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Pollutants in harvested water

The composition of stormwater is highly variable in space and time due to differences in land use and rainfall events. This variability is an extremely important consideration when evaluating the feasibility of a stormwater harvesting and use system and determining what level of treatment is necessary to achieve the water quality criteria of the end use.

Common pollutants in stormwater runoff include nutrients, sediments, heavy metals, salinity, pathogens, and hydrocarbons (Typical stormwater pollutants, summary of sources and potential concerns for harvest and use table below). Water quality of stormwater varies depending on the type of land uses in the drainage area, such as commercial, industrial, residential and parks/open spaces. Typical urban stormwater quality characteristics for the Twin Cities and two other cities are summarized in the Typical Annual and Snowmelt Urban Stormwater Quality Characteristics table below.

Typical stormwater pollutants, summary of sources and potential concerns for harvest and use
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Pollutant Sources Potential Concerns
Nutrients
  • Nitrogen
  • Phosphorus
  • atmospheric deposition, sediment (adsorbed nutrients)
  • organic debris
  • fertilizer runoff
  • animal feces
  • combined sewer overflows
  • Support growth of algae or unwanted microbial growth on the water surface in the storage unit.
Organic Matter
  • Organic debris (leaves, flowers, pollen, twigs, insect carcasses, etc.)
  • Decomposition in tank can result in low dissolved oxygen levels, nuisance odors, and release of pollutants from sediments
Suspended Sediment
  • paved surfaces
  • areas of bare soil/poor vegetative cover
  • construction activity
  • stockpiles
  • May clog pump intake or distribution
  • Increases maintenance of storage
Chlorides
  • De-icing chemicals
  • water softening chemicals
  • Corrosive to metal pipes/plumbing
  • Plant toxicity (irrigation)
Pathogens
  • animal feces (including bird feces on rooftops)
  • insects/vector organisms
  • drainage area activities such as waste management
  • sewage overflows or leaking sewers
  • Human health risk
Metals
  • Vehicle exhaust
  • Roofing materials
  • Drainage area activities that are potential sources of metals (e.g. vehicle fueling or repair)
  • Plant toxicity
Organic Chemicals
  • Pesticides/herbicides
  • Industrial chemicals and solvents
  • Petroleum-derived chemicals
  • Drainage area activities that are potential sources of organics (e.g. herbicide/ pesticide use or waste management)
  • Plant toxicity
  • Human health risk
  • Animal health risk


Typical concentrations of pollutants in stormwater runoff and snowmelt runoff for select cities
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Constituent (concentrations reported in mg/L) Annual Twin Cities Snowmelt4
Twin Cities1 Marquette MI2 Madison WI Storm Sewers Open Channels Creeks NURP5
Cadmium 0.0006 0.0004
Copper 0.022 0.016
Lead 0.060 0.049 0.032 0.16 0.2 0.08 0.18
Zinc 0.111 0.203
Biological Oxygen Demand 15
Chemical Oxygen Demand 169 66 169 82 84 91
Total Kjeldahl Nitrogen 2.62 1.50 3.52 2.36 3.99 2.35
Nitrate + Nitrite 0.53 0.37 1.04 0.89 0.65 0.96
Ammonia 0.2
Total Phosphorus 0.58 0.29 0.66 0.7 0.56 0.54 0.46
Dissolved Phosphorus 0.20 0.04 0.27 0.25 0.18 0.16
Chloride3 230 49 116
Total Suspended Solids 184 159 262 148 88 64
Volatile Suspended Solids 66 46 15

1 Event mean concentrations; Reference: Brezonik and Stadelmann 2002
2 Geometric mean concentrations; Reference: Steuer et al. 1997
3 Geometric mean concentrations; Reference: Waschbusch et al. 1999
4 Reference: Oberts, G. (Met Council). 2000. Influence of Snowmelt Dynamics on Stormwater Runoff Quality.
5 Reference: Median concentrations from more than 2,300 rainfall events monitored across the nation; EPA, 1983


Water Quality Considerations of Runoff from Specific Source Areas:

The source area from which stormwater is collected largely determines the water quality characteristics of harvested stormwater (Summary of pollutants typically found in stormwater by source area below). Most stormwater is collected from a mix of source areas, however stormwater harvested for use can often be collected from one dominant source area since the catchment area of the systems tend to be smaller than other larger scale stormwater BMPs. This section discusses the unique water quality considerations for stormwater harvest and use systems for the following source areas

  • Hard roofs
  • Green and brown roofs
  • Paved surfaces
  • Green spaces

Hard Roofs

The Typical Roof Runoff Quality in Minneapolis and Wisconsin table below provides a summary of typical roof runoff quality in Minneapolis and Wisconsin. High metal concentrations in rooftop runoff are a major water quality consideration for harvest and use systems (Concentrations of Zinc, Copper, and Lead in Roof Runoff Based on Roof Material Type Table). Hard rooftops may be composed of a variety of materials (ex. clay/concrete tile, asphalt/composite/wood shingles, metal, slate, or rubberized roofs). Although runoff collected from rooftops is generally high quality compared to other sources of stormwater (NAS 2016), certain roof materials may adversely affect the quality of harvested rainwater (Common roofing materials and water quality considerations Table). Other water quality concerns for rooftops include pathogens which may be found in bird or animal feces and organic litter from tree canopy which may contribute to biological oxygen demand.


Common roofing materials and water quality considerations
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Roofing Material Water Quality Considerations
Metal Roofs
  • Runoff may contain high levels of zinc, copper, and lead (see here)
Sheet Roofing (PVC)
  • Recommended for non-potable use only
Tile roofs (clay, ceramic, cement, fiberglass)
  • Need periodic cleaning - debris may accumulate between tiles
  • Recommended for non-potable use only
Shingles
  • asphalt
  • composite
  • three tab asphalt
  • Recommended for non-potable use only
  • Some shingles manufactured prior to 1980 may contain asbestos in trace amounts
  • May not be suitable for irrigation if shingles have been treated for mold eradication or with herbicides
Shingles – cedar shakes/wood shingles
  • Recommended for irrigation only
  • Shingles retain moisture, support mold, algae, and insects, may be treated with fire retardant or other chemicals


Typical Roof Runoff Quality in Minneapolis and Wisconsin
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Constituent Minneapolis1 Wisconsin2
E. coli (#/100 mL) 764
Total Solids (mg/L) 126
Total Solids (mg/L) 10 19
Total Hardness (mg/L) 44
Total Nitrogen (mg/L) 0.421
Ammonia-N (mg/L) 0.268
Nitrate-N (mg/L) 0.586
Total Phosphorus (mg/L) 0.104 0.24
Total Dissolved Phosphorus (mg/L) 0.076 0.11
Soluble Reactive Phosphorus (mg/L) 0.065
Cadmium (mg/L) 0.0004
Copper (mg/L) 0.0075 0.01
Lead (mg/L) 0.0032 0.01
Zinc (mg/L) 0.101 0.363

1 Arithmetic mean concentrations; Reference: Minneapolis Public Works, City of Minneapolis Neighborhood Rain Barrel Partnership Project, 2008 2 Highest geometric mean concentration reported; Reference: Roger T. Bannerman and Richard Dodds, Sources of Pollutants in Wisconsin Stormwater, 1992


Concentrations of Zinc, Copper, and Lead in Roof Runoff Based on Roof Material Type
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Metal Roof Materials Runoff Concentration (mg/L)
Zinc New uncoated galvanized steel 0.5-10
Old uncoated galvanized steel 1-38
Coated galvanized steel 0.2-1
Uncoated galvanized aluminum 0.2-15
Coated galvanized aluminum 0.1-0.2
Other (aluminum, stainless steel, titanium, polyester, gravel) <0.002
Copper Uncoated copper 0.002-0.175
Uncoated galvanized steel <0.003
Clay tiles 0.003-4
New asphalt shingles 0.01-0.2
New cedar shakes 1.5-27
Aged/patinated copper 0.9-9.7
Lead Uncoated galvanized steel 0.001-2
Coated and uncoated galvanized steel <0.0001-0.006
Painted materials <0.002-0.6

Zinc data: Clark et al. (2008a,b); Faller and Reiss (2005); Förster (1999); Gromaire-Mertz et al. (1999); Heijerick et al. (2002); Mendez et al. (2011); Schriewer et al. (2008); Tobiason (2004); Tobiason and Logan (2000); Zobrist et al. (2000)
Copper data: Clark et al. (2008a); Gromaire-Mertz et al. (1999); Karlen et al. (2002); Wallinder et al. (2009); Zobrist et al. (2000)
Lead data: Clark et al. (2007); Davis and Burns (1999);Förster (1999); Gromaire-Mertz et al. (1999); Good (1993); Gumbs and Dierberg (1985); Mendez et al. (2011); Shriewer et al. (2008)


Green and Brown Roofs

Filtrate from green and brown roofs may require little or no treatment since green and brown roofs are effective at removing sediment, although soluble nutrient concentrations (nitrogen and phosphorus) may be elevated and water may be colored.

Paved Surfaces

Paved surface source areas include parking lots, sidewalks, driveways, and roadways. The Urban Stormwater Quality Characteristics from Paved Surfaces table below provides a summary of water quality characteristics for several types of paved surfaces. Runoff from paved surfaces can contain higher levels of chlorides, solids, and hydrocarbons. Harvest and use systems collecting runoff from paved surfaces will likely require some sort of first flush diverter to bypass very high concentrations of pollutants in spring snowmelt, and potential toxic spills in the drainage area, Treatment may also require filtration units capable of removing fine solids and hydrocarbons.

Urban Stormwater Quality Characteristics from Paved Surfaces
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Constituent (concentrations reported in mg/L) Wisconsin Data1 Twin Cities Highways2
Arterial Street Feeder Street Collector Street Collector Street Residential Driveway
Cadmium 0.0028 0.0008 0.0017 0.0012 0.0005 0.0025
Chromium 0.026 0.007 0.013 0.016 0.002
Copper 0.085 0.025 0.061 0.047 0.02 0.023
Lead 0.085 0.038 0.062 0.062 0.02 0.242
Zinc 0.629 0.245 0.357 0.361 0.113 0.123
Nitrate-Nitrite 0.77
Total Phosphorus 1.01 1.77 1.22 0.48 1.5 0.43
Total Dissolved Phosphorus 0.62 0.55 0.36 0.07 0.87
Chloride3 11.5
Total Suspended Solids 993 1152 544 603 328
Suspended Solids 875 1085 386 474 193

1 Arithmetic mean concentration; Reference: Roger T. Bannerman and Richard Dodds, Sources of Pollutants in Wisconsin Stormwater, 1992
2 Reference: University of Minnesota Water Resources Center, Assessment of Stormwater Best Management Practices Manual, 2008
3 Data represents chloride concentrations during monitoring season, typically April through October. Chloride concentrations in winter snowmelt grab samples have been found to be as great as 3,600 mg/L.


Green Spaces

Green space source areas include lawns and park areas (see ‘Open Space’ land use in the Concentrations of contaminants found in stormwater table). Green spaces typically have lower concentrations of pollutants compared to stormwater source areas. Due to the presence of pets and/or wildlife (particularly Canadian geese), these areas may have very high concentrations of pathogens and require disinfection treatment for certain end uses.

Summary of pollutants typically found in stormwater by source area
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Source Area Solids Total Suspended Solids Particulate Nutrients Dissolved Nutrients Bacteria Metals Chlorides Grease, Oil Pesticides Other Chemicals
Hard Roofs
Green and Brown Roofs
Paved Surfaces
Green Spaces
Sedimentation Basins and Detention Ponds

● = relatively high concentrations
○ = relatively low concentrations



Seasonal Considerations

In addition to variability in stormwater quality from different source areas, stormwater quality also varies with season. The Typical Annual and Snowmelt Urban Stormwater Quality Characteristics table above illustrates water quality characteristics of snowmelt in the Twin Cities Metropolitan Area. Seasonal considerations include the following.

  • Snowmelt can have very high concentrations of chlorides and sediment from winter road de-icing practices (see Typical Annual and Snowmelt Urban Stormwater Quality Characteristics table above).
  • In spring, organic litter including pollen may increase BOD and residual solids on pavement may increase TSS.
  • In fall, leaf litter may contribute to BOD, nutrients and solids.