Theo's quote "IF I understand the wire you plan on using correctly, 8-10-12 gauge black and 10 gauge white, I think the limiting factor would be the stretch of 12 gauge black, not a whole length of 10 gauge white. I believe the exact answer would depend on the length of the 8, 10, and 12 gauge segments, but unless the 12 gauge is very, very short, it's going to be the overall load limiter (not the 10 gauge white)."

Yo to what Theo said. Local ground good.
Can you run 220 from the box and split the phases at the pond? I'm pretty sure you will be OK If your total load is 15 amps. If the voltage drop is more than 5 volts there are other solutions. I wouldn't want to discuss it at this time. Your little air compressor sure wont load it down. it draws like a 1/2 amp. Or did you get the Gast 0523? It only draws about 3-4 amps.