Let d be the distance of you from water level in well (depth of water level).
The total distance travelled by sound wave =2d
When the wave comes back after reflection so the distance will be doubled
To figure it out total distance, we need velocity and the total time. So please provide it.
If you have velocity it should be Total distance = Velocity • 0.1
I've also calculated your velocity algebrically, (Distance/Time)
Distance/0.1 down