SET-WET: A WETLANDSIMULATION MODEL TO OPTIMIZE

NPS POLLUTION CONTROL

ERIK RYAN LEE

Thesis submitted to the Faculty of theVirginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

Master of Sciencein

Biological Systems Engineering

Saied Mostaghimi, ChairTheo A. Dillaha

Raymond B. ReneauJohn V. Perumpral

September 15,1999Blacksburg, VA

Keywords: Wetlands, Model, Nonpoint Source Pollution,Biological, Nutrients

Copyright 1999, Erik R. Lee

SET-WET: A WETLAND SIMULATION MODEL TOOPTIMIZE NPS POLLUTION CONTROL

Erik Ryan Lee

(Abstract)

A dynamic, compartmental, continuously stirred tank reactor, simulation model (SET-

WET) was developed for design and evaluation of constructed wetlands in order to optimize

non-point source (NPS) pollution control measures. The model simulates the hydrologic,

nitrogen, carbon, dissolved oxygen, bacteria, vegetative, phosphorous and sediment cycles of a

wetland system. Written in Fortran 77, SET-WET models both free water surface (FWS) and

sub-surface flow (SSF) wetlands and is designed in a modular manner which gives the user the

flexibility to decide which cycles and processes to model. SET-WET differs from many existing

wetland models in that it uses a systems approach, and limits the assumptions made concerning

the interactions of the various nutrient cycles in a wetland system. It accounts for carbon and

nitrogen interactions, as well as effect of oxygen levels upon microbial growth. It also directly

links microbial growth and death to the consumption and transformations of nutrients in the

wetland system. Many previous models have accounted for these interactions with zero and first

order rate equations that assume rates are dependent only on initial concentrations. The SET-

WET model is intended to be utilized with an existing NPS hydrologic simulation model, such as

ANSWERS or BASINS, but may also be used in situations where measured input data to the

wetland are available.

The model was calibrated and validated using limited data collected at Benton, Kentucky.

A non-parametric statistical analysis of the model's output indicated eight out of nine examined

outflow predictions were not statistically different from the measured observations. Linear

regression analysis showed that six out of nine examined parameters were statistically similar,

and that within the expected operating range, all of the examined outflow parameters (9) were

within the 95% confidence intervals of the regression lines. A sensitivity analysis showed the

most significant input parameters to the model were those which directly affect bacterial growth

and oxygen uptake and movement. The model was applied to a subwatershed in the Nomini

iii

Creek watershed located in Virginia. Two year simulations were completed for five separate

wetland designs, with reductions in percentage of BOD5 (4%-45%), TSS (85%-100%), total

nitrogen (42%-56%), and total phosphorous (38%-57%) comparable to levels reported by

previous research.

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Acknowledgements

I would first like to thank my advisor Professor Saied Mostaghimi, who gave me

countless advice and information on how to do proper and professional thesis work. To my

committee members Professor Theo Dillaha and Professor Ray Reneau, your advice and tutelage

were sage and wise. To our Department head, Professor John Perumpral, I would like to give

thanks for helping me adjust to Virginia and making me feel at home. Big thanks to Kevin

Brannan and Shreeram Inmandar, who knew that when I knocked on their door they were going

to be interrupted for an hour. To Theresa Wynn I give thanks for all the help on my model when

you were dog-tired and the advice about life and other important things.

My parents, Priscilla and Collin Wong, were very encouraging and I am glad that they

made me learn how to cook and clean, because Ive seen plenty of very helpless people in

college. My grandmothers, Lin Kim Lennie Lee and Susie Lum have always been supportive

and understanding. To my brothers, Daryl, William, and Alex, I thank you for giving me the

motivation to study because I wanted to get better grades than you. To my aunts and cousins

who have sent me cookies through my college years, my roommates and I thank you. Of course,

even though I am about to graduate that tradition may continue.

I would also like to acknowledge every one in my family and all of my friends. Now that

I have my Masters in Biological Systems Engineering, I hope that you can finally remember

what the title of my degree is.

vTable of ContentsI. INTRODUCTION ............................................................................................................................................. 1

A. GOAL AND OBJECTIVES ................................................................................................................................. 2

II. LITERATURE REVIEW ................................................................................................................................. 3

A. NPS POLLUTION ............................................................................................................................................ 3B. BEST MANAGEMENT PRACTICES (BMPS) .................................................................................................... 5C. WETLANDS ..................................................................................................................................................... 7

1. Classification ............................................................................................................................................. 8a. Natural Wetlands.........................................................................................................................................................8b. Constructed Wetlands..................................................................................................................................................9

2. Constructed Wetland Design................................................................................................................... 103. Nitrogen Cycle in Wetlands..................................................................................................................... 15

a. Nitrogen Transformation Processes .........................................................................................................................17i. Mineralization (ammonification) ..........................................................................................................................17ii. Nitrification ..........................................................................................................................................................18iii. Denitrification.......................................................................................................................................................19iv. Nitrogen Fixation.................................................................................................................................................19v. Assimilation: Plant and Bacterial Uptake ............................................................................................................20

b. Other Nitrogen Fluxes ..............................................................................................................................................21i. Atmospheric Nitrogen Inputs ................................................................................................................................21ii. Ammonia Volatilization .........................................................................................................................................21iii. Adsorption ............................................................................................................................................................22iv. Burial of Organic Nitrogen ..................................................................................................................................22v. Biomass Decomposition.......................................................................................................................................22

4. Phosphorous Cycle in Wetlands ............................................................................................................. 22a. Importance of Sediment Sorption/Desorption .......................................................................................................23b. Precipitation ..............................................................................................................................................................24c. Biomass: Growth, Death, Decomposition, Uptake and Storage ..............................................................................25

5. Bacteria in Wetlands ............................................................................................................................... 256. Vegetative/Carbon Cycle in Wetlands ..................................................................................................... 277. Modeling Wetland Processes .................................................................................................................. 28

a. General Modeling Practices......................................................................................................................................29b. Modeling of Specific Wetland Processes ..................................................................................................................31

i. Hydrology .............................................................................................................................................................31Overall Water Budget ...........................................................................................................................................31Surface Water Flow...............................................................................................................................................33Evapotranspiration ................................................................................................................................................35Groundwater Flow ................................................................................................................................................37

ii. Nitrogen ................................................................................................................................................................37iii. Phosphorous .........................................................................................................................................................41iv. Sediment................................................................................................................................................................43v. Vegetation .............................................................................................................................................................45

c. Selected Wetland Models...........................................................................................................................................46D. LITERATURE REVIEW SUMMARY..................................................................................................... 55

III: MODEL DEVELOPMENT............................................................................................................................ 58

A. MODEL OVERVIEW:............................................................................................................................... 581. FWS vs. SSF Modeling ........................................................................................................................... 60

B. MODEL COMPONENTS: ........................................................................................................................ 621. Wetland main program: .......................................................................................................................... 622. Base submodel: ....................................................................................................................................... 633. Hydrology submodel: .............................................................................................................................. 644. Vegetation Submodel: ............................................................................................................................. 695. Nitrogen/Carbon/DO/Bacteria relations: ............................................................................................... 726. Carbon submodel: ................................................................................................................................... 73

vi

7. Nitrogen submodel:................................................................................................................................. 808. Dissolved oxygen submodel: ................................................................................................................... 879. Bacteria submodel: ................................................................................................................................. 91

a. Autotrophic Dynamics...............................................................................................................................................91b. Heterotrophic bacteria ..............................................................................................................................................93

10. Sediment submodel: ................................................................................................................................ 9611. Phosphorous submodel:.......................................................................................................................... 9812. Deltaht submodel: ................................................................................................................................. 10113. SET-WET Flow Chart .......................................................................................................................... 102

C. MODEL DEVELOPMENT SUMMARY ............................................................................................... 102

IV. MODEL EVALUATION............................................................................................................................... 106

A. MODEL CALIBRATION AND VALIDATION .................................................................................................. 1061. Study Area ............................................................................................................................................. 1062. Model Calibration: ................................................................................................................................ 1073. Model Validation: .................................................................................................................................. 125

B. STATISTICAL ANALYSIS: ............................................................................................................................ 132C. SENSITIVITY ANALYSIS:............................................................................................................................. 136D. MODELING APPLICATION .......................................................................................................................... 141

1. Study/Application Area ......................................................................................................................... 1412. Simulation Runs.................................................................................................................................... 1433. Simulation Results ................................................................................................................................ 146

E. MODEL EVALUATION SUMMARY ............................................................................................................... 152

V. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS ............................................................... 154

VI. CITED WORK:............................................................................................................................................. 158

VII. APPENDICES........................................................................................................................................... 166

A. APPENDIX A: MODEL PARAMETERS......................................................................................................... 167B. APPENDIX B: DATA ENTRY TO MODEL..................................................................................................... 180C. APPENDIX C: MODEL FORTRAN CODE FOR THE SET-WET MODEL ...................................................... 190D. APPENDIX D: SYMBOL DESCRIPTION FOR FIGURES 8 THROUGH 22 ........................................................ 239E. APPENDIX E: REGRESSION GRAPHS ......................................................................................................... 240F. APPENDIX F: SENSITIVITY ANALYSIS TABLES.......................................................................................... 244

VIII. VITA.............................................................................................................................................................248

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List of Tables

TABLE 1: NUTRIENT REMOVAL RATES FOR NATURAL WETLAND SITES RECEIVING WASTEWATER INPUTS.9TABLE 2: GENERAL HYDROPERIOD TOLERANCE RANGES FOR SELECTED WETLAND PLANT

COMMUNITIES..14TABLE 3: WETLAND DESIGN PARAMETERS ...........................................................15TABLE 4: A PARTIAL LIST OF PREVIOUS WETLAND MODELS..................................................30TABLE 5: MEASURED INFLOW VALUES TO WETLAND CELL 2 IN BENTON, KENTUCKY USED FOR

VALIDATION AND CALIBRATION OF SET-WET MODEL.........107

TABLE 6:INPUT PARAMETER VALUES AND SOURCES FOR CALIBRATION AND VALIDATION PERIODS.109TABLE 7: MEASURED, PREDICTED, AND DIFFERENCE BETWEEN THE MEASURED AND PREDICTED

VALUES FOR THE HYDROLOGY, AND VARIOUS WETLAND EFFLUENT CONCENTRATIONS FORTHE CALIBRATED, PREDICTED VALUES..123

TABLE 8: MEASURED, PREDICTED, AND DIFFERENCE BETWEEN THE MEASURED AND PREDICTEDVALUES FOR THE HYDROLOGY, AND VARIOUS WETLAND EFFLUENT CONCENTRATIONS FORTHE VALIDATED, PREDICTED VALUES130

TABLE 9: P-VALUES AND RESULTS OF THE WILCOXON SIGNED RANK TEST PROCEDURE FORDIFFERENCES BETWEEN THE MEASURED AND VALIDATED, PREDICTED VALUES OF WETLANDEFFLUENT IN BENTON, KENTUCKY ....133

TABLE 10: LINEAR REGRESSION DATA FOR OBSERVED (Y-AXIS) AND PREDICTED (X-AXIS) WETLANDEFFLUENT...134

TABLE 11: SENSITIVITY ANALYSIS RESULTS OF SET-WET MODEL AS APPLIED TO THE BENTONWETLAND FOR (+/-) 50% CHANGE IN BASE VALUES ...137

TABLE 12: INITIAL INPUT PARAMETERS TO SET-WET MODEL FOR FIVE HYPOTHETICAL SIMULATIONRUNS FOR POTENTIAL FWS CONSTRUCTED WETLAND IN QN2 SUBWATERSHED OF NOMINI CREEKWATERSHED146

TABLE 13: INFLUENT, EFFLUENT, AND % REDUCTION OF NUTRIENTS FOR VARIOUS NUTRIENTS FOR2 YEAR PERIODS OF WETLAND SIMULATIONS FOR QN2 SUBWATERSHED DATA.....150

TABLE 14: RANGE OF POLLUTANT REMOVAL EFFICIENCIES REPORTED FOR CONSTRUCTED WETLANDSYSTEMS152

TABLE F.1: SENSITIVITY ANALYSIS RESULTS OF SET-WET MODEL AS APPLIED TO THE BENTONWETLAND FOR (+/-) 10% CHANGE IN BASE VALUES ...244

TABLE F.2: SENSITIVITY ANALYSIS RESULTS OF SET-WET MODEL AS APPLIED TO THE BENTONWETLAND FOR (+/-) 25% CHANGE IN BASE VALUES.....246

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List of Figures

FIGURE 1: BREAKDOWN OF NPS POLLUTION EMANATION FOR RIVERS IN VIRGINIA....................................4FIGURE 2: CROSS SECTION OF A FWS WETLAND.........................................................................................10FIGURE 3: CROSS SECTION OF A TYPICAL SUBSURFACE FLOW WETLAND. .................................................11FIGURE 4: NITROGEN TRANSFORMATIONS IN WETLANDS. .........................................................................17FIGURE 5: PHOSPHORUS TRANSFORMATIONS IN WETLANDS......................................................................24FIGURE 6: WETLAND DESCRIPTION FOR SET-WET MODEL WETLANDS.......................................................59FIGURE 7: RELATIONSHIP OF SET-WET MAIN CODE TO SET-WET SUBMODELS ...........................................63FIGURE 8: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE HYDROLOGIC CYCLE

SUBMODEL FOR FWS WETLANDS IN THE SET-WET MODEL..................................................................66FIGURE 9: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE HYDROLOGIC CYCLE

SUBMODEL FOR SSF WETLANDS IN THE SET-WET MODEL ...................................................................67FIGURE 10: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE VEGETATION CYCLE

SUBMODEL OF THE SET-WET MODEL...................................................................................................71FIGURE 11: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE CARBON CYCLE

SUBMODEL FOR FWS WETLANDS OF THE SET-WET MODEL .................................................................74FIGURE 12: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE CARBON CYCLE

SUBMODEL FOR SSF WETLANDS OF THE SET-WET MODEL...................................................................75FIGURE 13: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE NITROGEN CYCLE

SUBMODEL FOR FWS WETLANDS OF THE SET-WET MODEL .................................................................81FIGURE 14: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE NITROGENCYCLE

SUBMODEL FOR SSF WETLANDS OF THE SET-WET MODEL...................................................................83FIGURE 15: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE OXYGEN CYCLE

SUBMODEL FOR FWS WETLANDS OF THE SET-WET MODEL..88FIGURE 16: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE OXYGEN CYCLE

SUBMODEL FOR SSF.............................................................................................................................89FIGURE 17: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE AUTOTROPHIC

BACTERIA CYCLE IN FWS WETLAND SURFACE WATER.........................................................................92FIGURE 18: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE AUTOTROPHIC

BACTERIA CYCLE IN FWS AND SSF WETLAND SUBSTRATE ..92FIGURE 19: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE HETEROTROPHIC

BACTERIA CYCLE IN FWS WETLAND SURFACE WATER.........................................................................94FIGURE 20: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE HETEROTROPHIC

BACTERIA CYCLE IN FWS AND SSF WETLAND SUBSTRATE...94FIGURE 21: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE SEDIMENT CYCLE

SUBMODEL FOR FWS WETLANDS OF THE SET-WET MODEL .................................................................97FIGURE 22: RELATIONSHIPS BETWEEN MODELED PROCESSES THAT AFFECT THE PHOSPHOROUS

CYCLE SUBMODEL FOR FWS WETLANDS OF THE SET-WET MODEL100FIGURE 23: FLOW CHART FOR CALLING ORDER OF SET-WET MODEL FROM MAIN CODE THROUGH

SUBROUTINES....................................................................................................................................103FIGURE 24A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR

HYDROLOGIC OUTFLOW FROM THE WETLAND..................................................................................114FIGURE 24B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR

HYDROLOGIC OUTFLOW FROM THE WETLAND..................................................................................114FIGURE 25A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR AMMONIUM

EFFLUENT CONCENTRATIONS FROM THE

WETLAND...115FIGURE 25B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR AMMONIUM

EFFLUENT CONCENTRATIONS FROM THE

WETLAND...115

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FIGURE 26A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR NITRATEEFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................................................116

FIGURE 26B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR NITRATEEFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................................................116

FIGURE 27A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR ORGANICNITROGEN EFFLUENT CONCENTRATIONS FROM THE WETLAND. .......................................................117

FIGURE 27B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR ORGANICNITROGEN EFFLUENT CONCENTRATIONS FROM THE WETLAND. .......................................................117

FIGURE 28A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR DISSOLVEDOXYGEN EFFLUENT CONCENTRATIONS FROM THE WETLAND. ..........................................................118

FIGURE 28B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR DISSOLVEDOXYGEN EFFLUENT CONCENTRATIONS FROM THE WETLAND. ..........................................................118

FIGURE 29A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR BOD5EFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................................................119

FIGURE 29B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR BOD5EFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................................................119

FIGURE 30A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR TOTALSUSPENDED SOLIDS EFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................120

FIGURE 30B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR TOTALSUSPENDED SOLIDS EFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................120

FIGURE 31A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR DISSOLVEDPHOSPHOROUS EFFLUENT CONCENTRATIONS FROM THE WETLAND. ................................................121

FIGURE 31B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR DISSOLVEDPHOSPHOROUS EFFLUENT CONCENTRATIONS FROM THE WETLAND. ................................................121

FIGURE 32A: OBSERVED AND CALIBRATED PREDICTED VALUES (4/27/88 TO 7/27/89) FOR TOTALPHOSPHOROUS EFFLUENT CONCENTRATIONS FROM THE WETLAND. ................................................122

FIGURE 32B: OBSERVED AND CALIBRATED PREDICTED VALUES (1/24/88 TO 4/26/89) FOR TOTALPHOSPHOROUS EFFLUENT CONCENTRATIONS FROM THE WETLAND. ................................................122

FIGURE 33: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR HYDROLOGICOUTFLOW FROM THE WETLAND ........................................................................................................125

FIGURE 34: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR AMMONIUMEFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................................................126

FIGURE 35: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR NITRATEEFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................................................126

FIGURE 36: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR ORGANICNITROGEN EFFLUENT CONCENTRATIONS FROM THE WETLAND. .......................................................127

FIGURE 37: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR DISSOLVEDOXYGEN EFFLUENT CONCENTRATIONS FROM THE WETLAND. ..........................................................127

FIGURE 38: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR BOD5EFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................................................128

FIGURE 39: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR TOTALSUSPENDED SOLIDS EFFLUENT CONCENTRATIONS FROM THE WETLAND..........................................128

FIGURE 40: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR DISSOLVEDPHOSPHOROUS EFFLUENT CONCENTRATIONS FROM THE WETLAND. ................................................129

FIGURE 41: OBSERVED AND VALIDATED PREDICTED VALUES (7/27/88 TO 1/24/89) FOR TOTALPHOSPHOROUS EFFLUENT CONCENTRATIONS FROM THE WETLAND. ................................................129

FIGURE 42: SIMULATED AND OBSERVED VALUES FOR DISSOLVED PHOSPHOROUS CONCENTRATIONS,PLOTTED WITH THE DETERMINED LINEAR REGRESSION, AND IDEAL 1:1 LINE. ................................135

FIGURE 43: LOCATION OF THE NOMINI CREEK WATERSHED IN VIRGINIA WITH RESPECT TORICHMOND, VA AND THE CHESAPEAKE BAY. ..................................................................................142

FIGURE 44: NOMINI CREEK WATERSHED (QN1) WITH SUBWATERSHED (QN2; SHADED) .......................142

xFIGURE 45: LINEAR REGRESSION OF RECORDED TOTAL BOD5 AND HYDROLOGIC INFLOW TO QN2SUBWATERSHED OF NOMINI CREEK WATERSHED FOR MARCH 26, 1992 TO MARCH 25, 1994.........144

FIGURE E.1.: SIMULATED AND OBSERVED VALUES FOR OUTFLOW, PLOTTED BESIDE THE DETERMINEDLINEAR REGRESSION WITH PREDICTION INTERVAL, AND IDEAL 1:1 LINE. ........................................240

FIGURE E.2.: SIMULATED AND OBSERVED VALUES FOR AMMONIUM CONCENTRATIONS, PLOTTEDBESIDE THE DETERMINED LINEAR REGRESSION WITH PREDICTION INTERVAL,AND IDEAL 1:1 LINE. .........................................................................................................................240

FIGURE E.3.: SIMULATED AND OBSERVED VALUES FOR NITRATE CONCENTRATION, PLOTTED BESIDETHE DETERMINED LINEAR REGRESSION WITH PREDICTION INTERVAL, AND IDEAL 1:1 LINE. ...........241

FIGURE E.4.: SIMULATED AND OBSERVED VALUES FOR ORGANIC NITROGEN CONCENTRATIONS,PLOTTED BESIDE THE DETERMINED LINEAR REGRESSION WITH PREDICTION INTERVAL, ANDIDEAL 1:1 LINE. .................................................................................................................................241

FIGURE E.5.: SIMULATED AND OBSERVED VALUES FOR DISSOLVED OXYGEN CONCENTRATIONS,PLOTTED BESIDE THE DETERMINED LINEAR REGRESSION WITH PREDICTION INTERVAL, ANDIDEAL 1:1 LINE. .................................................................................................................................242

FIGURE E.6.: SIMULATED AND OBSERVED VALUES FOR BOD5 CONCENTRATIONS, PLOTTED BESIDETHE DETERMINED LINEAR REGRESSION WITH PREDICTION INTERVAL, AND IDEAL 1:1 LINE. ...........242

FIGURE E.7.: SIMULATED AND OBSERVED VALUES FOR TOTAL SUSPENDED SOLID CONCENTRATIONS,PLOTTED BESIDE THE DETERMINED LINEAR REGRESSION WITH PREDICTION INTERVAL, ANDIDEAL 1:1 LINE. .................................................................................................................................243

FIGURE E.8.: SIMULATED AND OBSERVED VALUES FOR TOTAL PHOSPHOROUS CONCENTRATIONS,PLOTTED BESIDE THE DETERMINED LINEAR REGRESSION WITH PREDICTION INTERVAL, ANDIDEAL 1:1 LINE. .................................................................................................................................243

1SET-WET: a wetland simulation model tooptimize NPS pollution control.

I. Introduction

Nonpoint Source (NPS) pollution accounts for more than 50% of the nations total water

quality problems (Novotny and Olem, 1981) and over 65% of the total pollutant load to inland

surface waters (USEPA, 1993). Therefore, developing practices for controlling NPS pollution is

of major importance to the health of humans and wildlife. Various types of best management

practices (BMPs) have been developed to address this acute problem, one of which is the use of

wetlands.

Wetlands filter out pollutants and act as sinks for nutrients through physical, chemical

and biochemical processes (Novotny and Olem, 1994). Unfortunately humans have not always

perceived wetlands to be beneficial, and wetlands have been converted to other uses such as

agriculture, mining and development at an alarming rate in the United States. The U.S. Fish and

Wildlife Service estimates that over 50 percent of U.S. wetlands have been destroyed during the

last two centuries (Environmental Law Institute, 1993). Iowa alone has lost 99% of its original

natural marshes, while California has had 91% of its wetlands converted to other uses (Tiner,

1984). Nonetheless, wetlands still comprise over 6% of the entire land based area on the planet

Earth (Novotny and Olem, 1994).

In an effort to restore converted wetlands, many Federal management agencies have

active programs to restore wetlands under their jurisdiction and are encouraging private

landowners and other agencies to do the same (Whitacker and Terrell, 1993). Legislation in

Florida requires any natural wetland removal to be replaced with constructed or restored wetland

sites that are at minimum, two times the amount of lost wetland area.

The use of wetlands to control NPS pollution is a relatively new concept (Raisin and

Mitchell, 1995; Teague et al., 1997). Wetland restoration has taken place in northeastern Illinois

(Hey et al. 1989), and constructed wetlands have been established in Massachusetts (Daukas et

al., 1989) with encouraging results, as significant nutrients and sediments have been retained by

these systems. Research has supported the use of wetlands to treat NPS pollution, but the

2question is whether these wetlands are being properly designed to optimize a wetlands ability to

decrease NPS pollution.

The design of wetlands for NPS pollution removal can be optimized with the use of

models that accurately represent wetland systems processes. The ability to optimize wetland

design is beneficial for several reasons. Due to the no net loss policy developed at the

National Wetland Policy Forum in 1987, there should be no removal of wetlands without the

construction of replacement wetlands. There are no laws controlling the quality of these

replacement wetlands however, and many are poorly planned and constructed. To use

replacement wetlands effectively, there is a need to predict how effective these replacements will

be. It is pointless to replace an efficient waste removing wetland with a pond that

accomplishes little. Use of models allows comparisons among various designs, and

consequently improves the effectiveness of replacement wetland with respect to NPS pollution

control efforts.

A. Goal and Objectives

The overall goal of the study is to develop a simulation model that can be used as a

planning tool for the design of constructed wetlands for effective control and treatment of NPS

pollution. The specific objectives are to:

1) Develop a user-friendly, dynamic, long-term, lumped parameter model for the design of

constructed wetlands to optimize NPS pollution control measures.

2) Evaluate the proposed model by comparing its predictions with field data collected from

representative constructed wetland site(s).

3II. Literature Review

In this section a basic overview of the problems associated with NPS pollution is

presented. It describes various best management practices (BMPs) that are utilized to minimize

NPS pollution, but focuses mainly upon the use of wetlands as a pollution controller. A

description detailing the biological and chemical processes in a wetland is also presented,

followed by a general overview of modeling. The concluding section presents specific

descriptions of models previously developed for constructed wetlands.

A. NPS Pollution

The definition of nonpoint source pollution is tied to the definition of point source

pollution. Todays statutory definition of point sources of pollution is as follows (Water Quality

Act, Sec.502-14, U.S. Congress, 1987):

The term point source means any discernible, confined, and discrete conveyance, including but

not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling

rock, concentrated animal feeding operation, or vessel to other floating craft from which pollutants

are or may be discharged. This term does not include agricultural stormwater and return flow

from irrigated agriculture.

Nonpoint sources are defined as everything else and can be characterized as follows (Novotny

and Olem, 1994):

Nonpoint discharges enter the receiving water at intermittent intervals in a diffuse manner

and are highly correlated with the occurrence of meteorological events.

Pollution arises from an extended area of land and is in transit overland before it reaches

receiving waters or infiltrates into shallow aquifers.

Nonpoint sources lack a specific point of origin.

Unlike point sources where treatment is the most effective method of pollution control,

prevention of NPS pollution focuses on land and runoff management practices.

Waste emissions and discharges cannot be measured in terms of effluent limitations

4 The extent of NPS pollution is related to certain uncontrollable climatic events (rain, floods,

hurricanes, etc.) as well as geographic and geologic conditions.

There are five major forms of NPS pollution: sediments, nutrients, toxic substances,

pathogens, and oxygen demanding substances. Sediments are soil particles carried by runoff into

streams, bays, lakes, and rivers. Nutrients such as nitrogen (N) and phosphorous (P) are

necessary for plant and animal growth, but their usefulness has a plateau after which all excess is

potentially detrimental to the environment. Toxic substances such as pesticides, formaldehydes,

household chemicals, and motor oil, among others could cause human and wildlife health

problems. Pathogens are disease causing microorganisms that are present in animal and human

waste. Oxygen demanding substances decrease dissolved oxygen (DO) concentrations in aquatic

environments through degradation of organic materials.

There are approximately 45 nonpoint sources of pollution identified in the

Commonwealth of Virginia (DCR, 1996). Rivers receive a vast majority of its NPS pollution

impact from farms (64%), urban areas (6%), forest land (6%), and construction areas (6%), as

presented in Figure 1. All other sources of NPS pollution account for only 18% of the total NPS

pollution impact. Therefore, to maximize the use of limited resources (money and people), NPS

pollution control efforts should be directed towards highly contributive areas such as farms,

forest land, urban areas, and construction areas.

Farms 64%

Other Sources 18%

Urban 6%

Forest Land 6%

Construction 6%

FIGURE 1: BREAKDOWN OF NPS POLLUTION EMANATION FOR RIVERS IN VIRGINIAAdapted from DCR (1998)

5B. Best Management Practices (BMPs)

Methods, measures, or practices for preventing or reducing nonpoint source pollution to a

level compatible with water quality goals are termed BMPs (Novotny and Olem, 1994). By

definition, BMPs must be economically and technically feasible and can be categorized as

structural, vegetative, or management. Selection of BMPs is based on either controlling a known

or suspected type of pollution from reaching a particular source, or to prevent pollution from a

category of land-use activity (such as agricultural row crop farming) (Novotny and Olem, 1994).

Various BMPs exist, but selection of a BMP is dependent upon the particular pollutants

and the forms in which they are being transported. The following process can be used when

selecting which particular BMP to implement (USDA, Soil Conservation Service, 1988):

1) Identify the water quality problem (e.g., eutrophication in a lake).

2) Identify the pollutants contributing to the problem and their probable sources.

3) Determine how each pollutant is delivered to the water source (e.g., runoff from a feedlot).

4) Set a reasonable water quality goal for the resources and determine the level of treatment

needed to meet that goal.

5) Evaluate feasible BMPs for water quality effectiveness, effect on groundwater, economic

feasibility, and suitability of the practice to the site.

Various structural BMPs such as terraces and sediment basins have been developed.

Structural BMPs help control NPS pollution with changes to the landscape that either capture

and contain, or slow pollutant movement. A terrace is an earthen embankment, channel or a

combination of ridges and channels constructed across a slope to intercept runoff (Novotny and

Olem, 1994). Terraces decrease the effective slope of the land, which decreases runoff velocity.

A decreased runoff velocity allows soil particles and adsorbed pollutants to settle out, thus

preventing transport from the field to the receiving water source. Terraces can remove up to 95%

of sediment, up to 90% of sediments associated adsorbed nutrients, and between 30% to 70% of

dissolved nutrients (Novotny and Olem, 1994). Sediment basins, sediment control basins, and

detention-retention ponds are earthen embankments that are generally designed as large pools

that control water outflow. These structures retard water flow, allowing heavier particulates to

6settle out. Sediment basins can remove 40%-87% of the incoming sediment, up to 30% of the

adsorbed N and 40% of the total P (Novotny and Olem, 1994). Detention-retention ponds are

generally more effective than sediment basins due to the uptake of nutrients by associated

vegetation.

Vegetative BMPs include cropping practices, and vegetative filter strips. Cropping

practices such as conservation tillage and cover crops stress maintenance of vegetative cover

during critical times (heavy rains and strong winds) of NPS pollution generation (Novotny and

Olem, 1994). Conservation tillage is any tillage practice that leaves at least 30% of the soil

surface covered with crop residue after planting. Cover crops are close growing legumes,

grasses, or small grain crops that cover the soil during critical erosion periods for the area. Both

practices reduce NPS pollution by reducing erosion through decreased soil detachment, which

also decreases adsorbed pesticide and nutrient movement. Cover crops also store nutrients that

would otherwise be lost during fallow periods. Conservation tillage has been found to be highly

effective in sediment reduction (30-90%), but has very little effect on controlling soluble

nutrients and pesticides (Novotny and Olem, 1994). Cover crops have been found to be 40-60%

effective in reducing sediment, and 30-50% in removing total P (Novotny and Olem, 1994).

Vegetative filter strips utilize strips of closely growing vegetation, such as bunch grasses, sod, or

small grain crops with the primary objective of water quality protection. They are generally

placed between the source of pollution and the receiving water body. Vegetative filter strips are

designed to slow water velocity from sheet runoff and allow sediment and adsorbed pollutants to

deposit. They are effective in removing sediment and sediment-bound N (about 35-90%) but

much less effective in removing P, fine sediment, and soluble nutrients (Novotny and Olem,

1994).

Management BMPs focus on the use of potential pollutants and include integrated pest

management (IPM) and nutrient management. The combination of practices to control crop

pests (insects, diseases, weeds) while minimizing pollution is termed IPM. It works primarily by

decreasing the amount of pesticide or crop-protection chemical available for runoff by choosing

resistant crop varieties, modified planting dates, and selection of the least toxic, least mobile and

least persistent chemicals (Novotny and Olem, 1994). By decreasing the available chemical

amounts, pollution potential is reduced. The effectiveness of IPM is still being debated, with

some estimates being extremely high and others low. Nutrient management works with the same

7concept of decreasing availability of excess nutrients through improvements in timing,

application rates, and location/selection of fertilizer placement. A more precise application rate

minimizes the potential pollutant availability and has been shown to reduce N and P

concentrations by 20-90% (Novotny and Olem, 1994).

Wetlands are another BMP used for NPS pollution control. This approach is explained in

detail in the following section.

C. Wetlands

Wetlands provide many important ecological functions. Wetlands provide flood storage

and conveyance; stream flow modification; erosion reduction and sediment control; groundwater

recharge/discharge; wildlife habitat; recreation and enjoyment; and pollution control (Novotny

and Olem, 1994). In many aspects, wetlands are excellent BMPs because they provide so many

benefits to the environment and can also be appreciated by wildlife and humans alike. For the

purpose of this study however, the focus will be on wetlands abilities towards pollution control.

Mitsch and Gosselink (1993) described wetlands as the kidneys of the landscape.

Wetlands filter out pollutants and act as sinks for nutrients by purifying the water through

physical (sedimentation, filtration), physical-chemical (adsorption on plants, soil, and organic

substrates), and biochemical processes (biochemical degradation, nitrification, denitrification,

decomposition, and plant uptake) (Novotny and Olem, 1994). The mild slopes of wetlands serve

to slow the velocity of water, which consequently allows sediment and absorbed nutrients to

settle; enhances bacterial die-off due to longer retention times; allows wetland vegetation to

uptake nutrients; and provides a carbon source for microbial action (Novotny and Olem, 1994).

A precise definition which satisfactorily describes all wetland types is not possible due to

the varying types of wetlands (Mitsch and Gosselink, 1993); however, the most comprehensive

definition for wetlands was advanced by the U.S. Fish and Wildlife Service (Cowardin et al.,

1979):

Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually

at or near the surface or the land is covered by shallow water. Wetlands must have one or more of the

following attributes: (1) at least periodically, the land supports predominately hydrophytes; (2) the

substrate is predominately undrained hydric soils; or (3) the substrate is nonsoil (organic matter) with

water or covered by shallow water at some time during the growing season each year.

8As seen by this definition, the hydrology, soil type, and vegetation play significant roles

in determining the functionality and effectiveness of wetlands in retaining pollutants. This

significance will be explored more thoroughly in the section dealing with the design of

constructed wetlands.

1. Classification

There are various ways to classify wetlands but a consistent method has not been

developed to describe them. The easiest way to differentiate wetlands are to divide wetlands

between natural and constructed types, but beyond this simplistic categorization, a clear cut

classification scheme for wetlands does not exist. The confusion in terminology stems from the

vast diversity of wetland types that exist throughout the world and the lack of direct equivalent

translations between various languages (Mitsch and Gosselink, 1993).

The U.S. Fish and Wildlife Service (Shaw and Fredine, 1956) developed the first

classification scheme in 1956. In this classification, twenty types of wetlands were described

under the following four categories; 1) inland fresh areas, 2) inland saline areas, 3) coastal

freshwater areas, and 4) coastal saline areas. Presently, the classification scheme used in the

United States, as part of the National Wetlands Inventory (Cowardin et al., 1979) is very formal

and all encompassing, but very difficult to use. The classification system is based on a

taxonomic separation scheme, in which all wetland and deep-water habitats are divided into five

systems (marine, estuarine, riverine, lacustrine, and palustrine), and further subdivided into

various subsystems and classes. Mitsch and Gosselink (1993) divide wetland types into two

initial systems (coastal and inland) and then further subdivide these systems into seven separate

categories that encompass most, but not all wetland types.

a. Natural Wetlands

Natural wetlands originate in geological settings due to water movement and

accumulation. The major geological settings in which wetlands form are areas of 1) slope

discontinuity, 2) topographic depression, 3) stratigraphic features which inhibit infiltration, and

4) permafrost (Widener, 1995). Wetlands that are formed in lowland areas tend to be underlain

by glacial outwash, clay and silt, or alluvial outwash comprised of sand or a mixture of sand and

9TABLE 1: NUTRIENT REMOVAL RATES FOR NATURAL WETLAND SITES RECEIVING WASTEWATER INPUTS

Loading Nutrient RemovalType of (Population (percent)Wetland Location /Hectare) Substrate Total N Total PNorthern PeatlandBog Wisconsin 30 O 98 78

Nontidal freshwatermarshCattail marsh Wisconsin 17 O 80 88Lacustrine marsh Ontario n/a n/a 38 24Deepwater marsh Florida 99 O n/a 97Lacustrine marsh Hungary n/a n/a 95 n/aRiverine swamp South Carolina n/a O n/a 50

Tidal freshwater marshDeepwater marsh Louisiana n/a O 51 53Complex marsh New Jersey 198 I 40 0

Tidal salt marshBrackish marsh Chesapeake bay n/a O/I 0 1.5Salt marsh Georgia Sludge O/I 50 n/aSalt marsh Massachusetts Sludge O/I 85 n/aSource: Compiled by Mitsch and Gosselink (1986)Note: O= organic substrate; I= inorganic substrate; n/a= information not availablea Load given in g/m2-year

gravel, while wetlands formed in upland areas tend to be underlain by bedrock and glacial till

(Baker, 1973). Mitsch and Gosselink (1986) compiled data on the performance of natural

wetlands for removal of nutrients. As indicated in Table 1, retention of nutrients varies greatly

among different areas. This variability complicates modeling of wetland processes as further

explained in the modeling section.

b. Constructed Wetlands

Constructed wetlands are man-made systems designed to imitate the functions of natural

wetland systems. There are two fundamental types of constructed wetlands, the free water

surface (FWS) system, and the subsurface flow system (SSF) (Novotny and Olem, 1994). The

FWS system usually consists of basins or channels with a natural or subsurface barrier of clay or

impervious geotechnical lining to prevent seepage (U.S. EPA, 1988). The basins are then

filled with soils to support the accompanying planted vegetation (Figure 2). The water level in a

10

FWS wetland is above the soil substrate with water flow occurring primarily above ground. A

SSF system consists of a trench or bed underlain with an impermeable layer of clay. The trench

is back filled with media that usually consists of crushed stone, rock fill, gravel, and different

soils. Water flows through the medium and is purified through filtration; absorption by

microorganisms; and adsorption onto soils, organic matters, and plant roots (U.S. EPA, 1988)

(Figure 3). Hence, the performance of the wetland depends on the detention time of incoming

pollutants, the loading rates, the biotic condition within the system, and oxygen availability.

2. Constructed Wetland Design

Hydrology is the most important wetland design variable. With proper hydrologic

conditions, the potential chemical and biological elements necessary for a properly functioning

wetland exist. Hydrologic conditions can directly modify or change physical and chemical

properties, such as soil salinity, pH, sediment properties, substrate anoxia, and nutrient

availability (Mitsch and Gosselink, 1993). Hydrology is less forgiving than other biological

components, and if improperly accounted for, can cause a constructed wetland to fail.

FIGURE 2: CROSS SECTION OF A FWS WETLAND.Adapted from Novotny and Olem (1994)

11

FIGURE 3: CROSS SECTION OF A TYPICAL SUBSURFACE FLOW WETLAND.Adapted from EPA (1988)

Ultimately, the hydrologic conditions determine success of a wetland system, for it determines

the depth, residence time, and hydroperiod. The hydraulic residence time is the average length

of time a volume of water is detained in a wetland before exiting the system (Novotny and Olem,

1994), and can be estimated as:

Q

VpHRT

*= (1)

where HRT is the hydraulic residence time for a FWS system (T); p is the porosity ((ratio of

water volume)/(total volume); 0.9-1.0 for FWS); V is the active volume of the wetland (L3); and

Q is the average flow rate (L3/T).

The hydroperiod is the seasonal pattern of water level in a wetland or the water depth

above or below wetland surface level over time (Mitsch and Gosselink, 1993). The hydroperiod

is the dominant factor controlling the plant community composition of wetlands (Duever, 1988).

When hydrologic conditions in a wetland change even slightly, the biota may respond with

massive changes in species richness, composition, and ecosystem productivity.

12

The hydrologic conditions for a wetland are affected by various inputs, outputs and

storage patterns. The general balance between water storage and the outflows and inflows can

best be expressed with the following equation (Kadlec, 1996):

AETPQQQQQQdt

dVgwbosmci )( +++= (2)

where A is the wetland surface area (L2); ET is the evapotranspiration rate (L/T); P is the

precipitation rate (L/T); Qb is the bank loss rate (L3/T); Qc is the catchment runoff rate (L

3/T);

Qgw is the percolation to groundwater (L3/T); Qi is the input stream flow rate (L

3/T); Qo is the

output stream flow rate (L3/T); Qsm is the snowmelt rate (L3/T); t is the time step (T); and V is

the volume of water storage in wetland (L3).

The underlying soil strata play a very important role in wetland development. It

functions both as the medium in which many of the wetland chemical transformations take place

and as the primary storage of available chemicals for wetland vegetation (Mitsch and Gosselink,

1993). The soil is often described as hydric, defined by the U.S. Soil Conservation Service

(1987) as a soil that is saturated, flooded, or ponded long enough during the growing season to

develop anaerobic conditions in the upper part. Wetland soils usually have very high organic

matter content. Highly permeable soils are not suitable for wetlands that are not fed by

groundwater because a high permeability does not allow sufficient water storage for hydric soil

conditions to establish. Permeability must be kept below a certain threshold value, which may

vary according to site-specific and geographic conditions (Novotny and Olem, 1994).

Wetlands plants may be characterized as submersed (i.e., completely submerged),

emergent (i.e., those plants with a root system and stem below the water, but which reaches to

or above the surface), or terrestrial (land based) (Dennison and Berry, 1993). Due to the

anoxia, wide salinity range, and water fluctuations characteristic of an environment that is

neither aquatic nor terrestrial, wetland conditions can be physiologically harsh. The constant

fluctuations in living environment can be taxing to organisms as the changing conditions requires

limited energy supplies to be directed toward growth, and more towards survival practices.

Aquatic organisms can not easily adjust to the periodic drying that occurs in many wetlands and

terrestrial organisms could become stressed by long periods of flooding (Mitsch and Gosselink,

13

1993). To deal with anoxia, wetland plants have developed aerenchyma, or air spaces that run

from the stems to the roots, allowing the diffusion of oxygen from the aerial portions of the

plants to the roots. This adaptation allows plants to generate the required energy needed for

survival (Mitsch and Gosselink, 1993). Other adaptations are used by the species of woody trees

(mangroves, cypress, tupelo, willow and a few others) that have successfully adapted to the

wetland environment. Many woody trees have developed adventitious roots above the anoxic

zone, which allow them to attain the necessary air diffusion requirements for biological

processes. A whole plant strategy adopted by many wetland plants concerns the timing of seed

production and transport. Seed production occurs in the nonflooding season and is accompanied

by either delayed or accelerated flowering (Bloom et al., 1990); the production of buoyant seeds

that float until they lodge on unflooded, higher ground; and seed germination while fruit is still

attached to the trees (Mitsch and Gosselink, 1993). All of these mechanisms increase the

probability of plant survival in a wetland environment. Table 2 lists the general depth and

hydroperiod for selected wetland plant communities.

TABLE 2: GENERAL HYDROPERIOD TOLERANCE RANGES FOR SELECTED WETLAND PLANT COMMUNITIESAverage Water Average

Wetland Type Typical Species Depth (m) Hydroperiod *Floating Deep Hyacinths, pennywort

Floating rooted Water lily, water dock, 0.5 -02 70-100 aquatic water shield

Submerged hydrills, egeria, water 0.5-3.0 80-100 aquatic millfoil, naiad

Emergent Cattails, pickelrelweed, 0.1-1 40-100 marsh bulrush, sedgem maidencane

Floodplain Red maple, black gum, cabbage, 0.2-0.3 10 to 50 palm, pond cypress, oaks, pines, bald cypress, ash

Swamp Forest Bald cypress, ash, black gum, 0.3-1.0 50-80 tupelo, gum, red

lCypress dome Pond cypress, red maple, black 0.1-0.3 50-75

gum, dahoon holly

Wet prairie St.Johns wort iris, sagittaria 0.1-0.2 20-50

* The average % of the year the wetland water surface is above wetland ground level.Source: Adapted from Novotny and Olem (1994)

14

Constructed wetlands, as compared with natural wetlands, provide a better chance for

management and control of NPS pollution for two reasons; 1) government regulations, and 2)

location. In the Unites States, natural wetlands are considered natural receiving surface-water

bodies like oceans and lakes; hence they are protected from excessive pollution discharges, and

any discharge requires a permit (Novotny and Olem, 1994). There are limits on how much

pollution can be released to a wetland and this consequently reduces its use for water treatment.

Unlike natural wetlands, constructed wetlands do not have these restrictions placed upon them

and can therefore receive higher pollutant loadings for treatment. Consequently, constructed

wetlands are used more often for water quality improvement. In addition, constructed wetlands

can be created wherever the proper hydrologic, chemical and biological requirements can be

established. This allows constructed wetland systems to be more flexible for NPS pollution

treatment for they can be created where water treatment is necessary.

Novotny and Olem (1994) have summarized the basic principles of wetland design:

1. Design the system for minimum maintenance, where the system of plants, animals, microbes,

substrate and water flows are self-maintaining.

2. Design a system that utilizes natural energies, such as gravity flow and the potential energy

of streams.

3. Consider the landscape for system design. Do not overengineer wetland design with

unnatural basin shape, structures, uniform depths, and regular morphology. Try to mimic

nature.

4. Design the entire system as an ecotone, including the use of buffer strips around the site.

5. Consider the surrounding lands and future land-use changes.

6. Hydrologic conditions are paramount. A detailed surface and groundwater study is

necessary.

7. Give the system time to develop. Wetlands are not created overnight.

8. Soil surveys should be conducted, as highly permeable soils do not support wetland systems.

Table 3 lists wetland design parameters for constructed wetlands and compares them to

natural systems.

15

TABLE 3: WETLAND DESIGN PARAMETERSConstructed Constructed

FWS SFS Natural

Minimum Size

requirement 2 to 4 1.2 to 17 5 to 10

(ha/1000m3/d)

Hydraulic Loading 2.5 to 5 5.8 to 8.3 1 to 2

(cm/day)

Maximum water 50 water level below 50; depend on

depth (cm) ground surface native vegetation

Bed depth (cm) n/a 30 to 90 n/a

Minimum hydraulic

residence time (days) 5 to 10 5 to 10 14

Minimum aspect 2 to 1 n/a 1 to 4

ratio

Minimum Primary; secondary Primary Primary; secondary;

pretreatment is optional nitrification; TP

reduction

Configuration Multiple Cells in Multiple beds in multiple discharge

parallel and series parallel series

Distribution swale, perforated Inlet zone (0.5m) swale, perforated

pipe of large gravel pipe

Maximum Loading,

(kg/ha-day)

BOD5 100 to 110 80 to 120 4

Suspended Solids up to 150

TKN 10 to 60 10 to 60 3

Phosphorous ? ? 0.3 to 0.4

Additional Mosquito control Allow flooding Natural hydroperiod

Consideration with mosquitofish; capability for should be >50%; no

remove vegetation weed control vegetation harvest

Source: Novotny and Olem (1994).

3. Nitrogen Cycle in Wetlands

The transformations and interactions of the various forms of N in soils, sediment of

surface waters, and substrates of wetlands is very complex. The basic forms of N in soils and

sediments are ammonium ion (NH4+), nitrate (NO3

-), organic phytonitrogen in plants and plant

residues, and protein N in living and dead bacteria (Novotny and Olem, 1994). As a negatively

16

charge ion, NO3- is not subject to adsorption by negatively charged soil particles like the

positively charged NH4+ ion, and is thus more mobile in solution. In flooded soils and sediments,

the organic forms of N predominate, while NH4+ is the predominant inorganic N form (Reddy

and Patrick, 1984). Some researchers refer to N content in an area as either Total Kjeldahl N

(TKN) or as total N (TN). Total Kjeldahl N is a measure of reduced N equal to the sum of

organic N and NH4+-N (Kadlec and Knight, 1996). Total N is a measure of all organic and

inorganic forms and is essentially equal to the sum of TKN, NO3- and NO2-N (Kadlec and

Knight, 1996).

Sources of N that contribute to wetland sites include: a) precipitation on the surface of

flooded soils and sediments; b) N fixation in the water and the sediments; c) inputs from surface

and ground water infiltration/percolation; d) application of fertilizers; e) N release during

decomposition of dead aquatic vegetation and animal community inputs; and f) discharge of

waste water effluents (Reddy and Patrick, 1984).

A number of processes can transport or translocate N compounds from one point in a

wetland to another without molecular transformation. These transfer processes are physical in

nature and include: 1) particulate settling and resuspension, 2) diffusion of dissolved forms,

3) litterfall, 4) plant uptake and translocation, 5) NH3 volatilization, 6) sorption of soluble N on

substrates, 7) seed release, and 8) organism migrations (Kadlec and Knight, 1996).

Important processes that transform the basic forms of N in soils and sediments are

presented in Figure 4. These processes are mineralization (ammonification), nitrification,

denitrification, nitrogen (N2) fixation, and assimilation (plant and bacterial uptake).

Understanding the N transfer and transformation processes is very important to the design

of a wetland system. If these processes are not understood, the design of constructed wetland

systems will be negatively affected. The following sections describe the transformations and

transport processes of the N cycle in further detail.

17

FIGURE 4: NITROGEN TRANSFORMATIONS IN WETLANDS.SON =soluble organic nitrogen. Adapted from Mitsch and Gosselink (1993).

a. Nitrogen Transformation Processes

i. Mineralization (ammonification)

Mineralization is the biological transformation of organic N to NH4+ that occurs during

organic matter degradation (Gambrell and Patrick, 1978). Mineralization occurs through

microbial breakdown of organic tissues containing amino acids, hydrolysis of urea and uric acid,

and through excretion of ammonia directly by plants and animals (Kadlec and Knight, 1996).

Mineralization occurs under both anaerobic and aerobic conditions but proceeds at a slower rate

in anaerobic conditions due to the decreased efficiency of heterotrophic bacteria in these

environments (Reddy and Patrick, 1984).

The mineralization rate is affected by temperature, pH, carbon to nitrogen (C:N) ratio of

the substrate, available nutrients in the soil, and soil properties such as texture and structure

(Reddy and Patrick, 1988). The effect of these factors on mineralization in well-drained soils is

fairly well understood, but less is known about their effects in flooded soils. Reddy et al. (1979)

18

concluded that the rate of mineralization doubles with a temperature increase of 10 C, while the

optimum temperature of mineralization was found to be between 40 to 60 C (Reddy and

Patrick, 1984), a rare field condition. The optimal pH range for the mineralization process is

between 6.5 and 8.5 (Reddy and Patrick, 1984), a condition found under most flooded conditions

because the oxidation of organic material produces CO2, which buffers the system.

Measured mineralization rates in natural wetlands range from 0.3 to 35 mg N/m2/d

(annual average of 1.5 g/m2/yr)) in a swamp forest in central Minnesota (Zak and Grigal, 1991),

and 4.3 to 5.9 g/m2/yr in a Minnesota bog (Urban and Eisenrich, 1988). Higher rates were

reported in organic soils in Florida by Reddy (1982), with rates of 41 to 125 g/m2/yr.

ii. Nitrification

After NH4+ ions are formed through the mineralization process, it can take several

pathways. It can be absorbed by plant root systems or taken up by anaerobic microorganisms

and converted to organic matter; immobilized through ion exchange by soil particles; or it can

undergo nitrification (Mitsch and Gosselink, 1993).

Nitrification is the biological oxidation of ammonium-N to nitrate-N with nitrite-N

(NO2-) as an intermediate product. Nitrification is accomplished with the help of two groups of

chemoautotrophic bacteria that allow the oxidation process to occur. The first step (Mitsch and

Gosselink, 1993):

energyHOHNOONH ++++ + 42232 2224 (3)

is accomplished with the Nitrosomonas sp. The second step:

energyNOONO ++ 322 22 (4)

is conducted by the Nitrobacter sp.

Anaerobic conditions in wetland soils limit the amount of nitrification that can occur, as

nitrification requires oxygen. In a wetland system, nitrification can occur in; 1) the water

column above wet soils (Reddy and Patrick, 1984), 2) the thin oxidized layer at the surface of

19

wetland soils, and 3) the oxidized rhizosphere of plants (Mitsch and Gosselink, 1993).

Nitrification can still occur at low levels of about 0.3 mg/L of DO (Reddy and Patrick, 1984).

iii. Denitrification

As stated before, NO3- is far more mobile in solution than NH4

+. If NO3- is not

assimilated by plants or microbes or lost to groundwater flow through rapid movement,

denitrification may occur. Denitrification is the biological reduction of NO3--N to gaseous N

forms such as molecular N2, NO, NO2 and N2O (Novotny and Olem, 1994). Under anaerobic

(oxygen free) conditions and in the presence of available organic (carbon) substrate, denitrifying

organisms such as bacillus, micrococcus, alcaligenes, and spirillum, can use NO3- as an electron

acceptor during respiration. These organisms oxidize a carbohydrate substrate by converting

NO3- to carbon dioxide, water, N gas and other gaseous oxides that can result from denitrification

as indicated above (Reddy and Patrick, 1984):

OHNCOHNOOCH 22232 725445 +++++ (5)

This chemical reaction is irreversible in natural conditions.

Several factors are known to influence the rate of denitrification including the absence of

O2; presence of readily available C; temperature; soil moisture; pH; presence of denitrifiers; soil

texture; and presence of overlying floodwater (Reddy and Patrick, 1984). Denitrification rate

has been shown to increase with temperature and researchers (Reddy and Patrick, 1984) have

concluded that a 1.5 to 2.0 fold increase will occur with a 10 C rise in temperature.

iv. Nitrogen Fixation

Nitrogen fixation is the process by which atmospheric N2 gas diffuses into solution and is

reduced to organic N by autotrophic and heterotrophic bacteria, blue-green algae, and higher

plants (Kadlec and Knight, 1996). N fixation is an adaptive process that provides N for

organisms to grow in conditions that are otherwise depleted of N. N fixation is inhibited by high

concentrations of available N; and is generally not observed in N rich ecosystems.

20

In wetlands, N fixation can occur in overlying waters, in the anaerobic or aerobic soils

layers, in the oxidized rhizosphere of the plants and on the leaves and stem surface of plants

(Mitsch and Gosselink, 1993). Observations of N fixation values vary greatly from differing

wetland sites. Dierberg and Brezonik (1984) observed fixation rates ranging from 1.2 to 19.0

kg/ha/yr in a Florida cypress dome receiving municipal wastewater, but fixation was concluded

to be an insignificant contributor to total N loading.

v. Assimilation: Plant and Bacterial Uptake

Nitrogen assimilation refers to a variety of biological processes that convert inorganic N

forms into organic compounds that serve as building blocks for cells and tissues (Kadlec and

Knight, 1996). The two most commonly used forms of N are NH4+-N and NO3

--N. NH4+ is

more reduced energetically than NO3-, thus it is the more preferred source for assimilation by

plants and bacteria.

Depending upon the loading rate to the wetland, plant N assimilation can involve a

significant fraction of the total N load. Adcock et al. (1994) determined that a SSF treatment

wetland in Australia had 65% of the N load contained in macrophyte biomass due to its low N

loading rate (25 to 40 g/m2/yr). At sites with higher loading rates, the amount of N lost to

assimilation is a smaller overall percentage.

In temperate climates, plant assimilation is a spring-summer phenomenon. Depending on

location, plant species can either be sinks or sources of N. During the spring and summer when

growth is taking place, plants uptake N, but during the winter months when vegetation dies,

uptake ceases and decomposition occurs.

Microorganisms assimilate nutrients for growth, as NH4+ is readily incorporated into

amino acids by many autotrophs and microbial heterotrophs (Kadlec and Knight, 1996). The

amino acids are transformed into proteins, purines, and pyramidines that are used as energy. The

magnitude of the uptake process has not been quantified for treatment wetlands (Kadlec and

Knight, 1996).

21

b. Other Nitrogen Fluxes

There are numerous other pathways that N compounds can follow besides the previously

described molecular transformations. These processes may be important when designing

wetland systems and can contribute or subtract from the TN content of a wetland system. These

processes include (1) atmospheric N inputs through rainfall and dryfall, (2) NH3 volatilization,

(3) NH4+ adsorption, (4) burial of organic N, and (5) biomass decomposition (Kadlec and Knight,

1996). Brief descriptions of each process follow.

i. Atmospheric Nitrogen Inputs

Atmospheric deposition of N contributes measurable quantities of N to land areas. All

forms of N are involved including particulate, dissolved, inorganic and organic. Wetfall (rain or

snow) contributes more than dryfall, and rain contributes more than snow (Kadlec and Knight,

1996).

Nitrogen concentrations in rainfall are highly variable and dependent on atmospheric

conditions, air pollution and geographic location. A typical range of TN concentrations

associated with rainfall is 0.5 to 2.0 mg/L, with about 50% of this present as NO3- and NH3-N

(Kadlec and Knight, 1996). Atmospheric sources are usually negligible contributors to the

overall wetland N budget.

ii. Ammonia Volatilization

Un-ionized NH3 is relatively volatile and can be removed through mass transfer of NH3

from the water surface to the atmosphere (Kadlec and Knight, 1996). Volatilization has limited

importance for wetlands. Volatilization practically ceases if pH is at or below 7 (Novotny and

Olem, 1994). Typically, volatilization is an insignificant factor when discussing the N cycle in

wetlands. However, in wetlands with a high concentration of NH3-N (20mg/L) and a pH greater

than 8, volatilization can play a significant role (Kadlec and Knight, 1996).

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iii. Adsorption

Adsorption is the adherence of chemical ions to the surface of a solid. NH4+ can be

removed from solution through a cation exchange adsorption reaction with inorganic sediments

and detritus (Kadlec and Knight, 1996). The adsorbed NH4+ is loosely bound to the substrate

and can be released when water chemistry conditions change. Most forms of N are very soluble

and do not attach to sediment and other particle types; therefore adsorption plays a limited role in

the overall N balance.

iv. Burial of Organic Nitrogen

A fraction of the organic N incorporated in detritus and plants may eventually become

unavailable for additional nutrient cycling due to burial and peat formation. Burial of N can be

important for light N loading conditions, but becomes insignificant for high N loads (Kadlec and

Knight, 1996). For example, Reddy et al. (1991) reported a N burial rate of 14 to 34 g N/m2/yr

for a lightly fertilized zone of wetland, while the N burial rate was 365 g N/m2/yr in a treatment

wetland.

v. Biomass Decomposition

The N that is assimilated by macrophytes, microflora, and microfauna is partially

released during decomposition. Turnover times for leaf litter can vary from several months to

over 2 years in colder climates, but decomposition rates during warmer months do not vary much

with geographical conditions (Kadlec and Knight, 1996). The decomposition process is typified

by a rapid initial weight loss that is followed by an exponential loss of the remaining weight to

an irreducible residual which contributes to sediment and soil building (Kadlec and Knight,

1996).

4. Phosphorous Cycle in Wetlands

Due to the general scarcity of P in the natural environment and the absence of significant

atmospheric inputs, natural ecosystems such as wetlands, have numerous adaptations to

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sequester this element (Kadlec and Knight, 1996). P is rendered relatively unavailable to

microconsumers and plants when (Mitsch and Gosselink, 1993): a) insoluble phosphates

precipitate with ferric iron, calcium, and aluminum under aerobic conditions; b) chemical

sorption of phosphate to clay particles, organic peat, and other minerals occurs; and c) P

incorporates into the living biomass of wetland biota. Phosphorous is not particularly mobile in

soils and phosphate ions do not readily leach, thus P transport is mostly from plant uptake or

through soil transport (Novotny and Olem, 1994). Figure 5 details the basic transport modes and

reactions for P in a wetland.

Phosphorous occurs as insoluble and soluble complexes in both organic and inorganic

forms in wetland soils. The principal inorganic form is orthophosphate, which includes the ions

PO4-3, HPO4

=, and H2PO4- (Mitsch and Gosselink, 1993). The phosphorous cycle is sedimentary

rather than gaseous (i.e., N); therefore, commonly a major portion of a wetlands P content is tied

up in organic peat and litter and in sediment (Mitsch and Gosselink, 1993). Removal efficiencies

range from 0 to 90% (Watson et al., 1989).

a. Importance of Sediment Sorption/Desorption

As stated before, the P cycle is sedimentary-based, therefore, sediment movement plays a

vital role in determining P transport and concentrations. Dissolved P in both inorganic and

organic forms usually interacts with suspended and bed sediments. Many of these interactions

are heterogeneous in nature and it is therefore likely that the kinetics of the processes rather than

the chemical equilibrium determine the P division (Grobbelaar, and House, 1995). The nature of

specific interactions for many systems is still unknown, because (Grobbelaar and House, 1995):

The wide range of affinities of P for sediments, combined with the uncertainties in

sedimentary materials composition makes it difficult to identify the key processes,

Dissolution/precipitation, adsorption/desorption and biological uptake and release are

difficult to separate for measurement purposes, and

The transformations of organic P to inorganic P are not well understood.

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FIGURE 5: PHOSPHORUS TRANSFORMATIONS IN WETLANDS.SOP = soluble organic phosphorous. Adapted from Mitsch and Gosselink, 1993.

In many wetlands, P cycling tends to follow sediment deposition and resuspension. This

is due to the high sorption rates associated with P. However, there is a common misconception

that wetlands provide P removal only through sorption processes on settling sediments.

Although most sediments do have sorptive capacity for P, this storage will become saturated

under constant P loading rates (Kadlec and Knight, 1996).

b. Precipitation

Precipitation of P in wetland systems is very complicated and is highly dependent on pH

in the system. At higher pH values, the P precipitates mostly in combination with calcium.

Below a pH of 7, which is characteristic of soils with high clay and organic matter (such as

wetlands), P reacts predominately with the iron and aluminum ions in soils (Novotny and Olem,

1994). Depending on soil pH, the dissolved P concentrations may decrease to values of 0.01

mg/L or less.

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c. Biomass: Growth, Death, Decomposition, Uptake and Storage

The amount of P sustainably removed by a wetland is usually much less than the P taken

up by plants during a growing season. All wetland biota undergo a constant cycle of growth,

death and partial decomposition. This results in the decay of plant life and the subsequent

release of assimilated P. Therefore, increases in biomass should not be counted towards the

long-term sustainable P removal capacity of wetlands (Kadlec and Knight, 1996). Although

plants may temporarily remove P from the wetland water and soils, in the long term, it provides

very little retention.

Determining P removal is dependent upon the accretion of biomass residuals and

minerals because this is the only sustainable storage mechanism for P removal (Kadlec and

Knight, 1996). Burial of material removes P from the plant growth/death cycle; therefore the

more plant growth/death cycles, the more chances for burial. Turnover rate is defined as the

number of times the above ground biomass is replaced per year. In northern climates the

turnover rate is lower than in southern climates because southern areas have a longer growing

season (Kadlec and Knight, 1996). Since turnover rate is higher in southern climates, there are

increased chances for accretion and a higher probability of nutrient retention.

5. Bacteria in Wetlands

Many nutrient transformations in wetlands are due to microbial metabolism and are

directly related to microbial growth (Tanji, 1982). There are theories that state that

decomposition and ammonification rates are linked to microbial energy requirements, the C:N

ratio of the organic matter and the growth rate of microbes in the substrate (Parnas, 1975; Fyock ,

1977; Patrick, 1982). Nitrogen and C are both necessary as a source of energy, while C is

required for building microbial biomass (Parnas, 1975). Growth rates of microbes are a function

of both the environmental conditions and substrate availability.

Energy is obtained by the transference of electrons from an electron donor to an electron

acceptor. Examples of electron donors would be complex organics and NH4+, while oxygen and

NO3- are acceptable electron acceptors (Gidley, 1995). Most of the treatment in wetlands is due

to heterotrophic and autotrophic bacteria (Mitsch and Jorgensen, 1989). Particulate and soluble

labile organics are used as a C source and electron donor by heterotrophic bacteria (Gidley,

26

1995). Equations 3, 4, and 5 show how the microbial transformations generate energy, whose

yield differs for each process. Aerobic degradation of organic materials yields more energy per

mass of electron donor, than either organics degradation or nitrification.

Microbes also utilize N and C to build cell mass. A common formula for microbes is

C5H7O2N (Parnas, 1975). Nitrogen comprises more than 12% of cells, while C accounts for

more than 50% of cell mass. Since microbes use C and N organics; growth of heterotrophs are

influenced by the C:N ratio of the materials they degrade ( Reddy and Patrick, 1983). Aerobic

heterotrophs require organics with a C:N ratio of about 23.5 (Parnas, 1975). Part of the C is used

as an electron donor, while the rest is incorporated into cell mass. Anaerobic decomposition is

not as efficient, therefore more C is required to generate equal amounts of energy. Anaerobic

heterotrophs optimize organic use at a C:N ratio of about 80. Consequently, ammonification is

greater under anaerobic conditions (Reddy and Patrick, 1983). If the C:N ratio is lower than 23.5

or 80, for aerobic and anaerobic conditions respectively, growth will be C limited and the excess

N is wasted as NH4+. If the C:N ratio is higher than these ratios, growth is N limited and the C:N

ratio of the organic materials increase as N is incorporated into cell mass. If the excess N is

NH4+, microbes utilize NH4

+ and the C:N ratio remain the same (Parnas, 1975).

Microbial growth rate is determined by the availability of electron donors and acceptors,

the amounts of C and N, and environmental conditions (temperature, pH, space, etc.) (Grady and

Lim, 1980; Reddy and Patrick, 1983). While heterotrophs are responsible for ammonification,

nitrification is inhibited when the DO concentrations drop below 2 mg/L (Bowmer, 1987).

Conversely, the rate of denitrification is reduced in the presence of oxygen.

Optimal conditions for bacterial growth are generally reported as being between a pH of

six and nine, and at temperatures ranging from 15 C and 40C (Fyock, 1977; Reddy and Patrick,

1983; Bruno and Tomasso, 1991). Growth of microbes still occurs outside of these ranges but

the rates are reduced (Broderick et al., 1988). The pH of submerged soils is generally neutral

because the oxidation of organic material produces CO2, which buffers the system (Reddy and

Patrick, 1983). When organic loading is high, heterotrophs out-compete autotrophs and

nitrification is reduced (Grady and Lim, 1980).

The N and C cycles in a wetland are not mutually exclusive as other bacteria may require

the byproducts of one microbial process. For example, heterotrophic bacteria obtain energy from

organics and produce NH4+, which is in turn used by autotrophs as an energy source. The NO3

-

27

formed by the aerobic heterotrophs is then used by anaerobic heterotrophs as an electron

acceptor (Gidley, 1995). Heterotrophs rely on plants to provide organic substrates and a suitable

environment for survival, while plants are dependent on microbial decomposition for nutrient

recycling (Reed and Brown, 1992). At the same time though, plants and microbes both compete

for nutrients during the growing season (Good and Patrick, 1986). All of these interactions form

a complex system that is difficult to manage, model and recreate (Gidley, 1995).

6. Vegetative/Carbon Cycle in Wetlands

The C cycle in wetlands is dominated by wetlands plant life. Wetland plants follow a

cycle of growth and nutrient uptake, death, and lastly, decomposition, nutrient release, and soil

accretion (Gidley, 1995). Plant growth and death follow seasonal patterns, while processes such

as decomposition and soil accumulation may take years. Wetlands usually have a seasonal

pattern of nutrient retention in the summer, followed by a nutrient release in the fall and early

spring floods when lower temperatures reduce biological activity (Mitsch and Jorgensen, 1989;

Hantzsche, 1985).

During the summer, vegetation grows and uptakes nutrients. Boyd (1978) found the

mean C and N content were 45% and 1.01%, respectively, for Typha latifolia, and 48% and

1.36% for Juncus effusus, in a natural wetland. Tanner (1996) found the range of N and P

content were 1.5% to 3.2% and 0.13% to 0.34%, respectively, for eight emergent plant species in