at Moline, IUClinton, IA 190647 17 0.4 1 

1980-96 157 1.72 

Mississippi River at Grafton, IL 1899- 1900 123 0.40 

1980-96 131 2.63 

Mississippi River at New Orleans, LAISt Francisville, LA 1 900-0 1 9 0.14 

1905-06 52 0.56 

1955-65 308 0.65 

1980-96 1 82 1.45 

Missouri River at Fort Bellefontaine, MOlHermann, MO 1899-1900 63 0.54 

1980-96 227 1.23 
24 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

The longest uninterrupted data set on nitrate concentrations in the MARB is from the Mississippi 

River at St. Francisville, Louisiana. Samples have been collected at this site each year since late 1954. 

From 1954 to 1967 samples were collected daily and composited at 10- to 30-day intervals for analysis. Compositing was discontinued in late 1967, and all subsequent analyses were on discrete samples. A similar data set is available for the Ohio River at Grand Chain, Ihois, for 1954-97, but no 

samples were collected in several years during this period. The St. Francisvdle data set has been used 

extensively by scientists to estimate nitrate flm to the Gulf of Mexico and determine long-term 

changes in Mississippi River water quality (Turner and Rabalais 1991; Bratkovich et al. 1994; Rabalais et al. 1996; Goolsby et al. 1997). Figure 3.4 shows the long-term patterns in nitrate concentrations at these two sites. The average annual nitrate concentration at St. Francisville has more than 

doubled since 1954-60. The minimum and maximum annual concentrations have also more than 

doubled. In contrast, nitrate concentrations appear to have changed very little in the lower Ohio 

River over the last four decades. These long-term data indicate that the increase in nitrate concentrations at St. Francisville must be caused primarily by increased nitrate concentrations in water entering the Mississippi River from sources other than the Ohio River Basin. 

As noted in section 2.1 and Table 2.1, a significant change in the analytical method for nitrate occurred in the early 1970s. Two lines of evidence indicate this method change did not contribute to 

the upward trend in nitrate concentrations observed at St. Francisville (Figure 3.4A). First, the upward trend in nitrate concentrations occurred gradually from about 1970 to 1980. Most of the concentration change occurred after the switch to the cadmium reduction method. The concentration 

change would have occurred abruptly if it were caused by the change in analytical methods. Second, 

samples collected from the Ohio River at Grand Chain (Figure 3.4B), were analyzed for nitrate by 

the same methods used on the samples from St. Francisville. These samples do not show the trend 

in nitrate concentration shown in the St. Francisville data. 

3.3 CURRENT PHOSPHORUS CONCENTRATIONS 

No data on phosphorus (P) concentrations in the MARB were found prior to 1972. Data on ortho P 

and total P concentrations at large basin sites for 1980-96 are summarized in Table 3.1. Phosphorus 

concentrations in the 42 interior basins are summarized in Table 3.2 and Figure 3.5. Orthophosphate is the principal form of dissolved P and the only form of P that that can be utilized by algae, 

bacteria, and plants (Correll 1998). It typically constitutes one-third to one-tenth of the total P in 

small and large streams of the MARB. The remaining P is mostly in particulate form, which must be 

converted to orthophosphate by biogeochemical processes to become available to aquatic plants. 

The median concentration of ortho P in most of the 42 interior basins (Figure 3.5A) is less than 0.1 

mg/L. However, the median concentrations in those basins having a high density of people and/or 

cropland were 0.1-0.25 mg/L. These, in general, are the same basins that had high nitrate concentrations. The spatial distribution of total P concentrations (Figure 3.5B) is similar to that of ortho P, 

except basins with high concentrations of suspended sediment also tend to have high total P concentrations. There is no apparent long-term trend in either ortho P or total P concentrations or in 

the ratio of ortho to total P in the Mississippi River at St. Francisville since the period of record began in the early 1970s. 
Cha~ter3: Nutrient Concentrations 25 

I maximum 

mean 

minimum 

YEAR 

1970 1980 

YEAR 

FIGURE 3.4. Long-term patterns in nitrate concentrations in (A) the Mississippi 

River at St Francisville, Louisiana, and (B) the Ohio River at Grand Chain, Illinois. 
TOTAL PHOSPHORUS 

IN MILLIGRAMS PER LITER 

ORTHOPHOSPHATE AS 

IN MILLIGRAMS P
Chapter 3: Nutrient Concentrations 27 

3.4 CURRENTAND HISTORICALDISSOLVED 

SILICA CONCENTRATIONS 

Dissolved silica is present in natural waters primarily as slltcic acid H4Si04 or Si(OH)4 (Sturnrn and 

Morgan 1981). This report presents concentrations of dissolved silica as IS^. Silica concentrations are 

summarized in Table 3.1 for the large basins and in Table 3.2 and Figure 3.6A for the 42 interior 

basins. 

The highest silica concentrations are in basins 31 (Platte River, Nebraska), 30 (James River, South 

Dakota), and 12 (Minnesota River, Mmnesota). Basins 19 and 21 in Iowa and basin 35 in Arkansas 

also have above-average silica concentrations. The Minnesota River (12) and the two basins in Iowa 

have a high percentage of cropland, but the other basins that have high silica concentrations are not 

associated with cropland. It is more likely that the soils and hydrology of the basins, especially 

ground-water contributions, are more closely related to silica concentrations than are human activities. 

The long-term trend in silica concentrations in the Mississippi River at St. Francisville, Louisiana, is 

shown in figure 3.6B. Average silica concentrations in the mid- to late 1950s were 4 to 5 mg/L and 

were similar to those reported by Dole (1909) in 1905-06. Silica concentrations appear to have 

gradually declined from 4-5 mg/L in the 1950s to about 3 mg/L in the mid-1970s and have remained near that level through 1997. The reasons for this downward trend prior to the mid-1970s 

are not known for certain. Turner and Rabalais (1991) and Rabalais and others (1996) &st reported 

this trend and hypothesized that it may have been caused by increased diatom production in the basin as a result of increased phosphorus inputs to streams due to increasing fertiltzer use. The diatoms 

could remove dissolved silica from the riverine system and convert it to biogenic silica that could be 

deposited in river sediments or transported to the Gulf in particulate form. The decrease could also 

be due, in part, to dilution caused by increased streamflow (see section 4.2). 
28 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

BASIN IDENTIFICATION NUMBER 

maximun 1 mean 

I minimum 

YEAR 

FIGURE 3.6. (A) Boxplots showing the distribution of silica concentrations in the 42 

interior basins, and (B) long-term pattern in silica concentrations in the Mississippi 

River at St. Francisville, Louisiana, 1955-96. 
CHAPTER 4 

Nutrient Flux and Sources 

Previous investigators (Turner and Rabalais 1991; Smith et al. 1993; Alexander et al. 1996; Dunn 

1996; Howarth et al. 1996; Goolsby et al. 1997) have reported on the annual flux of nitrogen and/or 

phosphorus to the Gulf. Antweiler et al. (1995) estimated the flux of nitrate and determined its predominant source areas in the Mississippi Basin during 1991-92. Lurry and Dunn (1997) estimated 

the long-term (1974-94) average annual flux of total N and total P at about 40 gaupg stations 

within the MARB. Smith et al. (1997) estimated the total N and total P flux at 414 NASQAN stations in the United States and used a spatial referencing model (SPARROW) to estimate the N and 

P yields at more than 2,000 U.S. stream locations. These studies provided much information on nutrient flux in the MARB and were invaluable in developing the nutrient flux estimates presented in 

this report. 

Flux estimates are presented in this report for the entire MARB, 9 large basins, and 42 interior basins. The locations of these basins and the stations used to develop the flux estimates were previously described in section 2.2 and in Tables 2.2 and 2.3 and are shown in Figures 2.1 and 2.2. Flux 

estimates were made for nitrate, total nitrogen, orthophosphate, total phosphorus, silica, and chloride. The chloride flux estimates were used in validating nutrient balances and are not discussed in 

this report. 

The mass flux of a solute past a measurement station is defined as the product of the solute concentration (expressed in mass per volume) and the water discharge (volume per time), yielding the solute mass per unit time. Water discharge is measured frequently enough (every 15 minutes to one 

hour) that it is essentially a continuous measurement. The accuracy of discharge data varies from 

station to station but is usually within 10°/o of the reported value. The accuracy of discharge for each 

USGS gaupg station is published in USGS annual water data reports for each state. Accuracy information is not available for discharge data obtained from other sources, but the accuracy is assumed to be similar to that of USGS data. Solute concentrations, however, are measured much less 

frequently because of the high costs of sample collection and chemical analysis. The accuracy of solute concentrations are also quite high, usually within a few percent, based on quality control data. 
30 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Three basic approaches can be used to interpolate concentration between measurements: 

Averaging. The concentration of all samples collected during the period of interest can be averaged (generally a flow-weighted average is calculated). The average concentration is multiplied by the total water discharge during the period to estimate flux. The standard error of 

the average concentration can be used to estimate the precision of the flux estimate. This 

approach assumes that the samples were collected in a "representative" fashion. The reliability of the flux estimates using this approach depends on the number of observations during 

the period. 

Interpolation. Concentrations can be estimated by linearly interpolating between observations 

(or by using some other interpolation method, such as a cubic spline). No estimate of the 

standard error is possible using this approach, as concentrations are assumed to be smoothly 

varying between observations. Because the reliability of this estimate is also strongly dependent upon the frequency of the observations, this method is best used when sampling frequencies are hgh relative to the frequency of forcing functions that determine solute 

concentration. 

Maltiple regression. A multiple-regression model is developed to relate concentration (or flux) 

to more frequently measured variables, such as stream discharge. This approach has the advantage of not being so dependent on the sampling frequency because the model's parameters are estimated using all available data. Furthermore, if certain statistical assumptions are 

met, the standard error of the flux estimate can be readily calculated. This approach, however, requires substantially more effort and is subject to a variety of statistical considerations. 

Because the CENR hypoxia analysis required short-term (annual or seasonal) estimates of 

flux and the sampling frequency in our data set was generally monthly or less frequent, we 

decided that this was the best approach to flux estimation. 

4.1. I Model Structure 

Consistent with many past studies (e.g., Cohn et al. 1992), a seven-parameter model was fit of the 

form 

is the natural logarithm of the argument in brackets 

is the flux of-thesolute (C*Q) 

is the solute concentration 

is the daily average discharge 

is a centering term (a constant) to ensure that the linear and quadratic flow terms are independent 

is time, expressed in decimal years and 

is a centering term (a constant) to ensure that the linear and quadratic time terms are independent 

is the error term 

are the fitted parameters in the multiple regression model 
Chapter 4: Nutrient Flux and Sources 3 1 

This model captures the dependence of concentration on dscharge, season (the sine and cosine 

terms), and any long-term trend. Quadratic terms were included to account for curvature that remained after transformation. Model parameters were estimated using the SAS system (SAS Institute 

Inc., 1990a, 1990b). Standard diagnostics (e.g., plots of observed vs. predcted values, and various 

residual plots) were calculated and examined for all models. All terms were retained in the models 

even if the model parameters were not sipficant to simplify calculation of models across all sites 

and solutes. Inclusion of the insignificant terms does not change the flux estimates appreciably, and 

the estimation of the additional parameter caused a small proportional decrease in the degrees of 

freedom in the regression because of the large number of observations available. 

Slight modifications were made to the model at some of the large river sites. Discharges of upstream 

tributaries were substituted for at-site dscharge because these values better captured the variations 

in concentration. For example, the solute flux models for the Mississippi at St. Francisville, Louisiana, used discharges from the mouths of the Missouri and Ohio and the discharge of the Mississippi 

above its confluence with the Missouri because each of the rivers has different solute concentrations. When upstream discharges are used, they are lagged to account for the time required for the 

water to flow between the sites. Two stations (Cedar River at Cedar Falls, Iowa-basin 17, and Raccoon River at Van Meter-Des Moines, Iowa-basin 20) had gaps in concentration data in the middle 

of the study period. For these stations, the quadratic time term was removed to prevent a spurious 

model form to be fit. The coefficients of determination (R') for these models are quite hgh because 

flux,which includes the independent variableQ, is being estimated. Although this poses no problem 

for parameter estimation, model diagnostics are misleading because the same variable is both an independent and a dependent variable. However, the RQoes correctly reflect that most of the variability in the calculation of flux comes from discharge (which is measured) and not from 

concentration (which is estimated). 

Daily streamflow was not measured at sites 17 and 21 Fable 2.3) and streamflow from a nearby 

gauging station was used in the regression model to estimate nutrient flux. For six sites (basins 4,27, 

35,36, 39, and 42-see Table 2.3), continuous measurement of streamflow was not available. For 

these sites, flux was estimated using the discharges made when samples were collected and flowweighted average nutrient concentrations. Annual runoff was estimated from the average of the 

measured discharges. 

As part of the model evaluation, outliers and points of high leverage were identified from scatter 

plots. These unusual points, which accounted for less than 6% of the points at any site, were eliminated from the analysis. In addition to eliminating outliers, in many cases it was necessary to remove 

values less than the detection limit when these affected model fit. When the percentage of samples 

that had concentrations below the detection limit was greater than 20°/0, a flow-weighted average 

instead of the regression method was used to estimate flux. Errors in the flux estimates are determined by calculating the mean square error of the flux estimates on an annual basis for every 

site/solute combination, using the approach by Gilroy et al. (1990). Error estimates for long-term 

average fluxes (multiple years) were determined by averaging the annual mean square errors. 

4.2 FLUX OF NUTRIENTS TO THE GULFOF MEXICO 

The average annual flux of nutrients from the MARB to the Gulf of Mexico for 1980-96 is surnrnarized in Table 4.1. The table also shows the standard errors of the flux estimates. This 17-year period 

included the drought of 1988-89when fluxes were very low and the flood of 1993when fluxes were 

very hgh. Thus, the fluxes in the table are believed to be representative of current average condi-
32 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

tions. The average flux of all forms of nitrogen was 1,567,900 +/-58,470 metric tonslyr (metric tons 

per year). This is almost identical to the flux estimate of 1,597,000 metric tons/yr by Dunn (1996) 

for 1972-93 and only slightly less than the 1982-87 estimate of 1,824,000 metric t~ns/~r made by 

Turner and Rabalais (1991). The total N flux is about 61% nitrate and 2% ammonia, and the remaining 37% is dissolved and particulate organic nitrogen. Normalized to drainage area the yield of total 

N is 489 kg/krn2/yr for the entire MARB, and the yield of nitrate is 297 kg/km2/yr (Table 4.2) for 

1980-96. About three-quarters of the N flux from the MARB enters the Gulf via the Mississippi 

River channel, and the remainder discharges through the Atchafalaya River. However, nearly all of 

the nitrogen discharging from the Atchafalaya comes from the Mississippi by way of the Old River 

diversion. Only about 4% of the nitrogen flux to the Gulf is from the Red and Ouachita River Basins. 

TABLE 4.1. Mean annual flux of nutrients from the Mississippi-Atchafalaya River Basin to 

the Gulf of Mexico, 1980-96. 

Nutrient Mean Flux Percent of Total Standard Error of Estimate 

Metric Tons Metric Tons % of Mean 

Nitrogen (N), Total 

Nitrate 

Ammonium 

Dissolved organic N 

Particulateorganic N 

Phosphorus (P), Total 

Orthophosphate 

Particulate phosphorus 

Silica (Si), Dissolved 

The average annual flux of total phosphorus was 136,500 +/-9,130 t, of which 31% was dissolved 

orthophosphate and the remaining 69% was in particulate form. This can be compared with Dunn's 

(1996) estimate of 143,100 metric tons/yr and Turner's and Rabalais's (1991) estimate of 106,500 

metric t~ns/~r. The flux of dissolved silica as Si averaged 2,316,800 +/-289,700 memc tons/yr. 

When normalized for the basin area, the yields are 42, 13, and 722 kg/krn2/yr for total P, ortho P, 

and silica (Si), respectively (Tables 4.4 and 4.6). A very large, but unknown, amount of silica was also 

present in the suspended and colloidal sediment transported to the Gulf. Most of the suspended silica is in the form of quartz and other relatively insoluble aluminosilicate minerals, but some is no 

doubt present as diatom remains. Some unknown portion of this suspended silica could decompose 

and become available in the Gulf. About 90% of the phosphorus and 87% of the silica entering the 

Gulf from the MARB comes from the Mississippi River Basin; the remainder is from the Red and 

Ouachita Basins. 

Nutrient fluxes varied over a vide range during each year and from year to year due to seasonal and 

annual variations in rainfall and runoff. Fgwe 4.1 is a plot of the regression model estimates of the 

daily flux of nitrate from the Mississippi River Basin to the Gulf during 1980 to mid-1998. This plot 

illustrates the dramatic seasonal pattern in nitrate flux to the Gulf that occurs each year. The daily 

flux of nitrate varies from a low of several hundred metric tons per day during low streamflowin the 
Chapter 4: Nutrient Flux and Sources 33 

fall to several thousand memc tons per day during high streamflow in the spring and summer. 'Ihe 

seasonal flux of phosphorus and silica follows similar patterns. 

The annual flux of nitrate to the Gulf increased sipficantly over the period 1955-96 as shown in 

Figure 4.2. This increase in flux parallels the increase in concentration shown in Figure 3.4A. A 

Kendall's tau test for trend (Helsel and Hirsch 1992) shows the increase in nitrate flux to be highly 

significant @ < 0.001), with a trend slope of about 19,000 memc tons/yr. For the first 15 years of 

this period (1955-70) nitrate flux averaged 328,000 metric tons/yr. However, for the last 17 years 

(1980-96) the nitrate flux averaged 952,700 metric tons/yr, almost a three-fold increase. Essentially 

all of this increase occurred between about 1970 and 1983. There is no statistically significant trend, 

upward or downward in nitrate flux since 1980, even if the flood year of 1993 is removed. Essentially all of the increase in total nitrogen (nitrate plus organic N) that has occurred since 1970 (Figure 

4.2) can be attributed to nitrate. The trend in the annual flux of organic nitrogen is not statistically 

significant @ = 0.23) for this period. The large year-to-year differences in flux are caused by variations in streamflow (Figure 4.2). The flux of nitrate was relatively low during the drought years of 

1987-89 (500,000-700,000 t), but was high (> 1,500,000 t) during the flood year of 1993. Nitrate 

flux was also hh during 1979 and during the early 1980s, when streamflow was abnormally high. 

However, nitrate flux was noticeably lower during a high streamflow period in the early 1970s than 

in later years. Both streamflow and nitrate flux have become much more variable in the last 25 years. 

The average 1980-96 total N yield for the entire MARB was estimated to be 489 kg/km2/yr. Howarth (1998) and Howarth et al. (1996) estimated the total N yield for the Mississippi Basin to be 2.5-

7.4 times more than the estimated "pristine" yield of 76-230 kg/km2/yr for the North Atlantic Basin. The average total N yields for the MARB determined from this assessment are 2.2-6.5 times 

more than the yields for "pristine" conditions and are almost identical to the yield increase for the 

Mississippi Basin suggested by Howarth et al. (1996). 

Figure 4.3 shows the annual flux of total phosphorus for 1972-96. Although there are significant 

year-to-year variations in the flux of phosphorus due to differences in streamflow, the Kendall's tau 

test (Helsel and Hirsch 1992) showed no statistically sigruficant @ = 0.24) long-term trend. One can 

hypothesize that the flux of P to the Gulf was considerably higher prior to completion of the Missouri River reservoirs in the 1950s than it is today. Nearly 70% of the phosphorus flux to the Gulf is 

associated with suspended sediment (Table 4.1), and the construction of these reservoirs cut the 

sediment flux to the Gulf nearly in half (Meade and Parker 1985). However, the P associated with 

the suspended sediment would have to be converted to dissolved ortho P in order for it to be available to algae and other aquatic plants (Correll1998). 
34 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

FIGURE 4.1. Hydrograph of the daily flux of nitrate in the Mississippi River at St. 

Francisville, Louisiana, 1 980-98. 

organic N 

\ 

FIGURE 4.2. Annual flux of nitrate and organic nitrogen and mean annual stream 

flow from the Mississippi River Basin to the Gulf of Mexico. 
Chapter 4: Nutrient Flux and Sources 35 

FIGURE 4.3. Annual flux of total phosphorus and mean annual streamflow from the 

Mississippi River Basin to the Gulf of Mexico. 

The annual flux of dissolved silica for 1955-96 is shown in Figure 4.4. As with phosphorus, the silica flux also varies considerably from year to year due to variations in streamflow. However, there is 

no statistically significant (p > 0.9) long-term trend in silica flux. Figure 3.6B showed that the concentrations of shca decreased 30% or more from the 1950s to the 1970s. However, there is no corresponding decrease in the annual flux of silica (Figure 4.4). The reasons for this apparent 

contradiction are not known. Part of the reason may be related to changes in streamflow since the 

early 1960s and the availability of dissolved silica that can be transported into streams in the MARB. 

There have been no known significant anthropogenic additions of silica to the basin over the past 40 

years. Thus, the supply of soluble silica available for transport into streams is controlled by the natural weathering of soils and minerals. Unless higher precipitation results in an increase in the weathering rate, sllica could be leached from the soil faster than it is produced by weathering processes. This 

would cause dilution of silica concentrations in the receiving streams. Thus, an increase in precipitation and streamflow could decrease silica concentrations with no net change in the annual silica flux. 

Removal of silica from streams by increased diatom production, as hypothesized by Turner and Rabalais (1991) and Rabalais et al. (1996), would also reduce dissolved silica concentrations, but would 

also reduce the flux of dissolved shca. 

In addition to the MARB, several other rivers along the Gulf coast discharge small amounts of nutrients to the Gulf of Mexico. Dunn (1996) estimated the total nitrogen and phosphorus inflows to 

the Gulf from 37 streams discharging to the Gulf between southwest Texas and southern Florida. 

Dunn's estimates included nine large rivers, in addition to the MARB, in the region from the Sabine 

River on the Louisiana-Texas border to Perdido River on the Alabama-Florida border. The combined average total nitrogen flux from these streams for 1972-93 was estimated to be 81,000 metric 

tonslyr. This is equal to about 5% of the total nitrogen discharge of the MARB. The estimated total 

phosphorus flux from these nine rivers was 8,890 metric tons/yr, which is about 6.5% of the Mississippi fiver's total phosphorus discharge. These results clearly show that the MARB is the principal 

source of nitrogen, phosphorus, and probably dissolved silica entering the Gulf of Mexico via 

streams. 
Chapter 4: Nutrient Flux and Sources 36 

FIGURE 4.4. Annual flux of silica and mean annual slreamflow from the Mississippi

River Basin to the Gulf of Memo. 

4.2.1 Climate Effects on Nutrient Flux 

The average annual streamflow increased significantly during 1955-97-the period that is the focus of this 

report. Streamflow was approximately 30% higher during 1980-96 than during 1955-70. A Kendall's tau 

test on the mean annual streamflow showed a statistically significant trend (p = 0.001) with a slope of 158 

rn 3 /s/yr. Some of this increase is the result of long-term climatic variation, and some is driven by shorterterm climatic cycles. Baldwin and Lall (1999) analyzed streamflow from the Mississippi River at Clinton, 

Iowa, for 1874-96 and reported a long-term, U-shaped trend in average annual discharge, with the beginning and end experiencing high flows. The period 195596 showed a particularly large increase in flows. A 

10-year Loess regression through the average annual discharge data showed decadal-scale trends. This 

value has a minimum of less than 1,132 cm in the late 1950s, and increases to over 1,700 cm by the late 

1990s. The higher flows in the latter half of the century are attributed to increased precipitation throughout 

the year, particularly to warmer, wetter springs (Baldwin and Lall 1999). Angel and Huff (1995) analyzed 

frequency characteristics of rainfall in the MARB from records dating back to 1901, and found a 20% increase in the number of extreme one-day rainfall events. 

The higher precipitation and streamflow in the later time period could influence nitrate flux in several ways. 

First, the volume of flow would be larger and more nitrate would be transported, unless concentrations 

decreased. Second, the higher precipitation could leach more accumulated nitrate from soils in the basin 

into tile drains and ditches, and would actually cause nitrate concentrations in streams to increase, as previously noted in section 3.1.1. Third, higher streamflow would decrease both the contact time of water in the 

river with bottom deposits and the rates of denitrification (Howarth et al. 1996). The combination of higher 

nitrate concentrations and higher streamflow, and possibly decreased denitrification during 1980-96, would 

produce significant increases in nitrate flux. 
Chapter 4: Nutrient Flux and Sources 37 

Sources of nutrients in the MARB were evaluated at two scales-the large-basin scale shown in Figure 2.1 and the smaller, interior-basin scale shown in Figure 2.2. At the large-basin scale it was possible to develop estimates of the nutrient load contributions from each basin to the total nutrient 

load dscharged from the MARB to the Gulf of Mexico. The interior basins provided a more precise 

indication of the watersheds and the land uses and human activities that were most significant in 

contributing nutrients to the Mississippi River and the Gulf. A summary of the nutrient flux data for 

nitrogen is presented in Table 4.2 for each of the large basins and in Table 4.3 for each of the 42 

interior basins. Each table shows the average annual runoff and nitrogen flux for 1980-96, along 

with the standard error of estimates, expressed as a percentage of the nitrogen flux and yields. Also 

shown is the average nitrogen yield, which is the nitrogen flux divided by the drainage area. This 

normalizes the flux and makes it possible to determine which basins are abnormally large contributors of nutrients per unit area. In addition, Table 4.2 estimates the percentage each of the nine large 

basins contributes to the nitrate and total nitrogen flux to the Gulf of Mexico. 

The estimated percentage nitrogen contributions presented in Table 4.2 assume no in-stream losses 

of nitrogen between the outflow point of each large basin and the Gulf. Three lines of evidence suggest in-stream nitrogen losses in large rivers are small. Nitrogen yield estimates for the large rivers 

and the smaller interior basins are within 7% when compared for the Upper and Lower Ohio; Upper, Middle, and Lower Mississippi; and Lower Missouri Basins. The Upper Missouri and Arkansas 

basins-whch contain large reservoirs, have low precipitation rates, and have very low yields-were 

excluded from this calculation. The total nitrogen yield for this area (1,668,400 km') determined 

from the large-basin fluxes in Table 4.2 is 826 kg/km2/yr. The total nitrogen yield for 30 interior 

basins, which comprise 62% of the area of the large basins, is 880 kg/km2/yr. This small difference, 

which is about 6% and well within the standard errors, indicates that no significant denitrification 

losses occur between the outlets of the interior basins and the Gulf of Mexico. In other words, most 

of the nitrogen that is discharged into the Ohio, Missouri, and Mississippi Rivers from smaller 

streams is ultimately transported to the Gulf. 

The second line of evidence is based on the results of a model that Howarth et al. (1996) applied to 

rivers draining to the North Atlantic Ocean. The model relates nitrogen retention, which is largely 

denitrification, to the ratio of mean depth to residence time of rivers. The deeper the rivers are, the 

less time nitrogen in the water column is in contact with benthic zones where denitrification could 

occur. They suggest that 5-20% of the nitrogen inputs to streams may be lost through denitrification in larger rivers. The Ohio, Missouri, and Mississippi Rivers used in the CENR assessment 

should be near the low end of their denitrification estimate. Short-term removal of nutrients in algal 

and plant biomass should be accounted for in the long-term flux estimates if these nutrients are later 

released in dissolved or particulate forms. The above discussion suggests that most of the nitrogen 

that enters the Ohio, Lower Missouri, and Mississippi Rivers is eventually discharged to the Gulf of 

Mexico. However, denitrificationprobably results in significant losses of nitrogen in streams smaller 

and shallower than the large rivers used in this assessment. Howarth et al.'s (1996) model would 

suggest that very large nitrogen losses via denitrificationwould occur within the interior basins used 

in this assessment. 
TABLE4.2. Average annual flux and yields of nitrogen from large watersheds in the Mississippi-Atchafala

during 1980-96, estimated from regression models. 

Entire Ohio River Basin 

Upper Ohio River Basin (I) 

Lower Ohio River Basin (C)(2) 

Entire Missouri River Basin 

Upper Missouri River Basin (3) 

Lower Missouri River Basin (C)(4) 

Entire Mississippi and Missouri 

River Basins above Ohio River 

Upper Mississippi River Basin (5) 

Entire MRB above Missouri River 

Middle MRB (C)(6) 

Entire Mississippi R., Including 

Flux Diverted into Atchafalaya R. 

Arkansas River at Little Rock (7) 

Lower MRB (C)(8) 

Atchafalaya River 

Mississippi River diversion into 

Atchafalaya River 

Red & Ouachita River Basins (C)(9) 

Entire MAR6 Flux to Gulf 

Mississippi River flux to Gulf at 

Tarbert's Landing, LA (C ) 

Atchafalaya River flux to Gulf at 

Melville, LA 

526,000 

25 1,200 

274,800 

1,357,700 

836,100 

52 1,600 

1,847,200 

22 1,700 

444,200 

267,800 

2,967,000 

4 10,000 

184,000 

Ind. 

24 1,700 

3,208,700 

Ind. 

Ind. 

Est. < 5 1,567,900 

1,18 1,600 

Est. < S 

NOTES:See Figure 2.1 for locations of flux estimation sites; (C) is calculated as the difference between sites; Ind. = Indeterminate, 
Chapter 4: Nutrient Flux and Sources 39 

TABLE4.3. Average annual flux and yields of nitrate and total nitrogen from the 42 interior 

basins during 1980-96, estimated with regression models. 

Basin Basin Nameand Data Runoff Nitrate Nitrate Nitnts Total Total Total 

ID Locationof Sampling Site Used Flux as Yield 

N 

Flux Std. 

Error 

N 

Flux 

N 

Yield 

NStd. 

Error 

yn mIyr mebic kglkn?lyr % metrf) kgIkn?lyr % 

tons tons ' 

Allegheny R at New Kensington, 16 60.8 l3,6 10 

PA 

Monongahela R at Braddock, PA 

Muskingham R at McConndsville, 

OH 

Kanawha R. at Winfield, W 

Scioto R. at Higby, OH 

Great Miami at New Baltimore, 

OH 

Kentucky R. at Lockport, KY 

Wabash R. at New Harmony. IN 

Cumberland R near Grand 

Rivers, KY 

Tennessee R near Paducah. KY 

Mississippi R. near Royalton. MN 

Minnesota R. at Jordan, MN 

St Croix R. at St Croix Falls. WI 

Chippewa R. at Durand, WI 

Wisconsin R. at Muscoda, WI 

Rock R. near joslin, lL 

Cedar R at Cedar Falls, IA 

Iowa R at Wapello, IA 

(includes Cedar R. Basin # 17) 

Skunk R. at Augusta, IA 

Raccoon R. at Van MeterIDes 

Moines, IA 

Des Moines at St Francisville, MO 

(includes Raccoon R. Basin #20) 

Illinois R. at Marseilles, IL 

Lower Illinois R. Basin 

Illinois R. at Valley City, IL 

(#22 & 23) 

Kaskaskia R. nr. Venedy Station. IL 

Milk R. near Nashua, MT 

Missouri R. near Culbertson, MT 

Bighorn R near Bighorn, MY 

Yellowstone R. near Sydney, MT 

Cheyenne R. at Cherry Creek, SD 

JamesR. near Scotland, SD 

Platte R. near Louisville, NE 

Kansas R. at Desoto, KS 

Grand R. near Sumner, MO 

Osage R. below St Thomas, MO 

St Francis Bay at Riverfront, AR* 

White R. at Clarendon, ARM 

Arkansas R. at Tulsa, OK 

Canadian R. at Calvin, OK 

Yazoo R. at Redwood, MS" 

Big Black R. near Bovina, MS 

Red R. at Alexandria, LA 

Ouachita R. near Columbia, LA* 

concentrotion 
40 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

The third line of evidence is from stable isotope data on the 615N and 6180 of nitrate in water samples collected in April 1998 from the Ohio River at Grand Chain, Illinois, and from the Mississippi 

River at Thebes, Illinois, and St. Francisville, Louisiana (see Table 2.2 and Figure 2.1 for locations). 

The isotopic ratios 615N and 6180 of nitrate measured at St. Francisville (+6.5 and +8.3 per mil) are 

essentially the same as would be predicted by simple mixing of water from the two sources (+6.5 

and +8.8 per mil) (Kendall et al. 1999). The estimated travel time from the Ohio River confluence 

with the Mississippi River to St. Francisville was about seven days. These preliminary results suggest 

there was no appreciable loss of nitrate from denitrification in this reach of the hhssissippi River 

during the seven-day period. Denitrification would result in an increase (enrichment)in the 615N and 

6180of the remaining nitrate. 

Given the assumption that nitrogen is conservative in large rivers, the data in Table 4.2 show that 

the Ohio River basin, on average, contributes about 34% of the nitrate and 32% of the total nitrogen discharged by the MARB to the Gulf. About 56% of the nitrate and 54% of the total nitrogen 

comes from the Mississippi River Basin above the Ohio River Basin. The Missouri Basin contributes 

about 15% of the total N, and the combined Upper and Middle Mississippi Basins contribute about 

39%. The Lower Mississippi Basin contributes less than 8% of the N and the combined Arkansas, 

Red, and Ouachita Basins contribute less than 8%. The Middle Mississippi Basin, with only 8.5% of * 

the MARB drainage area, contributes about 33% of the nitrate discharpg to the Gulf and the largest amount of nitrate and total nitrogen per unit area (Figure 4.5). The respective nitrate and total 

nitrogen yields from this basin are 1,150 and 1,690 kg/km'/yr, which are nearly 90% higher than the 

Ohio Basin and more than 100% higher than the Upper Mississippi Basin (Table 4.2). These yield 

estimates are sdar to those presented by Smith et al. (1997). As wdl be shown later in this report, 

the hgh nitrogen yields in this basin are primarily associated with intensive agriculture. Nitrogen 

yields in the western half of the MARB are relatively low-320 kg/km2/yr or less, and less than the 

entire MARB average of 489 kg/km2/yr (Table 4.2). This can be attributed largely to the drier climate, lower runoff, and different land uses in this part of the MARB. 

Figure 4.6 shows the temporal pattern in annual nitrogen yields for three large basins and the entire 

Mississippi Basin for 1970-96. The annual yields vary considerably, depending on precipitation. The 

Middle Mississippi Basin has the greatest variability. The nitrate yield from this basin ranged from 

about 250 kg/km2 in 1989 to more than 2,500 kg/km2 in the flood year of 1993. The large variability in nitrate yields, which comprises most of the nitrogen, discharging from this basin is an indication that large amounts of nitrate are available for leaching from the soils, unsaturated zone, and 

ground water of the basin. It also indicates that the amount of nitrate delivered to streams is largely 

determined by precipitation. This basin tends to dominate the amount of nitrogen discharged by the 

MARB to the Gulf, even though is comprises only about 8.5% of the area of the MARB. 
Cha~ter4: Nutrient Flux and Sources 4 1 

FIGURE 4.5. Spatial distribution of the average nutrient and chloride yields in nine large basins during 1980-96: (A) nitratenitrogen, (B) total nitrogen, (C) total phosphorus, (D) orthophosphate phosphorus, (E) silica as Si, and (F) chloride. 
42 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

FIGURE 4.6. Temporal patterns in nitrate yields in the Middle Mississippi, Ohio, Lower 

Missouri, and entire Mississippi River Basin, 1970-96. NOTE: See Figure 2.1 for site locations. 

The sources of nitrogen in the MARB are shown in greater detail with data on the average annual 

fluxes and yields presented in Table 4.3 for the 42 interior basins. The spatial patterns in average annual nitrate and total nitrogen yields, which are very sunilar, are shown graphcally in Figures 4.7A 

and 4.8A. The distribution of the annual yields of nitrate and total nitrogen for each basin are shown 

in the form of boxplots in Figures 4.7B and 4.8B for 1980-96. The hghest average annual total nitrogen yields range from 1,000 to more than 3,000 kg/km"yr and occur in a band extending from 

southwestern Minnesota across Iowa, Illinois, Indiana, and Ohio. Annual average yields of 1,800 to 

more than 3,100 kg/km2/yr occur in the Des Moines (20 plus 21), Iowa (17 plus 18), and Skunk 

River Basins in Iowa (Table 4.3 and Figure 4.8A); the upper Illinois River Basin (22), and the Great 

Miami Basin in Ohio (6). During years with high precipitation the total nitrogen yield from these 

five river basins (Iowa, Skunk, Des Moines, Illinois, and Great Miami) can be 3,000 to more than 

7,000 kg/km2/yr (Figure 4.8B. Discharge from these five basins alone can account for as much as 

21% of the total nitrogen discharge from the MARB during average years and more than 30% during flood years, such as 1993. The nitrate discharged from these basins during 1993 was equivalent 

to more than 37% of the nitrate discharged to the Gulf. The Minnesota River Basin (12), Rock River 

(16) and Lower Illinois River (23) in Illinois, Grand River (33) in Missouri, Wabash River (8) in Indiana, and Muskmgham (3) and Scioto (5) Rivers in Ohio have nitrogen yields of 1,000-1,800 

kg/km2/yr. Other basins adjacent to these, but not shown in the figures because of insufficient data, 

may have similar nitrogen yields. Nitrogen yields were generally 500-1,000 kg/kmZ/yr in basins 

south of the Oho River and generally less than 500 kg/km2/yr in the Missouri, Arkansas, and 

Lower Mississippi Basin. Many of the drier basins in the western part of the MARB had nitrogen 

yields of less than 100 kg/km2/yr. 
Chapter 4: Nutrient Flux and Sources 43 

BASIN IDENTIFICATION NUMBER 

FIGURE 4.7. (A) Spatial distribution of nitrate yields in the 42 interior basins, and (B) boxplots showing distribution of nitrate yields in the 42 interior basins. NOTE: Dashes (-) show 

median yields for several sites where other statistics could not be calculated. 
44 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

"""" maximum 

75th percentile 

25th percentile 

minimum 

0 

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 

BASIN IDENTIFICATION NUMBER 

FIGURE 4.8. (A) Spatial distribution of total nitrogen yields in the 42 interior basins, and 

(B) boxplots showing distribution of total nitrogen yields in the 42 interior basins. NOTE: 

Dashes (-) show median yields for several sites where other statistics could not be calculated. 
Chapter 4: Nutbient Flux and Sources 45 

The large range in annual yields of nitrogen shown in Figures 4.7B and 4.8B can be attributed largely 

to year-to-year variations in precipitation and leaching of nitrogen from nonpoint sources. During 

dry years there is little rainfall to transport nitrogen (mainly nitrate) from the soil and unsaturated 

zone to streams. Under these conditions nitrogen yields are low, and nitrogen inputs from point 

sources may dominate in some streams. During periods of hgh precipitation nitrate that has accumulated in the soil can be flushed into streams via agricultural drains, ground-water discharge, and 

overland flow. Basins with large point-source inputs, such as the Upper Illinois River (basin 22) with 

more than 300 people per km2, exhibit a different pattern in annual yields. The minimum annual 

yield is very hgh because of sustained year-round direct inputs to the stream. The range in nitrogen 

flux is small because this input is not greatly affected by precipitation. Much of the year-to-year vaiability that does occur in basin 22 may be due largely to varying amounts of precipitation leaching 

varying amounts of nitrogen from soils in the basin. Several other basins (5-Scioto, &Great Miami, 

8-Wabash, and 1GRock) that also have above-average population densities show this same pattern 

but to a lesser extent. 

The nitrogen flux and yield estimates presented in the foregoing discussion represent the amounts of 

nitrogen delivered near the mouths of the streams to larger rivers, usually the Ohio, Missouri, or 

Mississippi, and to the Gulf of Mexico. As previously Qscussed, they do not account for any instream losses such as denitrification or burial in reservoirs or on flood plains before water reached 

the sampling point. These processes would not significantly affect our estimates of nitrogen flux 

from the MARB to the Gulf of Mexico. However, if denitrification is significant in large rivers, this 

could affect our estimates of the percentage contributions from basins within the MARB (Table 4.2). 

The percentage of nitrogen contributed to the Gulf by the farthest upstream basins could be overestimated, and the percentage contributed by the farthest downstream basins could be underestimated. 

4.4 SOURCES OF PHOSPHORUS 

The average annual flux and yields of orthophosphate and total phosphorus are summarized in Table 4.4 for the large basins. The Mddle Mississippi and Ohio Basins are the largest contributors of 

both ortho- and total phosphorus to the Gulf of Mexico. The Middle Mississippi contributes about 

25% of the total phosphorus discharged by the MARB, and the Ohio contributes about 29%. About 

19% of the total phosphorus comes from the Missouri Basin, 6% comes from the Upper Mississippi 

Basin, and another 12% comes from the Lower Mississippi and Arkansas Basins. Phosphorus yields 

for the large basins are shown in Figures 4.5C and 4.5D. The Middle Mississippi Basin has the highest total phosphorus yield of 130 kg/km2/yr (Figure 4.5C). Phosphorus yields in the Upper and 

Lower Ohio Basin and Lower Mississippi Basin range from 56 to 96 kg/kmVyr, and yields are 8-53 

kg/km2/yr elsewhere in the MARB. The highest yields of orthophosphate, 35 and 36 kg/km2/yr, 

are in the Lower and Middle Mississippi Basin (Figure 4.5D). The phosphorus discharged from all of 

the large basins is predominantly in particulate form, with dissolved orthophosphate comprising 

only about 20-30% of the total phosphorus. 

The phosphorus flux and yields for the 42 interior basins are presented in Table 4.5. These estimates 

are considerably less precise than the phosphorus and the nitrogen flux estimates for the large basins 

(Table 4.4), as indicated by the large standard errors, which show that variables in addition to those 

used in the regression models are important in controlling the flux of phosphorus. The interior basins with the highest ortho- and total phosphorus yields (Figures 4.9A and 4.10A) are generally the 

same ones that had the highest nitrogen yields (Figures. 4.7A and 4.8A). They are also the ones that 

have the largest amount of variabihty in annual phosphorus 
TABLE4.4. Average annual flux and yields of phosphorus from large watersheds in the Mississippi-Atchafala

during 1980-96, estimated from regression models. 

Basin NarnetlD and Location of Area Runoff Ortho P Ortho P Ortho P Total Total P Total P 

Flux Estimation Sites Flux Yield Standard P Flux Yield Standard 

Error Error 

km2 cmlyr metric kglkm21yr percent metric kgI&m21yr percent 

tons tons 

Entire Ohio River Basin 

Upper Ohio River Basin (I) 

Lower Ohio River Basin (C)(2) 

Entire Missouri River Basin 

Upper Missouri River Basin (3) 

Lower Missouri River Basin (C)(4) 

Entire Mississippi and Missouri 

River Basins above Ohio River 

Upper Mississippi River Basin (5) 

Entire MRB above Missouri River 

Middle MRB (C)(6) 

Entire Mississippi R., Including 

Flux Diverted into Atchafalaya R. 

Arkansas River at Little Rock (7) 

Lower MRB (C)(8) 

Atchafalaya River 

Mississippi River diversion into 

Atchahlaya River 

Red & Ouachita River Basins (C)(9) 

Entire MARB Flux to Gulf 

Mississippi River flux to Gulf at 

Tarbert's Landing, LA (C ) 

Atchafalaya River flux to Gulf at 

Melville, LA 

Ind. 

24 1,700 30.7 

3,208,700 20.0 

Ind. 

Ind. 

10.1 

27.2 

4.4 

18.3 

5.0 

10.5 

9. I 

4.9 

16.9 

Est. <6 

7.7 

53 

42 Est. <7 

-- - -- - - -- - --

NOTES:See Figure 2.1 for locations of flux estimation sites; (C) is calculated as the dfierence between sites; Ind. = Indeteninate. 
Cha~ter4: Nutrient Flux and Sources 47 

TABLE 4.5. Average annual flux and yields of orthophosphate and total phosphorusfrom the 

42 interior basins during 1980-96, estimated with regression models. 

Basin Basin Name and Data Runoff Ortho Ortho Ortho Total Total Total 

ID Location of Sampling Site Used P Flux P Yield P Std. P Flux P P Std. 

Error Yield Error 

yn cmlyr metric kgIkm2lyr % metric rpjkm2lyr % 

tons tons 

Allegheny R. at New Kensington, 

PA 

Mononeahela R. at Braddock, PA 

~anawhaR. at Winfield, W 

Scioto R. at Higby, OH 

Great Miami at New Baltimore, OlKentucky R. at Lockport, KY 

Wabash R. at New Harmony, IN 

Cumberland R. near Grand 

Rivers, KY 

Tennessee R. near Paducah, KY 

Mississippi R. near Royalton, MN 

Minnesota R. at Jordan, MN 

St Croix R. at St. Croix Falls, WI 

Chippewa R. at Durand, WI 

Wisconsin R. at Muscoda, WI 

Rock R. near joslin, IL 

Cedar R. at Cedar Falls, IA 

Iowa R. at Wapello, IA 

(includes Cedar R. Basin # 17) 

Skunk R. at Augusta, IA 

Raccoon R. at Van MeterIDes 

Moines, IA 

Des Moines at St Francisville, MO 

(includes Raccoon R. Basin #20) 

Illinois R. at Marseilles, IL 

Lower Illinois R. Basin 

Illinois R. at Valley City, IL (entire 

(Illinois R. basins #22 & 23) 

Kaskaskia R. nr. Venedy Station, IL 

Mi1k.R. near Nashua, MT 

Missouri R. near Culbertson, MT 

Bighorn R. near Bighorn, MT** 

Yellowstone R. near Sydney, MT 

Cheyenne R. at Cherry'Creek, SD 

James R. near Scotland,'SD 

Platte R. near Louisville, NE 

Kansas R. at Desoto, KS 

Grand R near Sumner,.MO 

Osage R. below St Thomas, MO 

St Francis Bay at Riverfront, ARH 

White R. at Clarendon, AR** 

Arkansas R. at Tulsa, OK 

Canadian R. at Calvin, OK 

Yazoo R. at Redwood, MSH 

Big Black R. near Bovina, MS 

Red R. at Alexandria, LA 

Ouachita R. near Columbia, LA** 

**Estimated from mean concentrotions and discharge at time of sampling. 

NOTE:s = number of samples; na = not available; c = calculated as a dflerence between two sites. 
48 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

ylelds as shown by the boxplots in Figures 4.9B and 4.10B. These basins extend from north central 

Iowa eastward across Ohio. The total P yields from these basins range from about 50 to 190 

kg/kmvyr. The highest phosphorus yield is from the Upper Illinois Basin (#22), which has the 

highest population density and large point-source inputs from the Chicago area. These point-source 

inputs are also shown by the high median and minimum annual yield values in the boxplots (Figures 

4.9B and 4.10B) for basin 22. Orthophosphate comprises from 25% to more than 50% of the total 

phosphorus in most of these basins. The highest percentages of orthophosphate are generally in basins that have high population densities or a large percentage of the basin in cropland, or both. The 

total phosphorus yields were also high in basins 39 and 40 in Mississippi and basin 33 in northwestem Missouri (Figure 4.9). However, in these basins 85-90% of the phosphorus is in particulate form 

(Table 4.5), indicating that sediment is the principal source of phosphorus in these basins. The dissolved orthophosphate present in these streams and transported into the Gulf of Mexico is readily 

available for use by aquatic plants. However, the particulate forms of phosphorus must be converted 

to orthophosphate by chemical or microbiologcal processes before plants can use it. 

A comparison of phosphorus yields from the interior basins with yields from the large basins indicates that there is no significant net loss of phosphorus in the large rivers. The average yields from 

the Upper and Lower Ohio, Lower Missouri, and Upper, Middle, and Lower Mississippi Basins are 

61 and 19 kg/km2/yr for total phosphorus and orthophosphorus. The average yields measured for 

30 interior basins that comprise about 60% of the large-basin area are 67 and 22 kg/km2/yr for total 

phosphorus and orthophosphorus, respectively. The similarity of these values suggests that there is 

little net in-stream loss or gain in phosphorus over the long term in the large river basins. The main 

process for phosphorus removal would be deposition of sediment. This probably does not occur to 

any significant degree, except in basins with large mainstem reservoirs. 

4.5 SOURCES OF SILICA 

The average annual flux and yields of silica in the nine large basins are shown in Table 4.6 and Figure 4.5E. There is no clearly dominant source of silica at this scale. The fluxes are generally proportional to the amount of streamflow contributed by each basin. The silica yields fall within a fairly 

narrow range (1,170-1,510 kg/km2/yr), except for the Upper Mississippi Basin and the more arid 

western half of the MARB, where yields were lower because of less runoff. 

The flux and yields of silica from the 42 interior basins are shown in Table 4.7 and Figure 4.11A. 

Even at this scale, there are no clearly dominant source areas. Silica yields in most basins in the eastem pan of the MARB are about 1,000-2,320 kg/km2/yr. Basins with the highest yields (> 1,500 

kg/km2/yr) are scattered throughout this area and do not appear to be associated with any particular 

land use or human activity. Basins with the highest average annual silica yields (e.g., basins 13, 18, 

19, and 21) also generally have the largest variability in annual yields (Figure 4.11B). This suggests 

that leaching of silica from these basins is more affected by precipitation than in the other basins. 

The reasons for this have not been determined, but may include geochemical and hydrologic processes. Silica is derived from the dissolution of silicate minerals in soils and rocks. Therefore, the rate 

at which silica is transported into streams is more likely to be regulated by geochemical process, such 

as pH and the mineralogy of soil and rocks, and by hydrologic processes, such as ground-water contributions to streams, than by human activities. 
Chapter 4: Nutrient Flux and Sources 49 

FIGURE 4.9. 

miximum B 

median 

25th percentile 

Q - 0 

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 

BASIN IDENTIFICATION NUMBER 

(A) Spatial distribution of total phosphorus yields in the 42 interior basins, 

and (B) boxpl& showing distribution of total~pho~phorus~yields in the 42 interior basins. 

NOTE: Dashes (-) show median yields for several sites where other statistics could not be calculated. 
50 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

FIGURE 4.10. (A) Spatial distribution of orthophosphate yields in the 42 interior basins, 

and (B) boxplots showing distribution of orthophosphate yields in the 42 interior basins. 

NOTE: Dashes (-) show median yields for several sites where other statistics could not be calculated. 
Trutllr 4.6. Mean annual flux and yields of silica (Si) and chloride fh-n large watersheds in the Mississippi-Atc

Basin during 1980-96, estimated from regression models. NOTES:See Figure 2.1 for locations of flux estimatio

calculated as the difference between sites; Ind. = indeterminate. 

Basin NarndlDand Location of Airea Runoff Silica Silica Silica Chloride Chloride Chlorid

Flux Estimation Sites Flux Yi Std. Flux YreM Std. 

Error Emr 

kml cmlyr mt kglkmZlyr percent mt kglkm21yr percent 

Entire Ohio River Basin 

Upper Ohio River Basin (I) 

Lower Ohio River Basin (C)(2) 

Entire Missouri River Basin 

Upper Missouri River Basin (3) 

Lower Missouri River Basin (C)(4) 

Entire Mississippi and Missouri River 

Basin above Ohio River 

Upper Mississippi River Basin (5) 

Lower MRB above Missouri River 

Middle MRB (C)(6) 

Entire Mississippi River, Including 

Flux Diverted into Atchafalaya River 

Arkansas River at Little Rock (7) 

Lower MRB (C)(8) 

Atchafalaya River 

Mississippi River diversion into 

Atchafalaya River 

Red & Ouachita River Basins (C)(9) 

526,000 

25 1,200 

274,800 

1,357,700 

836,100 

52 1,600 

1,847,200 

22 1,700 

444,200 

267,800 

2,967,000 

4 10,000 

184,000 

Ind, 

24 1,700 

Entire MARB Flux to Gulf 3,208,700 20.0 2,3 16,800 722 Est. -12 14,457,700 4,506 Est. <4 

Mississippi River flux to Gulf at Ind. 1,582,800 na na 9,216,100 na na 

Tarbert's Landing, LA (C) 

Atchafalaya River flux to Gulf at Ind. 734,000 na 23.1 5,241,600 na 4.7 

Melville, LA 
52 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE 4.7. Average flux and yields of silica and chloride from the 42 interior basins during 

1 988-96, estimated with regression models. --

3asin Basin Name and Data Runoff Silica Silica Silica Chlor- Chlor- Chloride 

ID Location of Samplinn . - Site Used Flux as Yield Std. ide ide Std.

N Error Flux Yield Error 

yn cmlyr metric kgIkmllyr % metric b#kn?Ip % 

tons tons 

Allegheny R. at New Kensington, PA 16 39,190 

usk king ham R. at McConnelsville, OF 12 

25,760 

30,960 

Kanawha R. at Winfield, WW 40,900 

Scioto R. at Higby, OH 13,100 

Great Miami at New Baltimore, OH 15,210 

Kentucky R. at Lockport, KY 17,670 

Wabash R. at New Harmony, IN 98,860 

Cumberland R. near Grand 50,590 

Rivers, KY 

Tennessee R. near Paducah. KY 1 14.40 

0 

MississippiR. near Royalton, MN 19,650 

Minnesota R. at Jordan, MN 52,280 

St Croix R. at St.Croix Falls, WI 3 1,720 

Chippewa R. at Durand, WI 3 1,230 

W~sconsinR. at Muscoda,WI 2 1,600 

Rock R. near Joslin, IL 43,040 

Cedar R. at Cedar Falls, IA no data 

Iowa R. at Wapello, IA 75,090 

(includes Cedar R. Basin # 17) 

Skunk R. at Augusta, IA 19,820 

Raccoon R. at Van MeterIDes no data 

Moines, IA 

Des Moines at St. Francisville, MO 75,800 

(includes Raccoon R. Basin #20) 

Illinois R. at Marseilles, IL 29,720 

Lower Illinois R. Basin 40,430~ 

Illinois R. at Valley City, IL 70,150 

(#22 & 23) 

Kaskaskia R. nr. Venedy Station, IL 11,190 

Milk R. near Nashua, MT 1,490 

Missouri R. near Culbertson, MT 26,370 

Bighorn R. near Bighorn. MTW 10,670 

Yellowstone R. near Sydney, MT 44,560 

Cheyenne R. at Cherry Creek SD 3,500 

James R. near Scotland, SD 3,480 

Platte R. near Louisville, NE 95,390 

Kansas R. at Desoto, KS 43,450 

Grand R. near Sumner, MO 18,790 

Osage R. below St Thomas, MO 28,670 

St. Francis Bay at Riverfront AR** 24,500 

White R. at Clarendon, AR** 62,900 

Arkansas R. at Tulsa, OK 42,770 

Canadian R. at Calvin, OK 9,740 

Yazoo R. at Redwood, MSW 45.500 

Big Black R. near Bovina, MS 1 1,720 

Red R. at Alexandria, LA 9 1,220 

Ouachita R. near Columbia. LA** 

**Estimated from mean concentrations and discharge at time of sampling. 

NOTE:s = number of samples; no = not available; c = calculated as a difference between two sites. 
Chapter 4: Nutrient Flux and Sources 53 

8000 . 

maximum B 

75th percentile 

25th percentile 

minimum 

9 

0 

BASIN IDENTlFlCATlON NUMBER 

FIGURE 4.1 I. (A) Spatial distribution of silica (Si) yields in the 42 interior basins, and (B) 

boxplots showing distribution of Si yields in the 42 interior basins. NOTE: Dashes (-) show 

median yields for several sites where other statistics could not be calculated. 
54 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

The nitrogen flux estimates for the interior basins were used to develop rough estimates of how 

much nitrogen most states in the MARB contribute to the Gulf of Mexico. The estimates assume 

there are no sipficant in-stream denitrification losses in the large riiers and are un-affected by any 

denitrification losses that may occur within the interior basins. The total nitrogen yield was calculated for the area of each state covered by the 42 interior basins, and was then multiplied by the portion of each state that drains to the Mississippi. The resulting estimates were expressed as a 

percentage of the average annual nitrogen flux to the Gulf during 1980-96. 

The results, presented in Table 4.8, show that, on the average, the states of Iowa and Illinois each 

contributes 16-19% of the total annual nitrogen flux from the MARB to the Gulf. Minnesota, Indiana, Ohio and Missouri each contributes 6-9% of the annual flux. Contributions can be much higher 

during years with extreme events, such as the 1993 flood. For example, in 1993it is estimated that as 

much as 30% of the total nitrogen and 35% of the nitrate discharged from the MARB to the Gulf 

origmated in Iowa, which drains only about 4.5% of the MARB. Other states in the flooded area 

also contributed abnormally large amounts of nitrogen to the Gulf that year. The large fluxes of nitrogen during flood events, such as 1993, are an indication that large quantities of nitrogen in a mobile form (nitrate) are present in the soils, unsaturated zone, and shallow ground-water systems in 

these states. Agricultural drainage practices employing tile lines, etc., in these states may also be a 

factor in transporting large amounts of nitrate from source areas to streams more quickly than if the 

drainage practices were not in place. 

TABLE 4.8. Approximate percentage of total nitrogen flux to the Gulf of Mexico contributed by selected states in the Mississippi-Atchafalaya River Basin. NOTE: Percentages are 

based on 1980-96 average total nitrogen flux of 1,567,900 metric tons per year. 

States Percent of Total 

Nitrogen Flux 

Pennsylvania, Oklahoma, West Virginia, 

Nebraska, South Dakota 

< 2 

Kentucky, Louisiana, Mississippi, 

Tennessee, Wisconsin, Arkansas, Kansas 

2-5 

Minnesota, Indiana, Ohio, Missouri 6-9 

Iowa, Illinois 16-19 
Nutrient Inputs and Outputs 

Nutrients are chemical elements that are essential for plant and animal growth and development. 

The nutrient requirements for plants and animals vary by species and environment. Nutrients occur 

naturally in soils, but also are added to soils in commercial fertilizers and manure. This chapter focuses on the inputs and outputs of two nutrients: nitrogen (N) and phosphorus 0,in the MARB. 

Silica (Si) is also an important nutrient in the basin. However, anthropogenic inputs of silica, including Si-based ingredients in certain pesticides and fertilizers (Meister 1997), are likely to be insignificant relative to the natural inputs that include dissolution of certain rock types and clay minerals. 

This chapter is divided into four sections: inputs to the MARB from the atmosphere, inputs and 

outputs from agriculture, inputs from municipal and industrial point sources, and atmospheric inputs directly to the Gulf of Mexico. In general, nutrient sources were quantified using the best available data or the most current estimation technique. A geographc information system (GIs) was 

used to manage and manipulate nutrient input and output data. 

5.1 ATMOSPHERICINPUTSTO THE 

MISSISSIPPI-ATCHAFALAYARIVERBASIN 

Through human activities, the deposition of biologically available N from the atmosphere has increased to rates that are significant in relation to rates of natural fixation of N2 (Vitousek et al. 1997). 

In the northeastern United States, atmospheric deposition of N has been recognized as a major factor in the overfertilization of forest ecosystems (often termed N saturation) and the acidification of 

freshwater lakes and streams (Aber et al. 1995; Stoddard 1994). Atmospheric deposition of N has 

also been identified as a significant contributor to the eutrophication and hypoxia of Chesapeake 

Bay (Magnien et al. 1995). Assessment of nitrogen cycling in the Mississippi River Basin, therefore, 

requires the spatial and temporal quantification of N deposition rates from the atmosphere. 

5.1. I Methods 

The general approach for quanti5ing atmospheric deposition of N was to (1) apply existing data sets 

where possible, and (2) estimate deposition for regions where data were not available on the basis of 

empirical relations developed from the existing data and information from peer-reviewed publications. Atmospheric deposition models for refining N deposition estimates were not developed or 

applied in this study due to insufficient data for the watershed scales used in this assessment. 
56 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

5.1.2 Available Data 

In general, measurements of atmospheric deposition of N can be categorized as wet deposition 

(whlch falls as rain or snow), or dry deposition ('particles or vapor deposited from the atmosphere 

primarily during periods of no precipitation). 

Wet deposition is monitored year-round at approximately 200 sites through the National Acid 

Deposition Program/National Trends Network (NADP/NTN). The distribution of these sites is 

approximately uniform nationwide. At each site, precipitation is collected for chemical analysis in a 

polyethylene bucket that remains covered, except when precipitation is falling. Through this 

method, deposition of No3 and NH4 is determined weekly. Wet deposition data analyzed for this 

report were collected from 1984 through 1996. These data and further information on the 

NADP/NTN are available on the World Wide Web (http://nadp.sws.uiuc.edu; accessed 1998). 

Dry deposition is monitored at approximately 60 sites nationwide through several programs that operate under EPA's Clean Air Status and Trends Network (CASTNet; Clarke et al. 1997). Two-thirds 

of these sites are located east of the Mmissippi River; all but three of the remainder are located from 

the Rocky Mountains to the West Coast. Dry deposition is determined at these sites by measurements of air concentrations 10 m above the ground and an inferential model of deposition velocities, described in Hicks et al. (1985). Air concentrations are determined by a three-stage filter pack 

that contains a TeflonTh4filter, a nylon filter, and a cellulose filter, in sequence. Arr is continuously 

pulled through these filters at 1.50 L min-I at eastern sites and 3.00 L min-' at western sites. Particulate NO3 and NH4 are collected by the TeflonTMfilter; HN03 vapor, by the nylon filter; and SO?,by 

the cellulose filter, although this filter also collects indeterminate forms of N. Gaseous NH3 is not 

collected by filter pack. Meteorological and vegetation conditions are also monitored at each site to 

provide data necessary for modeling deposition velocities. Wet deposition was monitored at approximately one-third of the sites by the same method as used by the NADP/NTN. Dry deposition 

data analyzed for this report were collected from 1988 (the first complete year of operation for most 

sites) through 1994. Data from 1995 and 1996 were incomplete at most sites; therefore, these two 

years were excluded. 

5.1.3 Estimation Methods 

Wet deposition data from the NADP/NTN database were converted to GIs point coverages of annual wet deposition of NO3 and NH4. Inverse distance weighted (IDW) interpolation was performed on the points to create a grid of 6.25 kmhells over the coterminous United States. 

Deposition values were determined for each cell by IDW interpolation from a combination of Sample points. Zonal statistics were performed to sum the cell values by watershed for each year (1984-

96). For the purposes of presentation and budget estimates, the cells were aggregated into polygons 

to determine a single value for each area that represents an accounting unit (area based on drainage 

divides) or watershed used in this analysis (Figures 2.1 and 2.2). 

The locations of CASTNet monitoring sites were not suitable for interpolating a surface of dry 

deposition in the Mississippi Basin. Therefore, sites were selected within the basin at which both dry 

and wet deposition were monitored to determine if the spatial distribution of dry deposition could 

be estimated from the data collected at NADP/NTN sites (Figure 5.1). The single exception is the 

data from Wyoming, which include NADP data collected at Snowy Ridge, Wyoming (site code 

WYOO), and CASTNet data collected at Centennial, Wyoming (site code CNT169). These monitoring stations were paired because they are less than 100 km apart, and they represent the only loca-
Chapter 5: Nutrient Inputs and Outputs 57 

tion in the western part of i?e basin where dry and wet deposition measurements could be related. 

Comparisons between wet and dry deposition were possible at 12 sites east and 2 sites west of the 

Mississippi Rwer. Seasonal values were compiled from the weekly data of these 14 sites for December-February, March-May, June August, and September-November. Seasons with missing weekly 

values were omitted. This approach enabled 147 values of dry deposition to be directly compared 

with wet deposition. 

FIGURE5.1. Wet deposition of NO,-, averaged for 1990-96 data from the National Atmospheric Deposition Program (NADP, http://nadp.sws.uiuc.edu/) in each the 133 accounting 

units that make up the Mississippi River Basin. NOTE:Blue circles indicate where NADP and 

CASTNet (Clean Air Status and Trends Network) sites are co-located. 
58 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

5.1.4 Results-Wet Deposition 

No trend in the rates of wet deposition of either N03- or NH4+ was observed between 1984 and 

1996 for values representing the overall basin. The lowest deposition rates were recorded in the 

drought years of 1988 and 1989. The highest rates of wet deposition of NO3- within the basin were 

consistently in an area that extends from central Ohio eastward to the basin boundary (Figure 5.1). 

Wet deposition rates of Nos- generally decrease southward and westward from Ohio. The highest 

rates of wet deposition of NH4+ are centered in Iowa and generally decrease in all directions (Figure 

5.2); the lowest rates of wet NH4+ deposition are in Montana. 

110 100 90 80 

Base fmrn US Geolog~calSurvey d!gltal dala 1 2 OW OW 1980 

Albers Equal.Area Conr prolechon 

Standard parallels29 30 and 45 30 central mend~an-9600 

FIGURE5.2. Wet deposition of NH4+,averaged for 1990-96 data from the National Atmospheric Deposition Program (NADP, http://nadp.sws.uiuc.edul) in the 133 accounting 

units that make up the Mississippi River Basin. NOTE:Blue circles indicate where NADP and 

CASTNet (Clean Air Status and Trends Network) sites are co-located. 
Chapter 5: Nutrient Inputs and Outputs 59 

The average wet deposition for the interior basins also reflect this pattern. The highest wet NOj 

deposition was observed in interior basin 2, the Monongahela River at Braddock, Pennsylvania (427 

kg N km-9,whereas NH4+ deposition was hghest in interior basin 20, the Raccoon River at Van 

Meter, Iowa (344 kg N km-'; Figure 5.3 and Table 5.1). The highest total wet deposition was estimated at 665 kg N km-', in interior basin 22, the Illinois River at ~arseiiles,Illinois-more than six 

times that estimated for basin 27, the Bighorn River at Bighorn, Montana (105 kg N km-9.Wet 

deposition of NH4+ was about 80% of wet Nos deposition when averaged over the entire basin. 

EXPLANATION 

C3 NH, 

mNOJ 

-

-NH, error

NOgem 

" 1 2 5 3 22 6 8 7 2423 9 16 4 3534181910334236172021394015121441 11 133237303138328262725 

Basin ID Number 

FIGURE5.3. Wet deposition of Nor and NH4+in 42 sub-basins of the Mississippi River Basin, averaged for 1990-96 data from the National Atmospheric Deposition Program 

(NADP, http:llnadp.sws.uiuc.edul). NOTE: Vertical lines represent I standard deviation. 
60 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE5.1. Wet, dry, and total nitrogen deposition values (kg Nlkmz) for 

the overall Mississippi-Atchafalaya River Basin (MARB) and 42 interior basins averaged for 1990-96. 

Watershed Wet Deposition Dry Nor Total NOi 

Number Deposition and Organic N 

NO; NH,' Deposition 

MRB 200 203 140 440 
Chapter 5: Nutrient Inputs and Outputs 6 1 

5.1.5 Relations Between Wet and Dry Deposition 

A statistically signtficant positive correlation was observed between total dry and total wet deposition for the seasonal data at the 14 sites (Figure 5.4). However, significant variability in dry deposition measurements was not explained by wet deposition measurements (R" 0.18). The relation 

between wet and dry deposition varied at the individual sites from the moderately strong correlation 

observed at Parsons, West Virpa (R" 0.50), to statistically insignificant correlation at several 

sites. The average ratio of total dry deposition (particulate N03- and NH4+ plus HN03 vapor) to 

total wet deposition (N03- plus NH4+) for the 14 sites was 0.47. Individual values of the ratio of total dry deposition to total wet deposition ranged from 0.13 to 1.9, but 76% of these values were 

from 0.13 to 0.69 (Figure 5.5). The value of average dry Nos- deposition (particulate NO3- plus 

HN03 vapor) divided by the average wet Nos- deposition was 0.70. Wet N03- deposition did not 

explain a large amount of the variabihty in dry Nos- deposition (R2= 0.21). There was considerable 

variability in total dry deposition measurements among sites, and no geographical pattern was evident. If averaged for all sites and seasons, total dry N deposition was comprised of 81% HN03vapor, 16% particulate NH4+,and 3% particulate N03. 

0 

adj. 

WET DEPOSITION (kg N km'2) 

FIGURE5.4. Dry deposition data from CASTNet (Clean Air Status and Trends Network) 

as a function of wet deposition data from the National Atmospheric Deposition Program 

(NADP, http:llnadp.sws.uiuc.edul) at 14 sites in the Mississippi River Basin where dry and 

wet deposition measurement stations are co-located. NOTE:Values represent seasonal totals 

(winter, December-February; spring, March-May; summer, June-August; fall, September-November) 

from January 1989 through November 1994. (CASTNet data from Clarke et al. 1997.) 
62 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

RATIO OF DRY TO WET DEPOSITION 

(kg N (kg NY 

FIGURE 5.5. Distribution of the ratio of dry deposition data from CASTNet (Clean Air 

Status and Trends Network) to wet deposition data from the National Atmospheric 

Deposition Program (NADP, htt~:llnad~.sws.uiuc.edu/)at 14 sites in the Mississippi River 

Basin where dry and wet deposition measurement stations are co-located. NOTE:Values 

represent seasonal totals (winter, December-February; spring, March-May; summer, JuneAugust; fall, 

September-November) from January 1989 through November 1994. (CASTNet data from Clarke et a/. 

1997.) 

Average deposition for 1990-96, of wet N03- and NH4+,dry N03-, and total Nos- are summarized 

in Table 5.1 for the entire MRB and for the 42 interior basins. Examples of N-deposition fractions 

are shown in Figure 5.6 for 4 sites along a west to east transect in the basin for the period December 

1, 1992, through November 30, 1993. Depositions of all five fractions were lowest at the Wyoming 

site, and total N deposition at this site was less than half the deposition at the Oho site. Highest wet 

deposition of NO3- was measured at the West Virginia site, whereas hghest wet NH4+ deposition 

was measured at the Illinois site, and hghest HNO3 deposition was observed at the Ohio site. Total 

N deposition at the Illinois site was more similar to values at the West Virgmia site than at the 

nearby Ohio site. 

The dry fractions shown in Figure 5.6 represent N forms that are collected by the first two filters in 

the three-stage filter pack. The third filter, however, also collects a significant amount of N in compounds that are unidentified. Without knowledge of the chemical form of N collected by the third 

filter, a deposition velocity cannot be developed; therefore, deposition rates cannot be estimated. To 

evaluate the potential magnitude of N deposition that could be contributed by these unidentified 

forms, the deposition velocity developed for HN03 vapor was applied to the air concentrations of 

N measured by the third filter. Averaged for all sites and seasons, N collected by the third filter was 

46% of N collected as HN03 vapor by the second filter. If the deposition velocities for N particles 

had been applied, the deposition rate would have been considerably lower because the deposition 

velocities of HN03 vapor are at least an order of magnitude higher than the deposition velocities of 

N particles. 
Chapter 5: Nutrient Inputs and Outputs 63 

Snowy Bondville. Oxford. Parsons, 

Ridge, IL OH WV 

WY 

FIGURE5.6. Chemical species of wet deposition data from the National Atmospheric 

Deposition Program (NADP, http://nadp.sws.uiuc.edu/) and dry deposition data from CASTNet (Clean Air Status and Trends Network) measured at 4 sites on a west-to-east transect 

across the Mississippi River Basin for December 1992 through November 1993. (CASTNet 

data from Clarke et a/. 1997.) 

5.1.6 Discussion 

The regional patterns of wet deposition of NO3- and NH4+reflect the regonal patterns of emissions 

and atmospheric transport processes. The highest rates of Nos- deposition occur in Oho and Pennsylvania (Figure 5.1), northeast of the concentration of high-emitting electric uality plants located in 

southern Indana and western Kentucky (NAPAP 1993). Fossil fuel combustion is a known source 

of NO and NO?, which are oxidized in the atmosphere to form HN03 vapor and particulate N03-. 

Particulate N03- can have a long residence time in the atmosphere, whch facihtates long-range 

transport. Once formed, HN03 vapor has a hgh deposition velocity and a relatively short residence 

time, although it can react with other pollutants such as NH3 to form particles with low deposition 

velocities. Significant atmospheric transport of N from midwestem power plants to the northeastern 

states has been well established (NAPAP 1993). 

Wet and dry deposition of NH4+ is generally atuibuted to NH3 emissions from high concentrations 

of livestock and N fertilization of croplands (Vitousek et al. 1997). Dry deposition estimates do not 

include dry deposition of NH3, which could be significant relative to wet and dry deposition of 

NH4+ (Ferm 1998). Emissions from automobiles can also contribute atmospheric NH3, but in the 

South Coast Air Basin of California, which includes Los Angeles and surrounding developed areas, 

estimates of NH3 emissions from automobiles did not exceed agricultural sources (Fraser and Cass 

1998). The highest levels of wet NH4+ deposition in the Mississippi Basin are centered in Iowa, an 

intensively agricultural state (Figure 5.2). 
64 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

In contrast to NO and NO2 released from fossil fuel combustion, NH3 released to the atmosphere is 

already in a highly water-soluble form that is effectively scavenged by precipitation and vegetation. 

This characteristic results in deposition of NH3 closer to sources than deposition of other forms of 

N emitted to the atmosphere. Transport distance, however, depends on wind speed and reactions 

with other pollutants. Modeled estimates of NH3 transport by ~sman!and van Jaarsveld (1992) indicated that 46% of emitted NH3 was deposited within 50 km of the source; 40% as dry deposition 

and 6% as wet deposition. Results from a separate modeling effort described in Ferm (1998) indicated that 49% of NH3 emitted in a 22,000-km2region in Sweden was deposited within this same 

region, with 21% as dry deposition and 28% as wet deposition. 

Although a large fraction of emitted NH3 tends to be deposited near its source, reactions with 

H2S04 and HN03 to form particulate NH4+ can greatly increase transport. Therefore, high atmospheric concentrations of SO? and NO, significantly enhance transport of NH3. Deposition research 

in the Netherlands found that NH, deposition beyond 300 km of the source was halved approximately every 450 km, a pattern similar to that of SO, compounds (Ferm 1998). High emissions of 

SO2 and NO, in Illinois, Kentucky, Indiana, and Ohio most likely enhance the transport of NH4 

from the agricultural regions in the central part of the Mississippi Basin to eastern sections of the 

basin, as well as across the basin boundary. 

Based on NADP and CASTNet data, NH4+ deposition represents approximately 35% of total N 

deposition in the MARB, but the collection methods of both programs probably result in an underestimation of this fraction. Some of the NH4+ collected by NADP buckets may be converted to organic nitrogen through microbial assimilation between the time of deposition and the weekly 

collection (Vet et al. 1989). And the three-stage filter pack used in the CASTNet program is designed to collect NH4+ particles, but not NH3, which has a deposition velocity approximately five 

times higher than that of the NH4+ particles (Ferm 1998). Deposition of gaseous NH3, therefore, is 

likely to represent a significant fraction of dry deposition, but primarily in the vicinity of sources because of the short residence time of NH3 in the atmosphere. 

Other fractions of N deposited from the atmosphere include organic forms, such as peroxyacetyl 

N03-(PAN). Most of the studies of organic N in the atmosphere, however, have been investtgations 

of urban air quality. One exception is the recent study of Scudlark et al. (1998), in which deposition 

of organic N in precipitation was measured in the Chesapeake Bay watershed. This study showed 

that organic N comprised approximately 20% of total N in wet deposition, and that to obtain reliable estimates, samples needed to be collected daily. The N collected by the third filter of the 

CASTNet filter packs may include some of the same organic-N compounds measured in precipitation in the Scudlark et al. (1998) study. 

The wet deposition of Nos- and NH4+ are the least uncertain of the N deposition estimates discussed above. The wet-only bucket approach is a direct measurement of deposition that does not 

require additional meteorologcal measurements or modeling. The large number of sites dismbuted 

nationwide also enables realistic interpolation for regional or watershed assessments. However, 

monitoring of organic N in precipitation is needed to evaluate the temporal and spatialvariability of 

this fraction. 
Chapter 5: Nutrient Inputs and Outputs 65 

Dry deposition estimates have a higher degree of uncertainty than the wet deposition estimates, but 

the level of uncertainty is difficult to quantify. Eddy correlation techniques provide a direct measurement that can be compared with the filter pack/deposition velocity modeling approach, but this 

method can only be used for short measurement periods (30-120 minutes) and cannot be used to 

measure HN03 deposition. The uncertainty in the accuracy of dry deposition of HN03 vapor and 

Nos- particles has been subjectively estimated by Clarke et al. (1997) to be 40%. The precision of 

CASTNet measurements of deposition is approximately 12% for HN03 vapor and 17% for N03-

particles (Clarke et al. 1997).The current location of CASTNet monitoring sites is a severe limitation 

on efforts to accurately estimate dry deposition in the region between the Mississippi River and the 

Rocky Mountains. 

A limited number of investigations have shown that dry deposition can vary greatly over distances 

less than a kilometer, particularly in varied terrain (Clarke et al. 1997). The importance of these 

small-scale variations, however, may significantly decrease at some larger scale. Examples of this 

scale effect have been identified for measurements of streamflow (Wood et al. 1989) and stream 

chemistry (Wolock et al. 1997). Defining how spatialvariation of CASTNet measurements vary with 

scale would significantly increase the utility of these data. 

5.1.7 Budget Implications 

Despite the uncertainties of dry deposition estimates, the CASTNet measurement approach is sufficiently reliable to indicate that, in general, dry deposition is (1) positively correlated with wet deposition and (2) of similar magnitude to wet deposition. This information can be used in conjunction 

with NADP data to estimate total deposition of N (wet plus dry) to subregions of the basin for the 

purpose of N budget estimates. Wet and dry deposition of Nos- compounds should be considered a 

budget input because these compounds origmate largely from combustion of fossil fuels, which otherwise would be unavailable for biological utilization. Dry deposition of HNO3 and Nos- can be 

approximated throughout the basin by multiplymg wet deposition of N03- by the fraction dry deposition/wet deposition (0.70), determined at the 14 sites where wet and dry deposition measurement 

stations were co-located. Dinnel (1998) determined a value of 0.75 for this fraction with 1990-92 

CASTNet and NADP data. 

Organic N deposition is likely to contribute to atmospheric N inputs, although the magnitude of the 

deposition rate is highly uncertain. If the fraction of organic N/total N in wet deposition measured 

by Scudlark et al. (1998) is assumed to be similar to the fraction that occurs in the Mssissippi Basin, 

wet deposition of organic N in the basin can be estimated as 0.25 multiplied by total wet deposition. 

To determine an estimate of dry deposition that includes organic N, wet deposition can be multiplied by the fraction of dry deposition that includes N colle~tedby the third frlter/wet deposition 

(1.0), although the third filter of the filterpack air sampler may also collect inorganic forms of N. 

Wet and particulate NH4+ comprise a significant fraction of atmospheric N deposition throughout 

the basin. Results show (1) the region of highest NH3 deposition is in the center of the basin, (2) 

half or more of emitted NH3 is deposited within 300 km of the source, and (3) the lowest deposition 

is on the western (windward) side of the basin. Most of the NH4+ deposition within the basin, therefore, is likely to be the result of internal sources, which indicates that the overall basin can be considered a net source of NH3 emissions. The substantial variability of NH4+ deposition within the 

basin, however, means that some basins are net sources, whereas others are net sinks. Because most 

of the NH4+ emissions are either directly or indirectly the result of the use of fertilizers and manure, 

budget estimates that include fertilizer or manure inputs of N would overestimate total inputs, if at-
66 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

mospheric deposition of NH4 is also included (Howarth et al. 1996). The atmospheric deposition of 

NH4+,therefore, should be considered an internal transport process, rather than a basin input. 

5.2 AGRICULTURAL~NPUTSAND OUTPUTS 

Agricultural activities, such as row crop cultivation and livestock production, can be significant nonpoint-source inputs of N and P. The application of commercial fertilizer to cropland is the primary 

input of "new" N and P in most areas of the MARB. In many parts of the basin, fixation of atmospheric N by legumes is a significant input of "new" N, and animal manure is a significant input of 

"recycled" N and P. Mineralization of organic matter in agricultural soils can also be considered an 

agricultural input of recycled N and P, which is largely a combination of mineral N and P inherent 

to the soil, rnicrobially immobilized fertilizer or manure, and organic crop remains (Gentry et al. 

1998;Cambardella et al. 1999). 

Rates of mineralization are largely controlled by cover type and soil ullage. Nutrients can be removed in harvested crops. Nutrients in harvested crops can be exported from the basin in food or 

animal products, or can be consumed and cycled again within the basin. N can also be lost by volatilization from soils, manure, or plants during senescence, or by denitrificationin the soils, wetlands, 

and river bottoms. Both N and P can be immobilized in the soil zone and lost from cropped areas 

with soil erosion. Management practices on cropped land, such as conservation ullage and crop rotation, can reduce nutrient losses, whde tile drainage wiU most likely increase nutrient losses from 

cropped land (NRC 1993; Gentry et al. 1998). Nutrients are consumed by forested land and wetlands, so these landscapes will ultimately affect the nutrient budget by reducing nutrient losses from 

watersheds (Mitsch and Gosselink 1993; Aber et al. 1995; Gersburg et al. 1983; Nolan et al. 1997; 

Battaglin and Goolsby 1998). Use of riparian buffer strips has been suggested by many researchers 

as a means to intercept agricultural pollutants in sediment, runoff, and shallow ground-water flow 

before they can reach streams (NRC 1993). 

Estimates of annual N and P inputs from agricultural sources were compiled for the 20 states listed 

below, which comprised most of the agricultural land in the MARB during 1951-96. These estimates are used to show temporal trends in N and P inputs and outputs, and in developing state- and 

MARB-level N and P budgets, discussed later in this report. 

Arkansas Kansas Missouri South Dakota 

Colorado Kentucky Montana Tennessee 

Illinois Louisiana Nebraska West Virginia 

Indiana Minnesota Ohlo Wisconsin 

Iowa Mississippi Oklahoma Wyoming 

Estimates of N and P inputs and outputs associated with agriculture were also compiled by county 

for 1992. These estimates are used here to show a more detailed picture of the spatial distribution of 

inputs and outputs and, later in the report, to make comparisons with nutrient yield estimates at the 

three basin scales-the IMARB, the nine large basins, and the 42 interior basins. 
Chapter 5: Nutrient Inputs and Outputs 67 

5.2.1 Fertilizer 

Untd the 19th century, increased food production in the United States was largely the result of an . . expandmg cropland base, the addition of nutrients in animal manure, and the mmmg of soil nutrients. By the 20th century, soil fertdity and crop yields were maintained by the addtion of N and P 

containing natural waste materials, such as animal manure, seaweed, bonemeal, and guano. Bepning in the 1940s manufactured fertilizers, such as superphosphates, urea, and anhydrous ammonia, 

replaced most "natural" fertilizers (USDA 1997). Since the 1960s, the yields per acre for major crops 

have doubled. Some of this increase can be attributed to better plant hybrids, and some can be attributed to increased application of crop nutrients. Figure 5.7 shows the estimated N content of 

commercial fertilizers sold in the 20 basin states during 1951-96. The state-level fertilizer N inputs 

were compiled from Alexander and Smith (1990), Battaglin and Goolsby (1995), and USDA (1998). 

These estimates include both agricultural and nonagricultural fertilizer sales. Estimates of nonagricultural fertihzer use represent 5-20% of the total use (H. Taylor, USDA, written communication, 

1998). During 1951-96, annual ferdzer inputs increased form < 1 to > 6 don metric tons. 

atmossheric nitrate 

Year 

FIGURE 5.7. Annual nitrogen inputs to the Mississippi-Atchafalaya River Basin, 195 1-96 

NOTE:See text for sources of data and methods used to estimate inputs. 
68 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Ferultzer applications have a spatial pattern closely related to the pattern of crop production. Figure 

5.8A shows the spatial distribution of N inputs in kilograms N per square kilometer per year (kg 

N/km2/yr) from commercial fertilizer. These use estimates by hydrologic accounting unit (Seaber et al. 

1987) were generalized from 1992 county-level data developed from reported state sales totals and estimates of county-level expenltures for fertilizer from the 1992 Census of Agriculture (USDC 1995). In 

much of the upper Midwest, inclulng most of Iowa, Illinois, and Indiana, inputs of N from commercial 

ferultzer exceed 5,000 kg N/km2/yr (Figure 5.8A). In this same area, inputs of P from commercial ferttlizer exceed 1,000 kg P/km2/yr. Tables 5.2 and 5.3 contain estimates of the N and P inputs in fertilizer 

for the three basin scales. 

Plants only use a portion of the N and P in applied ferultzers. The unused N and P, whch can be 50% 

or more of the applied amount, is retained in the soil or lost from the soil through volatihzation, leaching, or erosion (Oberle and Keeney 1990; Barry et al. 1993; David et al. 1997; Cambardella et al. 1999). 

Fertihzer stabilizers, nitrification inhbitors, and slow-release formulations can help reduce nutrient loss 

by delaying nutrient mobihzation or timing nutrient release to better coincide with crop demands 

(USDA 1997;Diez et al. 1996; Serna et al. 1996). 

5.2.2 Legume Fixation 

Certain crops and native plants belonging to the legume family, such as clovers, alfalfa, and beans, establish a symbiotic relationship with microbes from the rhizobium family. These microbes reside in nodules 

on the roots of host plants and can fix atmospheric N, which is either used by the legume plant or remains in the soil where it can undergo minerahtion and nitrification. The amount of N fixed by crops 

varies as a function of the crop yield (Barry et al. 1993); soil conltions, such as the avadability of inorganic N, drainage, pH, and moisture content; and clunatic conltions. Rates of fixation range from < 

500 kg N/km*/yr for some types of beans and clover to > 60,000 kg N/kmVyr for alfalfa. Estimates of 

N furation in pastureland range from 100 to 1,500 kg N/km2/yr ('Jordan and Weller 1996). Legume 

crops use more N than they fur. Soybean crops will use symbiotically fured N, minerahzed soil N, and 

maybe even some organic N to meet their N requirements (Barry et al. 1993;David et al. 1997; Gentry et 

al. 1998). Even when other conditions are favorable, the presence of available N in the soil will lscourage N furation by legumes (Buckman and Brady 1969; Gentry et al. 1998). Some nonlegume species of 

plants can also fix nitrogen, but are not considered in this report, nor is the fixation of N by nonsymbiotic bacteria, which is estimated to be less than 700 kg N/km2/y (Barry et al. 1993). 

For this study, N inputs from fixation by legumes were estimated using crop and pastureland data by 

state from USDA (1998) and by county from the 1992 Census of Agriculture (USDC 1995). Table 5.4 

lists the N-fixation rates used in ths study and the range of estimates reported in the literature (Meisinger and Randall 1991; NRC 1993; Troeh and Thompson 1993). Many of the N-furation rates used in 

this study are the same as those used by Jordan and Weller (1996). Pasture and rangeland in Mmnesota, 

Iowa, Missouri, Louisiana, and states to the east were assigned a pasture N-furation rate, whde the rangeland N-fixation rate was used for pasture and rangeland in western states. Figure 5.7, whch shows the 

amounts of N estimated to have been fured annually by legumes and pasture in the 20 basin states for 

1951-96, indicates that during 1950-90,legume-N inputs increased by 2.5-4.0+ don metric tons/yr. 

Figure 5.8 shows an estimate of the N fured by all legumes in 1992 by hydrologic accounting unit. In 

much of the upper Midwest, including large parts of Iowa, Illinois, Missouri, Minnesota, and Indiana, 

inputs of N from legume fixation exceed 2,800 kg N/km2/yr (Figure 5.8B). Estimates of the total N 

fured by all legumes, alfalfa, nonalfalfa hay, pasture, and rangeland for the three basin scales are presented in Table 5.2. 
Chapter 5: Nutrient Inputs and Outputs 69 

FIGURE5.8. Nitrogen inputs in hydrologic accounting units in 1992 from (A) fertilizer, (B) 

legumes, (C) livestock manure, (D) soil mineralization, (E) industrial point sources, and (F) 

municipal point sources. 

Cha~ter5: Nutrient ln~utsand Out~uts 7 1 

TABLE5.3. Estimated annual inputs of phosphorus as P in the Mississippi-Atchafalaya 

River basin, 9 large sub-basins, and 42 interior basins (in metric tons). 

Basin 

ID 

Basin Name and Location Commercial 

Fertilizer 

Industrial 

Point 

All 

Manure Municipal Point 

Sources Sources 

Intire MARB 

P Large Sub-basins 

I Upper Ohio 

2 Lower Ohio 

3 

4 Upper Missouri Lower Missouri 

5 

6 

7 

Up er Mississippi

~tidleMississippi Arkansas 

8 

9 

Lower Mississippi 

Red and Ouachita 

I2 Interior Basins 

Allegheny River at New Kensington, PA 

Monongahela River at Braddock, PA 

Muskingham River at McConnelsville,OH 

Kanawha River at Winfield, WV 

Scioto River at Higby, OH 

Great Miami at New Baltimore, OH 

Kentucky River at Lockpoq KY 

Wabash River at New Harmonv. IN 

Cumberland River near Grand Rivers, KY 

Tennessee River near Paducah, KY 

Mississippi River near Royalton, MN 

Minnesota River at Jordan, MN 

St. Croix River at St. Croix Falls, WI 

Chippewa River at Durand, WI 

Wisconsin River at Muscoda, WI 

Rock River near oslin, IL 

Cedar River at 2edar Falls, IA 

Iowa River at Wapello, IA 

~ncludesCedar River Basin- 17) I'kunk River at Augusta, IA 

Raccoon River at Van MeterIDes Moines, IA 

Des Moines at St. Francisville, MO (includes Raccoon River Basin-20) 

Illinois River at Marseilles, IL 

Lower Illinois River Basin 

Entire Illinois River Basin (basins 22 and 23) 

Kaskaskia River near Vened Station, IL 

Milk River near Nashua. M? 

Missouri River near Culbertson, MT 

Bi horn River near Bighorn, MT 

~ilowstoneRiver near Sydney, MT 

Cheyenne River at Cher Creek SD 

Imes River near ~cotlanTSD 

latte River near Louisville, NE 

Kansas River at Desoto, KS 

Grand River near Sumner, MO 

Osa e River below St. Thomas, MO 

St. ?rancis Bay at Riverfront, AR 

White River at Clarendon, AR 

Arkansas River at Tulsa, OK 

Canadian River at Calvin, OK 

Yazoo River at Redwood, MS 

Big Black River near Bovina, MS 

Red River at Alexandria, LA 

Ouachita River near Columbia, LA 
72 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE5.4. Estimated rates of nitrogen fixation by legumes (in kg NlkmYyr). 

I Legume Estimate Used Low Estimate High Estimate 

Alfalfa 

Soybeans 

Other Hay 

Cowpeas 

Peanuts 

Lentils 

Dry Beans 

Pasture (Midwest) 

Rangeland (west) 

NOTE:Table is modified from Meisinger and Randall 1991; NRC 1993; and Troeh and Thompson 1993. 

5.2.3 Animal Manure 

Animal manure can be a significant source of N, P, and other nutrients needed for crop growth. If 

applied to fields, manure can also add organic matter, improve soil quality, increase water- and nutrient-holdmg capacities, and increase resistance to soil compaction. Improper use or disposal of manure can lead to the buildup of N and P & soils and the loss of N and P to surface or ground water 

(NRC 1993). The nutrients in most animal manure are "recycled," since they originate from feed 

produced in the basin and given to the animals. The nutrient content of manure is highly variable 

and depends on such factors as type of feed, type and age of livestock, type of bedding material, and 

storage and handling practices. A common average composition estimate for manure is 0.5% N, 

0.125% P, and 0.4% K proeh and Thompson 1993). These nutrient content percentages are about 

20 times smaller than those commonly found in commercial fertilizers. 

In a prior investigation, livestock inventory estimates from the 1987 Census of Agriculture (USDC 

1989) and manure nutrient content estimates from the Soil Conservation Service's Agn'czdtzraL Watte 

Management Field Handbook (1992) were used to estimate annual manure inputs in all U.S. counties 

(Puckett 1994). This manure was estimated to contain 5.9 million metric tons of N. Using the same 

data, Battaglin et al. (1997) estimated that about 3.5 million t of N and 1.2 don metric tons of P 

were generated annually in the MARB. These estimates did not account for losses of N (or P) from 

manure handling or storage, for animals with more than one marketing or life cycle per year, or for 

animals of differing size. More recently, Lander et al. (1998) calculated the amounts of nutrients 

available from livestock manure relative to crop requirements for U.S. counties. Although their estimates account only for animals in confined feeding operations, they do include multiple marketings 

per year and nutrient losses in storage and handling. These estimates are much smaller than those 

given by Puckett (1994). Using Lander et al.'s data, an estimated 0.5 million metric tons of N and 0.3 

million metric tons of P are made available from manure for crops (or leaching) annually in the 

MARB. 

In this study, the N and P inputs from manure were estimated by state for 1951-96 using livestock 

inventory data from the National Agricultural Statistics Service (NASS) (USDA (1998), and a strategy for estimating N and P in manure waste that is sunilar to the method used by Lander et al. 

(1998), Puckett (1994), and Hoeft (1998). This method was also applied to county-level data from 
Chapter 5: Nutrient Inputs and Outputs 73 

the 1992 Census of Agriculture (USDC 1995). Coefficients used to estimate animal N and P production and losses during storage and handling are from the Midwest Planning Service-Livestock Waste 

Subcommittee (1985) or the Soil Conservation Service's (now National Resource Conservation Service) Agrictltttral Waste Management Field Handbook: (1992). The method accounts for both multiple 

marketings per year and nutrient losses in storage, handling, and application. Estimates of manure 

nutrient inputs and losses were made separately for hogs, cattle, poultry, sheep, and horses, and were 

summed by county. 

The N and P content in hog and pig manure was estimated using year-end inventory numbers from 

NASS or the Census of Agriculture. Hogs and pigs have less than a one-year life cycle, but the inventory numbers were assumed to be sunilar to inventories during the rest of the year. Data on 

numbers of animals by weight class were available from NASS but not from the Census of Agriculture, so when Census inventory numbers are used, all hogs and pigs were assumed to produce N and 

P at the rates estimated for 60-119-pound animals Fable 5.5). 

TABLE5.5. Estimates of the nitrogen and phosphorus voided in animal manure (in kilograms per day). 

Animal Nitrogen Phosphorus 

(kglday) (kgldoy) 

Hogs and Pigs 

<60 Ibs. hogs 

60-1 19 Ibs. 

120-179 Ibs. 

>180 Ibs. hogs 

Milk Cows 

Beef Cows 

Dairy Heifers 

Steers and Bulls 

Slaughter Cattle 

Chickens and Hens 

Pullets and Broilers 

Tom turkeys 

Hen Turkeys 

Sheep and Lambs 

Horses and Ponies 

People 

\IOTE: Table is modified from Midwest Planning ServiceLivestock Waste 

iubcommittee 1985 and U.S. Soil Conservation Service 1992. NRC 

1 993;and Troeh and Thompson 1993. 

The N and P content in cattle manure was estimated using year-end inventory numbers from USDA 

or the Census of Apculture for milk cows, beef cows, steers and bulls, and heifers. Most milk and 

beef cows have a one-year or longer life cycle, so year-end inventory numbers are likely to be representative of inventories during the rest of the year. However, some heifers and steers are slaughtered 

during the year and may or may not be accounted for in these inventories. When Census of Agricul-
74 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

ture data were used, it was assumed that one-half the number of steers and heifers inventoried were 

slaughtered during the year. When NASS data were used, it was assumed that as all steers, two-thirds 

of the beef heifers and all other heifers were slaughtered during the year. Slaughter animals were assumed to generate N and P for 170 days. Cattle were assumed to produce N and P in manure at the 

rates given in Table 5.5. 

The N and P content in poultry manure was estimated using year-end inventory numbers from 

NASS or the Census of Agriculture on hens, pullets, broilers, and turkeys. Hens, pullets, and broilers 

all have a shorter than one-year life cycle, but year-end inventory numbers were assumed to be representative of inventories during the rest of the year. However, turkeys were assumed to be in residence for only part of the year (112-133 days). Poultry were assumed to produce N and P in manure 

at the rates given in Table 5.5. Again, the NASS and Census of Agriculture data categories did not 

match exactly, so some data-specific modifications to the calculations were made. For example, in 

NASS data, broiler chickens were reported as production over the year, whde in the Census of Agriculture, they were reported as a year-end inventory. 

The N and P content in sheep and horse manure was estimated using year-end inventory numbers 

from NASS or the Census of Agriculture on sheep, lambs, and horses, ponies. Sheep and lambs are 

likely to have a shorter than one-year life cycle, but year-end inventory numbers were assumed to be 

representative of inventories during the rest of the year. Sheep and horses were assumed to produce 

N and P in manure at the rates given in Table 5.5. 

Figure 5.7 shows the amounts of N estimated to have been produced by livestock manure in the 20 

basin states for 1951-96. It indicates that manure N inputs did not change sipficantly during this 

period. Figure 5.8C shows an estimate of the N produced in livestock manure in 1992 by hydrologic 

accounting unit, indicating that in parts of Iowa, Illinois, Wisconsin, Oklahoma, and Texas, inputs of 

N from manure exceed 2,000 kg N/krn2/yr. Estimates of the N and P inputs, in manure, for the 

three basin scales are given in Tables 5.2 and 5.3, respectively. 

5.2.4 Soil Mineralization 

The majority of the N and P in soils is in organic forms that are not readily available to higher 

plants. This organic N and P can be in the form of microbial biomass; crop remains, such as straw, 

stalks, and roots; or otherwise immobilued fertilizer and manure N and P. Mineralization is the 

process by which the organic N and P are converted to inorganic forms. These forms, such as 

ammonium, nitrate, and orthophosphate, can be used by plants and can leach to ground and surface 

water. Rates of N and P mineralization in soils are a function of many conditions, including soil 

moisture content, temperature, cover type, management practices, and soil organic content (Troeh 

and Thompson 1993; Powers et al. 1998; Gentry et al. 1998). 

Mineralized N from soil organic matter is a significant source of nitrate, particularly in areas where 

the organic matter content is high or the climate is warm. The total nitrogen content of agricultural 

soils averages about 333,000 kg/km2 in the upper 30 cm of soil (Troeh and Thompson 1993), of 

which all but about 1% is organic. Reported annual rates of N mineralization range from 0% to 50% 

of the organic N. Nitrogen mineralization rates in soils that are cultivated are generally much larger 

than rates in uncultivated soils but smaller than those on soils that are cultivated for the first time 

(Troeh and Thompson 1993). New crop residues decompose and mineralize more rapidly than old 

residues (Schepers and Mosier 1991). Some researchers have suggested that the addition of N fertilizer to soils increases the rate of organic N mineralization (Azam et al. 1993; Rao et al. 1991). Rates 
Chapter 5: Nutrient Inputs and Outputs 75 

of N mineralization in soils can range from near zero in very dry sandy soils to more than 40,000 kg 

N/kmVyr in soils cultivated for the first time (Troeh and Thompson 1993). N mineralization rates 

of 1-3% of the organic N are commonly used for agricultural regions of the U.S. Midwest (Oberle 

and Keeney 1990; Schepers and Mosier 1991; NRC 1993; Gentry et a1. ,1998). Schepers and Mosier 

(1991) suggest that rates of mineralization should be viewed with an uncertainty of 25-50%. 

In this study, the potentially mineralizable N in soils was calculated (Burkart and James 1999) using 

information in the STATSGO soils database (USDA 1994). First, the mass of organic matter (in 

kg/km? in the upper 30 centimeters (cm) of soil was calculated as the product of the soil bulk density, percent organic matter content, and volume. The soil N content was estimated as 3% of the 

organic matter (Stevenson 1994). The soil organic N was estimated to mineralize at a rate of 2% per 

year in cultivated soils (Buckman and Brady 1969; Schepers and Mosier 1991; Gentry et al. 1998). 

Total potentially mineralizable N estimates were computed for STATSGO map units and then generalized to counties using area-weighted averages. 

The mineralization model was applied only to cropped land. Research by Tate (1990) and Dodds et 

al. (1996) suggests that while mineralization occurs, little N is lost to ground water or streams from 

native tall-grass prairie land. Similarly, research by Friedland et al. (1991), Swank and Vose (1997), 

Kortelainen et al. (1997), and Miller and Friedland (1999) shows that although N mineralization occurs in forested soils, little of this N leaves the forest ecosystem. Therefore, data from the 1992 Census of Agriculture (USDC 1995) were used to estimate the percentage of each county that was 

cropped land. Then, total potential mineralizable N estimates, by county, were multiplied by the percent of cropped land in the county, to determine potentially mineralizable N (Burkart and James 

1999). Estimates of potentially mineralizable N from soil organic matter for the three basin scales 

are given in Table 5.2. Figure 5.8 shows the spatial distribution of potentially mineralizable N in hydrologic accounting units in the MARB. In much of the upper Midwest, including large parts of 

Iowa, Illinois, Mmnesota, and Indiana, potential inputs of mineralized soil N exceed 5,000 kg 

N/kmVyr. The rates of mineralization reported here are similar to minerahation rates measured 

beneath Illinois soybean and corn crops of 8,800 and 13,300 kg N/km2/yr, respectively (David et al. 

1997). 

The organic matter in soils also contains P in organic combinations, which can be mineralized by 

microbes or dissolved by water. The total phosphorous content of soils averages about 0.05%, or 

about 100,000 kg/km2 in the upper 30 cm of soil Froeh and Thompson 1993). Most of this P is 

bound in forms that cannot be readily used by higher plants. The availability of inorganic P in soils is 

a function of its solubility, which varies with soil pH, the presence of iron, aluminum- and manganese-containingminerals, organic matter content, and microbial activity, which varies with soil moisture content, and temperature. Unlike nitrate, phosphorus is not very water soluble, and when in 

contact with sediments or soils, dissolved phosphorus will tend to become bound by anion adsorption (NRC 1993). In nonagricultural soils the amount of P mineralized and available to plants at any 

one time is small and readily removed by the plants. In apcultural soils, P is frequently added in 

fertilizer or manure to meet the need of high-yield crops. Only a portion of the added P is available 

to and used by the crop; the remaining P is immobilized in the soil and available for mineralization. 

The loss of P from watersheds, both in solution and on sediment, is a function of P levels in watershed soils (NRC 1993). Most of the P lost from cropland is not in solution, but is bound to eroded 

soil particles (NRC 1993). 

Few data are available on the rate of P mineralization in soils, and inputs of P from mineralization 

were not estimated in this report. 
76 Flux and Sources of Nutrients in the Mississippi-Atchafalava River Basin 

5.2.5 Nutrient Removal in Crops 

Estimates of the amount of N and P removed from basins in harvested crops were calculated using 

crop acreage from NASS or the 1992 Census of Agnculture, and crop yields by state from NASS. 

Estimates of the N and P content of harvested crops are from Meisinger and Randall (1991), Troeh 

and Thompson (1993), and Lander et al. (1998). The crops included in the calculation are alfalfa, 

corn, sorghum, soybeans, wheat, other hay, pasture, and rangeland. 

The coefficients for N and P removal in harvested crops used in this study appear in Table 5.6. N 

and P removal in grazed pasture was calculated using grazed cropland and pasture acreage data from 

NASS or pasture and rangeland data from the 1992 Census of Agnculture. The coefficients for N 

and P removal per unit for pasture and rangeland were the same as for other hay, but yields were 

reduced to account for the lower productivities of these landscapes (Jordanand Weller 1996). In this 

study, one-half of the other hay yield was applied to grazed cropland, one-fourth to pasture, and 

one-tenth to rangeland. 

TABLE5.6. Estimated rates of nitrogen and phosphorus removal in harvested crops 

(in kg N or P per common yield unit). 

Crop Unit Nitrogen Phosphorus 

Alfalfa Ton 

Corn for Grain Bushel 

Corn for Silage 

Sorghum for Grain 

Sorghum for Silage 

Soybeans 

Wheat 

Ton 

Bushel 

Ton 

Bushel 

Bushel 

Other Hay 

Pasture 

Ton 

Ton 

Note: Table is modified from Meisinger and Randall 1991; Troeh and Thompson 1993; and Lander et al. 1998. 

Figure 5.9 shows the estimated amounts of N removed with harvested crops and grazed pasture and 

rangeland in the 20 basin states for 1951-96. It indicates that removals increased from about 4 to 

nearly 10 &on metric tons/yr during ths period. Figure 5.10A shows an estimate of the N removed in harvested crops and pasture in 1992 by hydrologic accounting unit. In much of the upper 

Midwest, including most of Iowa, Illinois, and Indiana, these outputs of N exceed 7,000 kg 

N/krn2/yr (Figure 5.10). Estimates of the N and P outputs in harvested crops and grazed pasture 

for the three basin scales are given in Tables 5.7 and 5.8, respectively. 
Chapter 5: Nutrient Inputs and Outputs 77 

Year 

FIGURE 5.9. Annual nitrogen outputs from the Mississippi-Atchafalaya River Basin, 195 1-

96. NOTE:See text for sources of data and methods used to estimate outputs. 
78 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

FIGURE5.10. Nitrogen outputs in hydrologic units in 1992 from (A) harvested crops, (B) 

fertilizer volatilization, (C) manure volatilization, (D) plant senescence, (E) soil denitrification, and (F) soil immobilization. 
Chapter 5: Nutrient Inputs and Outputs 79 

TABLE 5.7. Estimated annual outputs of nitrogen as N in the Mississippi-AtchafalayaRiver Basin, 9 

arge sub-basins, and 42 interior basins (in metric tons). 

Basin ID md Name, 

md Loution 

Crop 

Hamest 

Pasture 

Grazing 

Crop 

Senescence 

Soil 

Denitrifiution 

All Soil Fertilken 

Mmun lmmobili- VolatffiLosses zatbn adon 

lUver 

Flux 

Entire MARB 

D Large Sub-basins 

I Upper Ohio 

2 Lower Ohio 

3 Upper Missouri 

4 Lower Missouri 

5 Upper Mississippi 

6 Middle Mississippi 

7 Arkansas 

8 Lower Mississippi 

9 Red and Ouachita 

42 Interior Basins 

I Allegheny River at New Kensington. PA 

2 Monongahela River at Braddock, PA 

3 Muskingham River at McConneisville, OH 

4 Kanawha River at Winfield. WV 

5 Scioto River at Higby. OH 

6 Great Miamiat New Baltimore. OH 

7 Kentucky River at Lockpoq KY 

8 Wabash Riwr at New Harmony. IN 

9 Cumberland River near Grand Rivers. KY 

10 Tennessee River near Paducah. KY 

II Mississippi River near Royalton. MN 

12 Minnesota River at Jordan. MN 

13 St Croix River at St Croix Falls. WI 

14 Chippewa River at Durand, WI 

15 Wisconsin River at Muscoda, WI 

16 Rock River near joslin. IL 

17 Cedar River at Cedar Falls. IA 

18 Iowa River at Wapdlo. IA (includes Cedar 

River basin-1 7) 

19 Skunk River at Augusta. IA 

10 Raccoon R at Van MeterlDes Moines. IA 

2I Des Moines at St Francisville. MO (includes 

Raccoon River basin-20) 

22 Illinois River at Marseilles. IL 

23 Lower Illinois River Basin (IRB) 

-- Entire IRB (basins 22 and 23) 

24 Kaskaskia River near Venedy Station. IL 

25 Milk River near Nashua. MT 

26 Missouri River near Culbertson. MT 

27 Bighorn River near Bighorn, MT 

28 Yellowstone River near Sydney. MT 

29 Cheyenne River at Cherry Cr& SD 

30 James River near Scotland. SD 

3 1 Plane River near Louisville, NE 

32 Kansas River at Desoto, KS 

33 Grand River near Sumner, MO 

34 Osage River below St Thomas, MO 

35 St Francis Bay at Riverfront AR 

36 White River at Clarendon. AR 

37 Arkansas River at Tulsa, OK 

38 Canadian River at Calvin, OK 

39 Yazoo River at Redwood, MS 

40 Big Black River near Bovina. MS 

4 1 Red River at Alexandria. LA 

42 Ouachita River near Columbia, LA 
80 Flux and Sources of Nutrients in the Mississi~~i-AtchafalavaRiver Basin 

TABLE5.8. Estimated annual outputs of phosphorus as P in the Mississippi-Atchafalaya 

River Basin, 9 large sub-basins, and 42 interior basins (in metric tons). 

--

Basin Basin Name and Location Crop Pasture River 

ID Harvest Grazing Flux 

Entire Mississippi-Atchafalaya River Basin 

Large Sub-basins 

I Upper Ohio 

2 Lower Ohio 

3 Upper Missouri 

4 Lower Missouri 

5 Up er Mississippi

6 ~iBdleMississippi 

7 Arkansas 

8 Lower Mississippi

9 Red and Ouachita 

12 Interior Basins 

Allegheny River at New Kensington, PA 

Monongahela River at Braddock, PA 

Muskingham River at McConnelsville,OH 

Kanawha River at Winfield. WV 

Scioto River at Higby, OH 

Great Miami at New Baltimore, OH 

Kentucky River at Lockport, KY 

Wabash River at New Harmon IN 

Cumberland River near Grand kvers. KY 

Tennessee River near Paducah, KY 

Mississippi River near Royalton, MN 

Minnesota River at Jordan, MN 

St Croix River at St Croix Falls, WI 

Chippewa River at Durand, WI 

Wisconsin River at Muscoda, WI 

Rock River near oslin, IL 

Cedar River at c]edar Falls, lA 

Iowa River at Wapello, IA (includes Cedar River basin- 17) 

Skunk River at Augusta, IA 

Raccoon River at Van MeterIDes Moines, IA 

Des Moines at St. Francisville, MO (includes Raccoon 

River basin-20) 

Illinois River at Marseilles, IL 

Lower lllinois River basin 

Entire lllinois River basin 

Kaskaskia River near Vened Station, IL 

Milk River near Nashua. ~4 

Missouri River near Culbertson, MT 

Bi horn River near Bighorn, MT owst stone River near Sydney. MT 

Cheyenne River at Cher Creek, SD 

bmes River near scotIan7 SD 

lane River near Louisville, NE 

Kansas River at Desoto, KS 

Grand River near Sumner, MO 

Osa e River below St Thomas, MO 

St Ancis Bay at Riverfront, AR 

White River at Clarendon, AR 

Arkansas River at Tulsa, OK 

Canadian River at Calvin, OK 

Yazoo River at Redwood, MS 

Big Black River near Bovina, MS 

Red River at Alexandria, LA 

Ouachita River near Columbia, LA 
Chapter 5: Nutrient Inputs and Outputs 8 1 

5.2.6 Volatilization Losses 

Several forms of N are volatile, including molecular nitrogen (N2), ammonia (NHs), and N oxides 

(NO, N20). Bouwman et al. (1997) lists rates of NHs loss from synthetic fertilizers in temperate 

climates that range from 2% to 20% dependmg upon the fertilizer type, and rates listed by Meisinger 

and Randall (1991) range from 0% to 60%. Buckman and Brady (1969) suggest that volatilization 

losses of 2040% of the N in fertilizer applications to poorly drained soil would not be uncommon. 

Bouwman et al. (1997) list rates of NH3 loss from animal manure that range from 4% to 36%. N can 

also be lost by volaalization from soils or plants during senescence. 

A portion of the N in some feralizers, primarily urea, is lost during application due to volatilization 

of ammonia. The rate of volatilization varies by ferultzer type, application method, climate, and soil 

pH (Meisinger and Randall 1991; Bouwman et al. 1997). The loss of N via direct volatilization of 

ammonium fertilizers is probably minimal because these feridzers are almost always injected or incorporated into soils (Troeh and Thompson 1993). For this study, states are designated as dry (Colorado, Kansas, Oklahoma, Montana, Nebraska, New Mexico, North Dakota, South Dakota, and 

Wyoming), humid (Alabama, Arkansas, Georgia, Louisiana, Mississippi, North Carolina, Tennessee, 

and Virgma), or subhurnid (all other states). 

Table 5.9 gives the rates of fertilizer loss used in this report. These values are estimates based on the 

rates reported in Meisinger and Randall (1991) and Bouwman et al. (1997). Estimates of the N output in fertihzer volatilization for the three basin scales are given in Table 5.7. Figure 5.9 shows estimates of N volatilized from fertilizer in the 20 basin states for 1951-96 and indicates that these 

amounts increased slightly during this period. Figure 5.10B shows the spatial distribution of N output in feralizer volatilization in hydrologic accounting units in the MARB. In much of the upper 

Midwest, including parts of Iowa, Illinois, and Indiana, rates of N volatilization from fertilizer exceed 300 kg ~/km'/~r. 

TABLE5.9. Estimated rates of nitrogen (ammonia) volatilizationfrom fertilizer, as a percent. 

Fertilizer Type Percent Loss 

in Humid 

States 

Percent Loss 

in Subhumid 

States 

Percent Loss 

in Dry States 

Range of 

Reported Loss 

Estimates 

Urea 10 15 

Ammonium Nitrate 2 5 

Anhydrous Ammonia 2 3 

Nitrogen Solutions 3 4 

Other Forms 4 6 

NOTETable is modified from Meisinger and Randall 1991 and Bouwman et al. 1997. 

A sipficant portion of the N in animal manure is lost during storage, handling, and application.P is 

assumed not be lost from volatiluation during manure storage, handhg or application, but some P 

is likely to be lost from runoff and erosion. Less than half of the P voided in manure is economically 

recoverable by crops (NRC 1993). Most of the N loss is likely to be from the volaalization of ammonia. The losses of N were calculated separately for manure from hogs, cattle, poultry, sheep, and 

horses (only included in 1992 Census of Agriculture estimates) using loss coefficients from Meisinger and Randall (1991) or from the Midwest Planning Service-Lmestock Waste Subcommittee 
82 Flux and Sources of Nutrients in the Mississippi-Atchafalava River Basin 

(1985). The estimates of N loss by animal class were summed by county to get the "all manure loss" 

values in Table 5.7. 

Losses of N from manure are a function of the manure type; climate; and storage, handling, and application practices. This study includes the following assumptions and estimates: 

20% of the hog manure is stored and handled as a solid, 52% using pits, and 28% using lagoons; 42% of the N in hog manure is lost in storage and handling, and 12% of the remaining N is lost during application. 

88% of cattle manure is handled in open lots, and 12% is handled as a liquid; 42% of the N 

in cattle manure is lost in storage and handhg, and 21% of the remaining N is lost during 

application. 

40% of the N in poultry manure is lost in storage and handling, and 16% of the remaining N 

is lost during application. 

0 45 '10of the N in sheep and horse manure is lost in storage and handling, and 23% of the 

remaining N is lost during application. 

The storage and handling losses used for several states were modified slightly from the percentages 

given above to reflect local conditions. These modifications were based on the recommendations of 

state soil scientists and agronomists and are applied only to the nutrient budget calculations that 

used the NASS data (Figure 5.9). Estimates of the N output in manure volaalization for the three 

basin scales appear in Table 5.7. Figure 5.9 shows the amounts of N estimated to have been volatilized annually from livestock manure for the 20 states in the MARB for 1951-96. Figure 5.10C 

shows the spatial distribution of N output in manure volaalization in hydrologc accountkg units in 

the MARB in 1992 based on the Census of Agriculture data. 

Nitrogen can also be lost hectly from plants. Ths loss occurs primarily as ammonia volatilization 

from the senescing leaves of plants. Ths loss generally occurs toward the end of the growing season, 

and has been estimated for crops to be 0-8,000 kg N/km2 (Francis et al. 1993; Meisinger and Randall 1991; Schepers and Mosier 1991; Bouwman et al. 1997). Rates of N loss from plant volaalization are hkely to vary with crop type, crop health, climate, fertilization rate, and soil conditions; 

however, insufficient data are available to refine estimates by region or other factors (Bouwman et 

al. 1997). 

In hs study, N lost via volatilization from crops is estimated using crop acreage data from NASS or 

the 1992 Census of Agriculture and the volatdzation rates in Table 5.10 (Francis et al. 1993;Burkart 

and James 1999). These rates are generally higher than the 400-1,600 kg/N/km%sed by Meisinger 

and Randall (1991) and the 250 kg/N/km2 used by Bouwman et al. (1997). Estimates of the N output during plant senescence for the three basin scales appear in Table 5.7. Figure 5.9 shows the 

amounts of N estimated to be lost from plant senescence in the 20 basin states for 1951-96 and indicates that losses increased slightly during this period. Figure 5.10C shows the spatial distribution of 

N output during plant senescence in hydrologic accounting units in the MARB in 1992. 

N is also volaalized from manure of wild animals, soils under natural vegetation, noncrop vegetation, burning of crop or forest biomass, fossil fuel combustion, and some industrial processes 

(Bouwman et al. 1997). However, the N volatdzation from these processes is not estimated in this 

report. 
Chapter 5: Nutrient Inputs and Outputs 83 

TABLE5.10. Estimated rates of nitrogen (ammonia) volatilization from senescing plant leaves (in kg N per square kilometer). 

Senescence 

Rate 

Corn-All Types 6,000 

Soybeans and Other Beans 4,500 

Wheat and Other Grains 3,500 

Sorghum-All Types 900 

1 

NOTE:Table is modified fiom Francis et a/. 1993 and Burkart andlames 1999. 

5.2.7 Denitrification Losses 

Significant amounts of N are lost by denitrification, a microbial process that occurs in soils. Microbial respiration of nitrate produces N20 and N2 gases that eventually escape to the atmosphere. 

Rates of denitrification vary by soil drainage class, because drainage affects the potential for saturation or water retention. Denitrification occurs most rapidly under low-oxygen conditions associated 

with water-saturated or poorly drained soils, such as those found in wetlands and river bottoms. Denitrification losses can occur episodically when rainfall or irrigation saturates soils (Gentry et al. 

1998;Kellman and Hdlaire-Marcel1998). 

In this study, rates of denitrification in agricultural soils are estimated by state using estimates of average soil organic matter and extent of hydric soils (Burkart and James 1999), and tables of denitrification rates from Meisinger and Randall (1991). Table 5.11 shows the average percent organic 

matter and denitrification rates as a percentage of N inputs used in this study for the states entirely 

or partly within the MARB. The denitrification rates in Table 5.11were applied to 60% of the residual fertilizer N (adjusted for volatilization), 100% of the mineralized soil N, and 60% of the atmospheric nitrate (wet and dry NO3). The rates were doubled and applied to 90% of the residual 

(adjusted for storage, handling, and application loss) swine manure N, 75% of the residual poultry 

manure N, and 45% of the residual cattle and other manure N (Meisinger and Randall 1991). Estimates of the N output from denitrification in agricultural soils for the three basin scales appear in 

Table 5.7. Figure 5.9 shows the amounts of N estimated to have been lost from soil denitrificationin 

the 20 basin states for 1951-96, and indicates that losses increased slightly during this period. Figure 

5.10E shows the spatial distribution of N output from denitrification in hydrologic accounting units 

in the MARB in 1992. 

5.2.8 limmobilization 

Immobilization is the process by whch plants or microbes convert inorganic ions, such as nitrate to 

organic N-containing compounds. This process is often considered to be the reverse of mineralization, and over the long term these two processes should balance, unless nitrogen is removed from 

the system (Troeh and Thompson 1993; Gentry et al. 1998). Immobilization is generally slower than 

mineralization, except immediately after the addition of N in crop residues, manure, or fertilizer. 

The rates of mineralization and immobilization of N are generally small relative to the size of the 

soil organic N pool (Power and Broadbent 1989). Estimates of the rate of fertilizer N immobilized 

in soil from various cropping systems ranged from 20% to 40% of the input (Power and Broadbent 

1989;Peterson and Frye 1989). 
84 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE5.1 1. Estimated average soil organic content and denitrification rate for states in the Mississippi-Atchafalaya River Basin. 

Basin States Average Organic 

Content Percentage of Soil 

Estimated Denitrification 

Rate as a Percentageof 

Available N Inputs 

Alabama 

Arkansas 

Colorado 

Georgia 

lllinois 

Indiana 

Iowa 

Kansas 

Kentucky 

Louisiana 

Maryland 

Minnesota 

Mississippi 

Missouri 

Montana 

Nebraska 

New Mexico 

New York 

North Carolina 

North Dakota 

Ohio 

Oklahoma 

Pennsylvania 

South Dakota 

Tennessee 

Texas 

Virginia 

West Virginia 

Wisconsin 

Wyoming 

NOTE. Table is modified from Meisinger and Randall 1991 and Burkart andjames 1999. 

In th~sstudy we assumed that 40% of residual feralvler N after volatilization losses and 40% of the 

nitrate from atmospheric deposition (wet plus dry)are immobfized by organisms in the soil and are 

not readily available for denitrification or utilization by crops. Manure N is already largely in an itnmobilized form and is not added to the immobilization output. A portion of the N that is rnineralized from soils can be immobilized, and the N taken up by the crop but not removed in harvest is 

also immobilized. These two potential immobiluation outputs are not quantified in this report. Estimates of the N output from immobilization in agricultural soils for the three basin scales appear in 
Chapter 5: Nutrient Inputs and Outputs 85 

Table 5.7. Figure 5.9 shows the amounts of N estimated to have been immobdized in the soil in the 

20 states for 1951-96, indicating an increase of about 0.5-3.0 million metric tonsIyr during thts period. Figure 5.10F shows the spatial distribution of N output by immobilization in hydrologic accounting units in the MARB in 1992. Like estimates of N from mineralization of soil, estimates of N 

immobilization rates should be viewed with an uncertainty of 25-50%. 

P is also immobilized in the soil, but estimates of the rate at whch this process occurs could not be 

found. Several researchers suggest that nearly all of the added organic or inorganic P is adsorbed on 

soil minerals and unavailable for plant use (NRC 1993;Troeh and Thompson 1993). 

5.2.9 Erosion 

Some of the N and P in soil organic matter is also lost from soils by erosion. Soil erosion from forest land and grasslands is generally less than 100,000 kg ~oil/km"~r.Erosion losses from cropped 

soils are larger: losses of several million kilograms of soil per km2/yr are common, and some annual 

losses are exceed 20 million kg ~oil/km'/~r(Troeh and Thompson 1993). Only a small fraction of 

the eroded material from soils is N or P, but the eroded material is likely to contain more fine sediments and higher percentages of organic matter and N and P than the parent soil (Buckman and 

Brady 1969). Sediment-bound N losses from apcultural land typically are less then 400 kg/I~rnZ/~r 

(Schepers and Fox 1989). The N and P losses from soil erosion were not estimated here; some of 

these losses can be can be accounted for in the suspended N and P flux in rivers discussed in chapter 4 of this report. 

5.3 MUNICIPALAND ~NDUSTR~ALINPUTS 

Municipal sewage treatment plants and many industries-including plastics and nitrogen fertilizer 

manufacturers, refuse systems, beef cattle feedlots, wet corn milling, steel ds, and petroleum refineriesdscharge signihcant quantities of N and P into rivers. These discharges are often referred to - -

as "point sources" of contaminants. Specific industry discharges vary considerably in different regions of the MARB. 

In a prior investigation using data from the late 1970s, Gianessi and Peskin (1984) and Battaglin et 

al. (1997) estimated that municipal sewage treatment plants and industrial facilities respectively dlscharged about 264,000 and 106,000 metric tons of N annually in the MARB. In 1998, EPA initiated 

a study to determine the best estimate of the total N and P discharged annually by each of the National Pollutant Discharge Elimination System (NPDES)-regulated point sources in the MARB. The 

final version of the database contains estimates of 1996 total N and total P discharges for about 

11,500 facilities rangmg in size and sipficance from a campground (-0.01 metric tons of N/yr) to 

a Chicago municipal seivage treatment plant (- 10,000 metric tons of N/yr). 

The estimates of N and P in municipal and industrial discharge presented in this report only account 

for permitted discharges from facilities contained in the NPDES database. They do not account for 

dumping or other illegal discharges of N or P from any facility. Municipal and industrial facilities 

(and motor vehicles) also emit N-containing compounds to the atmosphere, a portion of which is 

returned to the basin in wet and dry atmospheric deposition (Jaworskiet al. 1997). Emissions of N 

to the atmosphere from municipal and industrial facilities and motor vehlcles are not estimated in 

this report. Some of the atmospherically emitted N that is returned to the basin is measured as 

atmospheric deposition (see section 5.1 of ths chapter). 
86 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

5.3.1 Methods 

The U.S. Environmental Protection Agency (EPA) provided annual N and P discharge estimates for 

each of the NPDES-regulated point sources in the MARB, using the methods and procedures presented in USEPA 1998a. The final version of the methods used and resulting database is presented 

in Tetra Tech, Inc. 1998. The EPA database contains estimates of total N and total P discharges in 

1996 for about 11,500 facdities. The new discharge estimates were based largely on NPDES data 

found in EPA's Permit Compliance System. Where no other data existed, the information was estimated by applying typical pollutant concentrations and typical faciltty flows, as described by NOAA 

(1998). To the extent possible, data for N from all seasons were obtained for each facdity to get a 

reasonable assessment of the annual load, since the nitrogen cycle is temperature-dependent. 

For this study, estimates of N and P discharges by facdity are summed within 8-> hydrologic 

units (Seaber et al. 1987). Municipal and industrial discharges are kept separate. In some cases, location information was not avadable, and the facility could not be assigned to an 8-1st hydrologic 

unit. Nutrient discharges from these facdities are added only to the estimates of total input to the 

MARB, and are not assigned to any of the 9 large or 42 interior basins. The estimated N discharge 

from unassigned facilities is only 5.5% of the total N discharge in the basin, but more than 20% of 

the industrial N discharge is unassigned. An area-weighted summing algorithm was used to compute 

estimates of nutrient discharges from point sources for the three basin scales. 

5.3.2 Results 

Tables 5.2 and 5.3 provide estimates of the annual input of N and P for the three basin scales from 

municipal and industrial facihties. Estimates of the total N input from both municipal and industrial 

facilities and in hydrologic accounting units in the MARB appear in Figure 5.8. The current estimates are slmilar but are generally lower than the hstorical estimates made using data from the 

1970s (Figure 5.7) (Gianessi and Peskin 1984; Battaglin et al. 1997). An estimated 200,786 metric 

tons of N and 30,105 metric tons of P are discharged annually from municipal sewage treatment 

plants in the MARB, while an estimated 85,635 metric tons of N and 28,864 metric tons of P are 

discharged annually from industrial facilities (Table 5.12). It is unknown if the differences between 

current and historical estimates of nutrients point discharges are real or a function of improved data 

gathering and analysis. 

TABLE5.1 2. Estimated point-source nitrogen and phosphorus discharges for the Mississippi-Atchafalaya River Basin (in metric tonslyear). 

Source Category Estimated Estimated Error Historic Estimates 

Discharge (I 996) (late 1970s) 

N P N P N P 

Municipal Point Sources 200,786 30,105 40,800 6,100 264,000 55,000 

Industrial Point Sources 85,635 28,864 22,000 32,200 106,000 15,000 

Total 286,400 59,000 62,800 38,300 370,000 70,000 
Chapter 5: Nutrient Inputs and Outputs 87 

5.4 ATMOSPHERIC~NPUTSDIRECTLYTO THE GULFOF MEXICO 

Measurements of atmospheric deposition directly to the waters of the Gulf of Mexico are nearly 

nonexistent. A few precipitation events and fdterpack estimates from one cruise (Parungo and Miller 

1988; Parungo et al. 1990) provide a little wet and dry deposition data for the western half of the 

Gulf. Otherwise, all available data used in this summary were from land-based stations, primarily 

from the U.S. located along the Gulf coast. Available data are discussed in this section. 

5.4.1 Dry Deposition 

Dry deposition to Gulf waters can be estimated from available data with two approaches: (1) by using chemical concentration data collected aboard ship, coupled with appropriate deposition velocities, and (2) by extrapolating filterpack information from land-based sites, such as those managed 

through the EPA's Clean Air Status and Trends Program (CASTNet). Land-based deposition information represents a point measurement, typically valid for a small region of homogeneous fetch and 

land cover (Meyers and Sisterson 1990). In addition, the factors that control dry deposition in the 

coastal zone are believed to be vastly different from those for inland areas, and are largely controlled 

by poorly quantified processes involving sea salt aerosols (Keene et al. 1990). However, shoreline air 

concentration data can be quite indicative of nearby coastal regimes, since concentrations respond 

slowly to surface changes. 

Estimates of nitrogen deposition along the Gulf coast are extremely rare. Only three dry deposition 

monitoring sites are known to be operational within several hundred kilometers of the Gulf of Mexico, and data exist for only two of them. Even for these two, the data cannot yet be accepted without considerable caution because of the complicating effects of sea salt and local surface complexity, 

as mentioned above. The Sumatra, Florida, site (approximately 20 km north of Apalachicola, Florida) has been operated by CASTNet since December 1988. Data are available via an EPA web site 

(USEPA 1998b). Unpublished estimates of the ratio of dry deposition to total deposition of nitrogen 

generated using CASTNet data indicate that dry deposition of nitrogen constitutes about half of total deposition near the Sumatra site. Unpublished kriged estimates of all CASTNet data indicate that 

dry deposition of nitrogen ranges from about one-third to one-half of total deposition for the 

coastal Gulf regon east of Texas. These estimates should probably be ignored, given the paucity of 

data in this region. Extrapolation over the open waters of the Gulf cannot be done with any certainty. 

The Tampa Bay National Estuary Program has established precipitation chemistry and inferential 

method dry deposition filterpack (wet and dry) stations. Precipitation measurements are discussed 

below. In this particular case, deposition monitoring is performed according to a set of protocols 

designed to account for complications arising from sea salt aerosols-an approach different from 

that mentioned above for the CASTNet site. Dry deposition estimates to the water of Tampa Bay 

indicate that nitric acid deposition is approximately twice as great as nitrate deposition from precipitation--on the order of 7 kg/ha/yr (Greening 1998). Although this is a very large number, it is 

probably not a particularly relevant value because the site exposure is near the Gandy Bridge, which 

is close to a number of very large utility emission sources. The site is likely to be quite representative 

of Tampa Bay, but not the open waters of the Gulf. 
88 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

NOAA's Air Resources Laboratory is in the process of establishmg a deposition program near the 

Long Key Marine Laboratory on a jetty facing Florida Bay. Meteorological measurements, as well as 

aerosol and gas measurements of ammonia, oxides of nitrogen, nitric acid, aerosol nitrate, orthoand total phosphorus, and base cations will be made on a seven-meter tower. Major ions in precipitation, including nitrate and ammonium will also be measured nearby. The infrastructure is in place, 

and meteorological data are being collected. Chemical data should become available within the next 

year. 

5.4.1.4 IMPROVE DATA 

The IMPROVE (Interagency Monitoring of Protected Visual Environments) program operates a 

sampling site southeast of the Sumatra CASTNet site, near Chassahowitzka Bay, Florida. The Chassahowitzka data include estimates of ammonium nitrate concentration; no dry deposition estimates 

are available (Porter 1998). 

Dry deposition rates were estimated from concentration measurements made during a research 

cruise conducted in the summer of 1986 by a group of scientists from the United States and Mexico. 

The project was designed to investigate chemistry over the Gulf of Mexico (Parungo et al. 1990; Parungo and Miller 1988). This cruise generated the only known published estimates of deposition 

from samples in the region of interest collected in Gulf waters. 

Approximately 22 aerosol samples were collected in an area that stretches from coastal waters near 

New Orleans, Louisiana, along the coasts of Texas and Mexico to Progreso Merida (Yucatan) and 

back to New Orleans across the open Gulf waters. Mass concentrations of nitrate were less than 1 

pg/m3 throughout the Gulf, except near the port cities of Galveston, New Orleans, Veracruz, and 

Merida, where maximum values were all less than 4 pg/m3. It was determined that the large particles 

contained a large fraction of the nitrate. Because large particles have high rates of dry deposition and 

low residence time, nitrate concentrations decreased rapidly with distance from the shore. Based on 

the methodology of Slinn and Slinn (1980) and a number of assumptions regarding particle size distributions, it was estimated that the average dry deposition was approximately3.3 kg/ha/yr for Nos, 

and about 0.1 kg/ha for NH4+ to the entire Gulf. An area of 1.5 x lo7km2was assumed. 

5.4.2 Wet Deposition 

By far the greatest single source of potentially relevant data for this study is available through the 

National Atmospheric Deposition Program (NADP/NTN 1997). Ten stations are within approximately 100 km of the Gulf coast, nine operating according to the usual weekly wet-only sampling 

protocol pigelow and Dossett 1993) and one operating according to NOAA Atmospheric Integrated Research Monitoring Network (AIRMoN) protocol. The nine weekly sites are: Beeville and 

Attwater Prairie Chicken National Wildlife Refuge, Texas; Iberia Research Station and Southeast 

Research Station, Louisiana; Quincy, Bradford Forest, Chassahowitzka, Vema Well Field, and Everglades National Park, Florida. The AIRMoN station is located in Tampa Bay, Florida. Information 

regarding these and all other NADP stations is avadable via the NADP web site: 

http:/nadp.sws.uiuc.edu. 
Chapter 5: Nutrient Inputs and Outputs 89 

Using a least-squares method of interpolating between stations, NADP has developed a set of annual and seasonal concentration and wet deposition maps for the United States. For 1994,1995, and 

1996, these maps are posted on the NADP web site for major ions, including nitrate and ammonium. For each of these years, deposition values are typically higher in the regons surrounding New 

Orleans and perhaps Tampalst. Petersburg. Deposition values tend to b; lowest along the southern 

Texas coast and range from about 1 to 3 kg/ha of No3 (as N), and about 1.0 to 3.2 kg/ha of NH4+ 

(as N). Although a fairly broad range of estimated deposition is found, average deposition of total 

inorganic nitrogen via precipitation along the U.S. Gulf coast is typically on the order of 3-4 kg/ha 

N/yr, with Nor accounting for about 60% of total N deposited. 

If the NADP data are used to estimate deposition off the Louisiana coast, total N deposition would 

be adjusted upward, primarily because of increased deposition of NH4+.NH4+ deposition would be 

expected to be over 2.0 kg/ha/yr (as N), bringing total inorganic N deposition to nearly 5.0 

kg/ha/yr. Nos- deposition during the spring and summer typically accounts for well over half of the 

annual deposition, a time of year when prevailing winds are generally from the south. As with most 

coastal areas, NH4+ deposition during the spring and summer would be expected to account for up 

to about 75% of total annual ammonium deposition. Deposition of NH4+ is typically much lower 

during the fall and winter months. 

The AIRMoN-wet station located near the center of Tampa Bay has been operational since about 

August 1996. The NADP web page currently contains data through June 1997. Using the 11 months 

of available data and converting to elemental N, the Tampa Bay station indicates total N deposition 

for the period of about 3.4 kg/ha when weighted to cover an entire year. Given that this estimate 

was made independently of the NADP estimate, it is probably safe to conclude that coastal deposition of inorganic N via precipitation is less than 4 kg/ha Nlyr for much of the Gulf coast. 

It should be mentioned that estimates of ammonium deposition calculated from NADP weekly data 

are biased approximately 15% low, on an annual volume-weighted mean basis, relative to samples 

collected on a daily basis and are then preserved through chilling or analyzed quickly. Studies by Vet 

et al. (1989) and others have repeatedly shown that unpreserved weekly samples under-report NH4+. 

Unpublished data by R.S. Artz, NOAA, indicate that this is also the case when NADP weekly data 

are compared to dady AIRMoN-wet values. Corroboration of this phenomenon is possible through 

comparison of collocated NADP weekly and AIRMoN-wet data for such stations as State College, 

Pennsylvania, or Bondville, Illinois, using data posted on the NADP web site. 

Results of data collection from the Florida Atmospheric Deposition Study (FADS) were reported by 

Hendry et al. (1981). Total nitrogen (including organic nitrogen) was measured at 24 locations 

throughout the state, primarily using bulk (wet plus dry) collectors. Four of the stations used 

wet/% collectors; precipitation samples from the wet-side buckets were collected every two weeks. 

(The dry deposition methodology used in this program is generally unreliable because the polyethylene buckets used are a poor surrogate for most natural surfaces, whether they be water, soil, or biological; thus, the dry deposition estimates are ignored here.) The wet deposition estimates appear to 

be quite good. Inorganic nitrogen fluxes via wet-only precipitation measured in the FADS study 

range from 3.2 kg/ha/yr to 4.4 kg/haIyr, with a mean of 3.9 kg/ha/yr for the entire state, in good 

agreement with the NADP estimates. 
90 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Hendry et al. (1981) also noted that the deposition of inorganic nitrogen in Florida precipitation had 

apparently increased by approximately a factor of three between the time of an earlier study by 

Junge (1958, 1963) and the FADS program of 1978-79. No simdar increase has been observed in 

the past 20 years. It is believed that Junge's measurements were reasonably good, but this is probably 

impossible to prove. Based on values presented for the Gulf coast in Junge (1963), similar differences would be expected along other parts of the Gulf coast. 

The Parungo cruise (Parungo and Miller 1988; Parungo et al. 1990) also produced estimates of wet 

deposition based on 27 precipitation samples, many collected sequentially during about seven individual events. Samples were collected using simple Teflon funnel and polyethylene bottle systems, 

deployed in a manner to exclude dry deposition. Wet deposition calculated from the concentration 

and rainfall depth data indicated exceptionally high mean values of 19 kg/ha for NO3- and 3.0 kg/ha 

for NH4+ if an annual rainfall of 110 cm is assumed. This translates to an annual loading of total 

precipitation N of approximately 6.5 kg/ha, well over 50% greater than estimated for along the Gulf 

coast using NADP data. These values may be fairly accurate. However, given a nearly complete lack 

of information regarding quality assurance practices, the lack of a good statistical sample, and the 

fact that samples were captured during the summer, it is difficult to recommend that these values be 

extrapolated to provide annual estimates. 

5.4.2.4 XALAPA,VERACRUZ,MEXICO, PRECIPITATIONCHEMISTRYESTIMATES 

Baez et al. (1997) published the results of precipitation chemistry measurements collected between 

1993 and 1995. Sampling was performed in Xalapa, Veracruz, on the eastern flanks of the Sierra 

Madre Oriental facing the coastal prairies of the Gulf of Mexico. Unlike all of the other stations 

cited in this section, the Xalapa site was well above sea level, located at 1,370 MSL. Still, this station 

is the only site collecting data for any significant duration located near the Mexican coast. Based on 

data collected in 1994 and 1995 (1993 data were reported to be about a factor of five lower in deposition and were ignored in this analysis), the total annual loading of nitrogen is estimated to be approximately 3.7 kg/ha, but with a few interesting differences compared to U.S. precipitation. NH4+ 

in the Xalapa samples constitutes about '75% of total N; N03- is relatively much less significant. 

Also, deposition during the dry season (approximately 26% of total measured precipitation, November through April) is much cleaner than during the summer wet season. When compared to NADP 

data collected for a similar period near Beeville, Texas, NH4+ levels in Xalapa are roughly double 

those in Beeville, but NO3- levels are on the order of 80%. The net effect is that total N deposition 

appears to be fairly sidar between near-coastal areas of central Mexico and southern Texas on an 

annual basis. 

5.4.3 Wet Plus Dry Deposition 

As seen from the discussion above, there ae few data with which one can confidently estimate total 

deposition to coastal regions around the Gulf, let alone the open waters. Still, a few estimates are 

possible. First, wet and dry samples from the Parungo cruise provided an opportunity to estimate 

ratios of wet and dry deposition of nitrogen compounds over the open Gulf. By scaling the few 

events collected aboard shtp, Parungo et al. (1990) estimated that the wet/dry ratio is approximately 

5.8 for Nor and 20 for NH4+.Assuming that these numbers are reasonable, and both appear compatible with historical estimates derived from studies of radioactive fallout that typically yield a 

wet/dry ratio of about 10, the dry deposition of nitrogen compounds can be safely ignored over the 
Chapter 5: Nutrient Inputs and Outputs 9 1 

open ocean of the Gulf. Unfortunately, it is probably not safe to assume that these estimates are robust, 

given that the wet deposition values for total N are substantially larger than all of the other land-based 

estimates, and there is little reason to expect that wet deposition would be greater far downwind from 

major emission sources. 

5.4.4 Model Estimates 

In what is perhaps the first real summary regardmg the body of nitrogen cycling in the North Atlantic 

Ocean (Howarth 1996), output from four computer models was compared with values presented from 

various measurement programs for the North Atlantic Ocean (l'rospero et al. 1996). These models produced a value of 1.75 kg/ha of NO,- (as N) for the Everglades, Florida, compared with an estimated 

measured value for the period of 1.70 kg/ha N. For NH4+, agreement was almost as good with the 

models, indicating a loading of 1.3 kg/ha N and the measurements showing 1.36 kg/ha. The range in 

the model estimates for Nos- was about a factor of two, but was only about 10°/o for NH4+. 

Prospero et al. (1996) also provided model estimates for the eastern Gulf of Mexico, the coastal region 

of the Mississippi River, and the western Gulf of Mexico. The estimate for N03- for the Mississippi 

River region is about 4.2 kg/ha/yr, compared with 2.5 and 2.8 kg/ha/yr, respectively, for the eastern 

and western Gulf. Reduced nitrogen species (NH4/NH3) in coastal areas of the Mssissippi River were 

estimated to be approximately 2.5 kg/ha/yr, compared to 2.0 kg/haIyr for the western Gulf. No estimate was provided for the eastern Gulf. 

5.4.5 Comparisons with Chesapeake Bay and 

the Open Atlantic Ocean 

If we discount the values measured from the Parungo cruise and ignore the local dry deposition contribution to Tampa Bay, the likely range of deposition the Gulf of Mexico is on the order of 3.5-5.5 

kg/ha/year, largely depending on the magnitude of the dry fraction. If, as Parungo et al. (1990) assert, 

dry deposition is nearly negligible over open waters of the Gulf, 3.5 kg/haIyr would be a reasonable estimate for areas with a few hundred kilometers of the shore. Nearer to shore, however, 5.5 kg/halyr is 

probably a better estimate. For Gulf waters near southern Louisiana, a value approaching 7 kg/ha/yr 

appears likely. 

For purposes of comparison, estimates are also given for direct deposition to Chesapeake Bay. Valigura 

et al. (1996 and references therein) estimated that wet plus dry deposition of inorganic nitrogen (excluding dry deposition of NH4+)is approximately 5.8 kg/ha/yr, just a bit larger than estimated for the Gulf. 

This assumes that the mean annual wet deposition of N03- is approximately 3 kg/ha/yr, and NH4+is 

approximately 1 kg/ha/yr. The balance is dry deposition, primarily of nitric acid. 

5.4.6 Meteorological Considerations 

Because few of these data were generated from samples collected over water (and none over an appropriate length of time following a rigorous quality assurance program), it is assumed that deposition over 

water, particularly near coastal waters, is similar to that from the continent. Obviously, this assumption 

typically will break down as a function of distance from land, and as a function of prevarling wind. If the 

wind does not blow from a land mass, the local land-based measurements become fairly worthless. 
92 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

To get a good fix on this issue, wind speed and direction were compiled using the 1995 U.S. Navy 

ChaticAtlas oftbe World for the area south of central Louisiana and are presented in Table 5.13. For 

the hypoxic area of the Gulf, it is clear that wind with a northerly component is common in the winter and less common in the summer. However, transport from either the east or the west is common 

any time of year, making it Qfficult to rule out the influence of atmospheric sources to the hypoxic 

zone from any of the northern Gulf states or from northern Mexico. These data give no understanding regarding additional complications from sea breezes. The values shown above provide a good 

first estimate of deposition; however, additional measurements made directly in the hypoxic zone 

may be considerably Qfferent. 

TABLE5.13. U.S. Navy Climatic Atlas of the World (1 995) grid point data for latitude 28.0 

N, longitude 92.0 W, near Morgan City, Louisiana. 

Wind Direction 

N NE E SE S SW W NW 

July 

Wind Speed 4.32 4.14 4.85 4.84 4.45 4.33 4.30 4.58 

(meterslsecond) 

Percent of Time 2.76 2.84 12.88 22.16 22.94 13.00 10.70 5.70 

January 

Wind Speed 7.86 7.28 6.65 6.0 1 5.97 5.70 7.82 8.80 

(rneterslsecond) 

Percent of Time 15.74 15.25 18.21 15.27 14.8 1 5.90 4.45 9.39 
Linking Nutrient Flux to Nutrient 

Inputs and HumanActivities 

Preceding chapters of this report have discussed the flux of nutrients from the MARB to the Gulf of 

Mexico and have shown which regions within the MARB contribute abnormally large amounts of 

nutrients to the Mississippi River system. Nutrient inputs and outputs of sipficance from all known 

major human activities and natural sources in the MARB have been estimated and discussed. The goals 

of this chapter are to examine the relations between nutrient flux and nutrient inputs associated with 

human activities in the 42 interior basins and to determine whch human activities are most significant in 

contributing nutrients to rivers in the MARB and the Gulf. These goals were approached in several ways. 

First, a graphic approach was used to compare nutrient inputs and outputs based on observations and 

data. Second, a mass balance for the entire MARB was constructed to graphically compare the relative 

importance of nitrogen inputs and outputs. The residuals from the N mass balance (N inputs minus N 

outputs) were examined through time from 1951 through 1996.Third, a statistical approach was used to 

relate nutrient outputs from the 42 interior basins (yields) to nutrient inputs and human activities using 

multiple regression analysis. Results from the regression models are used to estimate the N and P 

contributions to the Gulf from the major N and P sources. 

Graphics were developed to visually examine the relations between the amounts of N and P discharged 

from the 42 interior basins in streamflow (outputs) and the amounts of N and P added to the basins 

(inputs) from various human activities. Figures 6.1A-F are scatter plots showing the relation between the 

1980-96 mean annual N yields from the basins and N inputs from fertilizer, mineralized soil, legumes, 

animal manure, atmospheric deposition of nitrate (wet and dry), and point-source discharges. The basins 

have been coded to show whether they are in the Ohio Basin (black), Upper or Middle Mississippi Basin 

(red), Missouri Basin (blue), or Lower Mississippi Basin (green). The N inputs used in this figure were 

discussed in chapter 5 of this report and are summarized in Table 5.2. The N yields shown in this figure 

are from Table 4.3. 

These scatter plots show how the N yields vary with each of the six N inputs. The yield-input relation is 

best for fertilizer N, mineralized soil N, and legume N (Figures 6.lA, B, and C). These are the three 

largest inputs, covering a range from < 100 to > 7,000 kg/krn2/yr. These three figures show that the 

largest inputs of N are from fertilizer, mineralized soil, and legumes, and the hghest N yields are in the 

Upper and fiddle Mississippi River Basin (red). The relation is somewhat poorer for N inputs from 

animal manure (Figure 6.lD). The htghest manure N inputs are in the Upper and Middle Mississippi 

Basins. The relation is poorest for N inputs from 
94 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

FIGURE6.1. Scatter plots showing relationships between total nitrogen yield and nitrogen 

inputs from (A) fertilizer, (B) mineralized soil, (C) legumes, (D) manure, (E) atmospheric 

deposition of nitrate (wet plus dry), and (F) municipal and industrial point sources. 
Chapter 6: Linkinn Nutrient Flux to Nutrient Inputs and Human Activities 95 

atmospheric deposition and point sources (Figures 6.lE and 6.1F). The largest N inputs from 

atmospheric deposition are in the Oho Basin (black ). However, the hghest N yields are in the 

Upper and Middle Mississippi Basins, which have lower atmospheric N inputs. There appears to be 

a linear trend in the relation between N yield and atmospheric inputs, but the N yields for a number 

of basins plot far above the trend line (Figure 6.1E). These are basins ih Iowa and Illinois (Upper 

and Middle Mississippi Basin) that have large agricultural inputs of N. This figure indicates 

atmospheric deposition of N is an important source of N in parts of the Ohio River Basin but is less 

important in the Upper and Middle Mississippi Basins. As shown in Figure 6.lF, there is very little 

relation between N yields and point-source inputs, except for three basins with point-source inputs 

greater than 200 kg/km2/yr. These basins are the Muskingham (3), Great Miami (6), and Upper 

Illinois River (22). For the remaining basins, the N yield varies from near 0 to more than 3,000 

kg/km2/yr, with little variation in point-source inputs. 

Figure 6.2A shows the relation between the yields of nitrate and organic N from each of the basins 

and N inputs from point sources. The N yields and point-source inputs for the entire MARB are 

shown on the right side of the graph for comparison with the individual basins. The height of each 

bar represents the mean annual total N yield (organic N plus nitrate-N) of the 42 basins for 1980-

96, as estimated from the regression models (Table 4.3). The green part of each bar represents the 

amount of the annual total N yield that occurred as dissolved and suspended organic N. The 

remainder of the bar (red plus blue portions) represents the amount of the total N yield that 

occurred as nitrate. The blue part of each bar, which has been overlaid on the' nitrate portion of the 

bar, represents the maximum total N yield that could have been derived from point-source 

hscharges within the basin. The remainder of the N is from nonpoint sources. 

For example, the total N yield of basin 3 is about 1,060 kg/km2/yr. Of this amount, about 760 

kg/km2Iyr is nitrate, and the remainder, 300 kg/km2Iyr, is organic N. Point-source N inputs to this 

basin could account for as much as 270 kg/km2/yr. The remainder, 790 kg/km2/yr, is from 

nonpoint sources. The point-source yields (blue bars) shown in hs figure assume no loss of N 

between the points where the discharges occur and the terminus of the basin. The actual amount of 

point-source N discharging from each basin may be sipficantly less than that indicated by Figure 

6.2A due to in-stream losses, such as denitrification. If point-source losses are sipficant, then the 

yields of N derived from nonpoint sources would be larger than shown in Figure 6.2A. 

Figure 6.2A shows that except in a few basins, the point-source inputs comprise only a small part of 

the annual N yields of these streams. Exceptions include basins 3,6,10, and 22. Point-source inputs 

to basin 6, Great Miami River in Ohio, could account for as much as one-third of the N yield of this 

basin. Point-source inputs to basin 22, Upper Illinois River in Illinois, which receives discharges 

from the Chicago area, could account for as much as one-third of the N yield of this river. Pointsources inputs to basin 3, Muskingham River Basin in Ohio, could account for about 25% of the 

average N yield. The Tennessee River Basin in Kentucky and Tennessee (10) has large N inputs 

from industrial sources, and the combined municipal and industrial point sources in this basin could 

account for as much as 40% of the average N yield, assuming in-stream N losses are not sipficant. 
96 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

organic nitrogen 

(green part of bar only) 

A 

B / 

plus blue parts of bar) 

2000 

W 

1500 

5 c3g 1000 

t 

= 500 

0 

0 3 6 9 1 1 1 2 2 2 3 3 3 3 4 Entire 

2 5 8 1 4 7 0 3 6 9 2 Mississipp 

Basin 

14000 

atmospheric deposition IN: 120001 ,(wet + dry nitrate) B 

0 3 6 9 1 1 1 2 2 2 3 3 3 3 4 Entire 

2 5 8 1 4 7 0 3 6 9 2 miss is sip^ Basin 

14000 

N' 

E 12000 manure 5 /(adjusted for 

10000 volatilization losses) 5 

n potential input 8000 from mineralized soil 

Z

3 6000 

0 

4000 Z 

0 2000 

5 

9 0 

BASIN IDENTIFICATION NUMBER 

FIGURE 6.2. 4A) Nitrogen yields (river outputs from basins) and point-source nitro en 

inputs in the 2 interior basms and the entire Mississippi-Atchafalaya River Basin (MA&). NOTE:Heights of the bars show the total nitrogen yields organic N plus nitrate). Or nic N yield is the

green part of the bar. The remainder of the bar (red plus blue portions) is nitrate yieE The blue part of 

the bar shows the maximum amount of the N yield that could be derived from point sources. (B 

Inputs of newly fixed nitrogen to the 42 interior basins and the MARB from fertilizer, d 

fixation by legumes, and atmospheric deposition (wet and dry) of nitrate. (C) Inputs of 

rec cled nitro en derived from manure, potential mineralization of soil organic nitrogen 

andplant resifue, and atmospheric wet deposition of ammonium. 
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities 97 

For the entire MARB, municipal and industrial point sources, at a maximum, could only account for 

about 18O/o of the mean annual total N yield of the basin (Figure 6.2A). This assumes no in-stream 

removal of the point-source N, which is unlikely since some of these discharges are to small streams. 

Expressed in terms of the average annual N flux, point sources could, at the most, comprise about 

287,000 of the 1,567,900metric tons of N discharged annually from the ~ R to the B Gulf. 

The 42 interior basins examined in thls assessment comprise about two-hds of the area of the 

MARB and are believed to be representative of all point- and nonpoint-source N inputs in the entire 

basin. They include the inputs from most, but not all, large cities in the basin. Specifically, several 

large cities dischargmg N from point sources directly to the Mississippi River are not included at the 

42-basin scale. However, these cities are included at the Mississippi Basin scale and are included in 

the 287,000 metric tons of point-source N discussed above. The point-source N contributions to the 

Mississippi River are expected to remain relatively constant throughout the year. Consequently, 

during low-flow conditions point sources would contribute a larger percentage of the N yield of the 

Mmissippi River, and during high flows they would contribute a smaller percentage. If point-source 

inputs are relatively constant, then the large increases in N flux that occur in the Mississippi River 

most years during the spring and summer (see Figure 4.1) when streamflows are high must be from 

nonpoint sources. 

Figure 6.2B shows the annual inputs of new N to the 42 interior basins and to the entire MARB 

from fertilizer, N fixation by legumes, and atmospheric deposition of nitrate (wet plus dry). These 

inputs are referred to as new N because they represent new amounts of N added to the MARB each 

year (Jordan and Weller 1996; Howarth et al. 1996). The inputs of N from fertilizer and legumes 

were calculated from data in the 1992 Census of Agriculture and were presented in Table 5.2 of this 

report. These inputs have been normalized to drainage area to make it easier to compare N inputs 

among basins and with N outputs belds) in Figure 6.2A. Figure 6.2B shows that N from fertilizer is 

the largest input of new N to most of the 42 basins and to the entire MARB. Fertilizer N accounts 

for more than half of the new N added annually to these basins and to the MARB. On average, 

legumes account for about one-third of the new N inputs, and atmospheric deposition of wet and 

dry forms of nitrate accounts for slightly more than 10%. New N inputs to the 42 basins (Figure 

6.2B) are typically three to six times lakger than the N outputs (Figure 6.2A), but in general the 

basins with the largest new N inputs are also the ones with the largest N outputs in streamflow. 

Also, the basins with the highest fertilizer N inputs are generally the ones with the largest N outputs. 

These three inputs form part of the pool of inorganic N in MARB soils. Th~sinorganic N pool is 

subject both to removal by crops and such biochemical processes as denitrification and 

immobilization in soil organic matter, and to leaching to ground water and streams. 

Figure 6.2C shows the annual recycled N inputs to the 42 basins from mineralized soil organic 

matter and manure (adjusted for volatilization losses of NH3), and atmospheric deposition of 

ammonium N. These inputs are referred to as recycled N because they either were already in the 

basin in the form of soil organic matter or were added to the basin during the year in manure or 

atmospheric deposition of ammonium N. The manure N is largely derived from the N in fertilizer 

and mineralized soil N. Likewise, the ammonium N in atmospheric deposition is derived largely 

from animal manure, which may also have originated from fertilizer or mineralized soil N. 

Although mineralization of organic matter in the soil constantly removes N from the soil organic N 

pool, N is also being returned to the soil organic N pool in the form of plant litter, debris, and 

manure, and immobilization of inorganic N by soil microorganisms. Recent research (Drinkwater et 
98 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

al. 1998) suggests that cropping systems that use legumes and manure do not deplete soil N, but can 

actually increase both soil N and carbon. While the recycled N normally is not a new source of N to 

the basin, it does represent a source of N that can readily be mineralized to inorganic N and leach to 

streams and ground water as nitrate if not used by crops or denitrified. Thus, from the standpoint of 

potential sources of N to streams and the Gulf of Mexico, recycled N is just as important a source as 

the new N. 

The mineralized soil N in Figure 6.2C represents the amount of N that could be minerahzed to 

ammonium and nitrate from the pool of organic matter, and microbial biomass in the upper 30 cm 

of soil during one year, and that could become available for uptake by crops. The minerahzed soil N 

represents the majority of the recycled N. The manure N shown in Figure 6.2C is the amount 

available after volatilization losses. Typically, more than half the N in manure is lost through 

volatilization during storage and application. The manure N that is applied to cropland is largely in 

organic form and must be decomposed to inorganic N before it is available to crops or for leaching 

to water resources. The ammonium N shown in Figure 6.2C represents the amount of ammonia 

measured in atmospheric wet deposition (see Table 5.1). Its principal source is manure. Dry 

deposition of ammonium was not estimated for this assessment. However, the literature suggests dry 

deposition is a significant source of ammonium (see section 5.1). 

As figures 6.2B and C show, the amount of inorganic N potentially available each year from 

minerahzed soil organic matter is comparable to the new N added in fertilizer and is potentially a 

large source of N to streams. These figures also show that, in general, the basins with the large 

potential mineralized soil N inputs are also the basins with large fertilizer N input, and the ones with 

large N outputs bields) (Figure 6.2A). The data on N inputs and outputs presented in Figures 6.2AC provide compelling evidence that fertilizer N and mineralized soil N are major sources of N to 

streams. The N input from legumes is also significant (Figure 6.2B). However, it has been shown 

that the amount of N removed in harvested legumes, such as soybeans, generally exceeds the 

amount of N they symbiotically fur from the atmosphere. Additional inorganic N to meet crop 

needs may be derived from mineralized soil, since soybeans are fertilized less frequently and at lower 

rates than corn (see section 5.2.3; David et al. 1997). Thus, legumes that are harvested (e.g., 

soybeans)generally are not net contributors of N to the soil system. 

Figure 6.3A shows the yields of P from the 42 interior basins. The height of the bars represents the 

total P yield. The suspended P yield is represented by the green portion of the bar, and the Qssolved 

ortho P yield is represented by the red. Figure 6.3B shows the amounts of the total P that could be 

derived from municipal and industrial point sources. The inputs shown here assume no in-stream 

losses between the source and terminus of the basin; hence, they represent maximums. 
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities 99 

5 200 

9- suspended 

w

> 

cn 

3 

rr2 100 n 

cn

2 

C1. 

n0 3 6 9 1 7 1 2 2 2 3 3 3 3 4 

2 5 8 1 4 7 0 3 8 9 2 MARB 

BASIN IDENTIFICATION NUMBER 

0 3 6 9 1 1 1 2 2 2 3 3 3 3 4 2 5 8 1 4 7 0 3 6 9 2 MARB 

BASIN IDENTIFICATION NUMBER 

FIGURE 6.3. Bar graphs showing (A) yields of orthophosphate, suspended and total 

phosphorus and (B) phosphorus inputs from point sources in 42 interior basins and the 

entire Mississippi-Atchafalaya River Basin. 

Several basins, notably the Muskingham (3),Scioto (5), Great Miami (6), Tennessee (lo), and Upper 

Illinois (22), have relatively large point-source inputs of P. These same basins have relatively large 

point-source inputs of N (Figure 6.2A). In four basins-3, 6, 10, and 22-the reported point-source 

inputs equaled or exceeded the estimated annual yields of total P for the basins. The reasons for this 

discrepancy are not known but could include in-stream loss of P, errors in the phosphorus flux 

estimates, errors in the point-source flux estimates, or all three. Table 4.5 shows that the standard 

errors in the total phosphorus flux estimates for these basins are about +/- lo%, and Table 5.13 

shows the error in point-source estimates of phosphorus inputs could be more than +/- 50%. Thus, 

estimation errors can easily explain the discrepancy for these four basins. Some of the point-source 

phosphorus may be temporarily or permanently removed from the streams through biological 

processes or physiochemical processes, such as adsorption on sedunent particles and subsequent 

deposition of the sedunent. In most of the remaining interior basins, point sources represent a 
100 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

relatively small percentage of the total phosphorus yield of the streams. For the entire MARB the 

maximum point-source inputs of phosphorus (about 59,000 metric tons) are equivalent to about 

43% of the 136,500 metric tons of phosphorus discharged annually from the basin to the Gulf of 

Mexico. This is a maximum value and assumes no losses between the sources and the Gulf, whch is 

highly unlikely. 

Analysis presented in previous sections of this report indicates a strong linkage between nitrogen 

flux in the Mmissippi River and agricultural activities. In an attempt to examine this linkage more 

closely, an N mass balance was developed for the MARB. The goals of the mass balance were to (1) 

examine the relative importance of all significant N inputs and outputs in the MARB, (2) determine 

how the N balance has changed over the period 1951-96, and (3) determine if there is a relation 

between changes in the N mass balance and the increased flux of nitrate to the Gulf of Mexico. 

Unlike N mass balances developed by Howarth and others, (1996), and Jordan and Weller (1996), 

we have attempted to account for internal recycling of N within the MARB. Ths was done because 

our mass balance was developed annually for 45 years, and we wanted to analyze the long-term 

patterns in the balance. The mass balance was developed for 20 states that comprise the majority of 

the MARB, largely from data reported annually by the U.S. Department of Agriculture, National 

Agricultural Statistics Service, and data on fertilizer sales, atmospheric deposition of N, and point 

sources of N. The mass-balance approach was developed with the assistance of soil scientists and 

agronomists in the Midwest (Hoeft 1998). Specific details about the sources of the data and the 

methods used to estimate N inputs and outputs in the MARB were described in chapter 5 of this 

report. 

All known major inputs and outputs of N during 1951-96 in the 20 major states in the MARB are 

summarized graphically in Figures 5.7 and 5.9. These data were used to develop an N mass balance 

for the MARB each year during that period. All N inputs and outputs were summed for each year. 

Inputs included N additions from fertilizer, legumes, atmospheric deposition, manure, potentially 

mineralizable soil organic N, and point sources. Estimates of atmospheric deposition were not 

available prior to 1984. Therefore, atmospheric deposition for 1951-83 was estimated as the average 

of the 1984-90 atmospheric deposition (see Figure 5.7). This may have over-estimated atmospheric 

deposition inputs in the early part of this period. Point sources, which are minor, are assumed to be 

constant throughout the period. Outputs included N removal in harvested crops, manure and 

fertilizer volatilization, plant senescence, denitrification, and immobilization in soil. The residual N 

was calculated as the N inputs minus the N outputs for each year. The residual includes the N 

leached to ground water and discharged from the MARB to the Gulf of Mexico. It also includes all 

errors in N inputs and outputs. Unfortunately we were not able to develop an estimate of the 

magnitude of these errors. The largest errors are likely to come from mineralization of soil organic 

N and immobilization.The implications of potential errors in these variables in the N mass balance 

are provided in the following discussion. 

Mineralization of soil organic N was estimated from the organic matter content of the upper 30 cm. 

(12 in.) of soil as given in the STATSGO data base (USDA 1994; Burkaa and James 1999; also see 

section 5.2.4). The soil organic material was assumed to contain 3% N and to mineralize at a 

constant rate of 2% per year, producing a constant annual inorganic N input from the soil. 

However, the assumed constant mineralization rate is almost certainly not true over the 45-year 
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities 101 

period of our mass balance. Changes in tillage practices in recent decades and variations in soil 

moisture and temperature would affect mineralization rates. Also, the increased use of fertilizer is 

reported to increase soil mineralization rates (David et al. 1997;Jenkinson et al. 1985) and could also 

increase the N content of the soil organic matter through increased immobilization of N in 

microbial biomass. 

Research using 15N-labeledfertilizer indicates that application of fertilizer at high rates can lead to a 

buildup of an easily mineralizable pool of soil organic N (Stevens et al. 1993). Recent research in 

Illinois (Hoeft 1999) suggests that the N immobilized from fertilizer by soil microbes mineralizes the 

following year as much as seven times faster than the 2% per year use in our soil mineralization 

calculations. Figure 5.9 shows that the estimated immobilization of N increased from about 0.5 

million metric tons/yr in 1951 to 3 million metric tons/yr in 1996. These estimates account for the 

increased use of fertilizer during this period, but not any increase in mineralization rates. In addition, 

the amount of plant residue that remains after crop harvest has most likely increased with increased 

crop yields and changes in tillage practices. This recycled plant residue and immobilized N becomes 

pan of the soil organic N pool and is available for mineralization to nitrate in subsequent years. We 

have accounted for immobilization in the output side of our mass balance, but have not accounted 

for any mineralization of the immobilized N in our budget. If this input is sipficant, then our mass 

balance underestimates N inputs by not including the immobilized N as an additional source. 

Mineralization of additional N from this source would increase the input side of the N balance and 

could significantlyincrease the N residuals. 

N inputs, outputs, and residuals from the mass balance calculations are plotted against time (years) 

in Figure 6.4A for the early 1950s through 1996.The figure shows that both the inputs and outputs 

have increased dramatically since about 1951. Total N inputs have increased from about 13 million 

to nearly 22 million metric tons/yr, and total N outputs have increased from about 10 million to 

about 21 million metric tons/yr. Outputs have increased at a faster rate than inputs. From 1951 to 

1971 N inputs generally were 3-4 million metric tons/yr greater than outputs, but since 1978inputs 

generally are 1-2 million metric tons/yr greater than outputs. The change in the relation between N 

inputs and outputs is shown more clearly in the residuals (Figure 6.4A), which declined slightly from 

about 3.5 to 3.0 million metric tons/yr between 1955 and 1970, but then declined rapidly from 

about 3 to about 1million metric tons/yr between 1969 and 1978. Essentially no change in residuals 

has occurred since 1980, although they have become highly variable from year to year. The residuals 

decreased most rapidly when fertilizer use and N outputs in harvested crops increased most rapidly. 

Fertilizer use and N output in harvested crops leveled off around 1980, at about the same time the 

residuals leveled off. A decrease in the N residuals with time may be an indication of increased 

efficiency in use of N in crop production, or it may just be the result of underestimatinginputs, such 

as mineralization, or overestimating N outputs. 
CUMMULATIVE RESIDUAL FROM NITROGEN MASS, 

BALANCE, IN MILLIONS OF METRIC TONS 

NITROGEN INPUTS A

METRIC TONS PER Y

RESIDUAL (NITROGEN INPUTS M

METRIC TONS PER YE
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities 103 

Figure 6.4B shows the long-term trend in the cumulative residuals from the N balance for 1955-96. 

The upper line in the plot is the cumulative residual from the N mass balance, excluding the nitrate 

discharged from the MARB in streamflow. The bottom line is the cumulative residual with the 

nitrate lscharged from the MARB included as an output. The difference between the two lines is 

the cumulative nitrate discharge to the Gulf of Mexico The pattern in theAe plots suggests that there 

was a constant relation between N inputs and outputs before about 1970. Between 1970 and 1978 

hs relation changed, and since 1978 a new relation has developed with N inputs and outputs being 

more equal. 

One interpretation of the residuals pattern in Figures 6.4A and B is that during the 1950s and most 

of the 1960s the N inputs and outputs were at or near a steady state, but inputs exceeded outputs. 

From the late 1960s to about 1980, rapid changes occurred in the N balance as a result of a rapid 

increase in ferttlizer use and increased crop production (see Figures 5.7 and 5.9). Fertilizer use more 

than tripled from about 2 million to nearly 7 million metric tons/yr, and N output in crops increased 

from about 6 million to more than 9 million metric t~ns/~r. Changes also occurred in agricultural 

policies and practices that increased crop production and efficiency. Acreage limits established by 

the federal government were removed from corn and grain production, use of herbicides on corn 

and soybean increased, producing increased crop yields, and new hybrids crop species also 

expanded. In some states, such as Illinois, there was a shift away from livestock production to more 

corn and soybean production (Hoeft 1998). 

Crop yields increased as a result of these changes, and by about 1980 the relation between inputs 

and outputs was again constant (Figure 6.4B). The leveling off of fertilizer input, crop outputs, and 

the N residuals may inlcate that a new steady-state condition was estabished about 1980. Figure 

6.4B (bottom plot), which includes the output of nitrate in streamflow to the Gulf, indicates that the 

N inputs and outputs for the MARB have been approximatelyequal and at a steady state since about 

1980.Also since about 1980, the trend in nitrate flux to the Gulf has leveled off (Figure 6.5) and has 

become highly variable. The large degree of variabhty in both the N residuals and the nitrate flux to 

the Gulf (Figure 6.5) is an indication of the high degree of sensitivity of these variables to climatic 

conditions, which affect both crop yields and nitrate leaching to streams and ground water. This 

variability may also be a consequence of the much larger annual inputs, outputs, and storage of N in 

the MARB during the last two decades. The residuals associated with the larger inputs and outputs 

are likely to be more sensitive to crop-growing conditions, especially weather, than when inputs and 

outputs were lower. In years with good growing conditions and good crop production, large 

amounts of N are removed in the crops and the residuals are low, indicating that the N outputs are 

about equal to the N inputs. The N outputs actually exceed N inputs during some years, if the N 

flux in streamflow to the Gulf is added to the output. This is possible if soil mineralization is higher 

than the estimated constant rate of 2% per year and if crops use the additional soil N. Crops may 

also use N added to the soil system in prior years, but not used due to excess inputs or poor growing 

conditions during those years. This, too, could result in the annual N outputs being larger than the 

N inputs. 
104 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

YEAR 

FIGURE6.5. Graphs showing the relationship between the annual nitrogen residual 

from mass balance and annual flux of nitrate to the Gulf of Mexico for 195 1-96. 

Years with good crop yields and associated small N residuals generally are years with above-normal 

precipitation and high streamflow (Figure 6.6). However, the higher precipitation also results in 

increased infiltration and leachmg of nitrate from the soil profile to surface and ground water and 

higher fluxes of nitrate to the Gulf. Examples of years meeting these conditions are 1972-74, 1979, 

1982-86, and much of the 1990s (Figure 6.6). During drought years or years with poor growing 

conditions, crop production is down, residuals are larger (inputs >> outputs), but nitrate flux to the 

Gulf is also low due to reduced rainfall and less leaching of nitrate from the soil profile. Examples of 

years having these conditions are 1976-77, 1980-81 and 1987-88 (Figure 6.6). Much of the period 

from 1955 to 1970 also fits these criteria. The implications of this relation are that if precipitation is 

above normal in future years, crop yields wiU be good, but nitrate flux to the Gulf will also be high 

or higher than at present. Conversely, if the climate becomes drier, nitrate flux to the Gulf will 

decrease, and crop yields will likely be lower. The year-to-year variability in nitrate flux to the Gulf 

will remain hlgh because of the large inputs and storage of N in the MARB. 
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities 105 

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 

YEAR 

FIGURE6.6. Graphs showing the relationship between the annual nitrogen residual 

from mass balance and the average annual streamflow from the MississippiAtchafalaya River Basin to the Gulf of Mexico. 

A long-term average mass balance for the MARB for the period 1980-96 is presented in Table 6.1. 

The average mass balance results are also presented in Figure 6.7 in the form of a bar graph. The 

average annual N inputs to the MARB are about 21 million metric tons, of which about 60% is new 

N added to the basin each year, and the remainder represents recycled N. Fertilizer N accounts for 

slightly more than half of the new N inputs. N f~ationby legumes contributes about 35% of the 

new N, and atmospheric deposition of nitrate (wet and dry forms) accounts for about 12%. 

Mineralization of the soil organic N is the largest source (-75%) of recycled N, or N that was 

already in the system. Mineralized soil organic N is estimated to contribute about 6.5 million metric 

tons of inorganic N to the basin annually, an amount equivalent to the current annual input from 

fertihzer. However, as previously discussed, there is much greater uncertainty in the estimates of N 

from the soil than from fertilizer.Animal manure is another input of recycled N largely derived from 

crops produced in the basin. On average, more than half the N in animal manure is lost through 

volat&zation, mostly as ammonia, during storage and application. The manure value in Table 6.1 

represents the amount of N in applied manure after volatihzation losses. N in manure represents 

about 15% of the total recycled N input. Atmospheric wet deposition of ammonia N, which is 

presumed to have originated within the basin, mostly from volatilization of ammonia from manure, 

represents about 7% on the total recycled inputs. Inputs of N from municipal and industrial point 

sources represent about 3% of the total recycled N inputs and are small in the overall budget. 

However, because they go directly into streams they may constitute a signtficant fraction of the N 

transported to the Gulf. 
106 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

TABLE6.1. Nitrogen mass-balance data for the Mississippi-Atchafalaya River 

Basin for 1980-96 (except as noted for atmospheric deposition and point sources). 

NOTE:Units are thousands of metric tonslyr 

I ~otallnputs 20,93 1 

Total New Nitrogen 

Fertilizer 

Total Legumes 

Soybeans 

Alfalfa and other hay 

Pasture and rangeland 

Atmospheric Deposition (1990-96 average) 

(includes wet + dry nitrate and organic nitrogen) 

Total Recycled Nitrogen 

Manure (total adjusted for volatilization losses) 

Potentially Mineralizable from Soil 

Atmospheric Deposition-Wet Ammonia 

Point-Source Inputs to Streams 

Municipal (I 996 data) 

industrial 

I Total Outputs 20,869 

Atmospheric Deposition on Gulf (-500 kg/km2/yr)for an Arbitrary 

Area of 30,000 km2(twice the size of the hypoxic zone) 

15 

Volatilization Losses 

Manure 

Fertilizer 

Crops and Pasture 

Harvested Crops 

Corn grain and silage 

Soybeans 

Alfalfa and hay 

Wheat 

Sorghum grain and silage 

Pasture 

Plant Senescence 

Denitrificationfrom Cropland Soil 

Immobilizationin Soil Organic Matter 

Mississippi-Atchafalaya River Discharge (1 980-96 average) 1,567 

Residual (Inputs-Outputs) (0.3 % of inputs for N) 62 
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities 107 

/

point source residual 

/ 

Inputs Outputs 

FIGURE 6.7. Bar graphs showing the average nitrogen mass balance for the 

Mississippi-Atchafalaya River Basin for 1980-96. NOTE:Mass balance includes estimates 

of all inputs and outputs known to be significant. Atmospheric deposition of nitrogen on the 

Gulf of Mexico is too small (15,000 metric tons) to be shown. 

The estimated average annual N output from the MARB is nearly 21 don metric tons/yr and is 

about equal to the N input. The largest output is N removal in harvested crops and pasture. This 

amounts to about 9.6 don metric tons/yr, or 46% of the total outputs, and is nearly 50% larger 

than the fertilizer inputs. Other outputs, in order of importance are plant senescence (16%), 

immobdization of N in soil organic matter (14%), deniuification (8'0)~and manure and fertilizer 

volatilization (7.8'0). Losses of N in stream discharge from the MARB to the Gulf are about 7.5% 

of the total N outputs and are a sipficant part of the N balance for the basin. Direct deposition of 

N on a 30,000-km2region of the Gulf is less than 0.1% of the N output Fable 6.1) and equal to 

about 1% of the MARB discharge of total N to the Gulf. 
108 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

The N mass balance was useful in examining the relations between N flux in the Mississippi River 

and agricultural activity in the basin. There is a definite linkage. However, much more analysis, and 

refinement of the mass balance--especially the minerahation, immobilization, denitrification, and 

plant senescence components-are needed to better understand these linkages. Ths analysis, which 

will require a collaborative approach involving hydrologists, sdl scientists, agronomists, and 

statisticians, could not be done within the time frame and constraints of this assessment. 

Multiple-regression analysis was used in an attempt to determine which inputs and which human 

activities were the most important contributors of N and P to the MARB and the Gulf. Models were 

developed using the estimated nutrient inputs and nutrient yields for the 42 interior basins, and were 

applied to the entire MARB. The following explanatory variables were considered in the regression 

models for N: fertilizer N, legume N, atmospheric deposition of wet plus dry nitrate N, mineralized 

soil N, manure N, point sources of N, and basin runoff. For P, the variables considered were: 

fertilizer P, manure P, point sources of P, and basin runoff. The source of the N and P input data 

was the 1992 Census of Agriculture data in Tables 5.2 and 5.3, and the basin runoff and N and P 

yield data were the 1980-96 averages in Tables 4.3 and 4.5. The nutrient inputs and outputs for each 

basin were normalized by dividing them by the basin area. Multiple-regression models were 

developed using the SAS Reg procedure (SAS Institute 1990b). Regression diagnostics and residuals 

were examined to ensure the validity of the results. 

6.3.1 Nitrogen Yield Models 

Multiple-regression models were developed to relate the yields of total N and nitrate to the 

normalized inputs of N in the 42 interior basins. Model results were used to help determine which N 

inputs were the most significant contributors to the N yields of these basins and to the Gulf 

Multiple-regression analysis proved to be problematic because of the high degree of correlation 

between many of the explanatory variables, which presented problems in developing a regression 

model. For example, there was a strong relation between N input from fertilizer and N input from 

mineralized soil organic matter (R2 = 0.73). Each was highly significant @ < 0.001) and about 

equally important as an independent variable in regression models if the other was not in the model. 

However, including both variables in a model caused problems with variance inflation, because both 

were apparently attempting to explain the same variation in total N yields. This problem was 

avoided by summing the N inputs from fertilizer and mineralization of soil organic N for each basin 

into a single variable, which will be referred to as the fertilizer-soil N pool. The model response to 

this new variable reflects the combined effects of both soil and fertilizer N inputs. Unfortunately, N 

inputs from fertilizer and soil are so closely interrelated that the individual effects of each source 

could not be separated with the multiple regression approach. 

In addition, preliminary results showed that some of the variability in the total N yield could be 

explained by the variability in runoff from the basins. In general, as runoff increased, so did the total 

N yield. As a result, runoff was included in the regression model, even though it was not directly a 

source of N. There was also a strong relation between basin runoff and atmospheric deposition of 

nitrate (R2 = 0.65). The highest atmospheric deposition of nitrate occurred in the Ohio River Basin 

(see Figure 6.1E), which also has the highest rainfall and runoff. Atmospheric deposition of nitrate 

was significantin the regression model if runoff was not included @ 
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities 109 

< 0.01). However, the addition of runoff to the model made atmospheric deposition statistically 

insipficant (p = 0.66). 

The variables used in the initial total N model were N inputs from the fertilizer-soil N pool (one 

variable), legumes, manure (adjusted for volatilization losses), atmospheric deposition of nitrate (wet 

and dry), point sources, and runoff. Since the primary goal of the regression analysis was to 

determine the relative contributions of the various N sources to the N flux to the Gulf, all input 

variables, except atmospheric deposition, were retained in the model. Atmospheric deposition of 

nitrate was not sipficant at the 0.5 probability level and was excluded. However, because of the 

strong correlation of atmospheric N with runoff, its effects inputs are represented in the model by 

runoff, as are other unmeasured N inputs, such as ground-water discharge and soil erosion. 

Parameter estimates for this model, units, and standard errors appear in Table 6.2. The final model 

with five explanatory variables is presented in equation 6.1, below. The model has an Rhf 0.88, 

indicating that it explains 88% of the variability in total N yields. 

(6.1) Total nitrogen yield = -384 + 0.134" (fertilizer-soil N pool) + 1.304" (point 

source) + 0.395" (manure) + 11.9" (runoff) - 0.115" (legume). 

A regression model was also developed for nitrate N yield, using the same approach discussed above 

for total N yield. The model selected contained the same explanatory variables as the total N model. 

Results for the nitrate yield model appear in Table 6.2 and in equation 6.2, below: 

(6.2) Nitrate yield = -358 + 0.14" (fertilizer-soilN pool) + 0.983" (point-source N) + 

0.391" (manure N) + 7.21" (runoff) - 0.212" (legume N). 

TABLE6.2. Regression model results for total nitrogen' and nitrate2 N yields from the 42 

interior basins. 

Independent 

Variable for 42 Inte- Estimate Error 

rior Basins Total Nitrate Total Nitrate Total Nitrate 

N N N N N N 

Intercept 

Fertilizer-Soil N Pool 

Point-Source N 

Runoff (represents 

atmospheric 

deposition, ground 

water, erosion, etc.) 

Manure N (adjusted for 

volatilization losses) 

Legume N I 

Atmospheric 

Deposition of Nitrate 

This variable was not used in the model. 

Itis included in the runoff variable. 

(wet plus dry) 

Units Mean Value Parameter Standard p Value 

' Model R2= 0.88; mean value of total N yield = 877 kglkm21yr;root mean square error = 322 kglkm21yr. 

Model R2= 0.85; mean value of nitrate N yield = 6 19 kglkm2/yr;root mean square error = 29 1 kglkm21yr. 
1 10 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Both models (equations 6.1 and 6.2 and Table 6.2) indicate that the fertiher-soil N pool is the most 

important source of total N and nitrate N transported from the 42 interior basins. The combined 

inputs of fertilizer and soil N to the basins are large (see Figure 6.2 and mean value in Table 6.2), 

and the regression coefficients indicate that on the average 13-14% of this input may be transported 

in streamflow. For point sources, essentially 100% of the input is transported out of the basins in 

streamflow. This is indicated by the parameter coefficients in equations 6.1 and 6.2, which are near 

unity. Both models indicate that basin runoff is a significant prehctor of N yields. However, runoff 

is not a source of N; rather, it represents undefined sources of N not in the model, including 

atmospheric deposition of N (see discussion at the beginning of this section), discharge of nitrogen 

from the ground water system, and perhaps N in soil erosion. The parameter coefficient in equation 

6.1 suggests that on average, 1 cm of runoff transports about 11.9 +/-3.1 kg/km"yr of total N to 

streams from undefined inputs. The regression coefficients for legume N inputs are negative in both 

the total N and nitrate regression models, and the models have negative intercepts. 

The negative coefficient for legume N inputs in the total N and nitrate regression models is puzzling 

at first. However, the following explanations suggest that legumes contribute little or no N to the 

Gulf of Mexico: 

First, in the total N regression model the legume N regression coefficient is not statistically 

significant @ = 0.35). But, it was retained in the model so that the total N and nitrate models 

would have the same variables. 

Second, the legume N inputs for the 42 basins are highly correlated @ < 0.001) with N 

inputs from fertilizer, soil, and manure, suggesting possible interactions in the regression 

models. For example, when the legume N input was entered into a nitrate regression model 

with either fertilizer or soil N input, the legume N regression coefficient was not statistically 

significant (p > 0.35). However, when legume N input was used in the nitrate yield model 

with the combined fertilizer-soil N input variable, it was significant @ = 0.06). The 

explanation for this must lie in the interaction among the legume-soil-fertilizer N inputs and 

nitrate yields in the regression model. In any case, the regression model results indicate that 

legumes make little or no contribution to the total N and nitrate yields in the MARB. 

Third, the data compiled for this assessment show that legume N outputs estimated for the 

MARB and the 42 interior basins significantly exceed the legume N inputs (see Table 6.1). 

For soybeans, the N output is about twice the N input from atmospheric N hxation. Thus, 

the net legume N inputs to these basins are negative, which means a large amount of N is 

taken from the soil N pool and removed in the harvested legume crops. If legumes were not 

harvested, but left on the cropland and the N returned to the soil, the net legume N inputs 

would be positive. 

Still another factor could be the corn-soybean rotation practice followed throughout the 

Corn Belt. In this practice, soybean is planted in a field one year, and corn is planted in the 

field the following year, because the residue from soybean crops provides a rotation benefit 

to corn crops. Typically, little or no fertilizer is applied to soybean crops. Thus, when 

soybean is grown, the N input from fertilizer is significantly reduced. This may indirectly 

contribute to a decrease in N inputs to streams and reduced N yields. 
Chapter 6: Linking Nutrient Flux to Nutrient Inputs and Human Activities I I l 

The negative intercept in equations 6.1 and 6.2 is believed to represent N losses and unmeasured N 

outputs. These include in-stream losses from denitrification wih the interior watersheds and 

temporary or permanent storage of N in stream sediments and on floodplains. Denitrification is 

probably a major sink for nitrate in small watersheds and wetland areas within the interior basins, 

whereas it is less sigmficant in large rivers. In small basins there is opportunity for much longer 

contact time between the overlying water column and stream sediments than in large streams. This 

provides more opportunity for denitrification to occur at the water-sediment interface, where 

anoxic or low-oxygen conditions can occur. The negative intercept may also be caused in part by 

unmeasured stream transport of N that has been assimilated into plant and animal biomass. 

Nitrogen transported in particles larger than about 2 millimeters escapes collection in water samples 

and, thus, is not measured or included in the yield estimates. Finally, the equation intercepts are not 

precisely known, as indicated by the large standard error of > 30%. 

Since the 42 interior basins generally represent the entire MARB, the parameter coefficients in 

equations 6.1 and 6.2 can be applied to the area normalized N inputs to the entire MARB with a 

reasonable degree of confidence. These results can be used to estimate the relative contributions of 

each input source to the N yield of the MARB and the N flux to the Gulf of Mexico. Results of 

these estimates for total N are presented in Table 6.3. 

TABLE6.3. Estimated contributions of nitrogen input sources to the total nitrogenyield of 

the Mississippi-Atchafalaya River Basin and total nitrogen flux to the Gulf of Mexico. 

Total Nitrogen Normalized -from Contribution Contribution 

Input Source Input to Entire Regression to MARB to Total N 

MARB Model 6.1 Yield Flux to Gulf 

(kglkm21yr) (percent) 

Fertilizer-Soil N Pool 4,039 kg/km2/yr 0.134 +/- .024 54 1 +/- 97 50 +/ -9 

Municipal and Industrial 

Point Sources 

89 kg/kmVyr 1.304 +/- 0.276 1 16 +I-25 11 +/- 2 

Other Inputs Represented 

by Runoff (including 

atmospheric deposition, 

ground water, soil erosion) 

22 cm/yr 1 1.9 +I-3.1 I 262 +/ -68 24+ / -6 

Manure (adjusted for 

volatilization losses) 

404 kg/kmVyr 0.395 +/- 0.26 160 +/- 105 15 +/- I0 

Legumes 1,348 kg/kmVyr -0.1 15 +/- 0.122 - 155 +/- 164 0 

Predicted Total N Contributions from All N Sources 

N Losses and Unmeasured Outputs (model intercept) 

N Losses (removal by legumes) 

Total N Yield of the MARB Predicted by Model 6.1 

Total N Yield of MARB Estimated in Table 4.2 

1,079+/-229 kg/km2/yr 

-384 +/- 127 kg/kmz/yr - 155+/- 164 kg/kmVyr 

540+/- 198 kg/km2/yr 

489+/- -25 kg/kmVyr 

Application of regression equation 6.1 to the entire MARB indcates that about 1,079 kg/kd/yr of 

total N is derived from input sources in the MARB. About 384 kg/km2/yr (model intercept) of this 

input is lost to undetermined sinks in the MARB, such as denitrification, unmeasured outputs, and 

storage. In addition, it is estimated that legumes decrease total N yield by about 155 kg/km2/yr. 

However, this value is very uncertain because of the high standard error for the regression 

coefficient. Reasons for the decrease in yield attributed to legumes were previously dscussed. 
I I2 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

The model predicts the total N yield of the entire MARB to be about 540 kg/km2/yr, which is close 

to the total N yield estimate of 489 kg/km2/yr developed from flux data dscussed in chapter 4 and 

summarized in Table 4.2. These results indicate that about 50% of the total N flux from the MARB 

to the Gulf is derived from the fertilizer-soil N pool. Point sources are estimated to contribute 

about 11% of the total N, which is about half the maximum contribution from point 

sources shown in Figure 6.1A and discussed in section 6.1. Animal manure may contribute about 

15% of the total N flux to the Gulf, although there is considerable uncertainty about this number 

(see Table 6.3). About 24% of the total N is estimated to be derived from sources not in the model 

but represented in the model by runoff. These sources include atmospheric deposition, groundwater discharge to streams, and perhaps N contained in sediment transported into streams by soil 

erosion. It should be noted that N input from ground water and represented in the model by runoff 

is largely derived from agricultural activities. It can take months to years before the N that leaches to 

ground water is transported into streams, and ground water can continue to contribute N to streams 

long after all N sources are removed. 

The contributions of N sources to the nitrate yield of the MARB and nitrate flux to the Gulf were 

estimated with equation 6.2 in the same manner as total N. The results, shown in Table 6.4, indicate 

that about 969 kg/km2/yr of nitrate is derived from sources within the MARB. 

TABLE6.4. Estimated contributions of nitrogen input sources to the nitrate-nitrogen yield 

of the MARB and nitrate-nitrogen flux to the Gulf of Mexico. 

Total Nitrogen Normalized Coefficient Contribution Contribution 

Input Source Input to Entire from to MARB to Total N 

MARB Regression Yield Flux to Gulf 

Model 6.2 (kglkm2/~) (percent) 

Fertilizer-Soil N Pool 4,039 kg/kmz/yr 0.14 +/- 0.022 565 +/- 89 58 +/- 9 

Municipal and Industrial 

Point Sources 

89 kg/km2/yr 0.98 1 +/- 0.249 87 +/- 22 9 +/- 2 

Other Inputs Represented 

by Runoff (including 

atmospheric deposition, 

ground water, soil erosion) 

22 cmlyr 7.22 +/- 2.8 1 159 +/- 62 16 +/- 6 

Manure (adjusted for 

volatilization losses) 

404 kg/km2/yr 0.39 1 +/- 0.234 158 +/- 94 16 +I- 9 

Legumes 1,348 kg/km2/yr -0.2 I2 +/- 0. I 1 -286 +/- 148 0 

Predicted Total N Contributions from All N Sources 

N Losses and Unmeasured Outputs (model intercept) 

N Losses (removal by legumes) 

Total NYield of the MARB Predicted by Model 6.2 

Total N Yield of MARB Estimated in Table 4.2 

About 358 kg/km2/yr of the nitrate (model intercept) is lost to undetermined sinks, such as 

denitrification or storage in ground water. Legumes decrease nitrate yield by 286 kg/km2/yr. (See 

earlier discussion for reasons for the decrease.) The model predicts a net basin nitrate yield of 325 

kg/km2/yr, which is close the estimate of 297 kg/km2/yr determined from flux data discussed in 

chapter 4 and summarized in Table 4.2. The N input from the fertilizer-soil N pool is estimated to 
Cha~ter6: Linking Nutrient Flux to Nutrient ln~utsand Human Activities 1 13 

account for about 58% of the nitrate N flux to the Gulf Fable 6.4). Point sources contribute about 

9% of the nitrate flux. Animal manure is estimated to contribute about 16% of the nitrate flux to the 

Gulf, and about 16% of the N flux to the Gulf is derived from undetermined inputs represented in 

the model by runoff. 

The regression model re'sults support the qualitative interpretations of Figures 6.1 and 6.2. The 

fertilizer-soil N pool appears to be the source of about 50-60% of the N transported to the Gulf of 

Mexico: about 10% is from point sources, 11-16% is from manure, and the remainder is from such 

sources as atmospheric deposition, ground-water discharge, and soil erosion. These results suggest 

that about 90% of the N flux to the Gulf is from nonpoint sources. 

Although much of the nonpoint-source N is derived from agncultural activities, some is of natural 

origin and would be present in the MARB, regardless of human activity. Mineralization of soil 

organic N and decomposition of vegetation were probably the only significant sources of nitrate and 

dissolved organic N to the MARB and the Gulf before human development in the basin. No doubt, 

such activities as tillage, drainage, and adhtion of fertilizer have sipficantly increased N 

contributions from the soil. Additional nonpoint N contributions come from manure and 

atmospheric deposition. Urban runoff contributes N to streams in some parts of the MARB, but 

was not specifically addressed in this report because of insufficient data. However, because urban 

land comprises less than 1% of the MARB, the contribution of urban runoff to the N flux to the 

Gulf is believed to be very small. The spatial distribution of N inputs discussed in section 4.2 

support this statement. With a few exceptions, the largest sources of N are basins dominated by 

agnculture, and not urban areas. 

6.3.2 Total Phosphorus Yield Model 

Multiple-regression analysis was used to examine the relation between total P yields and P inputs for 

the 42 interior basins. Explanatory variables used in the regression model were: P input from 

fertilizer, P input from point sources, P input from manure, and runoff. Runoff is not a P source, 

but it represents unmeasured inputs to streams, such as sediment, which contains the majority of the 

total P transported by streams. The results from this model are shown in Table 6.5 and in equation 

6.3. 

TABLE6.5. Regression model' results for total phosphorusyields from 42 interior basins. 

Independent 

Variable 

Units Mean Value for 

42 Interior 

Parameter 

Estimate 

Standard 

Error 

p Value 

Basins 

Intercept 

Fertilizer P 

Point Source P Inputs 

Runoff 

Manure P 

'Model RZ= 0.43; mean value of total P yield = 57 kglkm2/yr;root mean square error = 34 kglkmZlyr. 
1 14 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

Point sources are also contributors of P, but are less important than fertduer and sources 

represented by runoff. The model estimate of the coefficient for point-source input is 0.28, as 

opposed to near unity for point-source N inputs. This suggests that there could be considerable loss 

of P from the stream between the input sources and the terminus of the basin. P input from manure 

has a large uncertainty in the model, as inlcated by the large standard error and hgh p value (Table 

6.5). However, it is included so that the P contribution from hs source can be estimated. The 

model intercept is not significantly lfferent from zero (p = 0.8), suggesting there are no other 

sipficant inputs or losses of P that are not accounted for in the model. 

(6.3) Total P yield = -3.39 + O.O47*(fer&zer P) + 0.278" (point-source P) + 0.027* 

(manure P) + 0.905*(runofi). 

The total P model (equation 6.3 and Table 6.5) was applied to the area-normalized total P inputs for 

the entire MRB. The net total P yield estimated by the model for the entire MRB is 45 +/- 27 

kg/krnVyr (Table 6.6). Results of hs analysis were used to estimate the relative contribution of the 

P sources to the total P flux to the Gulf of Mexico. The P fertilizer inputs are estimated to 

contribute about 31% of the total P lscharged from the MARB to the Gulf (Table 6.6). Municipal 

and industrial point sources contribute about 10% of the P flux to the Gulf, which is about the same 

percentage that they contribute for total N. However, hs is significantly less than the maximum 

potential contribution from point sources of more than 40%, discussed in section 6.1 and shown in 

Figure 6.3B, and suggests sipficant in-stream losses, errors in the point-source estimates, or errors 

in the flux estimates. About 17% of the phosphorus flux is from manure, although hs value has 

much uncertainty, as inlcated by the large standard error and hgh p value. Unmeasured P inputs 

represented in the model by runoff contribute about 40% of the total P flux to the Gulf. The most 

significant of these unmeasured inputs is hypothesized to be P in sedunent from soil erosion. The 

estimated total P output from the MARB in suspended sediment is included in the total P yields of 

the streams; however, there is no estimate of the total P input to the streams in sedunent. 

TABLE6.6. Estimated contributions of phosphorus input sources to the total phosphorus 

yield of the Mississippi-Atchafalaya River Basin and total phosphorus flux to the Gulf. 

Phosphorus Input Source Normalized Coefficient Contribution Contribution 

Input to Entire from to MARB to Total P 

MARB Regression Yield Flux to Gulf 

Model 6.3 (kglkm21yr) (percent) 

Fertilizer-P 320 kglkm2lyr 0.047 +I-.02 15 +I-6 31 +I-12 

Municipal and Industrial 18 kgIkm21yr 0.278 +I-0.123 5 +I-2 I0 +I-4 

Point Sources 

Other Inputs Represented 22 cmlyr 0.905 +I-0.3 1 3 20 +I-7 42 +I-15 

I 

by Runoff 

Manure 3 1 I kdkm21yr 0.027 +I-0.036 8 +I-II 17 +I-23 

Predicted Total PContributions from All P Sources 

P Losses (model intercept) 

Total PYield of the MARB Predicted by Model 6.3 

Total PYield of MARB Estimated in Table 4.4 
Research Needs 

This assessment has identified several research needs and data gaps, which if addressed could 

provide a better scientific understanding of processes affecting the flux and sources of nutrients in 

the Mississippi-Atchafalaya River Basin. Improved understanding of these processes could lead to 

new practices, policies, and incentives targeted at reducing the loss of nutrients, such as nitrate, to 

surface- and ground-water systems within the basin. Reducing the loss of nutrients to streams in the 

basin could benefit the Gulf of Mexico by reducing the extent of hypoxia, and could also benefit the 

Upper Mississippi Basin by improving the quality of water in streams and aquifers. The identified 

research needs follow. 

The sources of most N discharging to streams in the basin and to the Gulf are the soils and 

unsaturated zones underlying cropland. This near-surface zone can serve as a huge storage reservoir 

for N derived from mineralization of soil organic N, agricultural activities, and atmospheric 

deposition. The annual N inputs to and outputs from this reservoir have doubled in recent decades 

with the increased use of fertilizer, and have substantially increased the amount of N potentially 

available for leaching. Precipitation can leach N present in the form of nitrate from this reservoir to 

streams in runoff, agricultural drains, and ground water. Research is needed to find ways to <<manage" the storage of N in this zone in a way that minimizes the accumulation of excess nitrate 

and minimizes the losses of nitrate from this zone to the hydrologic system. This research would 

include developing a better understanding of mineralization and immobilization processes, quick 

and easy ways to measure the amount and forms of N in the soil reservoir, and strategies to 

minimize leaching of nitrate from the soils to streams. 

Additional research is needed in small watersheds in both drained and undrained areas to better 

understand the dynamics and timing of nitrate transport from cropland to streams. Research is also 

needed to better define the extent and density of tile drainage and other agricultural drainage and to 

better understand the magnitude of the impact of these drainage practices on nutrient flux in large 

rivers. This could augment ongoing research in the MARB and should be designed to support the 

research needs outlined above in item (1). 
1 16 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

There is currently much uncertainty about the role of in-stream processes, such as denitrification in 

removing N from streams in the basin. Although denitrification does not appear to be a highly 

significant process in removing N from large streams, it may be very important in small streams. 

Research is needed to examine the significance of denitrification in removing N leached from 

agricultural land, and to find ways to enhance thls process in order to reduce leaching of nitrate to 

streams and ground water in the MARB. 

7.4 SPARROW MODEL 

The SPARROW model (spatially referenced regressions on watershed attributes; Smith et al. 1997) 

has been developed to estimate nutrient flux in unmeasured stream reaches. SPARROW uses a 

multiple-regression model based upon spatially referenced contaminant inputs, physical 

characteristics of the soil, and hydraulic properties of the stream reaches. In addition to predicting 

flux, the model allows the total flux to be apportioned among different input sources, such as 

fertilizer application, point sources, and atmospheric deposition, and to determine spatially where 

the flux comes from within a basin. Some work has been done to apply SPARROW to nutrient 

sources and transport in the Mssissippi Basin (Smith et al. 1997; Alexander et al. 1999; also see 

SPARROW on the worldwide web at: h~://wwwrvares.er.t/sgsgov/nawqa/~amw/). Further research 

and development on SPARROW are needed so that it can account for additional input and output 

terms, such as soil minerahation, crop export, immobilization, and annual variation in the location 

and quantity of precipitation. 

This assessment has made a first attempt at developing a nitrogen mass balance for the MARB. 

Some of the estimated inputs and outputs have large uncertainties. An effort should be made to 

improve upon this balance so that more precise estimates of the residual N that is available for 

leaching to surface and ground water can be developed. This can guide development of efforts to 

reduce N losses. Any effort to refine the N balance should be a multidisciplinary approach involving 

agronomists, soil scientists, hydrologists, and statisticians. 

Measurements of stable isotopes, such as 15Nand 180 in the nitrate (Nos) ion, may provide a means 

to identify specific sources of nitrate &charging to streams. Investigators have used isotopic 

techniques to determine mixing ratios of waters from different sources, to quantify such processes as 

denitrification, and to identify sources of N in water resources (Clark and Fritz 1997; Kendall 1998; 

Kellman and Hillaire-Marcel 1998). Most of the studies to date are from small study areas, and few 

have attempted to work with isotopes in large rivers (Kohl et al. 1971). A research effort to explore 

the utility stable isotopes for identifymg N sources is currently underway in the basin (Battaglin et al. 

1997), with support of EPA's Gulf of Mexico Program. 'Ihs research should be continued and 

expanded if the technique proves to be useful. 
Chapter 7: Research Needs 1 17 

7.7 ATMOSPHERICDEPOSITIONINTHE MARB 

Research is needed to improve estimates of wet and dry deposition of N compounds. Ths includes 

additional measurement stations to improve the spatial distribution of data needed to support spatial 

deposition models, such as described in Prospero et al. (1996). Improvements in air sampling 

techniques are needed to include NH3 and organic N. Techniques are also needed to identify all of 

the forms of N now being collected by three-stage filter packs. 

7.8 ATMOSPHERICDEPOSITIONINTHE GULFOF MEXICO 

There is currently very little data on the lrect atmospheric deposition of N on the waters of the 

Gulf of Mexico. The evidence that does exist suggests that atmospheric deposition of N is 

insignificant relative to other N inputs. Additional research on atmospheric deposition of N on the 

Gulf is needed to confirm or refute the current limited evidence. However, this research need 

should be given a lower priority than other needs listed in this chapter. 
CHAPTER8 

Concllusions and Recommendations 

This assessment has used available information to address two questions: (1) What are the loads 

(fluxes) of nutrients in the Mississippi-Atchafalaya fiver Basin and where do they come from, and 

(2) Which human activities are most significant in contributing the nutrients to the Mississippi fiver 

system? Nitrogen, phosphorus, and ska are addressed in this report. However most of the 

emphasis is on N, which is the nutrient of most concern to the hypoxia issue. 

8.1. I Flux and Sources 

Analysis of historical records shows that the concentrations of nitrate in the Mssissippi River and 

tributaries in the Upper Mississippi Basin have increased by factors of 2-5 since 1900. The current 

average annual N flux from the MARB to the Gulf of Mexico is about 1.6 million metric tons. The 

annual flux has approximately tripled during the last 30 years, with most of the increase coming 

between 1970 and 1983. Expressed as a yield, the average total N flux for 1980-96 is 489 kg/krn"yr 

and is estimated to be 2.2-6.5 times hgher than baseline "pristine" conditions for the North Atlantic 

Basin (Howarth 1998). The average flux has changed very little since the early 1980s, but there are 

large year-to-year variations in N flux caused by variations in precipitation. During wet years the N 

flux can increase by 50% or more due to flushmg of N that has accumulated in the soils and groundwater system in the basin. Episodic events, such as the 1993 flood, can and will continue to 

transport abnormally large quantities of nitrate to the Gulf. There has been no significant change in 

the flux of phosphorus since the early 1970s, when phosphorus records began, and no statistically 

significant change in the flux of sihca since the 1950s,when silica records began. 

The principal sources of N are watersheds in southern Minnesota, Iowa, Ilhois, Inlana, and Oh10 

that drain agricultural land. This region contributes several times more N per unit area to the 

Mississippi River than do basins outside this regon. Streams draining from two states, Iowa and 

Illinois, contribute on average, about 35% of the N discharged by the MARB, but comprise only 

about 9% of the total area of the MARB. In years with abnormally high precipitation they can 

contribute much more than hs. For example, in 1993 Iowa alone with 4.5% of the area of the 

MARB, contributed about 35% of the nitrate discharged from the MARB to the Gulf. 
Cha~ter8: Conclusions and Recommendations 1 19 

$. 1.2 Relative Importance of Human Activities in 

Contributing to Nutrient Flux 

Several analytical approaches were used to examine the relative importance of human activities, such 

as agriculture and point-source discharges, and atmospheric deposition, ih contributing nutrients to 

the MARB. These approaches included grapkcal comparisons, an N mass balance for the MARB, 

and multiple-regression models. 

Results indicate that about 90% of the N flux to the Gulf of Mexico is derived from nonpoint 

sources. Agricultural activities are by far the largest contributors of N to streams in the MARB. The 

N sources and their estimated contribution to the flux of total N to the Gulf are: (1) input from the 

fertilizer-soil N pool (50°/o); (2) inputs associated with basin runoff, such as atmospheric deposition, 

ground-water discharge, and soil erosion (24%); (3) animal manure (15%); and (4) municipal and 

industrial point sources (11°/o). The major contributors and their relative contribution to the flux of 

total P to the Gulf are: (1) inputs associated with basin runoff, such as soil erosion (4l0o);(2) P from 

fertilizer (31%); (3) animal manure (18'); and (4) municipal and industrial point sources (10%). The 

point-source discharges are believed to be relatively constant, indicating that the large increases in 

nutrient flux during above-normal precipitation come from nonpoint sources. However, within a 

few highly urbanized basins in the Upper Mississippi Basin, municipal and industrial point sources 

are very important sources of N and P. Atmospheric deposition appears to be a relatively small 

contributor of overall flux of nitrogen to the Gulf. This is in sharp contrast to Chesapeake Bay and 

elsewhere in the eastern United States, where atmospheric deposition has been reported to be a 

major source of N. 

Of the agricultural sources of N examined in this assessment (fertihzer, legumes, mineralization of 

soil, and manure) feralizer and soil organic N are the most important sources of N. They appear to 

contribute about equally to the N flux in streams, although their individual contribution could not 

be quantified with regression models. Legumes do not appear to be sipficant contributors to the N 

flux of the Mississippi River. More N is removed in the harvested legumes, particularly soybeans, 

than they fix from the atmosphere. However, the residue remaining from legume crops provides a 

rotation benefit to crops that follow them. If not used, the mineralized N can leach to streams and 

ground water. Some legume N is also contributed to streams indirectly through animal manure. 

Fertilizer, and to a lesser degree legumes, are the only two sources of N that have increased 

sipficantly since the 1950s. Fertilizer use has increased nearly seven-fold since 1960, and the 

amount of N removed in harvested crops has more than doubled since 1960,paralleling the increase 

in fertilizer use. 

8.1.3 Climatic Effects on Nutrient Flux 

The average annual streamflow of the Mississippi River has increased by about 30% since the 1955-

70 time period as a result of increased precipitation. This increase, in conjunction with increased N 

inputs, has resulted in increased leaching of nitrate, from agricultural land to ground water and 

streams, and has led to about a three-fold increase in N flux to the Gulf of Mexico. 

In future years the flux of nitrate to the Gulf most likely will continue to respond quickly and 

dramatically to variations in precipitation and runoff. Because of the readily available pool of nitrate 

in the soil-ground-water system, N fluxes wdl be high in wet years and low in dry years. However, 

because of the huge soil-ground-water reservoir available for storage of nitrate in the MARB 
I20 Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin 

system, the flux of nitrate to the Gulf willmost likely change very slowly in response to increases or 

decreases in N inputs. The N balance of the soil-ground-water system will have to adjust to changes 

in N inputs and outputs. The response time of the MARB to changes in N inputs and outputs is 

unknown, but may be several years or longer. 

At present no programs or mechanisms are in place to determine if changes in nutrient flux in 

streams occur as a result of voluntary actions and new policies. Nutrient monitoring is being carried 

out at a few sites on large rivers, such as the Mississippi and Ohio, by the USGS National Stream 

Quality Accounting Network, but there are no coordinated data-gathering efforts at the small basin 

scale, which will be most sensitive to changes in nutrient inputs. The following recommendations 

address these concerns. 

8.2.1 Nutrient Monitoring Program 

Establish a nutrient monitoring program in the MARB designed to determine the effects of 

voluntary actions, changes in nuuient management practices, and new policies aimed at reducing the 

nutrient flux to the Gulf of Mexico. Such a program should consider the re-establishment of 

monitoring in some of the 42 interior basins (former NASQAN stations) used in this assessment. 

These sites have the benefit of a long period of historical data. Monitoring at this scale should be 

augmented by nutrient monitoring in selected small basins, where the effects of changes in nutrient 

inputs will be most noticeable. Any nutrient monitoring program that is established must include a 

plan for data compilation and for timely synthesis and dissemination of data to all interested parties. 

8.2.2 Effluent Monitoring Program 

Establish an effluent monitoring program designed to systematically improve current estimates of 

nutrients discharged to streams from municipal and industrial point sources. 

8.2.3 MonitoringAtmospheric Deposition 

Continue current programs to monitor nutrients from atmospheric wet deposition in the MARB, 

and expand the current limited monitoring of nutrients in atmospheric dry deposition. This 

information is needed to determine if nuuient-reduction strategies affect precipitation chemistry. 

8.2.4 Interdisciplinary Forum 

EPA and USDA should provide a forum for discussing nutrient budgets in large watersheds. 

Participants should include hydrologists, soil scientists, ecologists, and agricultural engineers. The 

forum would provide a means to establish a dialog between researchers from different fields of 

expertise. This kind of interdisciplinary exchange offers the best hope of addressing the complex 

issue of understanding the links between nutrient sources, cycling, flux in large watersheds, and 

hypoxia in the Gulf, and developing strategies to reduce excess nutrients. 
Chapter 8: Conclusions and Recommendations I 2 I 

8.2.5 Long-Term Research 

Develop a long-term research effort that would collect the data and information needed to 

determine the relation between the three-dimensional extent of hypoxia in the Gulf (i.e., the volume 

of water in the Gulf affected by oxygen depletion) and the flux of nutrients from the MARB. This 

research would require collection of more extensive data on the extent of hypoxia in the Gulf than is 

currently being collected. That data would be quantified in terms of the volume of the Gulf affected 

and the amount of oxygen consumed. Existing and new nutrient monitoring data from the 

Mississippi River would be used to calculate the flux of nutrients entering the Gulf. Statistical or 

solute transport models, coupled with nutrient-dissolved oxygen models of the Gulf, would be 

developed and used to determine if there is a threshold nutrient flux for Mississippi River below 

which there is little or no problem from hypoxia. This research effort would logically be developed 

in conjunction with the proposed Global Ocean Ecosystems Dynamics (GLOBEC) program that 

NOAA has proposed for the Gulf of Mexico. 
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OTHER TITLES IN THE DECISION ANALYSIS SERIES 

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No. 2 

No. 3 

No. 4 

No. 5 

No. 6 

No. 7 

No. 8 

No. 9 

No. 10 

No. 11 

No. 12 

No. 13 

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Able, Kenneth W., and Susan C. Kaiser. 1994. Synthesis of Summer Flounder Habitat 

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Cohns, Elaine V., Maureen Woods, Isobel Sheifer, and Janice Beattie. 1994. Bibliography of 

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Hinga, Kenneth R., Heeseon Jeon, and Noelle F. Lewis. 1995. Marine Eutrophication Review. 

Lipton, Douglas W., Katherine Wellman, Isobel C. Sheifer, and Rodney F. Weiher. 1995. 

Economic Valuation of Natural Resources: A Handbook for Coastal Policymakers. 

Vestal, Barbara, Alison Reiser, et al. 1995. Methodologies and Mechanisms for Management of 

Cumulative Coastal Environmental Impacts. Part I-Synthesis with Annotated Bibliography; 

Part 11-Development and Application of a Cumulative Impacts Assessment Protocol. 

Murphy, Michael L. 1995. Forestry Impacts on Freshwater Habitat of Anadromous Salmonids in 

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Wdham F. IOer Associates. 1995. Watershed Restoration-A Guide for Citizen Involvement in 

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Valigura, Richard A., Winston T. Luke, &chard S. Artz, and Bruce B. Hicks. 1996. Atmospheric 

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Boesch, Donald F., et al. 1997. Harmful Algal Blooms in Coastal Waters: Options for 

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McMurray, Gregory R., and Robert J. Bailey, eltors. 1998. Change in Pacific Northwest Coastal 

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Rabalais, N.N., R.E. Turner, D. Justic Q. Dortch, and W.J. Wiseman, Jr. 1999. Characteriza-tion 

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&ver Basin: Topic 3 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. 

Brezonik, P.L., V.J. Bierman, Jr., R. Alexander, J. Anderson,J. Barko, M. Dortch, L. Hatch, G.L. 

Hitchcock, D. Keeney, D. Mulla, V. Smith, C. Walker, T. Whitledge, and W.J. Wiseman, Jr. 

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and the Gulf of Mexico: Topic 4 Report for the Integrated Assessment on Hypoxia in the Gulf 

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Mitsch, W.J., J.W. Day, Jr., J.W Gdham, P.M. Groffman, D.L. Hey, G.W. Randall, and N. Wang. 

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Doering, O.C., F. Diaz-Herrnelo, C. Howard, R. Heimlich, F. Hitzhusen, R. Kazmierczak, J. 

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and Benefits of Methods for Reducing Nutrient Loads to the Gulf of Mexico: Topic 6 Report 

for the Integrated Assessment on Hypoxia in the Gulf of Mexico. 
U.S. Department of Commerce 

National Oceanic and Atmospheric Administration 

Coastal Ocean Program 

13 15 East-West Highway 

Silver Spring, Maryland 209 1 0