If it is something you are adding to the aqueous solution to begin with the easiest way to keep track of things is to start with a "clear" solution and keep records of how much of the insoluble component that you have added (since it doesn't dissociate this approach can be sufficient). You can then figure out sort of an average concentration by taking the total amount the component and the total volume of water in your apparatus into consideration.
It is very difficult to measure the concentration of components in a non-homogenous solution once these components are actually in the solution. Usually, the way you would go about this is similiar to the way we measure the concentration of various components in the air. i.e. you would sample various portions of the solution, measure the concentrations of each component in each sample and take an average over all the solutions to find the average in any particular part. For the atmosphere or the ocean we can't control how much of a particular component is added to it and must use the tedious method, but for an industrial apparatus you probably can control and keep track of the amounts added from the get go.