hi jed, welcome to pondboss. i'm falling asleep at my computer so apologize up front for what may be an off target or abrupt response to yer questions...need to go to bed. most hardpans do transmit alot of water. thats how hardpans form to begin with in general. the water leaches through a soil horizon and deposits calcium or iron or silica or a combination thereof creating the hardpan.
test holes need to be dug to the (surveyed/estimated) depth of the pond site to both identify the soil types and identify on-site resources that may be useful to seal the pond.
clay is what you need to seal a pond, bottom and sides.
