I used a bitumen based sealant. Warmed it up to make it more liquid and
chose a hot sunny day so that the concrete was warm. Brushed as much
dust from the gap as possible and allowed the sealant to run down the
cracks. I then spread the sealant away from the cracks to make a T shape
over them (the downstick of the T is down the crack).

Allow the sealant to cure for a couple of days, then cover with pond paint,
preferably over the whole pond and a couple of coats. No sign of a leak
in a couple of years now. Make sure that neither the sealant nor the paint
has any kind of bactericide or algicide included or it will poison the pond.

You might like to make sure that the reason for the cracks has gone away
for good though or the stresses may return to create new cracks. In my
case it was a large tree root that had grown directly under the middle of
the
pond.

Good luck.

"Bernard Huckstep" wrote in message
...
Hi

I have a small pond (7' x 5' x1') with deep cracks in it. Pond water
gradually drains through the cracks. Does anybody know how to repair these
cracks and make the pond waterproof again?

Thanks in advance

Bonod