Causal Analysis/Diagnosis Decision Information System (CADDIS)

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CC.6. Ionic Strength

Ionic strength is the concentration of ionic charge in solution. Increased ionic strength and changes in ionic composition may lead to shifts in community composition and function based on factors such as taxa-specific preferences and adaptations (Section CC.6.1.4). Measurements of electrical conductivity, salinity, and total dissolved solids (TDS) are often used to represent the ionic strength of water and generally increase with increasing ion content (Section CC.6.2). This module provides advice for deciding whether or not to include increased ionic strength as a candidate cause of biological impairment.

Because ionic strength issues include a broad range of potential freshwater problems, we are forced to generalize about overall effects. There will be exceptions to these generalizations, especially in terms of taxa-specific reactions to various ion-specific stresses. Nevertheless, this stressor module introduces a common language and identifies some of the more widespread ion-related issues. Ultimately, causal assessors may need to dig deeper into site-specific characteristics and relevant literature.

Note that concentration of hydrogen ions (pH) and heavy metal ions (e.g., lead and zinc), while related to ionic strength and composition, are not included here. Metal toxicity is discussed in Section CC.1. Metals.

Advice for deciding whether to include ionic strength in your list of candidate causes is provided in Section CC.6.1. Ways to measure ionic strength are discussed in Section CC.6.2. You may go directly to a specific section of interest by clicking on the topic outline in the right navigation bar.

CC.6.1 What to Consider When Determining if Ionic Strength Should Be Included as a Candidate Cause

Increased ionic strength is addressed in this module as a proximate stressor that should be listed as a candidate cause when potential human sources and activities, site evidence, and biological effects support portions of source-to-impairment pathways as illustrated in the conceptual model diagram for ionic strength (Figure CC.6-1).

Ionic compounds are natural constituents of both inland and marine systems, and are not harmful unless levels exceed or fall below the tolerance range of aquatic organisms. Indeed, some constituents of ionic compounds are essential elements, necessary for the survival of aquatic organisms. Salts are ionic compounds composed of cations (positive charge) and anions (negative charge). Common salt ions include:

• Cations:    Na⁺    Ca²⁺    Mg²⁺    K⁺
• Anions:    Cl^-    HCO₃^-    CO₃^2-    SO₄^2-

Ionic strength varies naturally across aquatic ecosystems (Figure CC.6-2) and aquatic organisms generally prefer waters with specific ionic strength ranges, containing specific ions. When these parameters are changed, biota may be adversely affected. In recent years, several states (e.g., Florida, West Virginia) have adopted criteria which address the importance of ionic strength in determining water quality.

Ionic strength may significantly impact freshwaters through interactions with other stressors, and it may be difficult to distinguish among proximate stressors and interacting stressors. Potential interactions include:

• Increased salinity may foster or hinder uptake of toxic substances by organisms (Bidwell and Gorrie, 2006; Zalizniak et al., 2006; Environment Canada and Health Canada, 2001),
• Salts impact soil structure, potentially decreasing soil permeability (Rengassamy, 2002), increasing runoff volume, and increasing soil erosion (Environment Canada and Health Canada, 2001), and
• Salt solubility increases as temperature increases.

Salinity, along with temperature and altitude, also affects the solubility of oxygen in water. Freshwater (salinity generally < 0.5 parts per thousand) at 22°C and sea level can hold approximately 8.7 mg/L of dissolved oxygen (DO). Ocean water (salinity ≈ 35 parts per thousand) can hold only about 7.1 mg/L of DO at saturation. DO saturation might change by 6% over a 10 parts per thousand difference in salinity; therefore, with freshwater ecosystems generally falling below 0.5 parts per thousand, the associated change in DO over the spectrum of freshwater salinity levels is negligible (Stickney, 1979).

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CC.6.1.1 Checklist of Sources, Site Evidence, and Biological Effects

A checklist is provided below to help you identify key data and information useful for determining whether to include or exclude ionic strength among your candidate causes; the title for each column is linked to more detailed descriptions. The list is intended to guide you in collecting evidence to eliminate or enhance support for ionic strength as a candidate cause. You may be aware of other situations when ionic strength should be eliminated or included as a candidate cause; please send us your insights using the comment section.

Consider listing ionic strength as a candidate cause based on the presence of sources and activities, site evidence, and biological effects:

Consider contributing, modifying, and related factors as candidate causes when ionic strength is selected as a candidate cause:

• Flow alteration
• Metals
• pH
• Sediment
• Temperature
• Unspecified toxic chemicals

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CC.6.1.2 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 (Images CC.6-1 and CC.6-2), 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).

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 (Image CC.6-3) 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).

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; Image CC.6-4), 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.

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CC.6.1.3. Site Evidence that Suggests Listing Ionic Strength as a Candidate Cause

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 (Image CC.6-5),
• 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.

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CC.6.1.4. Biological Effects that Suggest Listing Ionic Strength as a Candidate Cause

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 CC.6-1, 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 CC.6-1. 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 (Image CC.6-6) 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)

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CC.6.1.5. 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 2.2 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.

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CC.6.2. Ways to Measure Ionic Strength

Ionic strength is a function of concentration and charge of all ions in a given solution (see Equation CC.6-1). Direct measurement of ionic strength is seldom used in ecological studies. Causal assessors are more likely to have access to measures which generally correlate with ionic strength.

• Electrical conductivity or conductance — Ability of a solution to conduct current (measured in μS/cm, mS/cm, μmho/cm, or mmho/cm), which is related to ionic strength, temperature, and the mobility of ions. One siemen (S) is equal to one mho, and these terms are used interchangeably. "Specific conductivity" indicates the measurement has been normalized to a reference temperature (usually 25°C). Conductivity increases approximately 2% for every 1°C increase in water temperature (Wetzel, 2001).
• Salinity — Salt content of water (measured in mg/L, g/L, parts per thousand, or ‰); parts per thousand can be interpreted as salt mass per 1000 units of water mass, or grams of salt per 1000 grams of water.
• Total dissolved solids (TDS) — Concentration of material dissolved in water (measured in mg/L, g/L, parts per thousand, or ‰).

Conductivity, salinity, and TDS do not fully account for variance in toxicity due to individual ions or ionic composition [refer to Mount et al. (1987) for more information on individual ions and related toxicity] especially for sub-lethal endpoints (Zalizniak et al., 2006). Different solutions with the same ionic strength, conductivity, salinity, and TDS may have different effective toxicities, specific to different organisms, if ionic compositions vary. Nevertheless, conductivity, salinity, and TDS may be useful measures of ionic strength, especially in the context of acute toxicity.

In some cases, it may be appropriate to use evidence regarding individual ions (e.g., chloride) if measurements are available and reliable for a particular study site, if the individual ion is a potential causal agent (i.e., concentrations of other ions are low relative to their toxicities), or if regional information describes potential biological effects as a function of exposure to that specific ion.

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Bidwell, JR; Gorrie, JR. (2006) The influence of salinity on metal uptake and effects in the Midge Chironomus maddeni. Environ Poll 139:206-213.

Black, RW; Munn, MD; Plotnikoff, RW. (2004) Using macroinvertebrates to identify biota-land cover optima at multiple scales in the Pacific Northwest, USA. J North Amer Benthol Soc 23:340-362.

Clark, ML; Miller, KA; Brooks, MH. (2001) US Geological Survey monitoring of Powder River Basin stream-water quantity and quality. Cheyenne, WY: U.S. Geological Survey.

Environment Canada and Health Canada. (2001) Priority substances list assessment report: road salts.

Evans, M; Frick, C. (2001) The effects of road salts on aquatic ecosystems. National Water Research Institute (NWRI), Saskatoon, Saskatchewan NWRI Contribution No 02-308.

Hassell, KL; Kefford, BJ; Nugegoda, D. (2006) Sub-lethal and chronic lethal salinity tolerance of three freshwater insects: Cloeon sp. and Centroptilum sp. (Ephemeroptera: Baetidae) and Chironomus sp. (Diptera: Chironomidae). J Exp Biol 209:4024-4032.

Horrigan, N; Choy, C; Marshall, J; et al. (2005) Response of stream macroinvertebrates to changes in salinity and development of a salinity index. Mar Fresh Res 56:825-833.

Horrigan, N; Dunlop, JE; Kefford, BJ; et al. (2007) Acute toxicity largely reflects the salinity sensitivity of stream macroinvertebrates derived using field distributions. Mar Fresh Res 58:178-186.

IUPAC (International Union of Pure and Applied Chemistry). (1993) Quantities, units and symbols in physical chemistry, 2nd ed. Oxford, UK: Blackwell Scientific.

Kaushal, SS; Groffman, PM; Likens, GE; et al. (2005) Increased salinization of fresh water in the Northeastern United States. Proc of the Natl Acad Sci USA 102:13517-13520.

Kefford, BJ; Papas, PJ; Nugegoda, D. (2003) Relative salinity tolerance of macroinvertebrates from the Barwon River, Victoria, Australia. Mar Fresh Res 54:755-765.

Mount, DR; Gulley, DD; Hockett, JR; et al. (1997) Statistical models to predict the toxicity of major ions to Ceriodaphnia dubia, Daphnia magna and Pimephales promelas (fathead minnows). Environ Manag 16:2009-2019.

National Land Water Resources Audit. (2001) Australian dryland salinity assessment 2000: extent, impacts, processes, monitoring and management options. Australia, National Land and Water Resources Audit, c/o Land & Water Australia On behalf of the Commonwealth of Australia.

Pond, GJ. (2004) Effects of surface mining and residential land use on headwater stream biotic integrity in the eastern Kentucky coalfield region. Frankfurt, Kentucky: Kentucky Department for Environmental Protection, Division of Water.

Remane, A. (1971) Ecology of brackish water. In: Remane A; Schlieper, C, eds. Biology of brackish water. New York, NY: John Wiley and Sons, New York.

Rengasamy, P. (2002) Transient salinity and subsoil constraints to dryland farming in Australian sodic soil: an overview. Aust J Exp Agri 42:351-361.

Rengasamy, P. (2006) World salinization with emphasis on Australia. J Exp Botany 57:1017-1023.

Roy, AH; Rosemond, AD; Leigh, DS; et al. (2003) Habitat-specific responses of stream insects to land cover disturbance: biological consequences and monitoring implications. J North Amer Benthol Soc 22:292-307.

Sikes, CS. (1978) Calcification and cation sorption of Cladophora glomerata (Chlorophyta). J Phycology 14:325-329.

Stickney, RR. (1979) Principles of warmwater aquaculture. New York, NY: John Wiley and Sons, Inc.

Wang, XH; Yin, ZY. (1997) Using GIS to assess the relationship between land use and water quality at a watershed level. Environ Int 23:103-114.

Wetzel, RG. (2001) Limnology, 3rd edition. San Diego, California: Academic Press.

Williams, DD; Williams, NE; Cao, Y. (1997) Spatial differences in macroinvertebrate community structure in springs in Southeastern Ontario in relation to their chemical and physical environments. Can J Zoo-Rev Can de Zoo 75:1404-1414.

Williams, WD. (2001) Anthropogenic salinisation of inland waters. Hydrobiologia 466:329-337.

Zalizniak, L; Kefford, BJ; Nugegoda, D. (2006) Is all salinity the same? The effect of ionic compositions on the salinity tolerance of five species of freshwater invertebrates. Mar Fresh Res 57:75-82.

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