Two of the more popular approved methods in the US are UV via a spectrometer or Ion chromatography via EPA method 300.0. There are drawbacks / advantages to both. Hach has a method for the spectrometer that utilizes cadimum reduction, a byproduct that will have to be disposed of by an outside environmental company. An IC , though at great expense, does not deal with this by product. There are ion selective probes, but this is difficult to utilize if salt water or a high ion content Is within your matrix due to the interferences .
There is also an 'Ion Specific Electrode' (ISE) method available for water. See APHA's (American Public Health Association) 'Methods for Water and Wastewater'.
In drinking water direct UV/visible spectrometry is as cheap and easy as it gets. Ion chromatography takes several minutes per sample and typically conductivity detection is not as sensitive as the direct UV method. The direct method isn't great for waters with organic matter, but you did state drinking water.