Leaded aviation fuel contains 1.2 to 2 grams of tetraethyllead, which works out to a little over 0.5 grams of lead per gallon. Given that this aviation fuel is used only by the approximately 170 thousand piston engined small aircraft in the US, it only accounts for 0.14% of the gasoline used in motors. (sourced using https://www.faa.gov/about/initiatives/avgas/, http://www.flyingmag.com/running-on-empty, and https://en.wikipedia.org/wiki/Avgas) This article strongly indicates you should believe almost all gas in the fuel stream is on the high end of 0.05 grams per gallon, which doesn’t hold up on the math end, even if you blended straight avgas into the rest of the gasoline supply.
We should certainly remove the lead from the gasoline in the aviation space, and it’s looking like the FAA is on track to do this, with certification of lead free fuel alternatives by 2018. This should hopefully alleviate the burden of this environmental toxin on neighbors of General Aviation airports and the world as a whole. If the current plan holds, leaded fuel will be completely out of the US in 2019.