Sand washed out under liner

[likesspace]

Hi guys,
I'm having a problem that I'm hoping someone can help me with.
We have a 24' above ground pool that is approx. 4 years old. The first year we had it we had a problem with the sand washing out from under the liner which we had fixed right away.
Later that year we had a problem with tree roots getting under the liner, as well as a mole that dug a little tunnel in another place.
Well when we replaced the liner and fixed the before mentioned problems we also had a poolkrete bottom installed and this has worked perfectly for the past 2 years.
Well a couple of nights ago, I found a depression in the poolkrete bottom exactly where the sand had washed out previously. There does not seem to be any water on the outside of the pool walls in this area but there is a small pile of sand that is visible on the outside of the wall in this area.
I also have not been having to add any water to the pool the entire year so it's highly unlikely that this is caused by a water leak in the liner. Also, the foam cove appears to be perfect in this area, as does the liner.
I guess my question is two fold....
First, what could be causing the sand to wash out in this one particular area and two.....what can I do about it?
I'd really hate to have to drain the water for the third time so I'm hoping for a quick, easy fix here. Our city water has very high alkalinity (above 400) so it's a real pain when I have to drain and refill.
Any help would be greatly appreciated.
Thanks,
Dave

[likesspace]

Anyone have any advice? Please?