[apham]

Here is the question: the concentration of naphthenic acids in oil-field wastes is often reported as mg/L. Because naphthenic acids is a mixture of cyclic and alicyclic carboxylic acids with a wide range of molecular weight, I wonder what mg/L means. mg/L of C? mg/L of carboxylic group?...

Thanks!

[TheUnassuming]

The mg/L they are using is probably an average based on the MW of the different acids present if I understand you correctly.
 
For various reasons, scientists that are non-chemists prefer using mass/volume to describe concentration rather than molarity, which would seem to give more information to those from a chemistry background.  My biologist collaborators for instance, prefer to work in mass/volume I think because it works best for protein work which they do primarily.

[kriggy]

Its problaby just mass of the acids combined per ml. Ie. if you isolated all napthenic acids from litre of oil waste, you would get xmg of product