Basic information
Sodium (Na) is a metallic element and highly reactive. Elemental sodium does not occur naturally on earth and reacts quickly in the air and water. Na occurs mainly as sodium ion (Na+) and is present is great quantities in the oceans and fresh water bodies. Its compounds are soluble.
Sources of Na come from natural sources and human related activities. Natural sources include atmospheric deposition (from sea salt), water-rock (halite) interaction and silicate weathering (albite component of plagioclase with the formula—NaAlSi3O8), sea water intrusion or brine and cation exchange of Ca2+ with Na+ on clay minerals. Na+ originating from human-related activities are for example, agricultural wastewater, industrial wastewater, municipal landfill and road de-icing and water softeners.
Measurement techniques
Na+ can be measured using either ion chromatography (IC) or inductively coupled plasma optical emission spectroscopy (ICP-OES)
Applications
Na is generally considered as a non-conservative cation and interacts with minerals within the soil zone and aquifers. However, it has proven to be an excellent tracer groundwater processes taking place along flow lines in aquifers. The main application of sodium is salinity characterization. Na concentration is often used with reference to Cl (Na/Cl) in surface and groundwater samples to differentiate different sources of salinity. Seawater has a molar Na/Cl of 0.85:1 and if Na/Cl values lie above the seawater-dilution line, this indicates that Na is derived from water-rock interaction - related to silicate weathering, cation exchange or some contamination. The relationship between sodium and other cations (e.g. Ca, Mg) are used to further characterise other sources and/or geochemical processes. During the freshwater diagenesis of carbonate aquifers, for example, the Na/Ca ratio will decrease as marine Na is replaced by Ca.
As a further example the combination of I/Na and Br can be used to separate precipitation, landfill leachate, leachate-affected groundwater and basin brine samples into distinct groups, with I/Na showing increase in landfill leachate. Precipitation samples have high I/Na and low Br. The relatively high Br concentrations and low I/Na ratio is characterized mostly by basin brine samples.
Na/(Na+Ca) ratio can be used to identify surface and groundwater chemistry controlled by precipitation or rock weathering or evaportranspiration/fractional crystallization (e.g. Gibbs, 1970).
References
- Gibbs, R.J., 1970. Mechanisms Controlling World Water Chemistry. Science, 170 (3962): 1088-1090.
- Neal, C. et al., 2005a. Water quality of treated sewage effluent in a rural area of the upper Thames Basin, southern England, and the impacts of such effluents on riverine phosphorus concentrations. Journal of Hydrology, 304(1-4): 103-117.
- Neal, C. et al., 2005b. Phosphorus concentrations in the River Dun, the kennet and Avon Canal and the River Kennet, southern England. Science of the Total Environment, 344(1-3): 107-128.
- Nimiroski, M.T. and Waldron, M.C., 2002. Sources of sodium and chloride in the Scituate Reservoir drainage basin. RI.USGeological Survey Report WRIR-02-4149.
- Panno, S.V., 2006. Characterization and identification of Na-Cl sources in ground water (vol 44, pg 176, 2006). Ground Water, 44(2): 129-129.