Just to put things into perspective.
A B747-400 would normally dump above 6000ft altitude, and yes preferably over the sea, however lets run some numbers. No fuel dump with flaps between 1 and 5 - personally I would have chosen flaps up. Minimum flaps up speed after a max weight take off is about 280 kts. The published dump rate is a max of 2000 kgs/minute
So, in one minute:
Aircraft travels 280kts = 8462 metres (280/60 * 1852)
Distance between jettison nozzles ~ 40 metres
Area covered = 338,480 sq metres (8462 * 40)
Jettison rate = 2000 kgs/min
Concentration at ground = 0.006 kg/sq metre (6 grammes)
Of course this assumes that the fuel comes down vertically without dispersion and evaporation. I would imagine the dispersion rate would be vast, even from 6000ft and I doubt that more than 1% of that would actually reach a given sq metre, ie less than 1/10th of a gramme.