From the Bay Watershed to Your Next Development Site

A Nutrient Primer for Stormwater Designers in the Chesapeake Bay Watershed

Key Themes

• Basics of eutrophication• The nitrogen and phosphorus cycles• Impacts to the Bay and its tributaries • Nutrient sectors in the watershed• Urban nutrient loadings and sources• Effect of stormwater practices on reducing

nutrient loads • Design approaches to maximize nutrient

reduction

Basics of Eutrophication: the role of excess nutrients in water

quality

Excess nutrients are carried into water bodies, where they create algal blooms. When the algae decay, it depletes the amount of oxygen in deeper waters (known as Hypoxia or Anoxia). The low oxygen levels can cause fish kills, harm aquatic life and shade submersed aquatic vegetation

The Nitrogen and Phosphorus Cycles

The Nitrogen Cycle Nitrogen can be added to the cycle through

nitrogen fixing bacteria, fertilizer lightning, wastewater and nutrients

Ammonia and nitrate are forms of nitrogen that are incorporated into plant and animal tissues, and become organic nitrogen

Different bacteria are involved in decomposition of organic matter

Nitrifying bacteria convert ammonia into nitrate Denitrifying bacteria convert nitrate into nitrogen

gas Nitrogen is often the limiting nutrient for algal

growth in estuarine and marine environments

Various microbial processes transform nitrogen into one of four different forms: organic nitrogen, ammonia, nitrate and nitrite.

Nitrogen is transformed many times throughout the nitrogen cycle

Phosphorus Cycle

• Land-based or sedimentary

• Bacteria less important

• Slow cycle – considered one-way land to ocean

• Limiting factor for plant growth in freshwater environments – a little means alot

Phosphorus Cycle

Elemental P very reactive – readily reacts with oxygen (O) Phosphates Orthophosphate, is the simplest

phosphate (P + 40 = PO4) Release of phosphorus from rock

weathering – low P content P availability controlled by degradation of

organic forms of P Biota persist as a result of a well-

developed recycling of P in organic forms

Phosphorus CycleDecay OM(organic P)

Mineralizationof organic Pby microbes

Inorganic P

Inorganic P(fertilizers)

Most phosphate compounds exists as particulatesDissolved or soluble P concentrations are low

Manure(organic, inorganic)

Plant uptake(as PO4

-3)

Impacts of Nutrient Over-enrichment on the Bay and its tributaries

Nutrient Enrichment and Streams

•Most streams are TP-limited•EPA recommends TP levels for streams less than 0.1 mg/l•P-enriched streams have:

•higher biomass•less biological diversity•More bottom algae and rock scum•Lower oxygen levels

Effect of Eutrophication on Bay

• Increased hypoxia and anoxia in main trench

• Decline is submersed aquatic plants

• Reduced water clarity

• Increased frequency of algal blooms

• Decline in benthic life

• Fish kills

• Harmful algal blooms (e.g., Pfsteria)_

Note how much of the main bay is now subject to hypoxic or anoxic conditions these days (panel b)

The main sources of nutrients to the Bay Watershed are:

•Runoff from Forests •Wastewater•Atmospheric Deposition to Open Water•Urban and Suburban Runoff•Agricultural Runoff •Septic Systems (N only)

Total Phosphorus Loads By Sector in Maryland Portion of Bay Watershed

Sector2009Load

Target Load % Reduction Needed to Meet Target

Million pounds per year

Forest 0.34 0.34 0

Atm. Deposition 0.04 0.04 0

Wastewater 0.67 0.70 0

Urban and Suburb an 0.68 (22%) 0.39 43%

Agricultural 1.37 1.25 9%

Septics -0- -0- 0

TOTAL 3.10 2.72 12%

Source: US EPA Chesapeake Bay Program, 2010

The EPA Bay Model Shows Where the Nutrients are Coming From: Note that Urban Runoff is a major Source of Phosphorus to Bay Watershed, and that cuts of 43% are needed to meet the Bay nutrient “diet” or TMDL.

Year to year nutrients loads reflect annual rainfall..in wet years loads increase due to stormwater runoff

Total Nitrogen Loads By Sector in the Maryland Portion of the Chesapeake Bay Watershed

Sector2009Load

Target Load % Reduction Needed to Meet Target

Million pounds per year

Forest 7.00 7.00 0

Atm. Deposition 0.69 0.69 0

Wastewater 13.9 10.94 21%

Urban and Suburban Runoff 5.88 (12%) 4.42 25%

Agricultural 18.05 13.59 25%

Septics 4.01 -0- 39%

TOTAL 49.53 2.46 21%

Source: US EPA Chesapeake Bay Program, 2010

Urban runoff is not quite as significant when it comes to nitrogen inputs into Bay

Years ago, much of the Bay nutrient load came from wastewater discharges….Now, due to improvements in treatment technology, wastewater and urban stormwater are roughly equal on the nutrient loads they deliver to the bay

Strengths of nitrogen and phosphorus in natural waters, stormwater and wastewater* (expressed in mg/L)

Parameter Natural Waters

Urban Stormwater

Untreated sewage

CSOs Treated sewage**

Nitrogen 0.1(as NO-3) 2 to 3 20 3-24 3

Phosphorus 0.005 – 0.05 (as dissolved P)

0.2 – 0.5 10 1-11 0.1

** current technology in Chesapeake Bay

Note that untreated urban stormwater runoff has about the same concentration as treated wastewater

Urban Nutrient Loads Are Fast Becoming a Big Slice of the Bay Pie

Year Total N Total P

1985 2% 5%

2000 9% 15%

2009 12% 22%

2030 ?? ??

Urban and suburban runoff is the only Bay nutrient load sector where we are seeing reverse progressIn load reductions- source OIG (2007)

So what are the implications of the bay-wide nutrient TMDL for localities?

EPA and the states will require localities to:

•Extensively retrofit existing development to achieve nutrient reductions

•Implement more stringent performance standards for new development so it doesn’t increase their nutrient load

Urban Nutrient Sources:

Where does it come from?

There are many sources of N and P in the urban environment

Relationship of Atmospheric Deposition to Urban Runoff Quality

NutrientAtmospheric Deposition 1

Stormwater Runoff Load 2

Pounds per impervious acre per yearTotal Phosphorus 0.7 2.0Total Nitrogen 13 to 17.0 3 15.41 measured rates during Washington NURP Study (MWCOG, 1983)2 Simple Method annual stormwater runoff loads for one acre of impervious cover (Schueler, 1987)3 About 40% of nitrogen deposition occurs through wetfall, which would presumably be quickly converted into runoff. 60% of nitrogen deposition occurs via dryfall, which is available for washoff in future storms, or may be blown over to pervious areas

Much of the nitrogen in urban runoff is derived from atmospheric deposition, either in the form of dryfall or wetfall

Other sources of nitrogen in urban runoff include:

•Washoff of fertilizers•Nitrogen attached to eroded soils and streambanks•Organic matter and pet wastes on IC

Nitrogen EMCs for different urban land covers

Urban Land Cover Total N (mg/l)

Lawns 9.70

Highway 2.95

Streets (Variable) 1.40

Parking Lots 1.94

Rooftops 1.50

Source; CWP, 2003

Runoff sampling shows that lawn runoff is very high in nitrogen. Also, rooftop runoff concentration shows effect of atmospheric deposition

Phosphorus EMCs for different urban land covers

Urban Land Cover Total P (mg/l)

Lawns 1.90

Highway 0.60

Streets (Variable) 0.50

Parking Lots 0.16

Rooftops 0.12

Source; CWP, 2003

The sources of phosphorus are more complex. While lawn runoff is high in nitrogen, atmospheric deposition is less important as a source of TP

Many sources of TP in urban runoff

• Blow in of organic matter onto impervious surfaces (leaves, pollen, clippings, flowers, etc.)

• Phosphorus attached to eroded soils and streambanks

• Fertilizer washoff

• Atmospheric deposition

What we know about turf and its management in the Bay Watershed

•3.8 million acres of turf•Represents 9.5% of watershed area•Exceeds area devoted to row crops (corn, wheat, soybeans)•75% of turf is home lawn

About 50% to 65% fertilize their yard

15 to 20% hire lawn care company

Average of two applications per year

50% of homeowners over-fertilize

Estimated N Fertilizer inputs by lawns: 215 million lbs/yr

What do we know about home lawns and nutrients?

Seasonal Changes in Phosphorus Sources

From Bannerman

SpringFall

0

0.2

0.4

0.6

0.8

1

1.2

To

ta

l P

, M

g/L

ma

y

ju

ne

ju

ly

au

gu

st

se

pt.

no

v.

Effect of Tree Canopy on Levels of Total P in Street Runoff

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80 100

Percent Tree Canopy

To

tal

P,

mg

/l

Source Pitt and Bannerman

Urban Nutrient Concentrations

Event Mean Concentrations (EMCs) The mean concentration of a pollutant over the

entire storm hydrograph measured over many storms in the field.

Variations in Urban EMCs

Variability from

Storm to Storm

Land Use

Urban Land Use

Urban Land Cover

Phosphorus EMCs by General Land Use

Land Use Total P

(mg/l)

Soluble P

(mg/l)

Urban 0.30 0.16

Cropland 0.25 0.10

Forest 0.05 0.01

Urban runoff has a TP concentration slightly higher than cropland runoff

Nitrogen EMCs by General Land Use

Land UseTotal N

(mg/l)

Soluble N

(mg/l)

Urban 2.0 0.70

Cropland 5.0 3.0

Forest 1.0 1.0

When it comes to total nitrogen concentrations, however, cropland runoff is much “stronger” than urban runoff

Total Phosphorus Concentration in Stormwater Runoff37 Residential Watersheds Across the United States

TP concentrations do not vary a lot across the country, although somewhat higher levels are seen in arid and snowmelt regions

Source: CWP, 1996

Phosphorus EMCs for different urban land uses

Urban Land Use Total P (mg/l)

Residential 0.30

Commercial 0.22

Industrial 0.26

Freeway 0.25Source: Pitt et al 2004

Residential runoff is slightly higher in TP concentration, which reflects the effect of vegetation and fertilization

Nitrogen EMCs for different urban land uses

Urban Land Use Total N (mg/l)

Residential 2.1

Commercial 2.1

Industrial 2.1

Freeway 2.3Source: Pitt et al 2004

Not a lot of difference among urban land uses for total nitrogen, which suggests that primacy of atmospheric deposition as a nitrogen source

Urban Nutrient Loads

The Universal Load Equation

Load = (F) x (C) x (TCF)Load = annual pollutant load in

pounds/year

F = flow in cubic feet per second

C = average pollutant concentration in milligrams/liter -- oops-slipped into metric units here

TCF = tricky conversion factor

The Simple Method (Schueler, 1987)

L = [ P Pj Rv/12 ] [ C A 2.72]Where:

L = Annual load (lbs)P = Annual rainfall (in)Pj = Fraction of storms producing runoffRv = runoff coefficient C = Pollutant concentration EMC (mg/l)A = Site Area (acres)2.72 = Unit conversion factor

Relationship Between Watershed Imperviousness (I)

and the Storm Runoff Coefficient (Rv)(Source: Schueler, 1987)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100

Watershed Imperviousness (%)

Ru

no

ff C

oe

ffic

ien

t (R

v)

Relationship of IC and Runoff Coefficient (Rv)

Rv = 0.05+0.009I

Urban Nutrient Loads

The nutrient EMC does not change as site impervious cover increases

The discharge of runoff, however, increases directly with impervious cover (IC)

Consequently, nutrient loads directly increase in relation to site IC

There can be other significant urban nutrient sources that are not directly to stormwater

• Septic Systems

• Sanitary Sewer Overflows (SSOs)

• Combined Sewer Overflows (CSOs)

• Illicit Discharges

• Channel Erosion

• Marinas

• Road Sanding

*These sources have a flow that is not directly relatedto land use area. Not all will occur in a watershed

Effect of Stormwater Practices on Reducing Nutrient Loads

Shifting Away from Percent Removal

• Calculations of percent removal are common in the stormwater world– Myth #1. EPA/CWA

requires 80% removal for stormwater.

– Myth #2. Percent removal is a good way to demonstrate BMP performance.

• Percent removal can be very misleading…

Why Can Percent Removal be Misleading?

?

Nevertheless, when percent removal is compared among different groups of

stormwater practices, it provides reveals general insights as to their

capability to remove nutrients, as the next two tables show

Typical Phosphorus Removal Efficiencies (CWP, 2008))

Practice Group TP (%) Sol P (%)

Dry Ponds 20 - 3

Wet Ponds 52 64

Wetlands 48 24

Infiltration* 70 85

Filtering Systems 59 3

Water Quality Swales*

24 -38

Typical Nitrogen Removal Efficiencies (CWP, 2008)

Practice Group TN (%) Sol N (%)

Dry Ponds 24 -9

Wet Ponds 31 45

Wetlands 24 67

Infiltration* 42 0

Filtering Systems 32 -14

Bioretention 46 43

Wet Pond Performance

TSS TP DP TN NO3 Cu Zn Bacteria

mg/L mg/L mg/L mg/L mg/L ug/L ug/L

Median Influent 37.73 0.21 0.09 1.64 0.38 9.84 60.75 na

Median Effluent 9.74 0.1 0.05 1.31 0.20 5.82 21.58 na

Median concentrations, International BMP Database (n=25)

Variations in Removal Efficiency

• Storm to Storm (negative to 100%)

• Among Different BMPs in the Same Group

• Studies only measure reductions within the practice (e.g. do not report untreated stormwater bypassed)

Note: Most monitoring studies are biased towards newer, well designed practices– we need to discount the ideal rate later to address poor installation or maintenance

There are clearly limits on how much current BMP can change the concentrations of nutrients

as they flow through a practice

• Fine particles may not settle out

• Internal re-packaging of nutrients

• Biological activity

• Nutrient leaching

“Irreducible” P Concentrations Discharged from Stormwater Practices

Practice Group TP (mg/L) Sol P (mg/L)

Dry Ponds 0.19 0.13

Wet Ponds 0.13 0.06

Wetlands 0.17 0.09

Filtering practices 0.16 0.06

Water Quality Swales

0.21 0.09

Untreated runoff 0.30 0.16

Source: Winer (2003)

“Irreducible” N Concentrations from Stormwater Practices

Practice Group TN (mg/L) Sol N (mg/L)

Bioretention 0.8 0.25

Wet Ponds 1.3 0.26

Wetlands 1.5 0.36

Filtering practices 1.1 0.60

Water Quality Swales

1.1 0.35

Untreated runoff 2.0 0.6

Source: Winer (2003)

Design Approaches to Maximize Urban Nutrient Reduction

Bioretention Removal Efficiencies

-150

-100

-50

0

50

100

150

TSS TP Sol P TN NOx Cu Zn Bacteria

Rem

oval

Eff

icie

ncy (

%)

The Interesting Case of Bioretention

Based on initial data, it looks as if bioretention has zero or negative removal rates

P leaching from early bioretentionsites

• Early bioretention media was 30% organic and topsoil had very high soil P index

• P leached through media so that it actually added more P to outflow

• Testing of low organic media (5%) and topsoil P testing showed sharp improvements in TP EMC reduction

• 2-4 feet media depth • 3-5% carbon source in media • Create anoxic bottom layer to promote denitrification• Increased hydraulic residence time through media (1-2 in/hr)• Test media to ensure low P-index

New research provides insights into bioretention design features that boost TN and TP removal

Volumetric Runoff Reduction Achieved by BioretentionLocation % Runoff

ReductionReference

Bioretention * CT 99% Dietz and Clausen (2006) Bioretention * PA 86% Ermilio (2005)Bioretention * FL 98% Rushton (2002)Bioretention * PA 80% Traver et al (2006)Bioretention * AUS 73% Lloyd et al (2002) Bioretention # ONT 40% Van Seters et al (2006)Bioretention # Model 30% Perez-Perdini et al (2005)Bioretention # NC 40 to 60% Smith and Hunt (2006)Bioretention # NC 20 to 29% Sharkey (2006) Bioretention # NC 52 to 56% Hunt et al (2006)Bioretention # NC 20 to 50% Passeport et al (2008)Bioretention # MD 52 to 65% Davis (2008)

Runoff Reduction Estimate 40 # to 80 *# underdrain design *infiltration design

Further research documents that bioretention can sharply reduce runoff volumes, which, in turn increases, the mass of nutrients which are removed

Bioretention Mass Load Performance

% Runoff Reduction

% Pollutant Removal (TP)

Total Removal

Level 1 = 40

Level 2 = 80

Level 1 = 25

Level 2 = 50

Level 1 = 55

Level 2 = 90

The performance of a BMP is now viewed as its combined capability to reduce runoff volume and reduce the nutrient EMC as it passes through the practice.

Together, these two factors determine how much mass of nutrients is reduced from a site

Lessons Learned from bioretentionresearch

• BMPs are not a black box; we can maximize nutrient removal through: – Internal design features

– Greater runoff reduction

– Increased hydraulic retention time

– Appropriate media recipe

BIORETENTION DESIGN

LEVEL 1 DESIGN LEVEL 2 DESIGN

RR = 40% TP = 55% TN = 64% RR= 80% TP= 90% TN = 90%

TV= (Rv)(A) TV= 1.25 (Rv)(A)

Filter media at least 24” deep Filter media at least 36” deep

One form of accepted pretreatment

Two or more forms of accepted pretreatment

At least 75% plant cover At least 90% plant cover, including trees.

One cell design Two cell design

Underdrain Infiltration design or underground stone sump

Both: Maximum organic material in media of 5% and hydraulic residence time of 1 inch per hour through media (10% fines)

We are now in an design era where we can isolate the design features that maximize runoff reduction and mass nutrient removal

The Role of Runoff Reduction in Increasing Mass Removal of Nutrients

Monitoring indicates that some practices are very effective in reducing the volume of runoff, which sharply increases the mass reduction of nutrients from sites

Runoff reduction is defined as the total volume reduced through canopy interception, soil infiltration, evaporation, rainfall harvesting, engineered infiltration, extended filtration or evapotranspiration

Runoff Reduction Rates (%)Infiltration 50 to 90 Bioretention 40 to 80Pervious Pavers 45 to 75 Green Roof 45 to 60 Dry Swale 40 to 60 Rain Tanks/Cisterns 40 Roof Disconnection 25 to 50 Grass Channel 15 to 30 Dry ED Pond 0 to 15 Wet Pond 0 Sand Filter 0

Source: CWP and CSN (2008)

Many Bay States are Now Shifting to Level 1 and 2 Design Criteria for BMPs

• Identifies Specific Design Features that Maximize Runoff Reduction

• Provides More Reliable Estimates of Expected TP and TN Removal

Practice DesignLevel1

TN LoadRemoval4

TP LoadRemoval4

Rooftop Disconnect 1 25 to 50 25 to 50 2 6 50 50

Filter Strips 1 25 to 50 25 to 502 6 50 to 75 50 to 75

Green Roof 1 45 452 60 60

Rain Tanks & Cisterns 1 15 to 60 15 to 602 45 to 90 45 to 90

Permeable Pavers 1 59 592 81 81

Infiltration Practices 1 57 632 92 93

Bioretention Practices 1 64 552 90 90

Dry Swales 1 55 522 74 76

Wet Swales 1 25 202 35 40

Filtering Practices 1 30 602 45 65

Constructed Wetlands 1 25 502 55 75

Wet Ponds 1 30 (20) 50 (45)2 40 (30) 75 (65)

ED Ponds 1 10 152 24 31

Future N and P Accounting Issues

• How do you account for decline in removal over time due to age and lack of maintenance?

• What sort of removal rates do non-structural practices have?

• What are reasonable nutrient reductions for stream restoration?