Development and Evaluation of a Biphasic Rain Garden for Stormwater Runoff Management

DEVELOPMENT AND EVALUATION OF A BIPHASIC RAIN GARDEN FOR STORMWATER RUNOFF MANAGEMENT

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Hanbae Yang, M.S.

Environmental Science Graduate Program

The Ohio State University

2010

Dissertation Committee:

Dr. Warren A. Dick, Advisor

Dr. Parwinder S. Grewal, Co-advisor

Dr. John Cardina

Dr. Edward L. McCoy

Hanbae Yang

2010 ABSTRACT

Rain gardens are bioretention systems that have the potential to reduce peak runoff

flow and improve water quality in a natural and aesthetically pleasing manner. In spite of

their popularity, results from column and field-scale studies show that level of pollutant

removal in rain gardens varies and is not always positive. To date, research has often

focused on a limited number of runoff pollutants. This study was conducted to develop and evaluate a new rain garden design for flow management and pollutant removal from stormwater runoff. Both column and field-scale biphasic rain gardens were designed and constructed to increase retention time and maximize removal efficiency of multiple runoff pollutants by creating a sequence of anaerobic to aerobic conditions. To evaluate hydraulic performance and pollutant removal capacity of the biphasic rain gardens, studies were conducted under actual and simulated runoff conditions with spiked concentrations of nutrients (nitrate-N and phosphate-P), and herbicides (i.e. atrazine (6-

chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine), glyphosate (N-

(phosphonomethyl)glycine), dicamba (3,6-dichloro-2-methoxybenzoic acid), and 2,4-D

(2,4-dichlorophenoxyacetic acid)). Both column and field-scale studies showed that the

biphasic rain gardens have the potential to be an effective best management practice for

reducing stormwater flow and pollutant loads. Peak flow and runoff volume were

effectively reduced five-fold in the biphasic rain gardens for actual and simulated runoff

ii events by holding runoff in the rain gardens (mainly in the anaerobic zone) until a

subsequent runoff event. The field-scale biphasic rain gardens were highly effective in

removing nitrate-N (~91%), phosphate-P (~99%), atrazine (~90%), dicamba (~92%), glyphosate (~99%), and 2,4-D (~90%) under high levels of pollution loading conditions simulated in both agricultural and urban runoff events. The column studies demonstrated that the biphasic rain garden, compared to the monophasic (conventional) rain garden, was better able to reduce peak flow, runoff volume, and pollutant loads by employing the sequence of anaerobic to aerobic (biphasic) conditions. In addition, higher organic matter content in soil media increased removal efficiency of nitrate-N through denitrification in the anaerobic zone. Overall, this study showed that increased retention time of runoff pollutants, as determined by design configuration and rainfall size, intensity, and interval, significantly increased overall nutrient and herbicide removal in the biphasic rain gardens.

Demonstrating more efficient hydraulic and pollutant removal capacity by the biphasic rain gardens, compared to the conventional monophasic rain gardens, is expected to stimulate the implementation of rain gardens as an effective stormwater best management

practice.

iii

To my parents and my partner in life and work, Sun-Jeong Park

iv ACKNOWLEGEMENTS

I would like to express my sincere gratitude and appreciation to my advisors, Dr.

Warren A. Dick and Dr. Parwinder S. Grewal for their support and invaluable advice with a continuous stream of suggestions, feedback, and encouragement that have guided me throughout the whole period of my Ph.D. research. I especially would like to thank them for always being accessible and making research their top priority.

I would also like to thank my committee members, Dr. Edward L. McCoy and Dr.

John Cardina for their constructive comments, suggestions and salient advice. Also,

thanks to Dr. Larry P. Phelan for providing me the opportunity to work in his lab for the

analytical method development of herbicides in this research.

The Center for Urban Environment and Economic Development of the Ohio State

University and the Ohio State University Alumni Grants for Graduate Research and

Scholarship, and the Ohio State University and the Ohio Agricultural Research and

Development Center (OARDC) are acknowledged for funding of this research.

I would like to thank all my past and present lab colleagues; Dr. Liming Chen, Dr.

Dave Kost, Dr. Jeongjin Kim, Dr. Serdal Bilen, Dr. Sindhu Jagadamma, Dr. Xiaolu Guo,

Sougata Bardhan, Natsuko Merrick, Darlene Florence, Dr. Ganpati Jagdale, Dr. Zhiqiang

Cheng, Dr. Ruisheng An, Dr. Xiaodong Bai, Dr. Alfred Alumai, Dr. Mamta Singh, Dr.

Jay Saimandir, Erin Morris, Priyanka Yadav, Harit Bal, and Patchareewan Maneesakorn

v for their friendship and contributions to my research. In particular, I want to give my

special thanks to Jeanne Durkalski, Kevin Power, and OARDC facility services staff for

their detailed and thoughtful comments, and tremendous aids in construction of the rain

gardens.

I would like to thank the faculty and staff in the Environmental Science Graduate

Program, School of Environment and Natural Resources, and Department of Entomology,

and OARDC for their continuous support and high quality education.

Many thanks to all students in the OARDC for creating a stimulating and supportive

environment, and all my friends at the Ohio State University, who have made another

part of my life full of joy and excitement.

My sincere warmhearted gratitude goes to my parents for their endless love,

understanding, patience and support. Without them, none of my accomplishments would

have been possible. Finally, my greatest thanks to my best friend, soul-mate, and wife, Dr.

Sun-Jeong (Sunny) Park for her love, support, and encouragement through these years.

vi VITA

1978 ………... Born, Jeju, South Korea

1998 – 2000 ……….. Sergeant, Korea Army, Pohang, South Korea

1996 – 1997, 2001 – 2003………... B.S. Environmental Engineering, Ajou University, Suwon, South Korea

2003 – 2005………... M.S. School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA.

2005 – present……… Graduate Research and Teaching Associate, Environmental Science Graduate Program, The Ohio State University, Columbus and Wooster, Ohio, USA.

PUBLICATIONS

Research Publications

Park, S-J., Cheng, Z., Yang, H., Morris, E.E., Sutherland, M., McSpadden-Gardener, B.B., Grewal, P.S., 2010. Differences in soil chemical properties with length of urbanization period and distance from roads in urban areas. Urban Ecosystems. In Press.

Yang, H., Florence, D.C., McCoy, E.L., Dick, W.A., Grewal, P.S., 2009. Design and hydraulic characteristics of a field-scale bi-phasic bioretention rain garden system for stormwater runoff management. Water Science and Technology 59, 1863-1872.

Epolito, W.J., Yang, H., Bottomley, L.A., Pavlostathis, S.G., 2008. Kinetics of zero- valent iron reductive transformation of the anthraquinone dye Reactive Blue 4. Journal of Hazardous Materials 160, 594-600.

Patent

Grewal, P.S., Dick, W.A., McCoy, E.L., Yang, H., 2009. Bi-phasic bioretention system. US and International Patent Application – US 2009/261026 A1 and WO 2009/129533 A2, US Department of Commerce and World Intellectual Property Organization.

vii FIELDS OF STUDY

Major Field: Environmental Science

Minor Field: Soil Science

viii TABLE OF CONTENTS

Page Abstract ……………………………………………………………………………. ii Dedication ………...……………………………………………………………….. iv Acknowledgements …...…………………………………………………………… v Vita ………………………………………………………………….………….….. vii List of Tables …………………………………………………………………….... xi List of Figures ……………………………………….…………………………….. xii

Chapters: 1. Introduction ………………………………………………………………..... 1 References ………………………………………………………………. 5

2. Design and Hydraulic Characteristics of a Field-Scale Biphasic Rain Garden for Stormwater Management ………………………………………….…….. 8 Abstract …………………………………………………………………. 8 Introduction ………….………………………………………….……..... 9 Materials and Methods ……..…………………………………………… 12 Results and Discussion …..……………………………………………… 16 Conclusions ……………....……………………………………………… 22 Acknowledgements ……………………………………………………... 23 References ………………………………………………………………. 24

3. Field Evaluation of a Biphasic Rain Garden for Flow Management and Pollutant Removal ……………………………………………..……...…….. 34 Abstract …………………………………………………………………. 34 Introduction ………….………………………………………….……..... 35 Materials and Methods ……..…………………………………………… 37 Results and Discussion …..……………………………………………… 42 Conclusions ……………....……………………………………………… 48 Acknowledgements ……………………………………………………... 49 References ………………………………………………………………. 50

4. Nutrient and Atrazine Removal from Simulated Urban and Agricultural Runoff in Column-Scale Monophasic and Biphasic Rain Gardens ……...... 62 Abstract …………………………………………………………………. 62 Introduction ………….………………………………………….……..... 63 Materials and Methods …….……………………………………………. 66 Results and Discussion …..……………………………………………… 70 Conclusions ……………....……………………………………………… 78 Acknowledgements ……………………………………………………... 78 References ………………………………………………………………. 79

5. Evaluation of Rain Garden Media for Stormwater Runoff Pollutant Removal ……………………………………………………………….…….. 90 Abstract …………………………………………………………………. 90 Introduction ………….………………………………………….……..... 91 Materials and Methods ……..…………………………………………… 93 Results and Discussion …..……………………………………………… 97 Conclusions and Recommendations .…..………....…………...………… 101 Acknowledgements ……………………………………………………... 103 References ………………………………………………………………. 104

6. Conclusions and Future Directions ……..…………………………………… 114

Bibliography ………………………………………………………………………. 118

x LIST OF TABLES

Table Page

2.1 The description of experimental site and biphasic rain garden …………… 26

2.2 Physical and chemical characteristics of biphasic rain garden soil medium ……………………………………………………………………. 27

2.3 Makeup of tracer and simulated stormwater runoff …………………...….. 28

2.4 Summary of tracer study results ………………………..……………...….. 29

3.1 Composition of simulated stormwater runoff ………………………….…. 54

3.2 Net mass removal (%) of pollutants from five sequential urban runoff events in the biphasic rain gardens …………………..…………………… 55

4.1 Composition of simulated storm water runoff ……………………………. 83

4.2 Mass balance and net mass removal (%) of pollutants from five sequential events in the monophasic and biphasic rain gardens ….……………….…. 84

4.3 Mass balance and net mass removal (%) of pollutants at different C/N ratios in the biphasic rain garden ………………………….………………. 85

5.1 Characteristics of the rain garden soil media ………………………....….. 107

5.2 Composition of simulated stormwater runoff …………………………..… 108

5.3 Mass balance and net mass removal (%) of pollutants among different soil media in the biphasic rain garden ……………………………..…….…….. 109

xi LIST OF FIGURES

Figure Page

2.1 Plan and cross-section views (a) and three-dimensional drainage configuration (b) of experimental biphasic rain garden: (1) drainage area, (2) influent, (3) micro-pool, (4) anaerobic zone, (5) reverse drainage pipe, (6) aerobic zone, (7) overflow inlet, (8) final discharge pipe, (9) aerobic zone effluent, (10) tipping-bucket rainfall gauge, (11) recharge zone access pipe, (12) bypass drainage pipe, (13) impervious liner, (14) plywood box, (15) anaerobic zone effluent, (16) mulch layer, (17) soil bed medium, (18) gravel, (19) recharge zone, and (20) underdrainage pipe. ………………………………………………………………..…….... 30

2.2 Cumulative volume (a) and flow rate (b) of influent and effluent hydrograph from the systems for a 180 mm/24 h simulated rainfall event. ………………………………………….…………………………... 31

2.3 Cumulative mass recovery (%) for bromide tracer and simulated runoff pollutants in the effluent from the aerobic zone (Note: phosphate-P and metals were not detected). ………………………………………………… 32

2.4 Breakthrough curves of bromide tracer (a), nitrate-N (b), and phosphate-P (c) in effluent from the anaerobic zone and the aerobic zone of the biphasic rain gardens. ……………………………………………………… 33

3.1 The monitored experimental biphasic rain garden system (a), adjacent to a concrete surface runoff pad on the Ohio State University’s Wooster campus, and three-dimensional drainage configuration (b). ………………. 56

3.2 Daily precipitation (a) and cumulative precipitation (b) during the experimental study period (1 April 2008 through 1 December 2009). Measurements are taken at The Ohio State University’s Weather Station in Wooster, Ohio located 1.5 km away from the experimental site using a tipping bucket rain gauge. ……………………………………………..….. 57

xii

3.3 Cumulative volume and flow rate of influent and effluent hydrographs from the biphasic rain gardens for selected representative medium (10.2 mm) (a) and heavy (23.6 mm) (b) rain events recorded at 5-min intervals. Values are averages of measurements taken at the two replicate rain gardens using tipping bucket rain gauges. ……………………………..….. 58

3.4 Cumulative volume (a) and flow rate (b) of influent and effluent hydrograph from the rain gardens for the 3rd simulated runoff event (180 mm/24 h rainfall). Values are averages of measurements taken at the two replicate rain gardens. Error bars represent one standard error. ………….. 59

3.5 Normalized mass (a) and cumulative mass recovery (b) of nitrate-N and atrazine in effluent from the aerobic zone of the biphasic rain gardens for the 3rd simulated runoff event (180 mm/24 h rainfall). …….……………... 60

3.6 Flow rate and cumulative volume of influent and effluent hydrograph (a), and cumulative loads and effluent concentrations of nitrate-N (b), dicamba (c), glyphosate (d), and 2,4-D from the anaerobic and aerobic zones of the biphasic rain gardens for five sequential rainfall events. Values are averages of measurements taken at the two replicate rain gardens. For each event, 25.4 mm rainfall was applied with simulated runoff pollutants over a 1 h at a constant influent rate of 1.58 m3/h. ………………………….….. 61

4.1 Schematic illustration of column-scale rain garden systems. The biphasic system (a) consisted of an anaerobic-aerobic column set and the monophasic system (b) consisted of an aerobic-aerobic column set. ….….. 86

4.2 Cumulative volume (a) and flow rate (b) of Column 1 effluent from monophasic and biphasic rain garden systems for five sequential runoff events. One L of influent was applied over 1 h at a constant flow rate. Symbols and error bars represent mean ± one standard deviation. ..……..... 87

4.3 Removal efficiency of NO3-N, PO4-P, and atrazine via monophasic and biphasic rain garden systems from five sequential runoff events. Loading for each event was 5.0 mg of NO3-N (a), 50 mg of NO3-N (b), 20 mg of PO4-P (c), and 0.16 mg of atrazine (d). Symbols and error bars represent mean ± one standard deviation. ……..…………………….……..………... 88

xiii

4.4 Removal efficiency of NO3-N in Column 2 effluent under the low (a) and high (b) level loading conditions at different input C/N ratio treatments and relationship between treatment C/N ratio and nitrate-N removal efficiency (c) in the biphasic rain garden system under the high level loading condition. Loading for each event was 5.0 mg of NO3-N for the low level and 50 mg of NO3-N for the high level. Symbols and error bars represent mean ± one standard deviation. The lines in (c) represent the linear regressions of each runoff event. y = 14.64 x + 29.24 for the 2nd runoff event (R2 = 0.969), y = 21.73 x + 25.37 for the 3rd runoff event (R2 = 0.989), y = 22.98 x + 31.83 for the 4th runoff event (R2 = 0.981), and y = 27.66 x + 30.40 for the 5th runoff event (R2 = 0.992). .…..………………... 89

5.1 Schematic illustration of a column-scale biphasic rain garden that consisted of an anaerobic-aerobic column set. ………………………..…... 110

5.2 Cumulative volume (a) and flow rate (b) of Column 1 effluent in the biphasic rain gardens among different soil media from the 2nd to 7th sequential runoff events. 500 mL of influent was applied over 1 h at a constant flow rate. Symbols and error bars represent mean ± one standard deviation. ………………………………………….…………………..…... 111

5.3 Removal efficiency of NO3-N in the biphasic rain gardens among different soil media from the 2nd to 7th sequential runoff events. Loading for each event was 2.5 mg (a) and 25 mg (b) of NO3-N. Symbols and error bars represent mean ± one standard deviation. …………..………………..…... 112

5.4 Removal efficiency of atrazine in the biphasic rain gardens among different soil media from the 2nd to 7th sequential runoff events. Loading for each event was 0.08 mg of atrazine. Symbols and error bars represent mean ± one standard deviation. ………….……….…………………..…... 113

xiv CHAPTER 1

INTRODUCTION

Stormwater runoff from impervious surfaces is one of the most problematic

environmental issues because it deleteriously affects both quantity and quality of surface water resources by altering hydrological patterns and by carrying a significant load of

various pollutants (US EPA, 1997; Allan, 2004). More than 600 organic pollutants have

been identified in urban stormwater runoff in significant quantities (Eriksson et al., 2007).

Most urban stormwater runoff contains sediments (e.g. total suspended solids), nutrients

(N and P), herbicides, polycyclic aromatic hydrocarbons, and heavy metals. Furthermore,

some of these compounds have been observed in urban stormwater runoff at

concentrations that could be detrimental to aquatic and soil inhabiting organisms (US

EPA, 2003).

Stormwater treatment has become an essential practice in urban areas to manage

stormwater runoff (Walsh, 2000; Dietz, 2007; Davis et al., 2009). However, source

control of runoff pollutants is difficult because of the numbers and varieties of point and

non-point sources. In response to these challenges, a number of decentralized stormwater

best management practices (BMPs) have been developed and deployed, which include

bioretention systems, green roofs, bioswales, and stormwater wetlands (US EPA, 2000;

1 Davis, 2005). Most of these best management practices are designed to reduce the risk

from stormwater flooding and to capture pollutants by employing infiltration/filtration

through porous media.

Some bioretention areas or biofilters are depressed landscape areas that are typically

constructed by placing a highly permeable soil medium in shallow trenches or basins and

planting various types of plants (Prince George’s County, 2002). These are often referred

to as rain gardens. Rain gardens have become increasingly popular over the past decade

as a stormwater BMP, specifically in urban areas (Dietz, 2007; Davis et al., 2009). The

rapid increase in the use of rain gardens is primarily due to their aesthetic value and low

cost compared to conventional treatment methods (Davis, 2005; Srivastava and

Majumder, 2008).

Rain gardens have potential to (1) reduce runoff volume and peak flows, (2) facilitate groundwater recharge, (3) increase evapotranspiration, and (4) reduce the amount of

runoff pollutants through a variety of processes that include sedimentation, adsorption,

infiltration, biological transformation (decomposition), and precipitation (USEPA, 2004).

In the past 15 years, rain gardens have been proven to effectively reduce large runoff

volumes and concentration of runoff pollutants such as sediments and metals (e.g. Cu, Pb,

and Zn) (Davis, 2005; Dietz, 2007; Davis et al., 2009). However, results from both

column and field-scale studies show that the level of pollutant removal in rain gardens

has varied and not always been positive. Particularly, poor removal and leaching of

nutrients such as nitrate and phosphorus remain a major obstacle in the widespread

adoption of rain gardens as an effective stormwater BMP (Davis et al., 2001; Hsieh and

Davis, 2005; Maurakami et al., 2008; Cho et al., 2009). Typical rain gardens also pose a

2 threat to groundwater contamination. This is mainly due to the fact that typical rain gardens are designed to drain runoff quickly with accelerated infiltration rates to maximize hydraulic performance. This can compromise pollutant removal efficiency because the accelerated infiltration rate results in reduced retention time and incompletion of biological processes for pollutant remediation/transformation.

Furthermore, most research has focused on a limited number of runoff pollutants (e.g. sediments, nutrients, or metals). Consequently, information on performance of rain gardens for other organic pollutants, particularly herbicides, is very limited even though the presence of herbicides in urban runoff has increased (Carlisle and Trevors, 1988;

Bucheli et al., 1998; Huang et al., 2004).

Recent efforts to improve removal efficiency of runoff pollutants in rain gardens have focused on the development of (1) alternative design configurations and (2) different soil media compositions (Dietz, 2007; Davis et al., 2009). Kim et al. (2003) reported that employing partial water saturated conditions at the bottom of the rain garden can increase nitrate removal through denitrification although it was only tested at relatively low nitrate loading conditions. The water saturated conditions, which impede the diffusion of oxygen, create a favorable oxidation-reduction (redox) potential for denitrification where nitrate-

N is converted to nitrogen gas (Maier et al., 2000). However, only a portion of the first flush can be retained in the saturated zone at the bottom of the rain garden and nitrates may not undergo complete biological denitrification if discharged after an insufficient retention time. It was also demonstrated that the proper layering of rain garden soil media could enhance the biological removal of ammonium-N through adsorption and nitrate-N through denitrification by increasing retention time of runoff (Hsieh et al., 2007).

3 Longer retention of water may cause overflow of the rain garden if large runoff

events occur in a short period of time. This is especially so if the soil medium has a low

infiltration rate (Cho et al., 2009). Therefore, development of alternative rain garden

designs and soil media for bioremediation of runoff pollutants is necessary. The overall

objective of this study was to develop and evaluate a new biphasic rain garden design for

flow management and pollutant removal from stormwater runoff. The specific objectives

were to:

(1) Design, construct, and hydraulically characterize a new field-scale biphasic rain

garden (Chapter 2).

(2) Evaluate field performance of the biphasic rain garden under actual and simulated

runoff conditions (Chapter 3).

(3) Compare and evaluate effectiveness of column-scale monophasic (conventional)

and biphasic rain gardens (Chapter 4).

(4) Evaluate and identify rain garden media configurations for more efficient

pollutant removal (Chapter 5).

This study will help in supporting the implementation of rain gardens as a more effective stormwater BMP by demonstrating more efficient hydraulic and pollutant removal capacity of the biphasic rain garden compared to the conventional rain gardens

currently employed.

4 REFERENCES

Allan, J.D., 2004. Landscapes and riverscapes: the influence of land use on stream ecosystems. Annual Review of Ecology, Evolution, and Systematics 35, 257-284.

Bucheli, T.D., Müller, S.R., Heberle, S., Schwarzenbach, R.P., 1998. Occurrence of pesticides in rain water, roof runoff, and artificial stormwater infiltration. Environmental Science and Technology 32, 3457-3464.

Carlisle, S.M., Trevors, J.T., 1988. Glyphosate in the environment. Water, Air, and Soil Pollution 39, 409-420.

Cho, K.W., Song, K.G., Cho, J.W., Kim, T.G., Ahn, K.H., 2009. Removal of nitrogen by a layered soil infiltration system during intermittent storm events. Chemosphere 76, 690- 696.

Davis, A.P., Shokouhian, M., Sharma, H., Minami, C., 2001. Laboratory study of biological retention for urban stormwater management. Water Environment Research 73, 5-14.

Davis, A.P., 2005. Green engineering principles promote low impact development. Environmental Science and Technology 39, 338-344.

Davis, A.P., Hunt, W.F., Traver, R.G., Clar, M., 2009. Bioretention technology: Overview of current practice and future needs. Journal of Environmental Engineering 135, 109-117.

Dietz, M.E., 2007. Low impact development practices: a review of current research and recommendations for future directions. Water, Air, and Soil Pollution 186, 351-363.

Eriksson, E., Baun, A., Scholes, L., Ahlman, S., Revitt, M., Noutsopoulous, C., Mikkelsen, P.S., 2007. Selected stormwater priority pollutants – a European prospective. Science of the Total Environment 383, 41-51.

5 Hsieh, C., Davis, A.P., 2005. Multiple-event study of bioretention for treatment of urban storm water runoff. Water Science and Technology 51, 177-181.

Hsieh, C., Davis, A.P., Needelman, B., 2007. Nitrogen removal from urban stormwater runoff through layered bioretention columns. Water Environment Researcg 79, 2404- 2411.

Huang, X., Pedersen, T., Fischer, M., White, R., Young, T.M., 2004. Herbicide runoff along highways. 1. Field observations. Environmental Science and Technology 38, 3263- 3271.

Kim, H., Seagren, E.A., Davis, A.P., 2003. Engineered bioretention for removal of nitrate from stormwater runoff. Water Environment Research 75, 355-367.

Maier, R.M, Pepper, I.L., Gerba, C.P., 2000. Environmental Microbiology. Academic Press, San Diego, CA, USA.

Maurakami, M., Sato, N., Anegawa, A., Nakada, N., Harada, A., Komatsu, T., Takada, H., Tanaka, H., Ono, Y., Furumai, H., 2008. Multiple evaluations of the removal of pollutants in road runoff by soil infiltration. Water Research 42, 2745-2755.

Prince George’s County. 2002. Bioretention Manual. Department of Environmental Resources, Landover, MD, USA.

Srivastava, N.K., Majumder, C.B., 2008. Novel biofiltration methods for the treatment of heavy metals from industrial wastewater. Journal of Hazardous Materials 151, 1-8.

USEPA, 1997. Managing urban runoff. EPA-841-F-96-004G. United State Environmental Protection Agency, Washington, DC, USA.

USEPA, 2000. Low impact development (LID), a literature review. EPA-841-B-00-005. United State Environmental Protection Agency, Washington, DC, USA.

6 USEPA, 2003. Nonpoint source program and grants guideline for states and territories. EPA-FRL-7577-6. United State Environmental Protection Agency, Washington, DC, USA.

USEPA, 2004. Stormwater Best Management Practice Design Guide: Volume 1 General Considerations. EPA/600/R-04/121. United State Environmental Protection Agency, Washington, DC, USA.

Walsh, C.J., 2000. Urban impacts on the ecology of receiving waters: a frame work for assessment, conservation and restoration. Hydrobiologia 431, 107-114.

7 CHAPTER 2

DESIGN AND HYDRAULIC CHARACTERISTICS OF A FIELD-SCALE

BIPHASIC RAIN GARDEN FOR STORMWATER MANAGEMENT

ABSTRACT

Field-scale rain gardens were constructed using a novel biphasic (i.e. sequence of anaerobic to aerobic) concept for improving retention and removal of storm water runoff pollutants. Hydraulic tests with bromide tracer and simulated runoff pollutants (nitrate-N, phosphate-P, Cu, Pb, and Zn) were performed in the rain gardens under a simulated continuous rainfall. The objectives of the tests were (1) to determine hydraulic characteristics of the rain gardens, and (2) to evaluate the movement of runoff pollutants through the rain garden. For the 180 mm/24h rainfall, the biphasic rain gardens effectively reduced both peak flow (~70%) and runoff volume (~ 42%). The breakthrough curves (BTCs) of bromide tracer suggest that the transport pattern of the rain gardens is similar to dispersed plug flow under this large runoff event. The BTCs of bromide showed mean 10% and 90% breakthrough times of 5.7 h and 12.5 h, respectively. Under the continuous rainfall, a significantly different transport pattern was found between each runoff pollutant. Nitrate-N was easily transported through the rain

gardens with potential leaching risk from the soil medium, whereas phosphate-P and

metals were significantly retained indicating sorption-mediated transport. These findings

support the importance of hydraulics, in combination with the soil medium, when

creating rain gardens for bioremediation that are effective for various rainfall sizes and

intervals.

INTRODUCTION

Expansion of urban areas has increased the type and amount of pollutants in

stormwater runoff. Roads and buildings create areas on the landscape where infiltration

of precipitation is impeded, thus increasing the volume and rate of stormwater runoff.

This excessive runoff not only degrades surface water resources, by altering hydrologic patterns in natural streams and groundwater, but diverse pollutants in the runoff can cause increased risk to human health and harm to biological components in ecosystems

(USEPA, 1997; Allan, 2004). In the past several years, a number of best management

practices have been developed and deployed for handling of stormwater such as

bioretention systems, green roofs, bioswales, and stormwater wetlands (USEPA, 2000;

Davis, 2005). Most of these best management practices are designed to reduce the risk for stormwater flooding and to capture pollutants by employing infiltration/filtration through porous media.

Rain gardens are bioretention practice. That is increasingly becoming adopted in urban and suburban areas due to their aesthetic appeal (Davis, 2005; Dietz, 2007). For this paper the term rain garden is used to refer to various types of bioretention systems.

Rain gardens are small constructed land depressions generally consisting of a porous soil medium layer, mulch layer, and vegetation. They can be distributed across the landscape to drain quickly and capture pollutants using constructed soils and an assemblage of plants that facilitate removal of runoff pollutants (Davis et al., 2001). The pollutant remediation in rain gardens has been attributed to several processes that include sedimentation, adsorption, filtration, biological transformation (decomposition), and precipitation (Prince George’s County, 1993).

In studies to date, successes of rain garden performance in improving water quality have been documented (Davis, 2005; Dietz, 2007), but other unexpected outcomes regarding their efficiency have also arisen. While high retention of metals (e.g. Cu, Pb, and Zn) was observed, poor and inconsistent treatment efficiencies (reduction or increase) of nutrients (i.e. nitrate-N and total P) indicate potential leaching risk in column and field-scale design (Davis et al., 2001; Hsieh and Davis, 2005; Dietz and Clausen,

2006; Hunt et al., 2006). These different outcomes may be explained by (1) variation of design configurations; (2) different types of soil media; and (3) different hydraulic conditions (e.g. flow/mixing pattern and retention time) of the systems.

To increase biological processes for nitrate-N retention (e.g. denitrification), an alternative design was developed by creating a saturated zone in the bottom of the rain garden (Kim et al., 2003). The water saturated conditions, which impede the diffusion of oxygen in rain gardens, would create a favorable oxidation-reduction (redox) potential for denitrification, where nitrate-N is converted to nitrogen gas (Maier et al., 2000). This modified design was tested in a field-scale study and showed improved removal

efficiency of nitrate-N (Dietz and Clausen, 2006). However, only a portion of the first

flush can be retained in the saturated zone at the bottom of the system and nitrates may

not undergo complete biological denitrification if discharged after only a short retention

time. For better handling of nitrate-N, the development of alternative rain garden systems

with improved treatment efficiency is still necessary.

Research on rain gardens has focused on evaluating treatment efficiency and

understanding mechanisms of pollutant retention (Dietz, 2007). However, research

describing hydrodynamics of rain gardens is very limited even though greater reliance on

treatment efficiency and operational optimization require a better understanding of the

hydraulics that can affect pollutant behavior. As a first step towards the evaluation of new

designs, tracer tests may be useful to understand hydraulic characteristics of the rain gardens.

This work is part of a broader project on the development of new biphasic rain gardens to maximize removal efficiency of multiple stormwater runoff pollutants. The objectives of the work presented here were (1) to hydraulically characterize a new biphasic rain garden and (2) to evaluate the movement of simulated multiple runoff pollutants through the rain garden. Further long-term removal efficiency of runoff pollutants will be reported in a subsequent communication. To the best of our knowledge, this is the first report of a tracer study applied to a rain garden for hydraulic characterization.

MATERIALS AND METHODS

Biphasic rain garden design and construction

The biphasic rain garden (Figure 2.1) is a new type of design that consists of a biphasic (anaerobic to aerobic) sequence and a recharge zone. The rain garden was designed to meet the main goal of increasing retention time of runoff pollutants to maximize removal efficiency. In the biphasic system, flow is directed first through the anaerobic zone, and then through the aerobic zone. The anaerobic zone is created by an impervious liner, which increases retention time for sustained water-saturated conditions

(Figure 2.1a). In the anaerobic zone, the first flush of runoff is captured, sediments are filtered, and adsorption and/or biological degradation of pollutants occurs. The U-shaped reverse drainage pipes maximize the retention time of the first flush runoff in the anaerobic zone and allow the treated overflow water to exit into the aerobic zone. The aerobic zone is designed as a capillary barrier profile to further retard flow for subsequent aerobic treatment (Figure 2.1a). This enables longer retention of stormwater runoff for more effective treatment of pollutants through both anaerobic and aerobic chemical and microbial processes. Finally, the treated water from the aerobic zone is discharged into the recharge zone located at the bottom of the system. The recharge zone filled with pea gravel is designed to facilitate groundwater recharge.

Two replicate field-scale biphasic rain gardens were built in April, 2008 at the Ohio

Agricultural Research and Development Center (OARDC), a campus of The Ohio State

University in Wooster, Ohio. The description of the experimental site and the field-scale biphasic rain garden is summarized in Table 2.1. The site is adjacent to a concrete surface

water runoff pad with surface area of 16.3 m by 8.53 m (139 m2) and a longitude slope of

4% downward to the north. To create two replicate runoff areas, the pad was equally divided by setting up berms on the pad. Each individual runoff area was 8.15 m by 8.53 m (69.5 m2) and was used as an impervious contact surface and was also associated with its own rain garden for collecting and treating both simulated and actual runoff.

In order to determine the size and storage volume of the rain gardens, precipitation

data in Wooster were obtained from the hydro-meteorological design studies center of the

National Oceanic and Atmospheric Administration (NOAA) weather service center

(NOAA, 2008). The average amount of precipitation for a 1-h rainfall in Wooster is

approximately 28.4 mm (1.12 in). The precipitation of a 1-hr intense rainfall with 10-year

return frequency was estimated to be 44.7 mm (1.76 in) by the NOAA Atlas 14 Model.

This latter value was used to calculate a target water quality volume, and the maximum

volume of water that can be received and treated in the rain gardens. The target water

quality volume generated from 44.7 mm rainfall was estimated to be 3.1 m3 based on the drainage area. The required size and infiltration rate of each zone in the rain garden was designed based on expected water volumes using the rational and mass continuity

methods (Davis and McCuen, 2005).

Each rain garden site was excavated to a depth of 1.4 m with surface area of 4.1 m by

3.4 m (13.9 m2). To make the aerobic zone effluent point accessible, a surface area of 1.1

m by 1.2 m (1.3 m2) was excavated at the final discharge area. The recharge zone access

pipe was placed between the final discharge area and main excavation area to direct the

aerobic zone effluent back into the recharge zone (Figure 2.1b). The recharge zone

located at the bottom of the main excavation area was filled with pea gravel (gravel size,

9.5 mm – 12.7 mm) to a thickness of 0.15 m and then covered by an impervious liner

(152-µm thick polyethylene sheeting, Sunbelt Plastic, Monroe, LA). The anaerobic zone

was constructed using a plywood box (2.6 m width, 2.6 m length, and 1.2 m depth). An

impervious liner was placed in the box and the reverse drainage pipes were set up in the

anaerobic zone (Figure 2.1b). The bypass drainage pipe connected to the reverse pipes was placed between the anaerobic zone and the final discharge zone for direct sampling.

Fine gravel (3.2 mm – 12.7 mm) was placed at the bottom to cover perforated portions of

the reverse drainage pipes. The soil medium was a mixture of sand, topsoil, and compost

(6:2:2, volume ratio) obtained from Kurtz Bros. Inc., Cleveland, OH. The medium was

placed above the gravel layer to a thickness of 1.0 m, leaving 20 cm freeboard in the box

for ponding of water in the anaerobic zone during runoff events. The aerobic zone was

layered using the same materials to create a capillary barrier profile. The underdrainage

pipes were placed at the bottom of the aerobic zone (Figure 2.1b) and the fine gravel and

soil medium were subsequently placed. The depth of the soil medium in the aerobic zone

was 0.5 m.

Six native plant species were planted at 0.2-m2 (2.25-ft2) spacing intervals based on

their growing habit. For the anaerobic (saturated) zone, selected plant species included

boneset (Eupatorium perfoliatum), spiderwort (Tradescantia ohiensis), and culver’s root

(Veronicastrum virginicum). For the aerobic zone, purple love grass (Eragrostis

spectabilis), Indian grass (Sorghastrum nutans), and purple coneflower (Echinacea

purpurea) were planted. A 5-cm mulch layer was then placed over both the aerobic and

anaerobic zones to cover the soil medium and improve runoff pollutant retention. The

mulch layer was made up of shredded bark and leaf litter.

The soil medium was submitted to a laboratory (N.W. Hummel and Co.,

Trumansburg, NY) accredited by the American Association for Laboratory Accreditation,

(Frederick, MD) for physical testing following prescribed methods (American Society for

Testing and Materials, 2007). Chemical characteristics were also analyzed at the Service

Testing and Research (STAR) Laboratory located on the OARDC Wooster campus

(STAR Lab, 2008). The physical and chemical characteristics of the soil medium used in the biphasic rain gardens are shown in Table 2.2.

Tracer study with bromide and simulated runoff pollutants

The field-scale bromide tracer tests were carried out with simulated runoff pollutants

(i.e. nitrate-N, phosphate-P, Cu, Pb, and Zn) in two replicate biphasic rain gardens during the summer of 2008. Prior to the tests, the anaerobic zones were initially drained by opening the bypass drainage pipes. In a 1-L volumetric flask, 15.5 g of bromide (22.9 g of potassium bromide, Fisher) solution was prepared. Each runoff pollutant solution was also prepared in a 1-L volume (Table 2.3). The entire volume (6 L) of the tracer and pollutant solutions was applied to the drainage area connected to the rain gardens within

1 min after starting four oscillating sprinklers to provide the simulated runoff. The simulated runoff was continuously applied for 24 h achieving 4-times the target water volume (total 12.5 m3 or 180 mm rainfall). For the first 4 h, the runoff was applied at approximately 1.58 m3/h as peak flow rate and was steadily decreased to 0.25 m3/h at the

end of the event. Inflow rates of runoff into the rain garden and outflow rates of the

anaerobic zone through reverse pipes (anaerobic zone effluent) were measured at 30-min

to 1-h intervals. Outflow rates of the aerobic zone (aerobic zone effluent) were measured with a tipping-bucket rain gauge. Tipping counts were recorded at 5-min intervals using a

CR3000 data logger (Campbell Scientific, Logan, UT). Samples were taken from the

inflow as it entered the rain garden and from the effluent of each zone using 250 mL

plastic bottles and stored at 4 °C until further analysis.

Concentrations of bromide, metals, nitrate-N, and phosphate-P in the effluent of each

zone were analyzed at the Service Testing and Research (STAR) Laboratory using a

Dionex DX-120 ion chromatograph for bromide and nutrients (APHA, 1992) and a

Teledyne Leeman Labs Prodigy inductively coupled plasma – atomic emission spectrometry for metals (APHA, 1992).

RESULTS AND DISCUSSION

Hydraulic performance

The anaerobic zone effluent through the reverse pipes and the aerobic zone effluent at

the final discharge pipe were monitored to investigate hydraulic performance of two

replicate biphasic rain gardens during the tracer study with a 24-h simulated runoff event.

Prior to the tests, both aerobic and anaerobic zones were initially drained. Figure 2.2

shows cumulative volume and flow rate of influent and effluent of the rain gardens

during the event. The cumulative volume of the influent during the event was

approximately 12.5 m3 (180 mm rainfall) with 1.58 m3/h of peak flow rate at the initial 4h.

Although the applied runoff volume was 4-times higher than the designed storage capacity for the rain gardens, both rain gardens contributed to a significant reduction in both peak flow rate and volume due to the initial storage conditions that existed in the rain gardens. The observed peak flow rate of the anaerobic zone effluent was approximately 0.56 m3/h at 3.5 h after initiation of the rainfall event and lasted for 10 h.

For the first 4 h, the aerobic zone effluent rate gradually increased to approximately 0.47 m3/h as peak flow rate and then it steadily decreased to 0.22 m3/h at 24 h.

The cumulative volume of each effluent at the end of the study was 8.94 m3 for the anaerobic zone and 7.23 m3 for the aerobic zone. The peak flow and volume reduction between influent and the aerobic zone effluent were 70.3% and 42.1%, respectively. The effluent from each zone in the rain gardens also exhibited a delayed response to the simulated runoff event. The average lag time between start of inflow to the anaerobic zone and the anaerobic zone effluent seen at the reverse pipes was approximately 73.0 min. The average lag time from the anaerobic zone effluent to the aerobic zone effluent was approximately 26.5 min. No direct overflow from either zone occurred during this

180 mm/24 h rainfall event.

Mass recovery

The tracer study of each biphasic rain garden was completed using a 24-h continuously simulated runoff event. Overall mass recoveries (%) of the tracer and runoff pollutants were calculated by integrating hourly flow rate and concentration data collected from the aerobic zone effluent. Figure 2.3 shows the cumulative mass recovery

plots for the bromide and runoff pollutants through the rain gardens. Some field-scale

variation between two replicate biphasic rain gardens was observed; biphasic rain garden

1 has a relatively longer mean retention time at 9.3 h compared to biphasic rain garden 2

at 6.6 h. The data show average bromide recovery of 95.9% and nitrate-N recovery of

113% at the aerobic zone effluent, showing that both bromide and nitrate-N were

transported directly through the rain gardens without adsorption. Higher than expected

nitrate-N recovery was noted due to nitrate leaching from the soil medium placed into the

rain gardens and the lack of time required for denitrifying microorganisms to build up in

the rain gardens. Similar results (~59% nitrate-N increases) were reported in column

studies (Hatt et al., 2007; Hsieh et al., 2007). However, no phosphate-P and metals (Cu,

Pb, and Zn) were observed at the aerobic zone effluent, indicating 100% retention in the

rain gardens (data not shown). The successful capture of about 96% of the applied bromide along with low variability between the experiments (5% mass loss) suggests that the test method used in this study was adequate in describing flow through the biphasic rain garden.

Movement of tracer and simulated runoff pollutants

Influent and effluent of the anaerobic and aerobic zones were monitored to investigate the movement of bromide tracer and runoff pollutants. The effluent collected from each zone is plotted in Figure 2.4a for the tracer, Figure 2.4b for nitrate-N, and Figure 2.4c for phosphate-P as a function of actual bed pore volume (PV) of the anaerobic zone in each biphasic rain garden. The actual PV of the anaerobic zone was determined by a

preliminary test. The anaerobic zone was initially water saturated without standing water

and then drained by opening the bypass drainage pipe. The volume of drained water was

determined and used as a measure of the PV of the anaerobic zone. The average PV of

the anaerobic zone was estimated to be approximately 1.05 ± 0.08 m3. In order to allow

direct comparison of each chemical, the concentration data have been normalized so that the area under the curve is equal to unity. The 10% and 90% mass breakthroughs of total bromide applied were calculated by integrating PV and normalized mass data from

Figure 2.4 and then expressing these data as a function of either PV or time. The overall results of the tracer studies for both rain gardens are summarized in Table 2.4.

The response to the bromide tracer pulses was close to symmetric curves and no extended tailings occurred (Figure 2.4a). This shows that the transport pattern of the biphasic rain garden is close to dispersed plug flow under the large runoff event applied in this study. It is expected that increased retention times would be affected by the size

and intensity of rainfalls and contribute to higher dispersed flows. For the aerobic zone

effluent, mean 10% and 90% breakthroughs of the total bromide applied were observed at

5.4 ± 0.7 PV (5.7 ± 1.3 h) and 7.8 ± 1.4 PV (12.5 ± 4.2 h), respectively. The effluent

concentrations of the tracer remained low until approximately the first 2.5 PV of

application. For the anaerobic zone effluent, pulse peaks were achieved at 5.0 PV in both

rain gardens, indicating the two anaerobic zones are identically replicated. However,

different pulse peaks were observed for the aerobic zone effluent. The pulse peaks were

reached at 7.0 PV for biphasic rain garden 1 and 6.0 PV for biphasic rain garden 2,

indicating field-scale variation in the aerobic zones (i.e. actual size and storage volume).

The movement of nitrate-N in the rain gardens shows a pulse response similar to the bromide tracer (Figure 2.4b). The observed similar pulse responses such as symmetric shapes and no extended tailings indicated insignificant sorption of dissolved nitrate-N through the rain gardens. It is likely that the soil medium does not provide sorption sites for negatively charged ions. Since biological processes (e.g. denitrification) require a C source and a certain time to occur, soluble nitrate-N may not be undergoing biological denitrification due to either short retention time or insufficient denitrifying microorganisms and C sources in the rain garden soil media. Kim et al. (2003) reported the effect of C source on denitrification reaction in laboratory rain gardens, showing shredded newspapers as an extra C source in rain garden soil media increased the removal of nitrate-N. Two previous studies (Kim et al., 2003; Dietz and Clausen, 2006) have demonstrated that the addition of a designed water saturated zone in rain gardens increases nitrate-N removal by creating favorable environmental conditions (i.e. anaerobic conditions and partially increased retention time) for denitrification. However, in our tracer study, increased nitrate-N was observed in the effluents from the anaerobic and aerobic zones in spite of attempts to increase biological denitrification activity in the anaerobic zone. This result supports the importance of designed storage volume and long retention times for biological processes during interval between rainfall events. Since the large simulated storm (i.e. 180 mm/24 h rainfall) generated a large volume of runoff, the water went through the rain garden rapidly without allowing significant retention time and denitrification within the rain garden. Hsieh et al. (2007) reported that increasing the retention time by using lower permeability media significantly increased nitrate-N

removal within column studies. Due to the fact that the biphasic rain garden design

focused on increasing retention time of runoff pollutants by holding first flush runoff

(25.4 mm rainfall) in the anaerobic zone until next rainfall event, it is expected that

nitrate-N removal would increase for rainfall events that are normally encountered due to

increased retention time in the rain garden and thus allowing more time for chemical and

biological transformation.

The response pulse of phosphate-P was relatively insignificant (Figure 2.4c). Also,

concentrations of metals were not observed in the effluents from the anaerobic and

aerobic zones (data not shown). Characteristics of phosphate-P pulse such as asymmetric shapes, extended tailings, and low pulse peaks (or mass recovery) can be explained by sorption-mediated non-equilibrium transport (Lee et al., 1988). Only 18.5% of total phosphate-P was recovered from the anaerobic zone effluent and no phosphate-P from the aerobic zone effluent indicated retention of phosphate-P in the rain gardens through sorption and/or precipitation processes. The absence of detectable metals in the anaerobic zone effluent showed that the metals were captured by the soil medium and mulch layer in the anaerobic zone. Well-known mechanisms related to the metal retention in the soil media include cation exchange (non-specific adsorption), precipitation, and organic complexation (Alloway, 1995).

Even though 100% of phosphate-P and metals were retained in the two rain gardens, the soil medium’s capacity to capture phosphate-P and metals may be limited in the long term due to the low cation exchange capacity (CEC) of the soil medium (Table 2.2). The long-term performance of the biphasic rain garden systems for pollutant removal should

be monitored over longer time periods and under real world situations to ascertain the

capacity for pollutant accumulation within the soil medium.

CONCLUSIONS

The tracer study with addition of simulated runoff pollutants was conducted using

two replicate field-scale biphasic rain gardens. The results of hydraulic performance

demonstrated that the biphasic rain garden can effectively reduce both peak flow (~70%)

and runoff volume (~ 42%) for a large 180 mm/24h rainfall event. Successful capture of

tracer mass for both rain gardens shows that the test method used in this study was

adequate in describing flow through the rain gardens. The pulse responses of bromide

tracer indicate that the transport pattern of the biphasic rain gardens is similar to

dispersed plug flow. The responses for two replicate rain gardens were fairly similar although some variation existed. Mean 10% and 90% breakthrough were 5.4 PV (5.7 h) and 7.8 PV (12.5 h), respectively. Substantial differences were observed in transport pattern of nitrate-N, phosphate-P, and metals (Cu, Pb, and Zn). Dissolved nitrate-N was easily transported through the rain gardens without any significant sorption, whereas dissolved phosphate-P and metals were significantly retained in the rain gardens indicating sorption-mediated non-equilibrium transport. The mean 13.0% increase of nitrate-N that exited the rain gardens also indicates potential leaching risk from the soil medium if runoff into the gardens is continuous and of high volume.

Overall, the results of this study provide understanding of hydraulic characteristics and information on the movement of runoff pollutants in the biphasic rain garden.

However, the lack of nitrate-N retention during the tracer study in the rain gardens can be

problematic. Soluble nitrate-N removal through rain gardens is greatly affected by

environmental conditions (e.g. C source and aerobic/anaerobic conditions) and retention

times for biological processes (e.g. denitrification). Thus the biphasic rain garden that can

be managed to increase overall retention time has the potential for being an effective best management practice in reducing flow and pollutant loads by employing the sequence of anaerobic to aerobic (biphasic) conditions. Further, long-term field evaluation of the biphasic rain garden for pollutant removal and development of rain garden soil media to provide a sufficient C source to enhance denitrification will be investigated in subsequent experiments.

ACKNOWLEDGEMENTS

The authors thank all members of Dr. Dick’s and Dr. Grewal’s labs and OSU-

OARDC facility services staff for aids in construction of the rain gardens. The authors especially thank Mr. Joshua Blosser for his assistance in this study. We also acknowledge

Dr. John Cardina and Mr. Donald Beam for selecting and establishing plant species in the rain gardens. This study was funded by the Center for Urban Environment and Economic

Development of the Ohio State University and by funds appropriated to The Ohio State

University and The Ohio Agricultural Research and Development Center.

A modified version of this chapter was published as:

Yang, H., Florence, D.C., McCoy, E.L., Dick, W.A., Grewal, P.S., 2009. Design and hydraulic characteristics of a field-scale bi-phasic bioretention rain garden system for storm water management. Water Science and Technology 59, 1863-1872. 23

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Alloway, B.J., 1995. Heavy Metals in Soils, Springer, USA.

APHA, AWWA, AEF, 1992. In: Standard Methods for the Examination of Water and Wastewater, 18th ed. American Public Health Association, Washington, DC, USA.

Davis, A.P., Shokouhian, M., Sharma, H., Minami, C., 2001. Laboratory study of biological retention for urban stormwater management. Water Environment Research 73, 5-14.

Davis, A.P., 2005. Green engineering principles promote low impact development. Environmental Science and Technology 39, 338-344.

Davis, A.P., McCuen, R.H., 2005. Stormwater Management for Smart Growth, Springer, USA.

Dietz, M.E., Clausen, J.C., 2006. Saturation to improve pollutants retention in a rain garden. Environmental Science and Technology 40, 1335-1340.

Dietz, M.E., 2007. Low impact development practices: a review of current research and recommendations for future directions. Water, Air, and Soil Pollution 186, 351-363.

Hatt, B.E., Fletcher, T.D., Deletic, A., 2007. Hydraulic and pollutant removal performance of stormwater filters under variable wetting and drying regimes. Water Science and Technology 56, 11-19.

Hunt, W.F., Jarrett, A.R., Smith, J.T., Sharkey, L.J., 2006. Evaluating bioretention hydrology and nutrient removal at three field sites in North Carolina. Journal of Irrigation and Drainage Engineering 132, 600-608.

Hsieh, C., Davis, A.P., 2005. Multiple-event study of bioretention for treatment of urban storm water runoff. Water Science and Technology 51, 177-181.

Hsieh, C., Davis, A.P., Needelman, B., 2007. Nitrogen removal from urban stormwater runoff through layered bioretention columns. Water Environment Research 79, 2404- 2411.

Kim, H., Seagren, E.A., Davis, A.P., 2003. Engineered bioretention for removal of nitrate from stormwater runoff. Water Environment Research 75, 355-367.

Lee, L., Rao, P., Brusseau, M., Ogwada, R., 1998. Nonequilibrium sorption of organic contaminants during flow through columns of aquifer materials. Environmental Toxicology and Chemistry 7, 779-793.

Maier, R.M., Pepper, I.L., Gerba, C.P., 2000. Environmental Microbiology, Academic Press, USA.

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Prince George’s County, 1993. Design Manual for Use of Bioretention in Stormwater Management. Prince George’s County, Department of Environmental Protection, Landover, MD, USA.

STAR Laboratory, 2008. Method references. Service Testing and Research Laboratory, http://www.oardc.ohio-state.edu/starlab/ (accessed 05 January 2009)

USEPA, 1997. Managing Urban Runoff. EPA-841-F-96-004G. United State Environmental Protection Agency, Washington, DC, USA.

USEPA, 2000. Low impact development (LID), a literature review. EPA-841-B-00-005. United State Environmental Protection Agency, Washington, DC, USA.

Parameters Specifications Drainage area Concrete pad, 69.5 m2 (748 ft2)

Anaerobic zone Surface area 6.8 m2 (72.8 ft2) Ponding depth 20 cm (8 in) Ponding volume 1.36 m3 ( 48.5 ft3) Soil medium depth 1 m (3.3 ft) Storage volume a 1.76 m3 (62.2 ft3) Reverse pipe 0.1 m (4 in) perforated pipe (bottom) 0.1 m (4 in) non-perforated pipe (side) Underdrain gravel bed Gravel (3.2 mm – 12.7 mm)

Aerobic zone Surface area 7.1 m2 (77.0 ft2) Soil medium depth 0.5 m (1.6 ft) Storage volume b 1.34 m3 (47.3 ft3) Underdrain pipe 0.1 m (4 in) perforated pipe Underdrain gravel bed Gravel (3.2 mm – 12.7 mm)

Recharge zone Surface area 14.0 m2 (151 ft2) Pea gravel depth 0.15 m (0.5 ft)

Soil medium Sand:top soil:compost 0.05 mm - 1.0 mm diameter (volume ratio, 6:2:2) with 2.5% clay a Estimation for 25.4 mm (1.0 in) rainfall storage based on the impervious contact surface area (only for storage within soil bed). b Estimation for 19.3 mm (0.76 in) rainfall storage based on the impervious contact surface area (only for storage within soil bed).

Table 2.1. The description of experimental site and biphasic rain garden

Physical analyses Chemical analyses Property Value Property Value Sand (%) 90.6 pH 7.2 Silt (%) 6.9 Organic matter (%) 0.7 Clay (%) 2.5 CEC (meq/100g) 3.1 d10 (mm) 0.11 Mg (µg/g) 37.3 d60 (mm) 0.46 P (µg/g) 9.3 Bulk density (Mg/m3) 1.67 K (µg/g) 22.0 Total porosity (m3/m3) 0.37 Ca (µg/g) 511 Air-filled porosity a (m3/m3) 0.14 Cu (µg/g) 14.2 Capillary porosity a (m3/m3) 0.23 Pb (µg/g) 8.0 Saturated conductivity (cm/h) 12.0 Zn (µg/g) 41.9 a Air-filled and capillary porosity determined at 300-mm tension.

Table 2.2. Physical and chemical characteristics of biphasic rain garden soil medium

Concentration Total Input Tracer/Pollutants Source (mg/L) a Mass (g) b Tracer Bromide (Br) 1.24 15.5 KBr Heavy metals Copper (Cu) 0.74 9.3 CuSO4 Lead (Pb) 0.74 9.3 PbCl2 Zinc (Zn) 2.5 31.0 ZnCl2 Nutrients Nitrate-N (NO3-N) 5.0 61.9 NaNO3 Phosphate-P (PO4-P) 5.0 61.9 NaH2PO4 a Average concentration based on the total volume of applied runoff b Each chemical solution was prepared in a 1-L volume

Table 2.3. Makeup of tracer and simulated stormwater runoff

Parameters Biphasic Biphasic rain garden 1 rain garden 2 Total simulated rainfall (mm) 178 183 Total generated run off (m3) 12.4 12.6 Total run time (h) 24.0 24.0 Influent rate range (m3/h) 0.27 - 1.57 0.23 - 1.59 Peak flow rate of effluent from anaerobic zone (m3/h) 0.57 0.54 Peak flow rate of effluent from aerobic zone (m3/h) 0.47 0.47 Bromide mass recovery (%) a 95.3 96.4 Nitrate-N mass recovery (%) a 107 119 10% tracer breakthrough (PV) a 4.9 (4.8 h) 5.9 (6.6 h) 90% tracer breakthrough (PV) a 8.8 (15.4 h) 6.8 (9.5 h) a At the aerobic zone effluent (final discharge pipe)

Table 2.4. Summary of tracer study results

Figure 2.1. Plan and cross-section views (a) and three-dimensional drainage configuration (b) of experimental biphasic rain garden: (1) drainage area, (2) influent, (3) micro-pool, (4) anaerobic zone, (5) reverse drainage pipe, (6) aerobic zone, (7) overflow inlet, (8) final discharge pipe, (9) aerobic zone effluent, (10) tipping-bucket rainfall gauge, (11) recharge zone access pipe, (12) bypass drainage pipe, (13) impervious liner, (14) plywood box, (15) anaerobic zone effluent, (16) mulch layer, (17) soil bed medium, (18) gravel, (19) recharge zone, and (20) underdrainage pipe. 30

14 (a) 12 ) 3 10

4 Cumulative Volume (m Cumulative 2

0 0 6 12 18 24 1.8 Time after start of the tracer experiment (hrs) 1.6 Biphasic rain garden 1 Influent (b) 1.4 Anaerobic zone effluent Aerobic zone Effluent

/h) 1.2

3 Biphasic rain garden 2 Influent 1.0 Anaerobic zone effluent Aerobic zone effluent 0.8

0.6 Flow Rate (m 0.4

0.2

0.0 0 6 12 18 24 Time after start of the tracer experiment (h)

Figure 2.2. Cumulative volume (a) and flow rate (b) of influent and effluent hydrograph from the systems for a 180 mm/24 h simulated rainfall event.

120

100

60 Biphasic rain garden 1 Br 40 NO3-N Mass Recovery (%) Mass Recovery Biphasic rain garden 2 20 Br NO3-N

0 0 6 12 18 24

Time after start of the tracer experiemnt (h)

Figure 2.3. Cumulative mass recovery (%) for bromide tracer and simulated runoff pollutants in the effluent from the aerobic zone (Note: phosphate-P and metals were not detected).

1.0

0.8 (a) Br

0.6

0.4

Normalized MassNormalized 0.2

0.0 024681012 1.0

(b) NO -N 0.8 3

0.6

0.4 Normalized MassNormalized 0.2

0.0 024681012 1.0

0.8 (c) PO4-P

0.6 Biphasic rain garden 1 Anaerobic zone effluent Aerobic zone effluent 0.4 Biphasic rain garden 2 Anaerobic zone effluent Normalized MassNormalized 0.2 Aerobic zone effluent

0.0 024681012 Pore Volume (PV)

Figure 2.4. Breakthrough curves of bromide tracer (a), nitrate-N (b), and phosphate-P (c) in effluent from the anaerobic zone and the aerobic zone of the biphasic rain gardens. 33

CHAPTER 3

FIELD EVALUATION OF A BIPHASIC RAIN GARDEN FOR FLOW

MANAGEMENT AND POLLUTANT REMOVAL

ABSTRACT

Field-scale rain gardens were constructed using a novel biphasic (a sequence of anaerobic to aerobic) concept for increasing retention time of runoff and providing improved environmental conditions for bioremediation. Hydraulic performance and

removal efficiencies of the biphasic rain gardens were evaluated in actual and simulated

runoff events. Influent and effluent of two replicate biphasic rain gardens from actual

runoff events were monitored during a 2-yr study. Three simulated agricultural runoff

events with high concentrations of nutrients (i.e. nitrate-N, phosphate-P) and the

herbicide atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) were undertaken during the summer of 2008. Five simulated urban runoff events with spiked concentrations of nutrients (i.e. nitrate-N and phosphate-P) and herbicides (i.e. glyphosate

(N-(phosphonomethyl)glycine), dicamba (3,6-dichloro-2-methoxybenzoic acid), and 2,4-

D (dichlorophenoxyacetic acid)) were applied to the rain gardens during the summer of

2009. Both peak flow and runoff volume were effectively reduced in the rain gardens for

actual and simulated events by holding runoff in the rain gardens (mainly in the anaerobic

34 zone) until the next runoff event. The biphasic rain gardens were highly effective in

removing nitrate-N (~91%), phosphate-P (~99%), atrazine (~90%), dicamba (~92%), glyphosate (~99%), and 2,4-D (~90%) under high levels of pollution loading with simulated runoff events. Increased retention time of runoff pollutants, as determined by design configuration and rainfall size, intensity, and interval, was found to significantly affect overall nutrient and herbicide removal in the biphasic rain gardens.

INTRODUCTION

Rain garden systems, often referred to as bioretention areas or biofilters, are being adopted in urban and suburban areas to reduce stormwater flow, volume, and pollutant concentrations (Davis, 2005; Dietz, 2007). While several possible design configurations exist, rain gardens are typically constructed by placing a porous soil medium in shallow trenches or basins and planting various types of vegetation (Prince George’s County,

2002).

Stormwater runoff pollutants can be retained and removed by rain gardens through

sedimentation, adsorption, infiltration, biological transformation (decomposition), and

precipitation (USEPA, 2004). Rain gardens have also been shown to effectively remove metals (e.g. Cu, Pb, and Zn) and sediment (Davis, 2005; Dietz, 2007; Davis et al., 2009), but are less effective in removing other pollutants such as nutrients (e.g. nitrate-N and

total phosphorus), and particularly when the nutrients are in dissolved forms (Davis et al.,

2001; Hsieh and Davis, 2005; Hatt et al., 2007; Maurakami et al., 2008). This is mainly because rain gardens are typically designed to drain runoff quickly due to accelerated infiltration rates. This results in reduced retention time and incomplete biological

35 processing for pollutant removal. Furthermore, most research has focused on a limited

number of runoff pollutants (e.g. sediments, nutrients, or metals) at low concentration

loadings sometimes below the National Primary Drinking Water Regulations - Maximum

Contamination Protection Level standards. Information on rain garden performance for other organic pollutants, particularly herbicides, is scarce even though the presence of herbicides in urban runoff has become problematic (Carlisle and Trevors, 1988; Bucheli et al., 1998; Huang et al., 2004).

To improve the removal efficiency of multiple runoff pollutants, a biphasic rain garden was designed to increase overall retention time of runoff and provide favorable environmental conditions for bioremediation (Grewal et al., 2009). The biphasic rain garden employs a sequence of anaerobic to aerobic conditions with various electron acceptors to support a wide array of degrading microorganisms (Rittmann and McCarty,

2001). We have previously reported hydraulic tests with bromide tracer in biphasic rain gardens at the field scale (Yang et al., 2009) and removal efficiencies of nutrients

(nitrate-N and phosphate-P) and atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5- triazine-2,4-diamine) at the column-scale (Yang et al., 2010 Submitted).

The objectives of the work presented here were to evaluate field performance of the biphasic rain garden under (1) actual and (2) simulated runoff conditions. Multiple loads of high concentrations of pollutants, including nutrients (nitrate-N and phosphate-P) and herbicides (atrazine, dicamba (3,6-dichloro-2-methoxybenzoic acid), glyphosate (N-

(phosphonomethyl)glycine), and 2,4-D (2,4-dichlorophenoxyacetic acid)) were applied to determine the efficiency of water flow management and pollutant removal in replicated biphasic rain gardens.

MATERIALS AND METHODS

Biphasic Rain Gardens

Two replicate biphasic rain gardens (Figure 3.1a) were constructed in the spring of

2008 at The Ohio State University’s Wooster campus. In these biphasic rain gardens,

runoff is first directed through an anaerobic zone and then through an aerobic zone. This

is accomplished by using U-shaped reverse drainage pipes that allow the treated overflow

water from the anaerobic zone to exit into the aerobic zone (Figure 3.1b). Each rain

garden was sized to handle 44.7 mm (1.76 in.) of runoff from a drainage area consisting

of a 99% impervious concrete surface pad with surface area of 69.5 m2. The surface area

of the anaerobic zone was 6.8 m2 and had a depth of 1.0 m, providing a storage volume of

approximately 1.58 m3 and a holding capacity of 25.4 mm runoff from the concrete

surface pad. The storage volume in the aerobic zone was 1.34 m3 with a depth of 0.5 m.

Six native species were planted in 0.2-m2 (2.25-ft2) spacing intervals. These intervals

were selected based on the growing habit of the selected plants. The anaerobic zone

included boneset (Eupatorium perfoliatum), spiderwort (Tradescantia ohiensis), and

culver’s root (Veronicastrum virginicum), while purple love grass (Eragrostis spectabilis),

Indian grass (Sorghastrum nutans), and purple coneflower (Echinacea purpurea) were planted in the aerobic zone. Design, construction, and hydraulic characteristics of the biphasic rain gardens have been described in detail previously (Yang et al., 2009).

37 Performance Under Actual Runoff Conditions

Actual rainfall events that occurred when temperatures were above 0oC were monitored from April 2008 to December 2009. The precipitation measurements are taken at the Ohio State University’s Weather Station (Wooster, OH) located 1.5 km away from the experimental site using a tipping bucket rain gauge that recorded at 5-min intervals.

This information was used to calculate the total volume of runoff generated in the drainage area. Inflow rates of runoff into the rain garden and outflow rates through a final discharge pipe from the aerobic zone (aerobic zone effluent) were measured at 5-min intervals with tipping-bucket rain gauges and logged on a CR3000 data logger (Campbell

Scientific, Logan, UT).

Actual rainfall events monitored during the study were categorized as light (<6 mm), medium (6-12 mm), or heavy (>12 mm) in a 24-hour period. Actual runoff data (i.e. runoff volume and peak flow) collected from the influent and aerobic zone effluent from the final discharge pipe were compared to evaluate hydraulic performance of the biphasic rain gardens. Monthly pan evaporation data were obtained from the Weather Station. This information was used to calculate the volume reduction through evaporation between the runoff events and compared actual volume reduction observed from the rain gardens.

Performance Under Simulated Runoff Conditions

Two types of simulated runoff conditions were applied in the summer months to the two replicate biphasic rain gardens. The 2008 runoff was made to represent agricultural conditions and the 2009 runoff was made to represent urban conditions.

38 Field evaluation of simulated agricultural runoff consisted of applying nitrate-N,

phosphate-P, and atrazine three different times five days apart (Table 3.1). Prior to the

experiments, the anaerobic zones were drained. For the first two runoff events, each

runoff pollutant solution was prepared in a 1-L volume, and then applied to the concrete

surface pad. One minute later, four oscillating sprinklers were turned on to provide the

rainfall to generate the simulated runoff. For each event, rainfall was applied for 1 h at a

constant rate to achieve a total simulated runoff volume of 0.79 m3 or 12.7 mm (0.5 in.)

rainfall. No pollutants were applied for the third simulated runoff and rainfall was

continuously applied for 24 h. For the first 4 h during the third event, the rainfall was

applied at approximately 1.57 m3/h as peak flow rate and was steadily decreased to 0.24

m3/h at the end of the event. Overall, a total of 12.7 m3 or 180 mm of rainfall was applied

during the third event (Table 3.1).

The spiked concentrations of pollutants in the first two runoff events and rainfall size

of the third runoff event are an exaggeration of real conditions. However, it was

necessary to provide an assessment of both hydraulic and pollutant removal performance

under “near worst case” scenarios. Since most atrazine losses from agricultural fields usually occur during the first significant rainfall event after application (Thurman et al.,

1991; Donald et al., 1998), the third runoff conditions were necessary for evaluating potential leaching of atrazine from the rain gardens. Between the simulated runoff events, sand bags and impervious liners were used to block any further runoff into the rain gardens caused by natural precipitation events. Natural precipitation on the rain gardens encountered during the experiments was estimated to be less than 3% of total simulated runoff volume. Runoff into the rain gardens and outflow from the anaerobic zone through

39 reverse pipes (anaerobic zone effluent) were measured at 30-min to 1-h intervals.

Outflow from the aerobic zone (aerobic zone effluent) was measured with the tipping-

bucket rain gauge as described above. Flow-weighted samples were taken from the

inflow as it entered the rain garden and from the effluent of each zone using 250 mL

plastic bottles at 30-min to 3-h intervals and stored at 4 °C until analysis.

Field evaluation of simulated urban runoff consisting of nitrate-N and phosphate-P,

and the herbicides dicamba, glyphosate, and 2,4-D was conducted by applying five

simulated runoff events once every seven days to the two replicate biphasic rain gardens

(Table 3.1). The anaerobic zones were drained prior to the first runoff event. Each runoff

pollutant solution was prepared in a 1-L volume, and then applied to the concrete surface

pad as described above. For each event, rainfall was applied for 1 h at a constant rate so

as to achieve a total surface runoff volume of 1.58 m3, equivalent to 25.4 mm (1.0 in.)

rainfall. The rainfall size and intensity used in this study were based on the weekly

average of precipitation from May to August (25.4 mm or 1.0 in.) and a 1-h intensity encountered as a 1-year return frequency storm (25.1 mm or 0.99 in.) in Wooster obtained from the hydro-meteorological design studies center of the National Oceanic and Atmospheric Administration (NOAA) weather service center (NOAA, 2009). Inflow and outflow from the anaerobic and aerobic zones were monitored and flow-weighted samples were taken from the influent and effluent of each zone as described above. Due to use of sand bags and impervious liners, direct input volume of natural precipitation into the rain gardens observed during the experiments was minimal (less than 5% of total simulated runoff volume).

40 Analytical Methods

Concentrations of nitrate-N and phosphate-P in the effluent of each zone were

analyzed at the Service Testing and Research (STAR) Laboratory located on the Ohio

State University’s Wooster campus (STAR Lab, 2009) using a Dionex DX-120 ion

chromatograph (APHA, 1992). Concentration of atrazine was measured using a Varian

3800 gas chromatograph equipped with a thermionic specific detector and a 30-m, 0.25-

mm inner diameter CP-Sil 8CB fused silica column (0.25 µm film thickness) using a

previously developed procedure (Yang et al., 2010 Submitted). Concentrations of

dicamba, glyphosate, and 2,4-D were analyzed after derivatization using an Agilent 6890

Gas Chromatograph equipped with a 5973 mass selective detector (Phelan et al., In preparation). Limits of detection were approximately 1.0 parts per billion (ppb) for all three herbicides.

To examine changes in pollutant removal efficiency over the repeated urban runoff events, regression analysis was performed on mean removal efficiency of each pollutant

using MINITAB v.15 (Minitab, Inc., State College, PA). Mean removal efficiency of

target pollutants was considered as a response variable and the number of runoff events

as a predictor. Regression slope was considered an indicator of increase (positive slope)

or decrease (negative slope) of removal efficiency over the events. When the p-value for

the regression slope was below 0.05, such changes were considered significant

(regression slope is significantly different from zero).

41 RESULTS AND DISCUSSION

Performance Under Actual Runoff Conditions

Measurable rainfall was recorded on 228 of the 600 d of the study (38%). Daily precipitation amounts ranged from 0.3 to 60.5 mm and cumulative precipitation measured during the study was 1453 mm (Figure 3.2). Between rainfall events, the anaerobic zone in each rain garden was partially or fully water saturated depending on the rainfall size and interval.

Figure 3.3 shows the average cumulative volumes and the flow rates of influent and effluent (aerobic zone effluent) of the rain gardens from a representative rainfall event within each medium and heavy precipitation category. No measurable effluent was observed during light rainfall events (< 6 mm) due to the storage capacity of the anaerobic zone. During the selected representative medium and heavy rainfall events, the biphasic rain gardens contributed to a significant reduction in both peak flow rate (84% for the medium rain and 88% for the heavy rain) and volume (59% for the medium rain and 54% for the heavy rain). The effluent from the aerobic zone in the rain gardens also exhibited a delayed response to each representative rainfall event. The average lag time between the start of inflow and the appearance of the aerobic zone effluent was approximately 180 min for the medium rain and 80 min for the heavy rain.

In general, the hydraulic performance of the biphasic rain gardens was affected by initial water conditions in the anaerobic zone. A greater reduction in both peak flow and volume was observed when the anaerobic zone was less saturated because of longer rainfall intervals and/or high ambient temperatures with high evapotranspiration rates.

The monthly pan evaporation obtained from the Weather Station was 12.79 ± 2.57 cm

42 (mean ± one standard error). Direct overflow/bypass of runoff without delay or retention

in the rain gardens never occurred during the 600 d of the study.

Performance Under Simulated Agricultural Runoff Conditions

Hydraulic Performance. No measurable effluent from the anaerobic zone was observed during the first two runoff events (12.7 mm rainfall or 0.79 m3 volume for each) due to the designed storage capacity in the anaerobic zone. Figure 3.4 shows the average cumulative volume and the flow rates of influent and effluent of the rain gardens during the third event. The cumulative volume of the influent for the third event was approximately 12.6 m3 (180 mm rainfall) with a peak flow rate of 1.57 m3/h during the

initial 4 h rainfall event. Although the applied amount of rainfall was 4-times higher than the designed storage capacity for the rain gardens, the biphasic rain gardens effectively

reduced both peak flow rate and volume. The peak flow rate of the anaerobic zone

effluent was approximately 0.57 m3/h at 6 h after initiation of the rainfall event and lasted

for about 11 h. For the first 8 h, the aerobic zone effluent rate gradually increased to

approximately 0.54 m3/h as peak flow rate and then after 16 hours steadily decreased to

0.20 m3/h at 24 h. The cumulative volume of each effluent at the end of the study was

9.62 m3 for the anaerobic zone and 9.07 m3 for the aerobic zone. The peak flow and

volume reduction between influent and the aerobic zone effluent were 67% and 28%,

respectively. Direct overflow/bypass of runoff without delay or retention in the rain

gardens never occurred during this 180 mm/24 h rainfall event.

43 Pollutant Removal Performance. Concentrations of pollutants in influent and effluent of the biphasic rain gardens were monitored during the third agricultural runoff event to evaluate pollutant removal efficiency. To allow direct comparison of the concentration profile for each pollutant, the data were normalized so the area under the curve was equal to unity. Overall mass recoveries (%) of the runoff pollutants were also performed by integrating hourly flow rate and concentration data collected from the aerobic zone effluent.

The effluent collected from the aerobic zone is plotted in Figure 3.5a for normalized

mass and Figure 3.5b for cumulative mass recovery of each pollutant as a function of

time after start of the third runoff event. Dissolved atrazine was significantly retained in

the two replicated rain gardens due likely to significant adsorption, whereas effluent concentrations of nitrate-N remained relatively high and pulse peaks were reached early

at 4 h after start of the third runoff. This indicates that dissolved nitrate-N can be easily

transported through the rain gardens if runoff is contiuous (Figure 3.5a). Higher than

expected mass recovery of nitrate-N (63.2 ±7.5%; mean ± one standard error) was

observed for the biphasic rain gardens even though the rain gardens employed a designed

anaerobic zone for biological denitrification (Figure 3.5b). Although simulated nitrate-N

in the first two runoff events was retained relatively longer in the anaerobic zone until the

third runoff event, there seems to be potential source of dissolved nitrate-N from the soil

medium through mineralization and nitrification of other forms of N that had a negative

effect on the overall removal efficiency of the nitrate-N under our “near worst case”

runoff conditions. Similar results were observed in the biphasic rain gardens during

44 hydraulic tracer tests, indicating potential leaching risk of dissolved nitrate-N from the

soil medium if runoff is continuous and of high volume (Yang et al., 2009).

No phosphate-P (data not shown) and only 9.4 % of atrazine (Figure 3.5b) were

recovered from the aerobic zone effluent, indicating effective retention in the rain

gardens through sorption, degradation, and/or precipitation processes. Under the

continuous runoff conditions (i.e. 180 mm rainfall/24h), leaching of the retained

phosphate-P and atrazine was not observed from the rain gardens. A limited number of

atrazine applications are annually used in agricultural areas (Thurman et al., 1991;

Donald et al., 1998) and the biotransformation processes would faciliate atrazine removal

by soil media in the long-term (Kaufman and Kearney, 1970; Mougin et al., 1994;

Bouquard et al., 1997; Martinez et al., 2001). Thus, the use of the biphasic rain garden

has a potential to effectively remove atrazine in agricultural runoff conditions over long

time periods without accumulation of atrazine in the soil medium. However, the soil

medium’s capacity to capture phosphate-P may be less efficient in the long-term due to

the potential accumulation of phosphate in the soil medium.

Performance Under Simulated Urban Runoff Conditions

Hydraulic Performance. The cumulative volume of the influent for each event at a

constant flow rate for 1 h was 1.58 m3 (approximately equivalent to 25.4 mm rainfall).

No effluent from the rain gardens was observed during the first runoff event. Hydraulic

performance of the rain gardens was consistent for the remaining runoff events, showing significant reduction of both runoff volume and peak flow (Figure 3.6a). The peak flow rate from the rain gardens over the events was 0.34 ± 0.01 m3/h (mean ± one standard

45 error) for the anaerobic zone effluent and 0.27 ± 0.02 m3/h for the aerobic zone effluent.

The cumulative effluent volumes from the anaerobic and aerobic zones for each event

were 1.09 ± 0.096 m3 and 0.84 ± 0.07 m3, respectively. The peak flow and volume

reduction between influent and the aerobic zone effluent for each event were

approximately 83% and 47%, respectively. The high volume reduction achieved for each

runoff event was most likely due to increase in the evapotranspiration rates with high

ambient temperatures during the experiments even though the anaerobic zone was fully

water saturated after each event. The weekly pan evaporation obtained from the Weather

Station was 4.16 ± 0.16 cm (mean ± one standard error). The differences between the estimated volume reductions through evaporation and the actual volume reductions observed from the rain gardens were less than 10% for all runoff events.

Pollutant Removal Performance. Rain garden influent and effluent concentrations of pollutants were compared to determine removal efficiency of each pollutant for the five sequential runoff events. Cumulative loads and effluent concentrations of the pollutants from the anaerobic and aerobic zones in the rain gardens are presented in Figure 3.6 and the overall results of net removal of each pollutant are summarized in Table 3.2.

High nitrate-N removals of 78 to 91% were observed in the biphasic rain gardens and these removal efficiencies were consistent without any significant difference among the events except for the first event (no effluent) (Table 3.2). Nitrate-N effluent concentrations were between 1.6 to 9.5 mg/L from the anaerobic zone and 0.7 to 6.9 mg/L from the aerobic zone (Figure 3.6b), indicating that nitrate-N removal occurred primarily in the anaerobic zone with some additional removal in the aerobic zone. This

46 agrees with the findings by Kim et al. (2003) and Dietz and Clausen (2006) who

investigated the effect of the presence of water-saturated conditions on nitrate removal in

rain gardens. It was concluded that the partially saturated conditions in the bottom of the

rain garden increases nitrate-N removal through denitrification although it was only tested at relatively low loadings (Kim et al., 2003). In this study a weekly average rainfall size (25.4 mm rainfall for 1 h) was chosen to reproduce actual rainfall events. However, the spiked input of nitrate-N was chosen to be high enough to represent “near worst case” condition of nitrate pollution. The results demonstrate that the biphasic rain garden systems can effectively remove high levels of nitrate-N by holding runoff in the anaerobic zone with significantly increased retention time. These results also support the importance of hydrodynamic conditions (i.e. rain size and intensity) that control retention times of nitrate-N in the anaerobic zone of the rain gardens, and sometimes negatively affect overall nitrate-N removal efficiency if runoff is continuous and of high volume as previously discussed in the agricultural runoff conditions.

Over the entire study period of the urban runoff conditions, phosphate-P was highly retained in the two replicate rain gardens, achieving between 94 to 99% removal (Table

3.2). Sorption of phosphate-P by the soil media in the rain gardens is assumed to be the primary factor for the retention of phosphate-P in stormwater runoff. Even though removal efficiency of phosphate-P was constant over the events, its retention in the rain gardens should be investigated over longer time periods to ascertain the capacity for pollutant accumulation within the soil medium.

Herbicide concentrations were significantly reduced in the effluent for each event and peak concentrations were below the National Primary Drinking Water Regulations -

47 Maximum Contamination Protection Level (MCL) standards except 2,4-D for all runoff events, indicating only a small percentage of the applied herbicides leached from the rain gardens (Figure 3.6). There was approximately 99% removal of glyphosate for each event, and removal of dicamba and 2,4-D was between 81 to 92% (Table 3.2). Since glyphosate has a high affinity to bind to soils compared to other two herbicides (Nowack, 2003), it is most likely that adsorption is the primary process for glyphosate removal in the short- term. For a long-term removal process for glyphosate, several studies also demonstrate that glyphosate can be degraded by soil microorganisms using either as a C, N, and/or P source to result in complete mineralization (Dick and Quinn, 1995; Krzysko-Lupicka and

Orlik, 1997; Haney et al., 2000). Although dicamba and 2,4-D are very water soluble and have the potential to be highly mobile (Grover, 1977; Tu et al., 2001), fairly effective and constant removal efficiencies were observed in the biphasic rain gardens over all runoff events. This is mainly due to the fact that the biphasic rain gardens held runoff in the anaerobic zone until next runoff event, allowing more time for chemical and biological degradation, which can require several days to remove most of the herbicides (Stearman et al., 2003). The results suggest that biphasic rain gardens have the potential to effectively remove all three herbicides (i.e. dicamba, glyphosate, and 2,4-D) for urban runoff conditions if a certain retention time of runoff is maintained for the removal processes in the rain gardens.

CONCLUSIONS

This 2-year field study found that the biphasic rain gardens contributed to a significant reduction in both peak flow and runoff volume over various rainfall events (i.e.

48 0.3–180 mm rainfall). The biphasic rain gardens were highly effective in removing

nitrate-N (~91%), phosphate-P (~99%), atrazine (~90%), dicamba (~92%), glyphosate

(~99%), and 2,4-D (~90%) under high level of pollution loadings simulated in both

agricultural and urban runoff events.

Overall the results of this study demonstrated the importance of increased pollutant

retention time (determined by design configuration, rainfall size, intensity, and interval)

that significantly affects overall nutrient (i.e. nitrate-N) and herbicide removal in rain

gardens. Since the sequence of anaerobic to aerobic (biphasic) conditions created in the

biphasic rain gardens can significantly increase overall retention time of runoff pollutants

and can be maintained in improved environmental conditions for bioremediation of

runoff pollutants (e.g. denitrification) by holding runoff until next event, the biphasic rain garden has the potential for being an effective best management practice in reducing flow and pollutant loads for both agricultural and urban runoff conditions.

ACKNOWLEDGEMENTS

This study was supported by the Center for Urban Environment and Economic

Development of the Ohio State University and by the Ohio State University Alumni

Grants for Graduate Research and Scholarship, and by funds appropriated to the Ohio

State University and the Ohio Agricultural Research and Development Center. The authors thank Dr. Sun-Jeong Park, Darlene C. Florence, Aaron Rehm, Zachary Robinson,

David Ellsworth, and Janna Pearson for their assistance in this study.

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53 Parameters Agricultural Runoff a Urban Runoff b Rainfall size (mm) 12.7 180 25.4 Rainfall duration (h) 1.0 24.0 1.0 Runoff volume (m3) 0.79 11.2 1.58 Pollutants Average Influent Concentration (mg/L) c Nutrients Nitrate 50.0 - 20.0 (NO3-N) Phosphate 20.0 - 10.0 (PO4-P)

Herbicides Atrazine 0.16 - - (C8H14ClN5) Dicamba - - 0.09 (C8H6Cl2O3) Glyphosate - - 2.0 (C3H8NO5P) 2,4-D - - 0.3 (C8H6Cl2O3)

a A total of three rainfall events, first two 12.7 mm rainfalls with simulated runoff pollutants and then 180 mm rainfall without runoff pollutants were applied at 5 day intervals. b A total of five rainfall events were applied at 7 day intervals. For each event, 25.4 mm rainfall with simulated runoff pollutants was applied. c Average influent concentrations were calculated based on runoff volume for each event.

Table 3.1. Composition of simulated stormwater runoff

54 Average Maximum Net Mass Removal (%) c Influent Effluent Pollutant Concentration Concentration Simulated Runoff Event (mg/L) a (mg/L) b 1st 2nd 3rd 4th 5th Nitrate 82 78 85 91 20 6.9 - (NO3-N) ± 10.0 ± 7.9 ± 2.0 ± 1.8 Phosphate 99 99 98 94 10 2.5 - (PO4-P) ± 0.1 ± 0.8 ± 1.4 ± 5.1 Dicamba 87 84 92 92 0.09 0.05 - (C8H6Cl2O3) ± 10.1 ± 11.4 ± 3.9 ± 4.8 Glyphosate 99 99 98 99 2.0 0.08 - (C3H8NO5P) ± 0.1 ± 0.1 ± 1.1 ± 0.1 2,4-D 85 81 86 90 0.3 0.25 d - (C8H6Cl2O3) ± 7.3 ± 7.5 ± 1.7 ± 3.9

a Average influent concentrations were calculated based on runoff volume (1.58 m3) of each event. b Monitored from the aerobic zone effluent (final discharge pipe). c Mass removal efficiency (%) for each runoff event. Mean ± one standard error. d Approximately 10% of runoff volume exceeded maximum contamination level of 2,4-D (0.07 mg/L) for each event.

Table 3.2. Net mass removal (%) of pollutants from five sequential urban runoff events in the biphasic rain gardens

Figure 3.1. The monitored experimental biphasic rain garden system (a), adjacent to a concrete surface runoff pad on the Ohio State University’s Wooster campus, and three- dimensional drainage configuration (b).

56 70 1500

60 (a) (b) 1200 50 900 40

30 600 20 300 Precipitation (mm) Precipitation 10

0 0 Cumulative precipitation (mm) precipitation Cumulative 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Days after the study initiation Days after the study initiation

Figure 3.2. Daily precipitation (a) and cumulative precipitation (b) during the experimental study period (1 April 2008 through 1 December 2009). Measurements are taken at The Ohio State University’s Weather Station in Wooster, Ohio located 1.5 km away from the experimental site using a tipping bucket rain gauge.

57 (a) Medium (10.2 mm rainfall)

) 1.5 2.0 3

/h) 1.5 1.0 3

1.0 Influent Effluent 0.5 0.5 Flow(m rate

Cumulative volume (m volume Cumulative 0.0 0.0 0246810 0246810 Time after initial rainfall (h) Time after initial rainfall (h) (b) Heavy (23.6 mm rainfall)

) 1.5 2.0 3

/h) 1.5 1.0 3 Influent Effluent 1.0

0.5 0.5 Flow rate (m

Cumulative volume (m 0.0 0.0 0 6 12 18 24 0 6 12 18 24 Time after intial rainfall (h) Time after initial rainfall (h)

Figure 3.3. Cumulative volume and flow rate of influent and effluent hydrographs from the biphasic rain gardens for selected representative medium (10.2 mm) (a) and heavy (23.6 mm) (b) rain events recorded at 5-min intervals. Values are averages of measurements taken at the two replicate rain gardens using tipping bucket rain gauges.

58 14 (a) Volume

12 ) 3 10

4 Cumulative volume (m volume Cumulative 2

0 0 6 12 18 24

1.8

1.6 (b) Flow rate

1.4

/h) 1.2 3 1.0 Influent Anaerobic zone effluent 0.8 Aerobic zone effluent

0.6 Flow rate (m 0.4

0.2

0.0 0 6 12 18 24

Time after start of the 3rd runoff event (h)

Figure 3.4. Cumulative volume (a) and flow rate (b) of influent and effluent hydrograph from the rain gardens for the 3rd simulated runoff event (180 mm/24 h rainfall). Values are averages of measurements taken at the two replicate rain gardens. Error bars represent one standard error.

59 0.15 (a) Normalized mass

0.12 Biphasic rain garden 1 NO3-N 0.09 Atrazine Biphasic rain garden 2 0.06 NO3-N Atrazine Normalized mass Normalized 0.03

0.00 0 6 12 18 24 100 Time after start of the 3rd runoff event (h) (b) Mass recovery 80

Mass recovery (%) Mass recovery 20

0 0 6 12 18 24 Time after start of the 3rd runoff event (h)

Figure 3.5. Normalized mass (a) and cumulative mass recovery (b) of nitrate-N and atrazine in effluent from the aerobic zone of the biphasic rain gardens for the 3rd simulated runoff event (180 mm/24 h rainfall).

60 ) 3 0.5 10 /h)

3 0.4 (a) Flow rate and volume 8 0.3 6 0.2 4 0.1 2

Flow rate (m 0.0 0

0 7 14 21 28 (m volume Cumulative 10 200 Time (d) 8 (b) NO3-N 150 6 100

-N (mg/L) 4 3 2 50 NO 0

0 (g) load Cumulative 0 7 14 21 28 100 1.0 80 0.8 g/L) (c) Dicamba Time (d)  60 0.6 40 0.4 20 0.2 Dicamba ( Dicamba 0 0.0 Cumulative load (g) load Cumulative 0 7 14 21 28 300 20 X Data

g/L) 250

 (d) Glyphosate 15 200 150 10 100 5 50

Glyphosate ( Glyphosate 0 0 Cumulative load (g) load Cumulative 0 7 14 21 28 500 3.0 X Data 400 (e) 2,4-D 2.5

g/L) 2.0

 300 1.5 200 1.0

2,4-D ( 100 0.5 0 0.0 Cumulative load (g) load Cumulative 0 7 14 21 28 Time (d) Anaerobic zone effluent Influent Anaerobic zone effluent Aerobic zone effluent Anaerobic zone effluent Aerobic zone effluent Aerobic zone effluent

Figure 3.6. Flow rate and cumulative volume of influent and effluent hydrograph (a), and cumulative loads and effluent concentrations of nitrate-N (b), dicamba (c), glyphosate (d), and 2,4-D from the anaerobic and aerobic zones of the biphasic rain gardens for five sequential rainfall events. Values are averages of measurements taken at the two replicate rain gardens. For each event, 25.4 mm rainfall was applied with simulated runoff pollutants over a 1 h at a constant influent rate of 1.58 m3/h.

61 CHAPTER 4

NUTRIENT AND ATRAZINE REMOVAL FROM SIMULATED

URBAN AND AGRICULTURAL RUNOFF IN COLUMN-SCALE

MONOPHASIC AND BIPHASIC RAIN GARDENS

ABSTRACT

Rain gardens are bioretention systems that have the potential to reduce peak runoff

flow and improve water quality in a natural and aesthetically pleasing manner. We

compared hydraulic performance and removal efficiencies of nutrients and atrazine in a monophasic rain garden design versus a biphasic design at a column scale using simulated urban and agricultural runoff. The biphasic rain garden was designed to increase retention time and removal efficiency of runoff pollutants by creating a sequence of anaerobic to aerobic conditions. We also evaluated the effect of C substrate availability

on pollutant removal efficiency in the biphasic rain gardens. Five simulated runoff events

with various concentrations of runoff pollutants (e.g. nitrate-N, phosphate-P, and atrazine

(6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine)) were applied to the

monophasic and biphasic rain gardens once every five days. Hydraulic performance was

consistent over the five simulated runoff events. Peak flow was reduced by

approximately 56% for the monophasic design and 80% for the biphasic design. Both

62 rain garden systems showed excellent removal efficiency of phosphate-P (89 to 100%)

and atrazine (84 to 100%). However, significantly (p<0.001) higher removal of nitrate-N

was observed in the biphasic (42 to 63%) compared to the monophasic rain garden

system (29 to 39%). Addition of C substrate in the form of glucose increased removal

efficiency of nitrate-N significantly (p<0.001), achieving up to 87% removal at a

treatment C/N ratio of 2.0. This study demonstrates the importance of retention time,

environmental conditions (i.e., anaerobic/aerobic conditions), and availability of C substrate for bioremediation of pollutants, especially nitrates, in rain garden systems.

INTRODUCTION

In recent years, the negative consequences of excessive stormwater runoff generated from urban and agricultural areas have increasingly caused concern. This excessive runoff not only degrades surface water resources by altering hydrologic patterns in natural streams and groundwater, but also increases loads of pollutants such as nutrients

(e.g. N and P), sediments, metals, pesticides, and pathogenic bacteria (USEPA, 1997;

Eriksson et al., 2007). Recognition of these consequences has led to the development of a variety of stormwater best management practices such as retention basins, stormwater wetlands, and rain garden systems (USEPA, 2000; Davis, 2005).

Rain gardens and other bioretention systems are relatively new best management practices designed to remove pollutants from stormwater runoff while providing an aesthetically pleasing landscape (Davis, 2005; Dietz, 2007). Rain gardens generally consist of a porous soil medium layer, mulch layer, and vegetation. They can be distributed across the landscape to capture pollutants using constructed soils and an

63 assemblage of plants that facilitate water transpiration and removal of runoff pollutants

(Davis et al., 2001). Pollutant removal mechanisms include sedimentation, adsorption,

filtration, biological transformation (decomposition), and precipitation (Prince George’s

County, 1993).

In studies to date, rain gardens have proven to reduce runoff volumes and concentration of multiple runoff pollutants (Davis, 2005; Dietz, 2007; Davis et al., 2009).

However, other problems and questions regarding their treatment efficiency remain.

While high removal efficiencies of total suspended solids and metals (e.g. Cu, Pb, and

Zn) have been reported, removal efficiencies of nutrients (i.e. nitrate-N and total P) are low and inconsistent (Davis et al., 2001; Hsieh and Davis, 2005; Dietz and Clausen, 2006;

Sun and Davis, 2007; Maurakami et al., 2008). Furthermore, most research on rain gardens has focused on low concentrations of runoff pollutants that are sometimes below the National Primary Drinking Water Regulations - Maximum Contamination Protection

Level standards. Potential applications of rain gardens for removal of pollution at “hot spots” require evaluation of removal efficiency of multiple runoff pollutants at high concentrations.

The efforts to improve removal efficiency of runoff pollutants in rain gardens have been focused on the development of (1) alternative design configurations and (2) different soil media compositions (Dietz, 2007; Davis et al., 2009). For example, Kim et al. (2003) developed a design to increase biological processes by creating water unsaturated conditions in the top and then partially saturated conditions in the bottom of the rain garden. This is done by setting up the underdrain pipe elevation higher than a traditional underdrainage configuration. The water saturated conditions, which impede

64 the diffusion of oxygen, create a favorable oxidation-reduction (redox) potential for denitrification where nitrate-N is converted to nitrogen gas (Maier et al., 2000). This modified design was tested in a field-scale study and was found to improve removal efficiency of nitrate-N (Dietz and Clausen, 2006). However, only a portion of the first flush can be retained in the saturated zone at the bottom of the system and nitrates may not undergo complete biological denitrification if discharged without sufficient retention time. Hsieh et al. (2007) reported that increasing the retention time by using lower permeability media increased nitrate-N removal within the traditional underdrainage approach at a column scale. However, this may cause overflow of the rain garden if large runoff occurs in a short period of time because of low infiltration rate of the soil media

(Cho et al., 2009). Therefore, development of alternative rain garden systems and soil media for bioremediation of runoff pollutants is necessary.

One of the possible options to achieve high removal efficiency of multiple runoff pollutants is a biphasic system. Such a system, also known as a sequencing batch reactor, can involve a sequence from anaerobic to aerobic conditions. Wastewater treatment plants have commonly removed multiple organic pollutants, nitrogen, and phosphorous by providing various environmental conditions and electron acceptors that support a wide array of degrading microorganisms (Rittmann and McCarty, 2001). In particluar, two separate units (i.e., one for anaerobic and another for aerobic conditions) have been found to be very effective for N removal via aerobic nitrification and anaerobic denitrification.

We adapted this concept in field-scale rain garden systems (Grewal et al., 2009) and have reported hydraulic tests with bromide tracer and simulated runoff pollutants (nitrate-N, phosphate-P, Cu, Pb, and Zn) under a simulated contiuous rainfall (Yang et al., 2009).

65 Results of the tracer tests revealed that dissolved nitrate-N was easily transported through

the newly established biphasic rain gardens without any significant sorption and/or

denitrification even though the biphasic system consisted of anaerobic conditions for

biological denitrification. This indicates potential leaching risk from the soil medium if

runoff is continuous and of high volume. Since removal of soluble nitrate-N and other

runoff pollutants through rain garden systems can be greatly affected by hydrodynamics

(e.g. retention times), environmental conditions (e.g. anerobic/aerobic conditions), and

available C substrates for biological processes, further investigation of these parameters is necessary for improved treatment efficieny in rain gardens.

In response to these needs for optimization of rain garden design, we conducted a laboratory study of 42 rain garden columns. We simulated urban and agricultural runoff applied to monophasic and biphasic rain gardens to compare hydraulic performance and removal efficiencies of nutrients and atrazine. We also evaluated pollutant removal efficiency from the biphasic rain garden as affected by C substrate availability. Multiple loads of high concentrations of nutrients (nitrate-N and phosphate-P), and the herbicide atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) were evaluate in simulated urban and agricultural runoff.

MATERIALS AND METHODS

Column Design and Set-up

Non-vegetated rain garden columns were constructed to compare pollutant removal efficiencies and hydraulic performance of two design configurations (Figure 4.1). One design consisted of a biphasic (anaerobic to aerobic) sequence with two polyvinyl

66 chloride (PVC) columns (Column 1; 91.4 cm long × 5 cm inner diameter and Column 2;

60.9 cm long × 5 cm inner diameter) (Figure 4.1a). Anaerobic conditions were created in

Column 1 by setting an effluent port at the level of the packed soil medium to increase retention time for sustained water saturated conditions. After Column 1, aerobic conditions were created in Column 2 with an underdrainage configuration at the bottom

of the soil medium (Figure 4.1a). The monophasic design (Figure 4.1b) also consisted of

two columns—each constructed using a traditional underdrainage approach. For direct

comparison of the two designs, the column set-up used in the biphasic design (i.e., soil

medium depth and number of columns) was applied to the monophasic design, creating

aerobic to aerobic conditions.

In both designs, 200 g of fine gravel (3.2 mm - 12.7 mm) was placed at the bottom of

the columns to a 5-cm depth. The columns were then packed with an air-dried soil

medium that was a mixture of sand, topsoil, and compost (6:2:2 volume ratio) obtained from Kurtz Bros. Inc., Cleveland, OH. The soil medium consists of 90.6% sand, 6.9% silt, and 2.5% clay with 0.7% organic matter and pH of 7.2. The physical and chemical characteristics of the soil medium have been described in detail elsewhere (Yang et al.,

2009). A total of 1,780 g of the air-dried soil medium were packed in Column 1, producing a 55.9-cm soil depth and 808 g of the air-dried soil medium in Column 2, producing a 25.5-cm soil depth with a bulk density of 1.57 Mg/m3 in both columns. Total

bed depth of each column was 60.9 cm for Column 1 and 30.5 cm for Column 2, leaving

30.5 cm of extended ponding depth. Before the main experiments, 1-L of deionized water

was applied four times to each column at 2-day intervals as a pretreatment. Effluents

from the pretreatments were analyzed for the targeted pollutants (e.g. nitrate-N,

67 phosphate-P, and atrazine) to estimate the leaching potential from the soil medium. Prior to the main tests, both Column 1 and Column 2 were drained.

Effect of Design

All experiments consisted of applying five simulated runoff events once every five

days to column sets constructed to represent both monophasic and biphasic rain garden

designs. For each runoff event, 1-L of simulated runoff with either zero (i.e. the control),

low or high concentration of urban and/or agricultural pollutants without additional C

substrates was applied to Column 1 over a 1-h period at a constant rate of 16.7 mL/min

(Table 4.1). This created a total of 18 rain garden experimental units (i.e. three pollutant

levels by two rain garden designs and three replicates for each combination). The 1-L

runoff volume corresponds to the amount that would be generated when a 25.4 mm

rainfall occurs and the rain garden surface area is 5% that of the drainage area. Column 1

effluent from each design was collected and volume was measured initially at 30-min and

over time at 2-h intervals. After Column 1 effluent was completely drained, final effluent

volume of Column 1 was measured and an aliquot sample representing less than 3% of

the total volume was taken using a 30 ml plastic bottle. The remaining effluent from

Column 1 was then applied to Column 2 over a period of 5 - 10 h as determined by the

drainage time of Column 1 for each design. Total effluent volume of Column 2 was

measured and an aliquot sample was taken. A total of 180 samples were collected and

stored at 4 °C until further analysis.

68 Effect of Treatment C/N Ratios

Glucose (Sigma Chemical Co., St. Louis, MO) was used as the C source to evaluate the effect of treatment C/N ratios on pollutant removal efficiency. Different treatment

C/N ratios of 0, 0.5, 1.0, and 2.0 (Table 4.1), based on initial input concentration of nitrate-N, were applied via simulated runoff to only biphasic rain garden columns. As in the previous experiment, the simulated runoff contained either a high or a low level of pollutants. Five runoff events were applied, as described above, with each event taking place five days after the last event was completed. For each event, samples were obtained from Columns 1 and 2 effluents and stored at 4 °C until further analysis. All of the experiments were conducted in triplicate and 240 samples were collected.

Analytical Methods

Concentrations of nitrate-N and phosphate-P in the effluent of each column were analyzed at the Service Testing and Research (STAR) Laboratory (STAR, 2009) using a

Dionex DX-120 ion chromatograph (APHA, 1992). Concentration of atrazine (Chem

Service, Inc., West Chester, PA) was measured using a Varian 3800 gas chromatograph

(GC) equipped with a thermionic specific detector (TSD) and a 30-m 0.25-mm I.D. CP-

Sil 8CB fused silica column (0.25 µm film thickness) (Varian Inc., Palo Alto, CA).

Atrazine was extracted from runoff samples using toluene. One mL of toluene was mixed with 1 mL of sample and the mixture shaken vigorously for 3 min. One µL of extracted sample was injected into the gas chromatograph. The oven temperature was initially maintained at 90 °C for 1 min and programmed to increase at 15 °C/min to 160 °C and

69 then 25 °C/min to 270 °C with an injector temperature of 280 °C. Helium was used as the carrier gas at a flow rate of 28 mL/min.

All statistical analyses were conducted using MINITAB v.15 (Minitab, Inc., State

College, PA). Mean percentages of removal were compared between treatments (e.g. design and C/N ratios) using one-way analysis of variance and followed by pair-wise comparison using Tukey’s honestly significant differences at an alpha level of 0.05.

Comparisons of treatments were conducted for separate runoff events. Total net removal values for the study are presented, but were not subjected to statistical analysis due to the limited number of data points.

RESULTS AND DISCUSSION

Hydraulic Performance

Effluent rates were monitored to investigate hydraulic performance of two rain garden systems during five sequential runoff events. Prior to the tests, each rain garden system was initially drained. The cumulative volume of the influent for each event was

1.0 L (approximately equivalent to 25.4 mm rainfall) at a constant flow rate of 16.7 mL/min. Approximately 55% of runoff volume reduction was observed in Column 1 of the biphasic system for the first runoff event due to the initial storage conditions that existed in the columns for this system (Figure 4.2a). No significant runoff volume reduction (~5%) was observed for the two rain garden systems with the remaining runoff events. However, significant reduction in peak flow rate, compared to influent rate was observed in both systems (Figure 4.2b). In the monophasic design, Column 1 effluent rate gradually increased to a peak flow rate of approximately 442 mL/h during the first 1 h

70 and then steadily decreased. Drainage of the columns was essentially complete after 5 h.

In the biphasic design, Column 1 effluent rate gradually increased to approximately 200

mL/h for the first 0.5 h and then steadily decreased. Drainage was completed after 10 h.

The different hydraulic flow profiles between the first runoff and the later runoff events

were observed in the biphasic system due to the initially drained and storage conditions

that existed in Column 1 at the time of the first simulated runoff treatment. The average

peak flow reduction over the five events between influent and Column 1 effluent was

56% for the monophasic design and 80% for the biphasic design. The reason for this

difference can be attributed to different design configurations. According to the Darcy’

law, Column 1 effluent rate was mainly controlled by the total head difference that is the

sum of the gravity head difference (Δz) and pressure head difference (Δh). Since no gravity head difference existed in Column 1 of the biphasic system, due to setting the effluent port at the same level as the packed soil medium, the biphasic system had a relatively smaller total head difference and was more effective in buffering peak flow compared to the monophasic system.

Effect of Design

Five simulated runoff events without additional C substrates (e.g. a C/N ratio = 0) were applied to both the monophasic and biphasic rain garden systems. Influent and effluent of the two systems were compared and pollutant mass balances were determined at two different runoff conditions representing low and high concentration loading of nutrients and atrazine. Mean removal efficiency from Column 1 and 2 was calculated based on the initial input (influent) mass of each pollutant. Pretreatment and control

71 experiments without pollutant addition showed no leaching of phosphate and atrazine.

However, approximately 0.7 mg/L to 1.3 mg/L of nitrate-N were detected in the pretreatment and control columns indicating potential leaching of nitrate from the soil medium used in the study. The overall results of pollutant mass balances and removal from five sequential runoff events are summarized in Table 4.2.

Nitrate-N removal efficiency for both types of rain garden systems from five sequential runoff events are presented in Figure 4.3a for the low concentration loading and Figure 4.3b for the high concentration loading conditions. Significantly (p<0.001) higher removal of nitrate-N was observed in the biphasic rain garden system. There was approximately 60% removal for the low concentration loading and 34% removal for the high concentration loading conditions. In comparison, the monophasic rain garden system had approximately 37% removal for the low concentration loading and 23% removal for the high concentration loading conditions. Previous column and field-scale rain garden studies have reported that removal of nitrate-N tends to be low in conventional systems with an underdrainage approach (Davis et al., 2001; Hsieh and Davis, 2005; Dietz and

Clausen, 2006; Maurakami et al., 2008). This is mainly due to the fact that these systems

(1) do not provide biogeochemically favorable conditions for nitrate removal (i.e., denitrification) that requires relatively long retention time and anaerobic conditions, (2) contain soil media that typically do not provide sorption sites for the negatively charged nitrate ion, and (3) release nitrate from soil media through mineralization and nitrification of other forms of N. In the biphasic system, however, highly improved nitrate-N removal was observed, which occurred primarily in Column 1 with some additional removal in

Column 2. Two previous studies (Kim et al., 2003; Dietz and Clausen, 2006) have

72 demonstrated that the addition of a designed water saturated zone in rain garden systems

increases nitrate-N removal by creating favorable environmental conditions (i.e.

anaerobic conditions and partially increased retention time) for denitrification. Since

Column 1 in the biphasic system was maintained under anaerobic conditions with

significantly increased retention time by holding runoff until the next event, the biphasic

rain garden system has potential for improved nitrate removal compared to the

monophasic system. Overall removal efficiency of nitrate-N in both systems, however,

decreased at the high concentration loading conditions (Figure 4.3b). At the higher

nitrate-N loading condition, there seemed to be insufficient amounts of available C

substrates to support complete denitrification.

Removal efficiency of phosphate-P and atrazine at the high concentration loading

conditions for five runoff events are presented in Figure 4.3c for phosphate and Figure

4.3d for atrazine. No phosphate-P and atrazine were leached from either rain garden

system at the low concentration loading conditions, indicating 100% retention and/or

mineralization in both systems under the simulated urban runoff conditions (Table 4.2).

At the high concentration loading conditions, both designs were fairly effective in

removing phosphate-P (89 to 94%) and atrazine (84 to 89%) (Table 4.2 and Figure 4.3).

However, the removal efficiency of both systems decreased over the runoff events, indicating the soil medium’s capacity to capture phosphate-P and atrazine was becoming less effective.

Sorption of phosphate-P and atrazine by soil media is assumed to be the primary factor for the retention of phosphate and atrazine from stormwater runoff. Therefore, we

conducted preliminary batch sorption-isotherm experiments to evaluate the potential of

73 the soil medium for phosphate and atrazine sorption using previously developed procedures (Correia et al., 2007; Goyne et al., 2008). The data for the soil medium used

was best-fitted with the Langmuir equation for phosphate (adsorption constant (b) =

-1 2 0.104 ± 0.021; adsorption maxima (Qm) = 175 ± 9.233 mg PO4-P kg ; R = 0.986) and

the Freundlich equation for atrazine (equilibrium constant (Kf) = 2.368 ± 0.221; degree of

nonlinearity (n) = 1.159 ± 0.115; R2 = 0.976), indicating that the soil medium has an

affinity for both phosphate and atrazine.

A limited number of atrazine applications are used annually in agricultural areas and

most atrazine losses from agricultural fields usually occur during the first significant

rainfall event after application (Thurman et al., 1991; Donald et al., 1998). Therefore, the

simulated runoff conditions for atrazine used in this study (e.g. high concentrations and multiple loadings with short intervals) would not to be expected to occur frequently.

Furthermore, the biotransformation processes would facilitate atrazine removal by the soil media in the long-term. Consequently, only the time that would pass before the medium in the columns was saturated with for phosphate was estimated. Based on the adsorption maxima, Qm, obtained from the Langmuir equation and the amount of soil

packed in the columns, the operational life-time for the rain garden systems was

estimated to be 6.2 years under simulated runoff conditions defined by storms of 25.4 mm per 1 hour spaced at 10 day intervals with low P loading (2 mg/L of phosphate-P) and 1.3 years under the same storm conditions but with high P loading (10 mg/L of phosphate-P). However, development of soil media for improved phosphate removal over the long-term and under real world situations should be investigated. One possible solution would be to replace P-saturated material in areas of the rain garden system where

74 stormwater runoff first enters by designing easily replaceable cartridges or socks

(Faucette et al., 2009).

Effect of Treatment C/N Ratios

Due to the highly improved pollutant removal observed in the biphasic rain garden

system compared to the monophasic system, further investigation of C substrate effect on

pollutant removal was only focused on the biphasic system. Five runoff events with

additional C substrate in the form of glucose were applied to the biphasic rain garden

system at different treatment C/N ratios. The overall results of pollutant mass balances

and net mass removal are summarized in Table 4.3.

Nitrate-N removal efficiency was positively correlated with treatment C/N ratios; the

linear slopes increased for each successive runoff event (Figure 4.4). Similar results were

observed for the low loading rates (data not shown). The removal efficiency for nitrate-N for each runoff event clearly shows that nitrate-N removal significantly (p<0.001) increased as treatment C/N ratio increased (Figure 4.4). The magnitude of differences in

nitrate-N removal as C/N ratios increased was greater under the high concentration

loading conditions and in the 5th runoff event (range of removal, 33% to 87%; Figure

4.4b) compared to the low concentration loading conditions and also for the 5th runoff event (range of removal, 53% to 72%; Figure 4.4a). This difference may be explained by the amount of nitrate leached from the soil medium (Table 4.3). The proportion of leached nitrate-N to the amount of nitrate-N added for the low concentration loadings over five events (i.e. 25 mg nitrate-N total) was approximately 13%. For the high concentration loading (i.e. 250 mg nitrate-N total), this proportion was only about 1.5%.

75 The increase in removal efficiency in the 5th runoff event compared to the 2nd runoff

event (Figure 4.4) is most likely due to the buildup of an active denitrifying microbial

population over time. Kim et al. (2003) also reported that shredded newspapers as an

extra C source in soil media increased the removal of nitrate-N in a laboratory rain

garden system. Available C substrate coupled with anaerobic conditions (i.e., low redox

potentials) is clearly a major driving force for activating biological denitrification,

especially under high levels of nitrate loading.

The effect of available C substrate on phosphate-P removal was insignificant (p>0.05)

in the short-term performance with exception of the 5th runoff event for phosphate-P

removal (p<0.001, data not shown). Concentrations of phosphate-P were not observed in

Column 2 effluent over the first four runoff events under high level loading conditions.

At the last runoff event, phosphate-P removal efficiencies varied with different treatment

C/N ratios. In general, there was higher removal efficiency at the higher treatment C/N ratios. A previous field-scale study showed that the primary factors for the retention of

phosphate-P in a biphasic rain garden system are sorption and/or precipitation processes

(Yang et al., 2009). However, further long-term effect of C substrates on phosphate

removal should be evaluated for other potential mechanisms.

The effect of available C substrate on atrazine removal was not initially apparent but

then became evident during the 3rd to 5th runoff events (data not shown). The first two

events yielded runoff that did not contain atrazine at detectable levels. However, the

removal of atrazine for the 3rd to 5th runoff events (variable y in the equation below) was

not as effective and was linearly and positively related to treatment C/N ratios (y = 4.35 x

+ 76.4; R2 = 0.992 for the 5th event). Thus the soil medium’s capacity to sorb or degrade

76 atrazine was less efficient in the long-term than in the short-term. Several studies have demonstrated that a large variety of soil microorganisms are known to partially degrade atrazine by N-dealkylation or dehalogenation (Kaufman and Kearney, 1970; Mougin et al., 1994; Bouquard et al., 1997; Martinez et al., 2001). Also, microbial growth has been observed by using atrazine either as a sole C or N source to result in complete mineralization (Radosevich et al., 1995; Chung et al., 1996; Rhine et al., 2003). The biological degradation rate of atrazine in the biphasic rain garden system did not increase over time, suggesting that atrazine was not efficiently used as either a C source or N source because (1) additional C substrate (i.e. glucose) at a relatively high concentration was a better source of C for microorganisms than atrazine and (2) nitrate-N present in the simulated runoff was a more easily utilized N source under anaerobic conditions (e.g.

Column 1) compared to atrazine.

Katz et al (2001) reported that the simultaneous removal of nitrate and atrazine could be achieved by applying a sequence of anaerobic to aerobic batch reactors. Initial anaerobic conditions are used to remove nitrate-N via denitrification followed by aerobic conditions to remove atrazine by serving as a N source for atrazine-degrading microbial populations. Since the biphasic rain garden design has both anaerobic and aerobic conditions, it has the potential for biological transformation/degradation of atrazine under aerobic conditions in Column 2 after nitrate-N removal under anaerobic conditions via denitrification in Column 1.

77 CONCLUSIONS

This study demonstrated that increased retention time and management of environmental conditions (e.g. available C substrate coupled with anaerobic/aerobic conditions) are key parameters for bioremediation of pollutants, especially nitrates, in rain garden systems. Both monophasic and biphasic rain garden designs were effective in removing phosphate-P and atrazine. However, significantly (p<0.001) higher removal of nitrate-N was observed in the biphasic design compared to the monophasic design.

Additional C substrate generally enhanced overall removal efficiency of runoff pollutants, especially nitrate-N under high loading rates. The biphasic, compared to the monophasic, rain garden system was better able to reduce peak flow, runoff volume, and pollutant loads by employing the sequence of anaerobic to aerobic (biphasic) conditions. High concentrations of both nitrate-N and atrazine can occur in many intensively managed urban and agricultural areas. Thus, the use of the biphasic rain garden system has the potential to simultaneously remove nitrate-N through biological denitrification under anaerobic conditions, and atrazine through adsorption and biodegradation primarily under aerobic conditions.

ACKNOWLEDGEMENTS

This study was supported by the Center for Urban Environment and Economic

Development of the Ohio State University and by the Ohio State University Alumni

Grants for Graduate Research and Scholarship, and by funds appropriated to the Ohio

State University and the Ohio Agricultural Research and Development Center. The

78 authors thank Dr. Sun-Jeong Park, Nathan Manges, and Zachary Wallace for their assistance in this study.

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Bouquard, C., Quazzani, J., Prome, J.C., Michel-Briand, Y., Plesiat, P., 1997. Dechlorination of atrazine by a Rhizobium sp isolate. Applied and Environmental Microbiology 63, 862-866.

Cho, K.W., Song, K.G., Cho, J.W., Kim, T.G., Ahn, K.H., 2009. Removal of nitrogen by a layered soil infiltration system during intermittent storm events. Chemosphere 76, 690- 696.

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Davis, A.P., Hunt, W.F., Traver, R.G., Clar, M., 2009. Bioretention technology: Overview of current practice and future needs. Journal of Environmental Engineering 135, 109-117.

79 Dietz, M.E., Clausen, J.C., 2006. Saturation to improve pollutants retention in a rain garden. Environmental Science and Technology 40, 1335-1340.

Dietz, M.E., 2007. Low impact development practices: a review of current research and recommendations for future directions. Water, Air, and Soil Pollution 186, 351-363.

Donald, W.W., Hjelmfelt, A.T., Alberts, E.E., 1998. Herbicide distribution and variability across Goodwater Creek watershed in North Central Missouri. Journal of Environmental Quality 27, 999-1009.

Faucette, L.B., Cardoso-Gendreau, F.A., Codling, E., Sadenghi, A.M., Pachepsky, Y.A., Shelton, D.R., 2009. Storm water pollutant removal performance of compost filter socks. Journal of Environmental Quality 38, 1233-1239.

Goyne, K.W., Jun, H-J., Anderson, S.H., Motavalli, P.P., 2008. Phosphorus and nitrogen sorption to soils in the presence of poultry litter-derived dissolved organic matter. Journal of Environmental Quality 37, 154-163.

Grewal, P.S., Dick, W.A., McCoy, E.L., Yang, H., 2009. Bi-phasic bioretention system. US patent application – US 2009/261026 A1. Publication date: 22 October.

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Hsieh, C., Davis, A.P., Needelman, B., 2007. Nitrogen removal from urban stormwater runoff through layered bioretention columns. Water Environment Researcg 79, 2404- 2411.

Katz, I., Dosoretz, C., Mandelbaum, R., Green, M., 2001. Atrazine degradation under denitrifying conditions in continuous culture of Pesudomonas ADP. Water Research 35, 3272-3275.

80 Kaufman, D.D., Kearney, P.C., 1970. Microbial degradation of s-triazine herbicides. Residue Review 32, 235-265.

Kim, H., Seagren, E.A., Davis, A.P., 2003. Engineered bioretention for removal of nitrate from stormwater runoff. Water Environment Research 75, 355-367.

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Prince George’s County, 1993. Design Manual for Use of Bioretention in Stormwater Management. Prince George’s County, Department of Environmental Protection, Landover, MD, USA.

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Rhine, E.D., Fuhrmann, J.J., Radosevich, M., 2003. Microbial community responses to atrazine exposure and nutrient availability: Linking degradation capacity to community structure. Microbial Ecology 46, 145-160.

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81 Sun, X., Davis, A.P., 2007. Heavy metal fates in laboratory bioretention systems. Chemosphere 66, 1601-1609.

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Low Concentration High Concentration Parameter Chemical (mg/L) a (mg/L) a Nutrients Nitrate 5.0 50.0 (NO3-N) Phosphate 2.0 20.0 (PO4-P) Atrazine Herbicide 0.020 0.16 (C8H14ClN5)

C/N ratiob 0:1 0.5:1 1:1 2:1 0:1 0.5:1 1:1 2:1

(w/w) Glucose 0 2.5 5.0 10.0 0 25.0 50.0 100 (C6H12O6)

a For each rainfall event, 1-liter of simulated runoff (i.e. the amount that would be generated when a 25.4 mm rainfall occurs and the rain garden surface area is 5% that of the drainage area) was prepared. b Carbon, as glucose, concentrations were calculated based on initial input concentration of nitrate-N.

Table 4.1. Composition of simulated storm water runoff

83 Output from rain gardens Net mass removal Input Output (mg) c (%) d from from soil Pollutant runoff medium a b Monophasic Biphasic Monophasic Biphasic (mg) (mg) design design design design

NO3-N 25 3.8 ± 0.3 17.7 ± 0.2 10.6 ± 0.2 39 ± 0.9 63 ± 0.7

250 3.8 ± 0.3 181 ± 1.7 146 ± 2.7 29 ± 0.7 42 ± 1.1

PO4-P 10 - - - 100 100

100 - 10.2 ± 0.6 6.0 ± 1.1 89 ± 0.6 94 ± 1.1

Atrazine 0.10 - - - 100 100

0.80 - 0.13 ± 0.004 0.09 ± 0.001 84 ± 0.4 89 ± 0.1

a Total mass input of each pollutant at different concentrations for five runoff events. b Determined by a control experiment with deionized water without pollutant addition from five sequential events. Mean ± one standard deviation. c Total mass output of each pollutant at different concentrations for five runoff events. Mean ± one standard deviation. d Net removal efficiency (%) = (1- Output from rain gardens/(Input from runoff + Output from soil medium))×100. Mean ± one standard deviation.

Table 4.2. Mass balance and net mass removal (%) of pollutants from five sequential events in the monophasic and biphasic rain gardens

84 c d Input from Output from soil Output from the rain garden (mg) Net mass removal (%) Pollutant runoff medium C/N ratio C/N ratio (mg) a (mg) b 0 0.5 1 2 0 0.5 1 2 3.8 10.6 9.5 8.6 7.5 63 67 70 74 NO -N 25 3 ± 0.3 ± 0.2 ± 0.1 ± 0.3 ± 0.2 ± 0.7 ± 0.3 ± 1.2 ± 0.8 3.8 146 133 110 60 42 48 57 76 250 ± 0.3 ± 2.7 ± 3.5 ± 0.6 ± 1.8 ± 1.1 ± 1.4 ± 0.2 ± 0.7

PO4-P 10 - - - - - 100 100 100 100 6.0 5.7 4.9 1.4 94 94 95 99 100 - ± 1.1 ± 0.6 ± 0.2 ± 0.4 ± 1.1 ± 0.6 ± 0.2 ± 0.4 Atrazine

85 0.1 - - - - - 100 100 100 100 0.09 0.07 0.06 0.04 89 91 93 95 0.8 - ± 0.001 ± 0.005 ± 0.005 ± 0.009 ± 0.1 ± 0.6 ± 0.6 ± 1.1

Table 4.3. Mass balance and net mass removal (%) of pollutants at different C/N ratios in the biphasic rain garden

Figure 4.1. Schematic illustration of column-scale rain garden systems. The biphasic system (a) consisted of an anaerobic-aerobic column set and the monophasic system (b) consisted of an aerobic-aerobic column set.

86 1000

800

600

400

200 (a) Volume Cumulative Volume (mL) Cumulative 0 1000

Monophasic System (Mean of 1st - 5th runoff events) 800 Biphasic System (1st runoff event) Biphasic System (Mean of 2nd - 5th runoff events)

600

400

Flow Rate (mL/h) 200 (b) Flow Rate

0 0246810 Time after start of the simulated runoff (h)

Figure 4.2. Cumulative volume (a) and flow rate (b) of Column 1 effluent from monophasic and biphasic rain garden systems for five sequential runoff events. One L of influent was applied over 1 h at a constant flow rate. Symbols and error bars represent mean ± one standard deviation.

87 (a) 5.0 mg of NO3-N Loading (b) 50 mg of NO3-N Loading 100 100 Column 1 effluent Column 1 effluent 80 80 Monophasic System 60 60 Biphasic System

40 40

20 20

0 0 100 100 Column 2 effluent Column 2 effluent 80 80

60 60 -N Removal Efficiency (%) Efficiency -N Removal -N Removal Efficiency (%) Efficiency -N Removal 3 40 3 40 NO 20 NO 20

0 0 1st 2nd 3rd 4th 5th 1st 2nd 3rd 4th 5th Simulated Runoff Events Simulated Runoff Events

60 60

40 40

20 20 Column 1 effluent Column 1 effluent 0 0 100 100

80 80

60 60 -P Removal Efficiency (%) Efficiency Removal -P 4 40 40 Conventional Design

PO Col 29 20 (%) Efficiency Removal Atrazine 20 Column 2 effluent Column 2 effluent 0 0 1st 2nd 3rd 4th 5th 1st 2nd 3rd 4th 5th Simulated Runoff Events Simulated Runoff Events

Figure 4.3. Removal efficiency of NO3-N, PO4-P, and atrazine via monophasic and biphasic rain garden systems from five sequential runoff events. Loading for each event was 5.0 mg of NO3-N (a), 50 mg of NO3-N (b), 20 mg of PO4-P (c), and 0.16 mg of atrazine (d). Symbols and error bars represent mean ± one standard deviation. 88 (a) 5.0 mg of NO3-N Loading (b) 50 mg of NO3-N Loading 100 100

80 80

60 60

40 40 Treatment C/N = 0 Treatment C/N = 0.5 20 Treatment C/N = 1.0 20 -N Removal Efficiency (%) Treatment C/N = 2.0 Efficiency-N Removal (%) 3 3 0 0 NO 1st 2nd 3rd 4th 5th NO 1st 2nd 3rd 4th 5th Simulated Runoff Events Simulated Runoff Events

5th runoff

80 4th runoff

3rd runoff 40 2nd runoff

2nd runoff event (Day 5) 20 3rd runoff event (Day 10) -N Removal Efficiency (%) Efficiency Removal -N 4th runoff event (Day 15) 3 5th runoff event (Day 20) NO 0 0.0 0.5 1.0 1.5 2.0 Treatment C/N Ratio

Figure 4.4. Removal efficiency of NO3-N in Column 2 effluent under the low (a) and high (b) level loading conditions at different input C/N ratio treatments and relationship between treatment C/N ratio and nitrate-N removal efficiency (c) in the biphasic rain garden system under the high level loading condition. Loading for each event was 5.0 mg of NO3-N for the low level and 50 mg of NO3-N for the high level. Symbols and error bars represent mean ± one standard deviation. The lines in (c) represent the linear regressions of each runoff event. y = 14.64 x + 29.24 for the 2nd runoff event (R2 = 0.969), y = 21.73 x + 25.37 for the 3rd runoff event (R2 = 0.989), y = 22.98 x + 31.83 for the 4th runoff event (R2 = 0.981), and y = 27.66 x + 30.40 for the 5th runoff event (R2 = 0.992).

89 CHAPTER 5

EVALUATION OF RAIN GARDEN MEDIA FOR STORMWATER RUNOFF

POLLUTANT REMOVAL

ABSTRACT

Different rain garden media were evaluated to identify media configurations for more

efficient pollutant removal from stormwater runoff. Hydraulic and treatment performance of four types of rain garden media—sand, soil-compost (SC) mix, and two leaf compost pellet (LCP) mixes (LCP mix 1 and 2) were tested in column-scale biphasic rain gardens consisting of an anaerobic to aerobic sequence that increases retention time and removal efficiency of runoff pollutants. Seven simulated runoff events with various concentrations of nutrients (i.e. nitrate-N, phosphate-P) and the herbicide, atrazine (6-chloro-N-ethyl-N'-

(1-methylethyl)-1,3,5-triazine-2,4-diamine) were applied to the biphasic rain garden columns once a week. The biphasic rain garden columns with different soil media reduced peak flow by 69% (sand and LCP mix 1) to 80% (SC mix and LCP mix 2). All soil media tested in the biphasic rain garden columns demonstrated good retention for nitrate-N (60 to 100%), phosphate-P (100%), and atrazine (96 to 100%). Higher organic matter content in soil media (i.e. LCP mix 1 and 2) increased removal efficiency of nitrate-N through denitrification in the anaerobic zone. However, high potential leaching

of nitrate-N from LCP mix 1 and 2 was noted, mostly from the aerobic zone, because of

the relatively high total N (%) contents of the two LCP media. Consequently, rain garden

media with high organic matter content in the anaerobic zone and media with low available N content in the aerobic zone could improve biological treatment processes, particularly denitrification without potential leaching of nitrate-N from the biphasic rain gardens.

INTRODUCTION

The excessive nutrient and herbicide concentrations in stormwater runoff from agricultural and urban areas are one of the most problematic environmental issues. It can lead to negative impacts on surface water bodies such as eutrophication and toxicity to biological components in ecosystems (USEPA, 1997; Davis et al., 2001; Huang et al.,

2004). Rain gardens are bioretention systems that are a cost-effective method for stormwater runoff treatment due to low cost of materials, installation, and maintenance

(Davis, 2005; Dietz, 2007). General features of a rain garden include a soil medium (a mixture of sand, soil, and/or compost), a mulch layer, and vegetation for infiltrating and treating stormwater runoff (Prince George’s County, 2002). Despite the promising performance, poor removal and leaching of nitrates still remains an obstacle to the widespread adoption of rain gardens as an effective stormwater best management practice

(Davis et al., 2001; Hsieh and Davis, 2005; Maurakami et al., 2008; Cho et al., 2009).

Efforts have been made to promote biological nitrate removal via denitrification in rain gardens by using alternative designs, whereby a portion of the rain garden media is

91 allowed to remain continuously submerged (Kim et al., 2003; Dietz and Clausen, 2006).

We have recently employed a sequence from anaerobic to aerobic zones to create biphasic, field-scale rain gardens to improve environmental conditions for nitrogen removal by maximizing retention time of runoff pollutants (Grewal et al., 2009; Yang et al., 2009). However, column studies revealed that an available C substrate is required to sustain this denitrification process in the anaerobic zone if runoff contains high levels of nitrate-N (Yang et al., 2010 Submitted). The results showed that 87% nitrate-N removal from simulated runoff was achieved by providing a C-to-nitrate-N ratio of 2.0; that is favorable for denitrification. Since different rain garden media compositions greatly affect pollutant removal mechanisms such as sedimentation, filtration, adsorption, precipitation, and biological transformation or degradation (Prince George’s County,

2002; USEPA, 2004), the efforts have focused on the development of different soil media compositions (Dietz, 2007; Davis et al., 2009). The proper layering of rain garden soil media could enhance the biological removal of ammonium through adsorption, and nitrate removal through denitrification by increasing retention time of runoff (Hsieh et al.,

2007). Zhang et al. (2008) also reported that phosphorus sorption on rain garden media was increased significantly with the incorporation of small amounts of fly ash.

Nonetheless, the hydraulic characteristics of rain garden media cannot be ignored.

Cho et al. (2009) reported that addition of layered soil media with high silt/clay content decreased the infiltration rate, significantly reducing the treatment capacity of rain gardens. Since the infiltration rate of the media primarily depends on soil texture, and because soils with high percentages of coarse fragments generally tend to have high

92 infiltration rates (Scott, 2000), a sandy medium is favored for rain gardens. However, a balance needs to be developed between the infiltration of rain garden medium and pollutant removal efficiency because high organic matter and clay content in soils are generally more active for chemical and biological processes (Hsieh and Davis, 2005).

Consequently, development of different media composition is critical to determining rain garden performance characteristics.

The objectives of this study were to evaluate hydraulic and treatment performance of different soil media compositions in the biphasic rain gardens and to identify improved media configurations for more efficient pollutant removal with a proper hydraulic performance. We conducted a laboratory study of 36 biphasic rain garden columns with four different rain garden media. Multiple loads of low and high concentrations of the nutrients, nitrate-N and phosphate-P, and the herbicide atrazine (6-chloro-N-ethyl-N'-(1- methylethyl)-1,3,5-triazine-2,4-diamine) were evaluated.

MATERIALS AND METHODS

Rain garden soil media

Four types of rain garden soil media, sand, soil-compost (SC) mix, and two different mixes of leaf compost pellet (LCP) with sand (LCP mix 1 and 2) were used to evaluate hydraulic performance and pollutant removal efficiency of column-scale biphasic rain gardens. Sand and SC mix were obtained from Kurtz Bros. Inc., Cleveland, Ohio and

LCP was obtained from The Ohio State University’s Center for Urban Environment and

Economic Development. Media characterization was done by the Service Testing and

Research (STAR) Laboratory located on the Ohio State University’s Wooster campus

(STAR Lab, 2009). SC mix was a mixture of sand, topsoil, and compost (6:2:2, volume

ratio). LCP mix 1 and 2 were prepared at different mixing ratios of the sand and LCP

(10:1 for LCP mix 1 and 3:1 for LCP mix 2, volume ratio). In these LCP media, the total

C, N, and organic matter contents were determined using the following equation

previously used in predicting sand content in soil mixtures (Taylor and Blake 1983):

C1V11  C 2V2  2 C t  (5 – 1) V11  V2  2

where: Ct = total C, N, or organic matter content in a soil mixture (kg/kg); C1 = total C, N,

or organic matter content of sand (kg/kg); C2 = total C, N, or organic matter content of

leaf compost pellet (LCP) (kg/kg); V1 = respective volume of sand added; V2 =

respective volume of LCP added; ρ1 = respective packed bulk density of sand 1.65

3 3 Mg/m ); ρ2 = respective packed bulk density of LCP (0.80 Mg/m ).

The characteristics of the soil media used in this study are summarized in Table 5.1.

Column design and set-up

Non-vegetated biphasic rain garden columns were used to compare pollutant removal

efficiencies and hydraulic performance by the four different soil media. The biphasic rain

garden columns consisted of an anaerobic to aerobic sequence with two polyvinyl

chloride (PVC) columns (Column 1; 91.4 cm long × 5 cm inner diameter and Column 2;

60.9 cm long × 5 cm inner diameter) (Figure 5.1). Anaerobic conditions were created in

Column 1 by setting an effluent port at the level of the packed soil medium to increase

retention time for sustained water saturated conditions. Aerobic conditions were created

in Column 2 with an underdrainage configuration at the bottom of the soil medium.

Two hundred grams of fine gravel (3.2 mm - 12.7 mm) were placed at the bottom of

the columns to a 5-cm depth. Each soil medium was poured into the columns and tapped

gently to form uniform layers until predetermined soil depths for Column 1 (55.9-cm)

and Column 2 (25.5-cm) were reached. Amount of the packed soil medium in Column 1

for each rain garden was 1,867 g of the sand with a bulk density of 1.65 Mg/m3, 1,777 g

of SC mix with a bulk density of 1.57 Mg/m3, 1,832 g of LCP mix 1 with a bulk density

of 1.62 Mg/m3, and 1,798 g of LCP mix 2 with a bulk density of 1.59 Mg/m3. The same

bulk density packed in Column 1 for each soil medium was used in Column 2, resulting

in 849 g of the sand, 808 g of SC mix, 833 g of LCP mix 1, and 822 g of LCP mix 2.

Total bed depth of each column was 60.9 cm for Column 1 and 30.5 cm for Column 2, leaving 30.5 cm of extended ponding depth (Figure 5.1). In all, 36 rain garden experimental units (i.e. three pollutant levels by four soil media and three replicates for each combination) were created.

Before the main experiments, 1-L of deionized water was applied four times to each column at 2-day intervals as a pretreatment and to obtain background information on potential leaching of nutrients from each medium. Effluents from the pretreatments were analyzed for the targeted pollutants (e.g. nitrate-N, phosphate-P, and atrazine) to estimate

the leaching potential from the soil media. Prior to the main tests, both Column 1 and

Column 2 were drained.

Application of simulated runoff and data collection

All experiments consisted of applying a total of seven simulated runoff events once

every seven days to biphasic rain garden column sets packed with different soil media.

For each runoff event, 500-mL of simulated runoff with either zero (i.e. the control), low

or high concentration of runoff pollutants was applied to Column 1 over a 30-min period

at a constant rate of 16.7 mL/min (Table 5.2). The 500-mL runoff volume corresponds to

the amount that would be generated when a 12.7 mm rainfall occurs and the rain garden

surface area is 5% that of the drainage area. Column 1 effluent from each soil medium

was collected and volume was measured initially at 30-min and over time at 2-h intervals.

After Column 1 effluent was completely drained, final effluent volume of Column 1 was

measured and an aliquot sample representing less than 6% of the total volume was taken

using a 30 ml plastic bottle. The remaining effluent from Column 1 was then applied to

Column 2 over a period of 6 h as determined by the drainage time of Column 1 for each

medium. Total effluent volume of Column 2 was measured and an aliquot sample was

taken. Altogether 432 samples were collected and stored at 4 °C until further analysis.

Sample Analysis

Concentrations of nitrate-N and phosphate-P in the effluent of each column were

analyzed at the STAR Laboratory using a Dionex DX-120 ion chromatograph (APHA,

1992). Concentration of atrazine (Chem Service, Inc., West Chester, PA) was measured using a Varian 3800 gas chromatograph (GC) equipped with a thermionic specific detector (TSD) and a 30-m 0.25-mm I.D. CP-Sil 8CB fused silica column (0.25 µm film

96 thickness) (Varian Inc., Palo Alto, CA) using a previously developed procedure (Yang et al., 2010 Submitted).

All statistical analyses were conducted using MINITAB v.15 (Minitab, Inc., State

College, PA). Mean percentage removals were compared between the soil media using one-way analysis of variance and followed by pair-wise comparison using Tukey’s honestly significant differences at an alpha level of 0.05. Comparisons of the soil media were conducted for separate runoff events. To examine changes in pollutant removal efficiency over the repeated runoff events, regression analysis was also performed on mean removal efficiency of each pollutant. Mean removal efficiency of target pollutants was considered as a response variable and the number of runoff events as a predictor.

Regression slope was considered an indicator of increase (positive slope) or decrease

(negative slope) of removal efficiency over the events. When the p-value for the regression slope was below 0.05, such changes were considered significant (regression slope is significantly different from zero). Total net removal values for the study are presented, but were not subjected to statistical analysis due to the limited number of data points.

RESULTS AND DISCUSSION

Effects of soil media on hydraulic performance

No measurable effluent from Column 1 was observed during the first runoff event

(500 mL or 12.7 mm rainfall) due to the designed storage capacity in Column 1. For the remaining runoff events, no significant runoff volume reduction (~10%) among the soil

media in the rain gardens was observed (Figure 5.2a). However, significant reduction in

peak flow rate, compared to influent rate was observed for each soil medium (Figure

5.2b). Column 1 effluent rate from the sand and LCP mix 1 columns gradually increased to a peak flow rate of approximately 310 mL/h during the first 1h then steadily decreased while the observed peak flow rate from SC mix and LCP mix 2 columns was approximately 200 mL/h. Drainage of each soil medium from Column 1 was essentially complete after 6h. The average peak flow reduction over the seven events between

influent and Column 1 effluent was 69% for the sand and LCP mix 1 and 80% for SC

mix and LCP mix 2. The reason for this difference can be attributed to different soil

media compositions. The increased amount of compost and/or soil into sand became less permeable than sand itself and thus SC mix and LCP mix 2 were more effective in buffering peak flow compared to the sand and LCP mix 1. It must be taken into account, however, that rain gardens should have a certain minimum infiltration rate (e.g. 2.5 cm/h)

if they are to be employed as a best management practice in urban areas (Davis and

McCuen, 2005). If infiltration rate is too low as a result of excessive addition of soil and

compost in soil media, overall treatment capacity of rain gardens may decline because of

potential direct overflow and/or bypass of runoff without delay or retention in rain

gardens (Hsieh and Davis, 2005; Cho et al., 2009).

Effects of soil media on pollutant removal performance

Influent and effluent of the rain garden columns were compared to determine removal

efficiency of each pollutant by different soil media from the seven sequential runoff

events. Mean removal efficiency from Column 1 and 2 was calculated based on the initial

input (influent) mass of each pollutant. Pretreatment and control experiments without

pollutant addition showed no leaching of phosphate and atrazine. However,

approximately 0.9 mg/L to 6.5 mg/L of nitrate-N were detected in the pretreatment and

control columns among different soil media, indicating potential leaching of nitrate from each soil medium used in the study. The overall results of pollutant mass balances and removal from the rain gardens (i.e. Column 2 effluent) are summarized in Table 5.3.

Nitrate-N removal efficiency in the biphasic rain garden columns at different media is presented in Figure 5.3a for the low concentration loading and Figure 5.3b for the high concentration loading conditions. Based on effluent concentrations from Column 1, all soil media in the biphasic rain gardens were fairly effective in removing nitrate-N (81% to 100%) and increased nitrate-N removal efficiency was observed over runoff events

(p<0.05). The effect of different soil media on nitrate-N removal from Column 1 effluent was initially apparent for the first three events (p<0.001), showing that higher organic matter content in soil media (i.e. LCP mix 1 and 2) increased removal efficiency of nitrate-N (Figure 5.3). However, after the 4th event there were no differences observed

among the soil media due to the fact that nitrate-N removals for all media became 100%

for both low and high loading conditions. The higher organic matter content in the media

(LCP mix 1 and 2) seemed to facilitate quicker buildup of a denitrifying microbial

population compared to the lower organic matter content media (sand and SC mix). The

denitrification rate has been reported to increase with the soil water content, showing that

a minimum 40% of volumetric water content in loamy soil is required for denitrification

(Klein and Logtesijn, 1994). Since fully water saturated conditions were created in

Column 1 of the biphasic rain gardens by holding runoff until the next event, high nitrate-

N removal through denitrification was achieved from Column 1 effluent among all soil media.

Under low loading conditions, significantly (p<0.001) decreased removal efficiency of nitrate-N was observed from Column 2 effluent among all soil media (except the sand medium), compared to that from Column 1 effluent until the 4th runoff event (Figure

5.3a). This decreased nitrate-N removal from Column 2 indicated potential leaching risk

of nitrate-N from each soil medium. The magnitude of potential leaching of nitrate-N was

greater from LCP mix 1 and 2 because they contained relatively high initial total N (%)

content compared to the sand and SC mix (Table 5.1). Even though leaching of nitrate-N

was initially observed from LCP mix 1 and 2, the removal efficiency of nitrate-N was

significantly (p<0.001) increased and reached 100% after 5th runoff event. Denitrification

in Column 2 (aerobic zone), where high organic matter content and partially water-

saturated conditions (e.g. microenvironment or interface layer between gravel and

medium) exist, is obviously facilitated by microbial activity (Rice et al., 1998; Dietz and

Clausen, 2006). However, removal efficiency of nitrate-N from Column 2 effluent was not decreased under high loading conditions, indicating no leaching of nitrate-N from the aerobic zone (Figure 5.3b and Table 5.3). Further studies should be investigated to clarify the main removal mechanisms of nitrate-N in the aerobic zone, especially under high loading conditions.

100

None of the tested soil media leached phosphate-P from the biphasic rain garden

columns, indicating 100% retention in the soil media under both low and high loading conditions (Table 5.3). No measurable effluent concentrations of atrazine from the rain

garden columns were observed under the low loading conditions, showing effective

retention and/or mineralization of atrazine through the soil media (Table 5.3). Even at the

high loading conditions, all soil media in the rain garden columns effectively removed

atrazine, achieving 96 to 100% removal (Table 5.3 and Figure 5.4). The effect of

different levels of organic matter contents on atrazine removal was not initially apparent,

but became evident at the 7th runoff event (p<0.05). As organic matter content in soil

media increased, the removal efficiency for atrazine increased under high loading

conditions. However, the overall removal efficiency of atrazine decreased over the runoff

events (p<0.005), indicating the soil media’s capacity to capture atrazine was decreasing over time (Figure 5.4). Even though potential biological processes for atrazine removal were demonstrated in previous studies (Radosevich et al., 1995; Chung et al., 1996;

Rhine et al., 2003), increased biological degradation rate of atrazine in the biphasic rain garden with different soil media was not observed over time.

CONCLUSIONS AND RECOMMENDATIONS

The biphasic rain garden columns with different soil media reduced peak flow by 69%

(sand and LCP mix 1) to 80% (SC mix and LCP mix 2) from Column l. All soil media demonstrated excellent retention/removal for nitrate-N (60 to 100%), phosphate-P

(100%), and atrazine (96 to 100%) in the biphasic rain gardens. In general, higher organic

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matter content in soil media (i.e. LCP mix 1 and 2) increased removal efficiency of

nitrate-N through denitrification in Column 1 (anaerobic zone). However, high potential

leaching of nitrate-N from LCP mix 1 and 2 was noted, mostly from Column 2 (aerobic zone) because of the relatively high initial total N (%) contents in the two LCP media.

Based on the results of this study, the following recommendations for soil media in the biphasic rain garden can be made:

 A proper runoff infiltration rate is desired for rain garden design to minimize

direct runoff overflow and/or bypass from rain gardens. For example, if surface

area of a rain garden is relatively small and it should capture a large volume of

runoff, soil media such as sandy soils or mixtures with a high infiltration rate

(minimum 2.5 cm/h) are recommended.

 If high nitrate-N removal is desired, soil media with high organic matter content

(%) in the anaerobic zone is recommend for promoting denitrification processes.

Since supplemental organic matter from mulch should be beneficial for

denitrification in the anaerobic zone, a regular maintenance of mulch layer is

recommended.

 To prevent potential leaching of nitrate-N mainly from the aerobic zone,

relatively low available N content (%) media may be employed in the aerobic

zone. However, it must be taken into consideration that the media should contain

a minimum N content to support plant growth and survival.

The high infiltration rate in rain gardens is an important design configuration to maximize hydraulic performance, which sometimes compromises removal efficiency of

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runoff pollutants because this accelerated infiltration rate results in reduced retention time

and incomplete biological processes. However, the biphasic rain garden systems

effectively controlled both runoff quantity (i.e. hydraulic performance) and quality (i.e.

pollutant removal) while maintaining high runoff infiltration rate. This is mainly due to the biphasic rain garden maximizing the retention time of runoff pollutants by employing a sequence of anaerobic to aerobic (biphasic) conditions that promotes biological processes.

ACKNOWLEDEMENTS

This study was supported by the Center for Urban Environment and Economic

Development of the Ohio State University and by the Ohio State University Alumni

Grants for Graduate Research and Scholarship, and by funds appropriated to the Ohio

State University and the Ohio Agricultural Research and Development Center. The

authors thank Dr. Sun-Jeong Park for her support in the statistical analysis. The authors

also thank Aaron Rehm and Zachary Robinson for constructing columns and water

sampling in this study.

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REFERENCES

APHA, AWWA, AEF, 1992. In: Standard Methods for the Examination of Water and Wastewater, 18th ed. American Public Health Association, Washington, DC, USA.

Cho, K.W., Song, K.G., Cho, J.W., Kim, T.G., Ahn, K.H., 2009. Removal of nitrogen by a layered soil infiltration system during intermittent. Chemosphere 76, 690-696.

Chung, K.H., Ro, K.S., Roy, D., 1996. Fate and enhancement of atrazine biotransformation in anaerobic wetland sediment. Water Research 30, 341-346.

Davis, A.P., Shokouhian, M., Sharma, H., Minami, C., 2001. Laboratory study of biological retention for urban stormwater management. Water Environment Research 73, 5-14.

Davis A.P., McCuen R.H., 2005. Stormwater Management for Smart Growth, Springer, USA.

Davis, A.P., Hunt, W.F., Traver, R.G., Clar, M., 2009. Bioretention technology: Overview of current practice and future needs. Journal of Environmental Engineering 135, 109-117.

Dietz, M.E., Clausen, J.C., 2006. Saturation to improve pollutants retention in a rain garden. Environmental Science and Technology 40, 1335-1340.

Dietz, M.E., 2007. Low impact development practices: a review of current research and recommendations for future directions. Water, Air, and Soil Pollution 186, 351-363.

Grewal, P.S., Dick, W.A., McCoy, E.L., Yang, H., 2009. Bi-phasic bioretention system. US Patent Application – US 2009/261026 A1. Publication date: 22 October.

Hsieh, C., Davis, A.P., 2005. Multiple-event study of bioretention for treatment of urban storm water runoff. Water Science and Technology 51, 177-181.

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Hsieh, C., Davis, A.P., Needelman, B.A., 2007. Nitrogen removal from urban stormwater runoff through layered bioretention columns. Water Environment Research 79, 2404- 2411.

Huang, X., Pedersen, T., Fischer, M., White, R., Young, T.M., 2004. Herbicide runoff along highways. 1. Field observations. Environmental Science and Technology 38, 3263- 3271.

Kim, H., Seagren, E.A., Davis, A.P., 2003. Engineered bioretention for removal of nitrate from stormwater runoff. Water Environment Research 75, 355-367.

Klein, C.A.M., Logtesijn, R.S.P., 1994. Denitrification in the top soil of managed grasslands in the Netherlands in relation to soil type and fertilizer level. Plant Soil 163, 33-44.

Prince George’s County. 2002. Bioretention Manual. Department of Environmental Resources, Landover, MD, USA.

Radosevich, M., Traina, S.J., Hao, Y.L., Tuovinen, O.H., 1995. Degradation and mineralization of atrazine by a soil bacterial isolate. Applied and Environmental Microbiology 61, 297-302.

Rice, C.W., Sierzega, P.E., Tiedje, J.M., Jacobs, L.W., 1998. Simulated denitrification in the microenvironment of a biodegradable organic waste injected into soil. Soil Science Society of America Journal 52, 102-108.

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Scott, H.D., 2000. Soil Physics: Agricultural and Environmental Application. Wiley- Blackwell, Ames, IW, USA

STAR Lab, 2009. Method references. Service testing and research laboratory. Available at http://www.oardc.ohio-state.edu/starlab/ (accessed 25 January 2010).

Taylor, D.H., Blake, G.R., 1983. Predicting sand content of modified soil mixtures from sand, soil, and peat properties. Agronomy of Journal 76, 583-587.

USEPA. 1997.. Managing Urban Runoff. EPA-841-F-96-004G, United State Environmental Protection Agency, Washington, DC, USA.

USEPA, 2004. Stormwater Best Management Practice Design Guide: Volume 1 General Considerations. EPA/600/R-04/121. United State Environmental Protection Agency, Washington, DC, USA.

Yang, H., Florence, D.C., McCoy, E.L., Dick, W.A., Grewal, P.S., 2009. Design and hydraulic characteristics of a field-scale bi-phasic bioretention rain garden system for storm water management. Water Science and Technology 59, 1863-1872.

Zhang, W., Brown, G.O., Storm, D.E., Zhang, H., 2008. Fly-ash-amended sand as filter media in bioretention cells to improve phosphorus removal. Water Environment Research 80, 507-516.

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Soil media Property Sand SC mix a LCP mix 1b LCP mix 2 c pH 7.2 7.2 7.2 7.3 Organic matter (%) 0.5 0.7 0.9 1.5 Total C (%) 1.8 1.0 2.7 4.5 Total N (%) 0.02 0.03 0.11 0.29 Infiltration rate (cm/h) d 16.0 10.2 15.1 9.9 a A mixture of sand, soil, and compost (6:2:2, volume ratio) b A mixture of sand and leaf compost pellet (LCP) (10:1, volume ratio). c A mixture of sand and LCP (3:1, volume ratio). d Observed peak infiltration rate from Column 1 at a constant influent rate of 500 mL/h.

Table 5.1 Characteristics of the rain garden soil media

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Low Concentration High Concentration Pollutants Reagent (mg/L) a (mg/L) a Herbicide Atrazine 0.02 0.16 C8H14ClN5 (C8H14ClN5)

Nutrients Nitrate 5.0 50.0 NaNO3 (NO3-N) Phosphate 2.0 20.0 NaH2PO4·H2O (PO4-P) a For each rainfall event, 500 mL of simulated runoff (i.e. the amount that would be generated when a 12.7 mm rainfall occurs and the rain garden surface area is 5% that of the drainage area) was prepared.

Table 5.2 Composition of simulated stormwater runoff

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Output from the rain garden (mg) c Net mass removal (%) d Input from Output from Soil media runoff soil media Soil Media Soil Media (mg) a (mg) b SC LCP LCP SC LCP LCP Sand Sand Mix Mix 1 Mix 2 Mix Mix 1 Mix 2 0.6 1.2 1.4 2.6 97 94 94 92 NO -N 17.5 1.7 – 13.0 3 ± 0.1 ± 0.1 ± 0.1 ± 0.2 ± 0.5 ± 0.4 ± 0.4 ± 0.6 8.2 3.5 95 98 175 1.7 – 13.0 - - 100 100 ± 0.2 ± 0.1 ± 0.1 ± 0.1

PO4-P 7.0 - - - - - 100 100 100 100

70 - - - - - 100 100 100 100

Atrazine 0.07 - - - - - 100 100 100 100 0.015 0.007 0.005 0.002 97 98 99 99 0.56 - ± 0.001 ± 0.001 ± 0.001 ± 0.001 ± 0.2 ± 0.2 ± 0.2 ± 0.2 109 a Total mass input of each pollutant at different concentrations for seven runoff events. b Determined by a control experiment with deionized water without pollutant addition from seven sequential events. The amount of nitrate-N leached from each soil medium over seven events was approximately 1.7 mg for sand, 3.4 mg for SC mix, 6.3 mg for LCP mix 1, and 13.0 mg for LCP mix 2. c Total mass output of each pollutant at different concentrations for seven runoff events. Mean ± one standard deviation. d Net removal efficiency (%) = (1- Output from rain gardens/(Input from runoff + Output from soil medium))×100. Mean ± one standard deviation.

Table 5.3 Mass balance and net mass removal (%) of pollutants among different soil media in the biphasic rain garden

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Figure 5.1. Schematic illustration of a column-scale biphasic rain garden that consisted of an anaerobic-aerobic column set.

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500 (a) Cumulative Volume

400

300

200

100 Cumulative volume (mL) volume Cumulative

0 0123456 500 (b)Time Flow after Rate start of the simulated runoff (h)

400

300 Mean of 2nd - 7th runoff events Sand 200 SC Mix LCP Mix 1

Flow (mL/h) rate LCP Mix 2 100

0 0123456

Time after start of the simulated runoff (h)

Figure 5.2. Cumulative volume (a) and flow rate (b) of Column 1 effluent in the biphasic rain gardens among different soil media from the 2nd to 7th sequential runoff events. 500 mL of influent was applied over 1 h at a constant flow rate. Symbols and error bars represent mean ± one standard deviation.

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(a) 2.5 mg of NO3-N Loading (b) 25 mg of NO3-N Loading 100 100

80 80

60 60

40 40

20 20 Column 1 Effluent Column 1 Effluent 0 0 2nd 3rd 4th 5th 6th 7th 2nd 3rd 4th 5th 6th 7th 100 X Data 100 X Data

80 80

60 60 Sand 40 Sand Nitrate-N Removal Efficiency (%) Efficiency Removal Nitrate-N Nitrate-N EfficiencyRemoval (%) 40 SC Mix SC Mix LCP Mix 1 LCP Mix 1 20 20 Column 2 Effluent LCP Mix 2 Column 2 Effluent LCP Mix 2 0 0 2nd 3rd 4th 5th 6th 7th 2nd 3rd 4th 5th 6th 7th Simulated Runoff Events Simulated Runoff Events

Figure 5.3. Removal efficiency of NO3-N in the biphasic rain gardens among different soil media from the 2nd to 7th sequential runoff events. Loading for each event was 2.5 mg (a) and 25 mg (b) of NO3-N. Symbols and error bars represent mean ± one standard deviation.

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100

20 Column 1 Effluent 0 2nd 3rd 4th 5th 6th 7th

100 X Data

60 Atrazine Removal Efficiency (%) Efficiency Removal Atrazine

40 Sand SC Mix 20 LCP Mix 1 Column 2 Effluent LCP Mix 2 0 2nd 3rd 4th 5th 6th 7th

Simulated Runoff Events

Figure 5.4. Removal efficiency of atrazine in the biphasic rain gardens among different soil media from the 2nd to 7th sequential runoff events. Loading for each event was 0.08 mg of atrazine. Symbols and error bars represent mean ± one standard deviation.

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CHAPTER 6

CONCLUSIONS AND FUTURE DIRECTIONS

A biphasic (anaerobic to aerobic) rain garden was developed and evaluated at column- and field-scales to maximize removal efficiency of runoff pollutants. The specific objectives were to: (1) design, construct, and hydraulically characterize replicated field-scale biphasic rain gardens (Chapter 2), (2) evaluate field performance of the biphasic rain gardens under actual and simulated runoff conditions (Chapter 3), (3) compare and evaluate effectiveness of column-scale monophasic (conventional) and biphasic rain gardens (Chapter 4), and (4) evaluate and identify media configurations for more efficient pollutant removal by the biphasic rain gardens (Chapter 5). The overall results of this study show that the biphasic rain gardens have the potential for being an effective best management practice in reducing stormwater flow and pollutant loads by employing a sequence of anaerobic to aerobic conditions. This biphasic condition increases overall retention time of runoff pollutants, thus providing improved environmental conditions for more efficient bioremediation of runoff pollutants (e.g. denitrification). Based on the results of the present study, the following specific conclusions can be made:

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1. The biphasic rain garden effectively reduced both peak flow (~70%) and runoff

volume (~ 42%) for a large 180 mm/24h rainfall event. However, a tracer study

showed that dissolved nitrate-N was easily transported through the rain gardens

without any significant sorption if runoff is continuous and of high volume,

whereas dissolved phosphate-P and metals (Cu, Pb, and Zn) were significantly

retained in the rain gardens (Chapter 2).

2. Field evaluation demonstrated that both peak flow and runoff volume were

effectively reduced in the biphasic rain gardens for actual and simulated runoff

events by holding runoff in the rain gardens (mainly in the anaerobic zone) until

next runoff event. The biphasic rain gardens were highly effective in removing

nitrate-N (~91%), phosphate-P (~99%), atrazine (~90%), dicamba (~92%),

glyphosate (~99%), and 2,4-D (~90%) under high levels of simulated pollution

loading conditions by both agricultural and urban runoff events (Chapter 3).

3. Column studies showed that the biphasic rain gardens, compared to the

monophasic (conventional) rain gardens, were better able to reduce peak flow,

runoff volume, and pollutant loads by employing the sequence of anaerobic to

aerobic (biphasic) conditions. Significantly (p<0.001) higher removal of nitrate-N

through denitrification was observed in the biphasic (42 to 63%), compared to the

monophasic rain gardens (29 to 39%), by creating anaerobic conditions. In

addition, available C substrate is required to sustain this denitrification process in

the anaerobic zone if runoff contains high levels of nitrate-N. Removal of 87%

nitrate-N could be achieved by providing a C-to-nitrate-N ratio of 2.0; that is

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favorable for denitrification in the biphasic rain gardens—especially in the

anaerobic zone (Chapter 4).

4. Higher organic matter content in soil media increased removal efficiency of

nitrate-N through denitrification in the anaerobic zone. However, relatively low

available N content (%) media are recommended in the aerobic zone to prevent

potential leaching of nitrate-N. Plant uptake and harvest could also enhance

nitrate-N removal and/or minimize leaching of nitrate-N from the aerobic zone

(Chapter 5).

Future studies should focus on the role of plants in stormwater flow management and pollutant bioremediation in biphasic rain gardens. Theoretically, roots will help to

promote runoff infiltration and also support microbial populations that may be beneficial

to pollutant degradation. Research on plant selection for phytoremediation may prove

beneficial in the decomposition of organic pollutants and in the uptake of metals. Studies

of soil microbial community responses to pollutant exposure in rain gardens using molecular methods will lead to better understanding of biological processes and will provide useful means to investigate the potential for possible bioremediation pathways of other organic runoff pollutants in future studies. Research on the use of specific media and design configurations to reduce certain target pollutants are other possible research

areas.

Overall, this study shows that increased retention time of runoff pollutants, as

determined by design configuration and rainfall size, intensity, and interval, significantly increased overall nutrient and herbicide removal in the biphasic rain gardens. By creating

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more efficient hydraulic and pollutant removal capacity of the biphasic rain gardens

compared to the conventional rain gardens, a more rapid implementation of biphasic rain gardens as an effective stormwater best management practice will be stimulated.

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Chung, K.H., Ro, K.S., Roy, D., 1996. Fate and enhancement of atrazine biotransformation in anaerobic wetland sediment. Water Research 30, 341-346.

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