An LID favorite: Does bioretention actually work for a ...

www.stormwater.bae.ncsu.edu

Jeffrey Johnson, PhD, EIExtension Associate

An LID favorite:Does bioretention actually

work for a long time?

NC State Stormwater Engineering Group Research Update – June 2020

This presentation is copyrighted property of North Carolina State University. All rights reserved.


The first BRCs…• 1990 – “Invention” in Prince George’s County, Maryland• 1993 – First BRC (rain garden) design guidance

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Conventional Design

Figure: Shawn Kennedy

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Fast forward 30 years…• BRCs are one of the most common SCMs in the United

States, Australia, and Asia

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Fast forward 30 years…• We know that BRCs work:

– Hydrologic & nutrient benefits• (Davis et al. 2009; Hunt et al. 2012)

– Metals, pathogens, & organics• (Davis et al. 2003; Gülbaz et al. 2015; Jang et al. 2005; Jones and Davis

2012; Li and Davis 2008; Paus et al. 2013; Sun and Davis 2007)

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Fast forward 30 years…• We know that BRCs work:

– But how long do these benefits last?

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Aging Bioretention HydrologyComing soon to an ASCEjournal near you…

• Examined hydrologic mitigation from 3 “aging” BRCs

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“Young” vs. “Old”Peak Flow

Young: 0.10 Old: 0.09

Volume Reduction:

Young: 0.35Old: 0.25

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Aging Bioretention Water Quality

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Results

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What’s Different?

Vegetation is our long-term friend

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• TN and TP are now below assigned values for nutrient reduction calculations from NC DEQ

• Not only does bioretention work, it can get even better with time and may even be undervalued

Period TN (mg/L) TP (mg/L)NC DEQ Credit 1.20 0.122002 – 2003 1.23 0.172017 – 2018 1.12 0.09

NC DEQ Credit Comparisons

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Aging Bioretention Media

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Mass Media Sampling

28 sampled BRCs

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Carbon:Nitrogen

y = 23.063e-0.035x

R² = 0.25760

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Car

bon:

Nitr

ogen

Bioretention Age(Years)

Target C:N for N removal = 20(Chapin et al. 2011)

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P-Index

• Note: P-Index estimated from TP

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Top Variable Importance: P-Index

Top Variable Importance: C:N18



Forebays: A Difference Maker

Good for P removal

Good for N removal

Target C:N

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Bioretention Take Home Points• If they installed correctly

and appropriately maintained…– BRCs can provide

long-term hydrologic mitigation and water quality improvement

– They can even improve over time thanks to bioturbation

– If feasible, forebaysare a good idea for future designs

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Wet Pond Improvements

Jeffrey Johnson, PhD, EIExtension Associate

NC State Stormwater Engineering Group Research Update – June 2020



20,000+ wet ponds in NC…23



And room for improvement…Pond Name Reference TSS Nitrate TKN TN OP TP

Echo FarmsMallin et al., 2002

-22% -83% -300% -41% -76% -35%

Ann McCrary 65% 5% 29% -4% 7% 23%

Silver Stream -37% 63% 83% 40% 77% 57%

LakesideWu et al., 1996

93% 31% 43%

Waterford 68%

Runaway Bay 64% 20% 33%

DOT Winston et al., 2013

88% 67% 25% 38% -50% 13%

Museum 82% 0% 50% 50% 22% 39%

However, remember that ER can be misleading

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And room for improvement…

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And room for improvement…

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Floating Treatment Wetlands

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(Relatively) Lots of

Research

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Hydroponic System29



A global phenomenon

New Zealand

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A global phenomenon

China

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Potential Advantages of FTWs• Does not compromise

current pond’s ability to mitigate peak flow

• Still allows sedimentation

• No heavy equipment necessary

• Improved habitat and biodiversity

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Wet Ponds: Pre-Retrofit

Museum of Life and Science I-85 DOT Pond

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Monitoring Design - DOT

InletOutlet

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Monitoring Design - Museum

Outlet

Inlet

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Floating Wetland Islands38



5 Months Later

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Total Nitrogen Results

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

DOT DOT with FWI Museum Museum with FWI

Conc

entr

atio

n (m

g/L)

Inlet Outlet

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TP – Effluent Concentrations

0.000.050.100.150.200.250.300.350.400.450.50

0 20 40 60 80 100

TP C

once

ntra

tion

(mg/

L)

Cumulative Probability (%)

DOT Outlet Pre DOT Outlet PostMuseum Outlet Pre Museum Outlet Post

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TSS – Effluent Concentrations

0

20

40

60

80

100

120

140

0 20 40 60 80 100

TSS

Conc

entr

atio

n (m

g/L)

Cumulative Probability (%)

DOT Outlet Pre DOT Outlet PostMuseum Outlet Pre Museum Outlet Post

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Final Thoughts

• FTW show significant (but minor) benefit to effluent TP & TSS concentration.

• FTW coverage needs to be >20% to observe a water quality benefit

• Also a biodiversity benefit (vertebrates, biofilms)

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Aquatic Shelf… Waiting to be retrofit?

Can the shelf be home to a retrofit?

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Littoral Shelf Filters• Drawbacks:

– Sacrifices some of pond’s volume

• Possible Benefits:– Potential nutrient

removal benefits (filtering)

45



Brid

gew

ood

Sapp

hie

Littoral Shelf Sand Filters46



Catchbasin Inlet Sand Filter Underdrain

Outlet Structure

Monitoring

Manual and tipping bucket rain gauges

Measured flow over weir installed in baffle box

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Bridgewood – TP Concentrations

Target TP Concentration

0.000.050.100.150.200.250.300.350.40

0% 20% 40% 60% 80% 100%

Concen

tration (m

g/L)

Cumulative Probability

Pond In Pond Out Littoral Shelf Out "Good" Threshold

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Bridgewood - TN Concentrations

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0% 20% 40% 60% 80% 100%

Concen

tration (m

g/L)



Target TN Concentration

49



Sapphire – TN Concentrations

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0% 20% 40% 60% 80% 100%

Concen

tration (m

g/L)



Target TN Concentration

50



Sapphire - TP Concentrations

0.00

0.10

0.20

0.30

0.40

0.50

0% 20% 40% 60% 80% 100%

Concen

tration (m

g/L)



Target TP Concentration

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Before Maintenance After Maintenance 52



Shelf Filters: Final Thoughts

• Littoral sand filters demonstrated reductions for both total nitrogen and total phosphorous.

• A maintenance schedule needs to be prepared and carried out in advance

• If the pond if submerged for long periods of time, extra maintenance is advised to remove potential surface clogging of the media

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Thanks to our project sponsors

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SCM Retrofit and Rehab Workshop55


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