From the Bay Watershed to Your Next Development Site. A Nutrient Primer for Stormwater Designers in the Chesapeake Bay Watershed

Transcription

1 From the Bay Watershed to Your Next Development Site A Nutrient Primer for Stormwater Designers in the Chesapeake Bay Watershed

2 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

3 Basics of Eutrophication: the role of excess nutrients in water quality

4 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

5 The Nitrogen and Phosphorus Cycles

6 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

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

9 Nitrogen is transformed many times throughout the nitrogen cycle

10 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

11 Phosphorus Cycle Elemental P very reactive readily reacts with oxygen (O) Phosphates Orthophosphate, is the simplest phosphate (P + 40 = PO 4 ) 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 welldeveloped recycling of P in organic forms

12 Most phosphate compounds exists as particulates Dissolved or soluble P concentrations are low Phosphorus Cycle Inorganic P (fertilizers) Manure (organic, inorganic) Decay OM (organic P) Mineralization of organic P by microbes Plant uptake (as PO 4-3 ) Inorganic P

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

14 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

15 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)_

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

18 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)

19 Total Phosphorus Loads By Sector in Maryland Portion of Bay Watershed Sector 2009 Load Target Load Million pounds per year % Reduction Needed to Meet Target Forest Atm. Deposition Wastewater Urban and Suburb an 0.68 (22%) % Agricultural % Septics TOTAL % 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.

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

21 Sector Total Nitrogen Loads By Sector in the Maryland Portion of the Chesapeake Bay Watershed 2009 Load Target Load Million pounds per year % Reduction Needed to Meet Target Forest Atm. Deposition Wastewater % Urban and Suburban Runoff 5.88 (12%) % Agricultural % Septics % TOTAL % Source: US EPA Chesapeake Bay Program, 2010 Urban runoff is not quite as significant when it comes to nitrogen inputs into Bay

22 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

23 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 Phosphorus (as dissolved P) ** current technology in Chesapeake Bay Note that untreated urban stormwater runoff has about the same concentration as treated wastewater

24 Urban Nutrient Loads Are Fast Becoming a Big Slice of the Bay Pie Year Total N Total P % 5% % 15% % 22% 2030???? Urban and suburban runoff is the only Bay nutrient load sector where we are seeing reverse progress In load reductions- source OIG (2007)

25 So what are the implications of the baywide 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

26 Urban Nutrient Sources: Where does it come from?

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

28 Much of the nitrogen in urban runoff is derived from atmospheric deposition, either in the form of dryfall or wetfall Relationship of Atmospheric Deposition to Urban Runoff Quality Nutrient Atmospheric Deposition 1 Stormwater Runoff Load 2 Pounds per impervious acre per year Total Phosphorus Total Nitrogen 13 to 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

29 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

30 Nitrogen EMCs for different urban land covers Urban Land Cover Source; CWP, 2003 Total N (mg/l) Lawns 9.70 Highway 2.95 Streets (Variable) 1.40 Parking Lots 1.94 Rooftops 1.50 Runoff sampling shows that lawn runoff is very high in nitrogen. Also, rooftop runoff concentration shows effect of atmospheric deposition

31 Phosphorus EMCs for different urban land covers Urban Land Cover Source; CWP, 2003 Total P (mg/l) Lawns 1.90 Highway 0.60 Streets (Variable) 0.50 Parking Lots 0.16 Rooftops 0.12 The sources of phosphorus are more complex. While lawn runoff is high in nitrogen, atmospheric deposition is less important as a source of TP

32 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

33 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

34 What do we know about home lawns and nutrients? 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

35 may june july august sept. nov. Total P, Mg/L Spring Fall Seasonal Changes in Phosphorus Sources From Bannerman

36 Total P, mg/l Effect of Tree Canopy on Levels of Total P in Street Runoff Percent Tree Canopy Source Pitt and Bannerman

37 Urban Nutrient Concentrations

38 Event Mean Concentrations (EMCs) The mean concentration of a pollutant over the entire storm hydrograph measured over many storms in the field.

39 Variations in Urban EMCs Variability from Storm to Storm Land Use Urban Land Use Urban Land Cover

40 Phosphorus EMCs by General Land Use Land Use Total P (mg/l) Soluble P (mg/l) Urban Cropland Forest Urban runoff has a TP concentration slightly higher than cropland runoff

41 Nitrogen EMCs by General Land Use Land Use Total N (mg/l) Soluble N (mg/l) Urban Cropland Forest When it comes to total nitrogen concentrations, however, cropland runoff is much stronger than urban runoff

42 Total Phosphorus Concentration in Stormwater Runoff 37 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

43 Phosphorus EMCs for different urban land uses Urban Land Use Total P (mg/l) Residential 0.30 Commercial 0.22 Industrial 0.26 Freeway 0.25 Source: Pitt et al 2004 Residential runoff is slightly higher in TP concentration, which reflects the effect of vegetation and fertilization

44 Nitrogen EMCs for different urban land uses Urban Land Use Total N (mg/l) Residential 2.1 Commercial 2.1 Industrial 2.1 Freeway 2.3 Source: 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

45 Urban Nutrient Loads

46 The Universal Load Equation Load = (F) x (C) x (TCF) Load = annual pollutant load in pounds/year F C = flow in cubic feet per second = average pollutant concentration in milligrams/liter -- oops-slipped into metric units here TCF = tricky conversion factor

47 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 runoff Rv = runoff coefficient C = Pollutant concentration EMC (mg/l) A = Site Area (acres) 2.72 = Unit conversion factor

48 Runoff Coefficient (Rv) Relationship of IC and Runoff Coefficient (Rv) 1 Relationship Between Watershed Imperviousness (I) and the Storm Runoff Coefficient (Rv) (Source: Schueler, 1987) Rv = I Watershed Imperviousness (%)

49 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

50 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 related to land use area. Not all will occur in a watershed

51 Effect of Stormwater Practices on Reducing Nutrient Loads

52 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

53 Why Can Percent Removal be Misleading??

54 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

55 Typical Phosphorus Removal Efficiencies (CWP, 2008)) Practice Group TP (%) Sol P (%) Dry Ponds 20-3 Wet Ponds Wetlands Infiltration* Filtering Systems 59 3 Water Quality Swales* 24-38

56 Typical Nitrogen Removal Efficiencies (CWP, 2008) Practice Group TN (%) Sol N (%) Dry Ponds 24-9 Wet Ponds Wetlands Infiltration* 42 0 Filtering Systems Bioretention 46 43

57 Wet Pond Performance Median concentrations, International BMP Database (n=25) TSS TP DP TN NO3 Cu Zn Bacteria mg/l mg/l mg/l mg/l mg/l ug/l ug/l Median Influent na Median Effluent na

58 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

59 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

60 Irreducible P Concentrations Discharged from Stormwater Practices Practice Group TP (mg/l) Sol P (mg/l) Dry Ponds Wet Ponds Wetlands Filtering practices Water Quality Swales Untreated runoff Source: Winer (2003)

61 Irreducible N Concentrations from Stormwater Practices Practice Group TN (mg/l) Sol N (mg/l) Bioretention Wet Ponds Wetlands Filtering practices Water Quality Swales Untreated runoff Source: Winer (2003)

62 Design Approaches to Maximize Urban Nutrient Reduction

63 Removal Efficiency (%) The Interesting Case of Bioretention 150 Bioretention Removal Efficiencies TSS TP Sol P TN NOx Cu Zn Bacteria Based on initial data, it looks as if bioretention has zero or negative removal rates

64 P leaching from early bioretention sites 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

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

66 Volumetric Runoff Reduction Achieved by Bioretention Location % Runoff Reference Reduction 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

67 Bioretention Mass Load Performance % Runoff Reduction Level 1 = 40 Level 2 = 80 % Pollutant Removal (TP) Level 1 = 25 Level 2 = 50 Total Removal 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

68 Lessons Learned from bioretention research 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

69 BIORETENTION DESIGN LEVEL 1 DESIGN LEVEL 2 DESIGN RR = 40% TP = 55% TN = 64% RR= 80% TP= 90% TN = 90% TV= (Rv)(A) Filter media at least 24 deep One form of accepted pretreatment At least 75% plant cover One cell design TV= 1.25 (Rv)(A) Filter media at least 36 deep Two or more forms of accepted pretreatment At least 90% plant cover, including trees. 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

70 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

71 Runoff Reduction Rates (%) Infiltration 50 to 90 Bioretention 40 to 80 Pervious 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)

72 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

73 Practice Design Level 1 TN Load Removal 4 TP Load Removal 4 Rooftop Disconnect 1 25 to to Filter Strips 1 25 to to to to 75 Green Roof Rain Tanks & Cisterns 1 15 to to to to 90 Permeable Pavers Infiltration Practices Bioretention Practices Dry Swales Wet Swales Filtering Practices Constructed Wetlands Wet Ponds 1 30 (20) 50 (45) 2 40 (30) 75 (65) ED Ponds

74 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 nonstructural practices have? What are reasonable nutrient reductions for stream restoration?

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