Barbara:

Since it's in the dam AND since you have had to haul in clay due to the porous native soil, I would recommend taking the precautions of using an anti-seep collar and also carefully repacking the trench. Your dam is the place you least want a leak.

Our dry hydrant has no collar, but it's located on the side of the pond with the steepest slope, going into nothing but clay. We put two anti-seep collars on each of the lines (8" drain pipe and 1" water line for livestock) which go downhill from the pond.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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