| Is there anything in it? Or does it just look like a bucket sunk into the ground, with some holes or slots in it?
In the first case, if you see some fixture(s) down in there (when the water is gone), it could be a water meter. These are often brass-colored, but can also be dirty and dark so that it's hard to tell. But since you say you shut the city water off at the meter, this seems very unlikely. It could be another meter for the pool, or maybe just a large-bore connection to the water main for filling the pool. In that case maybe you'll see a valve and some piping, maybe connecting to the pool systems.
If it's just an empty bucket (how far down does it go?), then it's what's known as a sump... as in "sump pump". It's supposed to be the place where you can put a float-switched pump to get water out of your basement, or wherever. Water is supposed to seep in through the holes, and fill it up until the pump comes on.
In any case, what you've described is a leak in the main water supply line, between your pool house and the main water meter. If you shut off the meter and the water stopped that pretty much sums it up right there. If you're lucky, the leak will be in the fitting or valve or whatever that's at the bottom of the bucket/sump/well. If not, you'll have to dig up the line back to the water meter... not cool given the slab for your poolhouse is over the first part. But from the way you describe it, sounds like the leak is either in the sump or very near it.
Good luck!
~aaron
ps: i'm NOT a plumber, so...... consider the source! |