I looked for axactly what you were talking about: genetic cross-linkage of eye color (which is geneticly very predictable) with density of cones and/or ratio of rods vs. cones in the optic nerve array.

If it's out there, it will have to peiced together at a full-blown medical library. MedLine didn't get me anywhere.

HOWEVER: I did run onto a couple of items of interest:

http://www.nlm.nih.gov/medlineplus/drinkingwater.html

Compemndium of everything you ever wanted to know about drinkingwater. Note especially the section on "Emergency Disinfection of Drinking Water."