You may want to steer clear of putting lemon wedges in your water when you're at a restaurant.
In a 2007 study from the Journal of Environmental Health, researchers tested 76 lemon samples from 21 different restaurants during 43 visits and found that many of the lemons contained microorganisms, including some pathogenic ones that may cause diseases.
To minimize your risk a bit, squeeze the lemon into your drink instead of dropping the whole wedge in. And if you see employees handling lemons with their bare hands, it might be best to stick to plain water.