Plant roots don't uptake lead as a mineral in the cation exchange in the soil.
Lead is relatively inert and doesn't like to combine with low energy compounds. So it likes to stay put. It doesn't travel easily and it's contamination vectors are easily traced. It does not move through soil layers easily and is not transported by water that is not also moving tracts of land.
Putting the dust in the water keeps it out of the air and lungs. Hand washing keeps it out of the mouth. Pouring the water in the drain keeps a tray of friable lead dust from being jostled around in the garage. Pouring it out on the lawn returns it to the ground, from whence it came.
(Though I don't recommend that. Shoe to floor contamination. That, and I have omnipotent, opressive control over my yard, Augusta National has nothing on my lawn...
)
You won't poison your neighbors with lead, unless ya shoot 'em...
A caviler attitude about lead is never prudent. I commend you on your concern for the neighborhood.
I have abated lead and asbestos for eleven years. I have shot and handloaded for five and three respectively. My blood level is still zero.