The ratio of sodium to cations is important in agriculture and human pathology.  Soil permeability can be harmed by a high sodium ratio.  Persons afflicted with certain diseases require low sodium concentrations.

Acceptable results:  No health standards but 30 ppm or less are considered ideal.

Sources:  Naturally occurring and from sodium exchange water softeners.  Also through human contamination such as septic systems, fertilizers, etc.

Recommended testing:  If water tastes salty or annually after installing a water softener (working properly).