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CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

Nitrogen in urban streams

One common water quality change associated with urban development is an increase in nutrient concentrations, especially nitrogen (Fig 24, Table 6). Wastewater inputs and stormwater runoff both contribute to increased nitrogen loading in urban catchments. Specific sources of nitrogen in urban systems include:

Human wastes
  • wastewater treatment plant effluents
  • leaky sewer and septic systems
Atmospheric deposition
  • vehicle exhaust
  • other forms of fossil fuel combustion

Fertilizers applied to lawns and golf courses

Pet wastes

Landfill leachates

Legacy sources (e.g., development of agricultural land)

In addition, riparian alteration can affect nitrogen uptake and cycling, and turn urban riparian areas into nitrogen sources (Groffman et al. 2002, 2003).

Courtesy of U.S. EPA

Although nitrogen loading to and export from urban streams typically are elevated, many studies also have found relatively high nitrogen retention [Groffman et al. 2004, Wollheim et al. 2005 (Table 6)] in these systems. Pervious surfaces such as lawns may act as nitrogen sinks in urban areas (Raciti et al. 2008), and help to mitigate at least some nitrogen loading increases. However, this mitigation may be limited as fertilizers often are over-applied in urban systems.

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Figure 24. Nitrate concentrations in three streams draining completely forested, suburban, and agricultural watersheds in Baltimore County, MD, October 1998–October 2001.
From Groffman PM et al. 2004. Nitrogen fluxes and retention in urban watershed ecosystems. Ecosystems 7:393-403. Reprinted with permission from Springer Science+Business Media.
Table 6. Nitrogen budgets for an urban and a forested headwater stream in Massachusetts, 2001-2002 water year.
Total N loading
(kg km-2 y-1)
Wet deposition (DIN) 494 496
Dry deposition (DIN) 290 290
Net waste N 350 586
Fertilizer N 1443 395
SUM 2578 1767
River N exports
(kg km-2 y-1)
(NO3 + NH4)
333 7.5
DON 51.5 51.6
SUM 384.5 59.1
N retention
85 97
Modified from Wollheim WM et al. 2005. N retention in urbanizing headwater catchments. Ecosystems 8:871-884.

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