If cadmium is used as the ion associated with a salt (eg Cd(NO3)2), it is desirable to express the total concentration of cadmium in the water as Cd+2.

hope you will find the following link helpful

https://safewater.zendesk.com/hc/en-us/articles/211403358-5-How-does-cadmium-get-into-my-drinking-water-

http://www.freedrinkingwater.com/water-contamination/cadmium-contaminants-removal-water.htm

http://www.who.int/water_sanitation_health/dwq/chemicals/cadmium.pdf

Cd2+, the free ion concentration, contributes approximately 10% to total concentrations which include sorbed, bound, complexed. In order to estimate the Cd2+ from total dissolved, I recommend the following tool:

Bootsma, H., J.P.M. Vink (2016). Simple equations for the calculation of free metal ion activities in natural surface waters. Deltares report 1210758, Utrecht. DOI: 10.13140/RG.2.1.1178.8408

Hi Daniel

You can use both, it does not matter which one, as long as you mention it

If you express it in molarity,  then the concentration is exactly the same regardless of which one you choose.

If you are preparing a working cadmium solution is best to do as Cd(NO3)2), because you need to bear in mind the contribution of the nitrate ions as well, sometimes this is not important but if you only mention Cd+2 no one will know if the salt you use is soluble at all