Nineteen years ago friend dug a 400m2 1.5 m deep pond on the slope of a
chalky hillside and puddled clay as a liner. After two years the water
started disappearing during the summer and now it is dry.

In the winter it partially refills from the bottom which suggests after
heavy rain when the soil is saturated the water table is higher than the
bottom of the pond.

I know that dewponds on chalk were filled by rainwater and retained
water because the evaporation was less than the rain input. Obviously
the clay seal has failed but I wonder if the hydrostatic pressure from
the water table could displace the clay and rupture it?

A plastic liner will be rather expensive and I have witness one of those
being blistered from water rising underneath in a pond belonging to a
denizen of this group.

I'm looking for pointers to solve the leak, attempts have been
unsuccessfully made to puddle the clay again and an exploratory dig
appears to show the clay is good and moist.

AJH