On 23/03/2015 00:49, sfrazi wrote:

I have drained the pond using a water pump and drilled a number of holes
at the bottom of the pond using a jack hammer. I thought this would have
meant any remaining water would quickly drain away, instead the
remaining water is still there. In some instances I have drilled at
least 7 to 8 inches below the concrete and the water just ends up
filling the hole I created.

A concrete pond wouldn't simply be built on soft earth or gravel etc the
hole would have to be firmly compacted. Its no wonder the water isn't
draining away.
If you just filled it in then it would get waterlogged over time.
As others have said, if it's clay sub-soil then without the pond there
the hole would fill up.
Can you chop out a slice from the top to the bottom with an angle
grinder? At least you'd get to see where the free draining soil starts
and you'd create a way of the water escaping if you're planning on just
filling it in rather than full demolition.