Jump to main content or area navigation.

Contact Us

CADDIS Volume 2: Sources, Stressors & Responses

Ionic Strength

Sources and activities that suggest listing ionic strength as a candidate cause

The ionic strength of surface waters is influenced by numerous human activities, in both waterbodies and their associated watersheds. The more extensive the relevant sources and activities, the more likely increased ionic strength will impair surface waters.

Dissolved ions enter waterways from both point and nonpoint sources. Ion transport occurs by overland flow; evaporation and transpiration of water in shallow soil zones, which increases concentrations of dissolved ions; and infiltration through soil into groundwater, with subsequent discharge to lakes, streams and rivers. Increased ionic strength is often positively correlated with watershed urbanization; watershed surfaces with reduced permeability (e.g., pavement) and storm drain networks transport ions efficiently to surface waters (Roy et al. 2003, Wang and Yin 1997). In addition to ion transport, concentration of ions through evaporation also may influence freshwater ionic strength. For most of the sources listed below, evaporation—due to anthropogenic activities or natural processes—might act as a step in the causal pathway.

Some soil types and geologic formations are natural sources of salts, and certain anthropogenic activities may mobilize and transport those salts to freshwater streams and rivers. Natural geologic variability among neighboring watersheds may result in profound—yet natural—differences in ionic strength of associated streams, especially in arid regions, such as the southwestern U.S. Causal assessors should characterize soil type and geology if ionic strength is being considered as a stressor, particularly if dryland salinity, mining, oil drilling, or irrigation occur in the watershed.

The atmosphere also can be a source of salts to streams and rivers. Rain, snow, and wind carry salts to freshwater systems, either in solution or as dry fallout. Along with natural salt sources, the atmosphere may mobilize domestic and industrial pollution with high salt content (Wetzel 2001).

Road salt applied to icy or snowy roads (Figures 3 and 4), road salt splashed beyond road shoulders, road salt stockpiles, and salt associated with piled waste snow may reach surface waters (Kaushel et al. 2005, Environment Canada and Health Canada 2001, Evans and Frick 2001).

Utility truck applying salt to road for winter driving safety.  Photo by Eric Vance, USEPA.
Figure 3. Utility truck applying salt to road for winter driving safety.
Courtesy of E. Vance, U.S. EPA.
Application of road salt to road near stream culvert. Photo by Eric Vance, USEPA.
Figure 4. Application of road salt to road near stream culvert.
Courtesy of E. Vance, U.S. EPA.
Dryland salinity, with salts mobilized to the ground surface in an agricultural field. Photo by Scott Bauer, USGS.
Figure 5. Dryland salinity, with salts mobilized to the ground surface in an agricultural field.
Courtesy of S. Bauer, USGS.

Land cover alteration, such as replacement of native vegetation with shallow root-zone vegetation or vegetation with decreased rainfall interception, can raise water tables and mobilize salts, particularly in regions with naturally saline groundwater, soil, or geology. Subsequently, salts may be transported to the ground surface (Figure 5) or to surface waters via groundwater discharge, a phenomenon sometimes referred to as "dryland salinity" (Rengassamy 2006, National Land and Water Resources Audit 2001).

Water withdrawal for human consumption or irrigation may reduce dilution of dissolved ions in surface waters, thereby increasing ionic concentration. Groundwater withdrawal in coastal areas may cause saltwater intrusion, whereby dissolved ions from marine sources reach otherwise freshwater aquifers. In turn, ions may reach surface waters by mechanisms related to land cover alteration (see above).

Streamside coal bed methane mining and related groundwater extraction. Photo by Scott Schenk, USGS.
Figure 6. Streamside coal bed methane mining and related groundwater extraction.
Courtesy of S. Bauer, USGS.

Irrigation may mobilize salts by mechanisms similar to the dryland salinity process described earlier. Additionally, irrigation water with high salt content may be introduced to watersheds and reach surface waters by overland flow.

Combustion effluents produced during or mobilized for coal, oil, or wood combustion, including cooling wastewater (blowdown) and ash sluice water, may have elevated ionic content.

Resource exploration and extraction can disturb geologic substrates and mobilize leachable minerals. Improperly cased oil wells may allow mobilization of saline waters (Pond 2004). Waters produced by or used in oil and gas development, mine dewatering, and related activities (e.g., coal bed methane mining; Figure 6), may be saline (Clark et al. 2001).

Sewage and industrial waste discharges often have higher ionic concentrations than receiving freshwaters, as modern wastewater treatment systems are not always equipped to reduce ionic content prior to discharge.

Top of Page


Site evidence that suggests listing ionic strength as a candidate cause

Leaf pack from an ion-rich aquatic habitat, showing leaves cemented to one another by minerals. Photo by Pond, 2004.
Figure 7. Leaf pack from an ion-rich aquatic habitat, showing leaves cemented to one another by minerals.
Courtesy of G. Pond, U.S. EPA Region 3.

In addition to observations of sources discussed above, visible or associative evidence suggesting that ionic strength should be included as a potential candidate cause include:

  • Signs of snow disposal into or near surface waters,
  • Crystalline deposits on channel surfaces such as banks, rocks, culverts, and drains,
  • Mineral precipitates appearing as floc, substrate discoloration, or cemented organic debris (Figure 7),
  • Loss of vegetation including, for example, macrophytes or riparian trees (Williams 2001),
  • Presence of salt-tolerant plants such as salt cedar, and
  • Decreased productivity of aquatic vegetation often associated with wilting, bleaching or loss of color, and/or stunted growth.

Top of Page


Biological effects that suggest listing ionic strength as a candidate cause

Isopod. Photo by Eric Vance, USEPA.
Figure 8. Isopod.
Courtesy of E. Vance, U.S. EPA.

There is debate among scientists as to the exact mechanisms responsible for toxicity associated with ionic strength. Toxicity due to ionic strength could result from disruption of organisms' osmotic regulation processes, decreases in bioavailability of essential elements, increases in availability of heavy metal ions, increases in particularly harmful ions, changes in ionic composition, absence of chemical constituents that offset impacts of harmful ions, a combination of the above, or other as yet unknown mechanisms. In some instances (perhaps the majority), increased ionic strength causes shifts in community composition rather than mortality; thus, specific conductivity, salinity, and TDS levels may be associated with biological impairment and yet be below mortality thresholds.

Toxicity due to ionic strength and composition is often site-specific, species-specific, and ion-specific. Horrigan et al. (2005, 2007) developed a numerical salinity index based on the presence/absence of salt sensitive and tolerant macroinvertebrate families. Additionally, Table 1, below, lists examples of potential biological effects due to changes in ionic strength. These examples do not apply to all situations; rather, they provide examples of what causal assessors may find in site-specific biological monitoring data and the type of information that might be found in the literature. Organisms listed in the table are primarily macroinvertebrates. Note that exceptions may exist for many of the summary statements listed below. For example, some amphipods may benefit from slightly increased levels of salinity, but this would depend on ionic composition and the specific taxa present. Before using these generalizations in specific causal assessment tasks, be sure to become familiar with the context of each study.

Table 1. Examples of potential biological effects due to changes in ionic strength.
Organism Effect
Caecidotea and Tipula Presence associated with elevated conductivity typical of agricultural and urbanized areas (Black et al. 2004)
Ceratopogonidae and Tipulidae Presence in spring-fed surface waters associated with elevated chloride levels attributed to road salt application (Williams et al. 1997)
Chlorophyta Green filamentous alga Cladophora glomerata reported to have affinity for calcium cations (Sikes 1978)
Ephemerellidae and Perlidae Dominance associated with low conductivity typical of forested sites (Black et al. 2004)
Gammarus pseudolimnaeus and Turbellaria Presence in spring-fed surface waters associated with low chloride levels (Williams et al. 1997)
Halophilic diatoms Some diatoms (e.g., Entomoneis) observed to increase with elevated salinity
Macrocrustaceans Amphipods, decapods, and isopods (Figure 8) with high acute lethal salinity tolerance relative to other freshwater taxa (Kefford et al. 2003); amphipods with optima at elevated conductivity levels (Black et al. 2004)
Mayflies Ephemeroptera declined along gradient of increasing conductivity (Pond 2004); some baetid mayflies with low acute lethal salinity tolerance relative to other freshwater taxa (Hassell et al. 2006, Kefford et al. 2003)
Orthocladiinae midges (brine- or shoreflies) Increased with ionic strength in multiple U.S. datasets
Soft-bodied non-arthropods Non-arthropod freshwater macroinvertebrates with soft-bodies (specifically, certain Oligochaeta, Gastropoda, Nematomorpha, Tricladida, and Hirudinea) with lower acute lethal salinity tolerance than other freshwater taxa, potentially due to increased permeability to dissolved ions (Kefford et al. 2003)

Top of Page


Site evidence that supports excluding ionic strength as a candidate cause

There are no site observations that specifically provide evidence of the absence of changes in ionic composition. General reasons for excluding a candidate from the list are described in Step 2 of the Step-by-Step Guide and in Tips for Listing Candidate Causes.

We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding ionic strength from your initial list of candidate causes, because different species have different ionic strength requirements and different sites have different naturally occurring levels of ionic strength.

Top of page


Jump to main content.