Sodium compounds constitute 2.83% of the crust of the Earth. Owing to the fact that most sodium salts are extremely soluble in water, any sodium that is leached from soil or discharged into streams will usually remain in solution. There are no important precipitation reactions that can maintain low sodium concentrations in water in the way that carbonate precipitation controls calcium concentrations.
In igneous rocks, sodium is only slightly more abundant than potassium, but in sediments, sodium is much less abundant. The amount of sodium held in evaporite sediments and in solution in the ocean is an important part of the total. Sodium is retained by absorption on mineral surfaces, especially by minerals with high cation exchange capacities such as clays. In resistate sediments, sodium may be present in unaltered mineral grains as an impurity in the cementing material or as crystals of readily soluble sodium salts deposited with the sediments or left in the sediments by saline water. These soluble salts go into solution readily.
In average soft waters, the equivalent percentage of sodium is second to that of calcium. In hard water, the proportion is less, usually following below calcium and magnesium concentrations. The most common form of sodium in natural waters is NaCl. In saline lakes such as Great Salt Lake, its salt content ranges from 24 to 26%.
Sodium saturated soils are greasy to feel. Sodium soil colloids swell, closing the pores of the soil and reducing soil permeability to water and air. Thus high sodium concentrations in soils are detrimental to water movement and plant growth.
The deterioration of soil quality because of sodium in the irrigation water is a steady cumulative process with impaired drainage resulting in even more highly concentrated soil solutions.
The exchangeable sodium percentage (ESP) is an index of the sodium status of soils. An ESP of 10 to 15% is considered excessive if a high percentage of swelling clay materials are present. The tolerance of crops to sodium in irrigation water can be estimated by the sodium absorption ratio.
Sodium in domestic water supplies may be harmful to persons suffering from cardiac, renal and circulatory diseases, and as much as 200 mg/L of sodium from drinking water may be injurious. The taste threshold of sodium in distilled water varies with the sodium salts used as follows:
Salt Tested Test Threshold of Sodium in mg/L
sodium acetate 140 c
sodium carbonate 34
sodium bicarbonate 290
sodium chloride 135
trisodium phosphate 75
Some of these concentrations of sodium in the water support good fish fauna. Ordinarily the concentration of sodium plus potassium is less than 6 mg/L in about 5%; less than 10 mg/L in about 50%; and less than 85 mg/L in about 95% of the waters.
From most studies it can be concluded that the toxicity of sodium salts depends largely on the anion involved. An example would be sodium chromate being exceedingly toxic and sodium sulfate being least toxic. Moreover, there appears to be no correlation between the toxicity displayed and the pH value. For each of five salts tested singly, the 100 hour toxicity thresholds and resulting pH values are in the following tabulation.
100 Hour Toxicity
Compound Tested Threshold in mg/L pH at Threshold
sodium bisulfite 1 02 7.3
sodium carbonate 524 9.5
sodium chromate 0.42 (huh?) 7.8
sodium silicate 247 9.1
sodium sulfate 4547 7.7
These toxicity levels are expressed for daphnia Magna, at 23° C.