i found an old document documenting .1 ppm in the detroit river near wyandotte. that's quite a bit higher, at about 1 mg/L - which would be acceptable as fluoridated drinking water. but, this is downstream from where we take the water, and south of zug island, which is the remaining industrial heart of detroit.
but, those are my likely bounds - between .08 and 1 mg/L.
if it's .08, i'd need to drink 40+ litres of water a day to get to the lower end of the rdi, at 3 mg, if water is my only source.
if it's at .3, i'd need to get to 10 L to get to 3 mg - and drinking 5 L, which is on the extreme end of what's actually plausible, would get me to 1.5 mg, which is almost 40%.
i could conceivably get into the habit of downing a big gulp of water before and after meals, which would be about 1.774 L. at .3 mg/L, that's .5322 mg - or 13.3% of the rdi. thrice that is nearly 40%. two cups of coffee is another 1.4 L of water, which is another 10%. it would be hard to drink more water than that, consistently, every day.
at .08 mg/L, it's unfortunately only .08*(3*1.774 + 1.4)/4 = 13%
at .4 mg/L, it's .4*(3*1.774 + 1.4)/4 = 67%
it was previously at .6 mg/L, and i'd then be at .6*(3*1.774 + 1.4)/4 = 100%, on the nose.
i can also get some water from the pasta cooking process.
so, we see what the bounds i have to actually work with actually are.