@Lady jane Indeed probably not, at least for now... the efforts to find an alternative for 100LL are ongoing and the aim is for it to replace the existing fuel (for existing engines), rather than just be an alternative for newer aircraft. As an average, at power, many small "light single" engine aircraft will burn around 25 litres per hour. They climb out at around 70-80 knots (80-90mph) therefore they will be about 2 miles away in around a minute, so within that mile or so they'll burn around 0.4l of fuel. At most, this is about 0.2g of TEL content, so on a busy day with say eight or nine departures, <2g of TEL burned close to the airfield.
I expect, averaged out over the year, the agricultural fuel burn will be far more significant.