Lithium in water 'curbs suicide'
news.bbc.co.uk
news.bbc.co.uk
However... note that these are epidemiological studies, and demonstrated (as I recall) that bipolar (aka manic-depressive) depressive illness is lower in areas of U.S. with higher natural lithium.
Even if it could be demonstrated that lithium-ingestion in water did reduce incidence of bipolar illness, this would not be expected to improve depression among those with the more common, uni-polar depression:: There has never been evidence that lithium is a "mood enhancer" or "psychoactive" - only that deficiencies of lithium may bring out or aggravate latent bipolar depressive illness. — A (non-clinical) psychology Ph.D.
While I also have reservations about dosing the water supply, I don't think flouride is such a bad thing; and the study suggests that micro-doses as described seem to reduce the rate of suicide by up to a third, which is nothing to sneeze at.
Some illness and suffering is caused by modern life an technology (on a personal level, I think our emphasis on getting up early and minimizing the amount of sleep we need is a very bad thing). While it needs a lot of further study, I wonder if rather than looking at something like this as a prophylactic, we should ask if it isn't rectifying an induced deficiency.
http://dustincurtis.com/screenshots/Lithium_levels_in_drinki...
http://norvig.com/experiment-design.html
Lithium's mechanism of action in patients who take it as a prescribed medication is not well understood, and it would take a great deal of follow-up to see if this epidemiological study is verified, especially as to its effect size reported here.
Re: the comment about unipolar versus bipolar depression posted earlier, the suicide risk is much higher for bipolar depression (it is highest for the pattern of mood called bipolar II), so reducing bipolar mood variation would seem to have great promise for reducing completed suicide.
If you ever go through Ashland, Oregon (you're driving north/south on I-5 for instance), be sure to visit here for a nice break:
http://en.wikipedia.org/wiki/Lithia_Park
It's a popular prank to bring visitors to the park and let them sample the water, which is really wretched.
Conversely, if high concentrations are linked with poor health that suggests it is toxic and water treatment in areas with high concentrations should seek to remove some of it.
If however the concentration in question does not already occur naturally in water drunk by large populations, you lack the vital data on the consequences of low-level, life time exposure. You are basically stuck and should leave well alone.