Mark,
Welcome to Pond Boss.
I assume you want to use kitty litter to seal the sand from leaking. Is the sand now holding water, and how does the water get into the pond? For instance, is it water-table water, well water, or from rain runoff, etc.?
Also, do you really mean 150 square feet (12.25' x 12.25') or do you mean 150 feet square (150' x 150' -- approx 1/2 acre)?
With a little more info, we should be able to get you some decent answers.
Regards,
Ken