I agree with the K.I.S.S. ...

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I agree with the K.I.S.S. method of backtracking the points of origin.  You could try the mentioned conductivity logging, or even pH logging.  If there are different colors of the solutions of flavors, you could also try optical spectrometry logging at a few key wavelengths.  If caustic dumping is the issue, then conductivity will catch that in spades, as would pH.