SALINITY FACTOR 281 
to return flow downstream. Thus, the Rio Grande has a range in 
salt concentration from approximately 180 ppm to 1,775 ppm 
at different points on the river. 
Ground waters from pumped wells constitute the principal 
source of irrigation supplies in many areas, and the range in salt 
concentration may be much greater than for surface waters. For 
example, analyses of a large number of wells in the Coachella 
Valley, California, indicated a range in soluble salts varying 
from 130 ppm to 8,500 ppm. In addition, wells in close proximity 
to each other may have very different salt concentrations. Two 
wells in this valley within a half mile of each other have salt con- 
centrations of approximately 400 and 8,500 ppm, the former 
well being 65 feet deep, the latter 180 feet. 
The principal ions found in natural waters are the cations 
calcium, magnesium, and sodium, and the anions bicarbonate, 
sulfate, and chloride. Potassium, nitrate, fluoride, boron, and other 
constituents may be present in low concentrations. Sulfate and 
chloride salts usually predominate, but occasionally waters may 
be high in bicarbonates and less frequently in nitrates. 
The sodium factor in irrigation water 1s related to the alkali haz- 
ard and is determined by the absolute and relative concentration 
of the cations. If the proportion of sodium is high, the alkali 
hazard is high and, if calcium and magnesium predominate, the 
hazard is low. The soluble cations in the irrigation water have a 
pronounced influence on the distribution of the exchangeable 
cations in the soil, and it is for that reason, in part, that the so- 
dium content in irrigation water is important. If sodium consti- 
tutes less than one-half of the cations in the irrigation water, 
there is ordinarily very little danger of unfavorable sodium soil 
conditions developing from the use of the water but, as the 
proportion of sodium increases, the hazard increases. 
In some instances, waters may be low in total salts but high in 
bicarbonate. This condition tends to aggravate the sodium prob- 
lem in soil where the amount of bicarbonate is considerably in 
excess of the calcium plus magnesium. In such a case, residual 
sodium carbonate 1s present and, as the irrigation water evaporates 
from the soil, calctum and magnesium carbonates precipitate, and 
the sodium percentage of the soil solution increases. This is 
