Dr. Nitish Priyadarshi
Sodium salts are found in virtually all food (the main source of daily exposures) and drinking water. Sodium levels in the latter are typically less than 20mg/l (milligram per litre) but can markedly exceed this in some countries. In some parts of Jharkhand state of India the concentration of sodium is more than 100 parts per million.
Sodium ion is soluble in water, and is thus present in great quantities in the Earth's oceans and other stagnant bodies of water. In these bodies it is mostly counterbalanced by the chloride ion, causing evaporated ocean water solids to consist mostly of sodium chloride, or common table salt. Sodium ion is also a component of many minerals.
Although it is generally agreed that sodium is essential to human life, there is no agreement on the minimum daily requirement. However, it has been estimated that a total daily intake of 120 to 400 mg will meet the daily needs of growing infants and young children, and 500 mg those of adults.
In general, sodium salts are not acutely toxic because of the efficiency with which mature kidneys excrete sodium. However, acute effects and death have been reported following accidental over doses of sodium chloride. Acute effects may include nausea, vomiting, convulsions, muscular twitching and rigidity and cerebral and pulmonary oedema. Excessive salt intake seriously aggravates chronic congestive heart failure, and ill effects due to high levels of sodium in drinking –water have been documented.
The relationship between elevated sodium intake and hypertension has been subject of considerable scientific controversy. Although short term studies have suggested that such a relationship does exist, most people in Western Europe and North America ingest a high-salt diet from infancy yet do not exhibit persistent hypertension until the fourth decade. Whereas reducing the sodium intake can reduce the blood pressure of some individuals with hypertension, this is not effective in all cases.
The feldspars contain the bulk of the sodium in the earth’s crust, while clinopyroxenes are the important carriers of sodium in the upper mantle. Sodium is major element in most igneous rocks. Clarke (1924) demonstrated that about 60 per cent of the minerals in igneous rocks are feldspars, either alkali feldspars or Na-Ca feldspars (Plagioclase), and the bulk of the Na and K in igneous rocks is therefore contained in feldspars. Average 2.40 per cent sodium has been reported in Precambrian Granites world wide.
The concentration of sodium in rain and snow is highest near the sea and near large cities since these are areas of sodium input to the atmosphere.
The concentration of elements in river water varies with time at any single sampling sites as a function of discharge, tributary supply and groundwater discharge. In areas where rivers are draining evaporite deposits, the sodium content can be very high.
The sodium content of groundwater is a function of weathering Sodium Plagioclase from bedrock followed by exchange of Ca2+ for Na+ on the surfaces of newly formed clay minerals. Sodium contents of the groundwater would rise due to weathering of plagioclase feldspars. In addition, groundwater can contain large amounts of sodium from the solution of evaporites from the salt water intrusion, and smaller amounts from cyclic sea salts in original precipitation.
What are the known sources of sodium in Groundwater?
All groundwater contains some sodium because most rocks and soils
contain sodium compounds from which sodium is easily dissolved. The most
common sources of elevated sodium levels in groundwater are:
• Erosion of salt deposits and sodium bearing rock minerals
• Naturally occurring brackish water of some aquifers
• Salt water intrusion into wells in coastal areas
• Infiltration of surface water contaminated by road salt
• Irrigation and precipitation leaching through soils high in sodium
• Groundwater pollution by sewage effluent
• Infiltration of leachate from landfills or industrial sites.
The sodium content of lake water is a function of river and groundwater supply followed by increase due to evaporation where this process exceeds input. The evaporation may reach a stage of sodium mineral precipitation.
In Jharkhand State of India where more and more people are now dependent on groundwater for the daily purpose, amount of sodium concentration in groundwater must be taken into priority basis. District wise sodium concentration in groundwater is given below (source Central Ground Water Board, India). Major source may be from the weathering of the feldspar from the bed rocks.
Dhanbad- 20 to 202 ppm (parts per million)
Giridih- 16 to 117 ppm.
Ranchi- 44 to 83 ppm.
Simdega- 14 to 17 ppm
Palamau- 1.1 to 64 ppm.
Lohardagga- 24 to 46 ppm.
National Research Council, 1989. Recommended dietary allowances, 10th ed. Washington, DC, National Academy Press.
Luft, FC et al. 1979. Cardiovascular and humoral responses to extremes of sodium intake in normal black and white men. Circulation, 60: 697-706.
Clarke, F.W. 1924. The data of geochemistry, 5th ed. U.S. Geol. Surv. Bull, 770.