You can use the cheapest available poly 3/4" at Home Depot you can get in 100' sections. If it heads down-slope towards the pond away from the pump so condensation can drain, you don't have to go more than a few inches deep. Using a ditch witch will help get it deeper, which is preferable to lessen the temperature swings.
Make sure and use stainless clamps, and in my case I am putting in an occasional omega shape in the line for expansion and contraction so temperature changes don't pull the fittings loose. In addition, being able to glue/weld the seams would be even better.
I hope to do this myself this fall, finance committee permitting (denied this spring thanks to car problems). Also it has been extraordinarily wet this year, and working the ground is really messy so a few projects had to be put off.
