Question: Can a DIY spectrometer be used to measure water turbidity?

By definition, turbidity measures the amount of light scattered at a 90 angle. So there would have to be a change in the optical layout from the basic spectrometer. You also need to check the specific method referenced. Some instruments are getting quite complex ( ratio recording , etc.). If a method specifies something like that, it can change your optical layout dramatically. Having said that, most turbidimeters are very simple and would be easy to duplicate.

Hm, good point -- actually going back to an old post there's some great stuff here:

https://publiclab.org/notes/donblair/08-25-2015/turbidity-001

So perhaps turbidity since it measures reflectivity, is not just an analogue for opacity, and Beer's Law may not apply for all contaminants?

So a design using the Lego Spectrometer, for example, might need a right-angle light source -- but that's not so bad, since already with fluorescence spectrometry we'd be doing this. Interesting!