Peach...I am not familiar with killing a pond, but I can not help myself to try and think of a good way to mix in your lime.
Do you have aeration? I don't recall.
I think the barrel approach may be a bit hard to control. The barrel would need to be stabilized very well, maybe a couple bags of cement in the bottom, now the barrel is heavy to handle. At least my imagination says that it would all be hard to control while keeping the nasty stuff out of your eyes and weezer.
How big is this pump?
Can it handle some trash like pebbles and such?
Could you place the lime in the pond in a shallow location (unbagged) and use the suction hose to pull pond water from that area and have the discharge hose moved around or at least taken to the far end? All the while someone is stirring the lime up so that it can be sucked up by the pump.
Maybe the pump can continue to operate in this fashion after all the lime is dissolved from the pile so that it can be circulated throughout the pond more evenly.
Just some things to ponder.
AND, make sure that the amount of lime is adequate for your total water mass or it could be in vane.
