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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

Urbanization & conductivity

Increases in conductivity or similar measures of ionic strength (see the Ionic Strength module for further discussion of different measurements) are among the most consistently documented water quality changes associated with urbanization. For example, Kaushal et al. (2005) examined salinization of suburban and urban streams in Maryland. They found that chloride concentrations exceeded thresholds for sensitive freshwater taxa at sites with greater than 40% impervious cover (Fig 23). In winter, chloride concentrations reached peaks of nearly 25% the concentration of seawater, and concentrations remained up to 100 times higher than at forested and agricultural non-impervious sites throughout the year.

This increase in dissolved solutes in urban streams has been attributed to several sources, including:

  • Road salt and other deicing agents (in northern regions)
  • Point source discharges (e.g., WWTP and industrial effluents)
  • Leaky sewer and septic systems
  • Concrete weathering

Some studies have shown that urbanization-associated changes in conductivity are related to shifts in biotic assemblages. For example:

  • Roy et al. (2003) found that specific conductance was a significant predictor of invertebrate responses to urbanization, negatively related to total invertebrate richness, EPT richness, total invertebrate density, and several benthic invertebrate indices.
  • Helms et al. (2009) found that streams with high concentrations of total dissolved solids were dominated by sunfish-based fish assemblages.

However, in many cases it is believed that conductivity is a general indicator of overall urban impact, rather than a direct cause of observed biotic effects.

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Figure 23. Relationship between impervious surface and mean annual chloride concentration in Baltimore Long Term Ecological Research (LTER) streams, 1998-2002. Dashed lines indicate thresholds for damage to certain land plants and for chronic toxicity to sensitive freshwater taxa (U.S. EPA 1988).
Reprinted from Kaushal SS et al. 2005. Increased salinization of freshwater in the northeastern United States. Proceedings of the National Academy of Sciences 102 (38):13517-13520. © 2005 National Academy of Sciences, U.S.A. Reprinted with permission.
Road salt and other deicers can contribute to elevated stream conductivity in northern urban catchments
Courtesy of U.S. EPA.

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