Salinity

The natural concentration of salts is largely influenced by the geologic formation underlying the area (James and Evison, 1979). Low salinity is expected in non-faulted areas underlain by igneous geologic formations (Perfetti and Terrel, 1989). High levels of dissolved solids often occur in areas underlain by ancient marine sediments. As time passes, the salts are removed from the sedimentary rocks by wind and water erosion. These elements remain dissolved in surface waters. Additionally, if an area is heavily faulted, marine sediments buried deep within the earth may contact ground water and form a brine. The fault may serve as a conduit for the brine, which may be introduced to surface water systems via springs.

Salt concentrations are expected to be high in arid or semi-arid areas where evaporation usually exceeds precipitation. As water evaporates from existing water bodies, salt concentrations increase. Because precipitation itself contains minute traces of salts, evaporation after a rain leaves salts in the soil. These salts may be carried in irrigation return flow or in overland flow during the infrequent rains (Perfetti and Terrel, 1989).

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Numerical Categories:

Health Effects: Sodium sulfate and magnesium sulfate levels above 250 mg/l in drinking water may produce a laxative effect. Excess sodium may affect those restricted to low sodium diets and pregnant women suffering from toxemia (EPA, 1986).

High levels of total dissolved solids may impart an objectionable taste to drinking water. Chloride, in particular, has a low taste threshold.

Industrial Effects: Dissolved salts may either encrust or corrode metallic surfaces. Salt in intake water may interfere with chemical processes within the plant (EPA, 1986).

Environmental Effects: Some freshwater organisms are able to tolerate low dissolved solids levels. If a total dissolved solids increase in the water body, a shift to more salinity-tolerant species can be expected (James and Evison, 1979). Salt- tolerant plants include greasewood, alkalai sacaton, fourwing saltbush, shadscales, saltgrass, tamarisk (salt cedar), galleta, western wheatgrass, mat saltbrush, reed canarygrass, and rabbitbrush (Perfetti and Terrel, 1989).

http://www.msnucleus.org/watersheds/ mudslough/images/saltgrass.jpg

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Estuarine aquatic life is generally tolerant of fluctuating salinity levels. Under natural conditions, estuarine water may fluctuate between fresh and brackish, depending on the flow rate of the river discharging into the estuary. Aquatic biota inhabit zones in the estuary according to preferred salinity levels. Thus, if the volume of fresh water entering the estuary fluctuates sufficiently to cause a change in the isohaline (areas of similar salinity) patterns, species may be displaced and the ecosystem disrupted (EPA, 1986).

Urban runoff containing high salt concentrations (e.g., from de-icing) may create saline layers in receiving lakes. Salt water has a higher density than freshwater and tends to sink and form a dense layer in the hypolimnion. This saline layer does not mix with remainder of the lake water, leading to decreased dissolved oxygen levels in the hypolimnion (Gower, 1980).

Irrigation Effects: Inadequate drainage or excessive evaporation from agricultural fields may lead to an accumulation of salts in the soil. The arid southwestern U.S. is especially vulnerable to this phenomenon because this area experiences intense evaporation and the upper layer of the soil is often baked to an impermeable crust-like state that prohibits the infiltration of water. Hence, the water ponds and then evaporates, leaving salts behind.

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Salt in the soil may harm crops. Certain salt constituents alone can prove toxic to some plant varieties. Also, high salt concentrations in the soil around plant roots may cause plant dehydration by reversing osmotic conditions (water will flow out of the plant in an attempt to achieve equilibrium). In some cases, rather than destroying a crop, elevated salt levels may simply reduce crop yields and leave the plants prone to disease (Sherrard et al., 1987).

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Sources:

1. Nonpoint source:
   • Natural: Natural sources include poorly drained soil, salty ground water (brine), erosion from geologic formations of marine origin, and wind-borne sea spray.
     
     http://www.loreoftheland.com.au/tour/thurra/cspray3.jpg
   • Agriculture: High salinity levels are often found in irrigation return flows and runoff from agricultural fields, especially in arid areas where intensive evaporation causes salt accumulation in soil.
     
     http://www.botgard.ucla.edu/html/botanytextbooks/lifeforms/images/halophytes/Saltcrust.jpg
   • Residential and Urban: The primary contributor of salts in residential and urban areas is runoff from urban areas, where salt is used to de-ice roadways and walkways.
2. Point source: Inorganic chemical industry may release dissolved cations in effluent waters (EPA, 1986).
   
   

   http://www.scdhec.net/eqc/water/html/landpage.html

   http://www.scdhec.net/eqc/water/html/landpage.html

Mode of Transport: Dissolved salts are transported in saturated flow, unsaturated flow, and overland flow. Salts may also be sorbed to wind-borne particulates.

Analytical Techniques: (Sittig, 1981; Yaron, 1981)

1. Electric Conductivity (EC): Uses a conductivity bridge calibrated with standard seawater solution. The resistance of a sample solution is measured with an electric potential. The solution's ability to transmit electricity is facilitated by increasing salt content. The EC is normally measured in mhos/cm, mmho/cm, or umho/cm (non-SI units) or siemen per meter (s/m) (SI units), depending on dissolved salt concentrations. (0.64 umho/cm= 640 mmho/cm= 640 dS/m= 64.0 cS/m=6.4 mS/m).

   EC values can be translated into the total quantity of dissolved salts with the following conversions:

   TDS (mg/l) = 640*EC (mmho/cm)
   TDS (mg/l) = 0.64*EC (umho/cm)
   
   

2. Density Method: Uses a precise vibrating flow densimeter.
3. Gravimetric Method (APHA 1992): As an example, magnesium is measured using the gravimetric method. Diammonium hydrogen phosphate precipitates magnesium in ammonical solution as magnesium ammonium phosphate. The test can be performed two ways. First, the ammonium salts and oxalate can be destroyed, followed by precipitation of magnesium ammonium phosphate. Second the diammonium hydrogen phosphate can undergo double precipitation without pretreatment (preferable option). Dry and weigh sample.

   Interferences: Presence of aluminum, calcium, iron, manganese, silica, strontium, and suspended matter might interfere with test. Solution should not contain more than 3.5 g NH4Cl.

NCSU Water Quality Group

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