On Wed, 18 Feb 2009 08:40:25 -0800 (PST), MC
wrote Re Septic Tank Question:

The house has been occupied continuously.

Could you guys please clarify something for me. If the tank is 1500
gallons, shouldn't the water level in the tank ALWAYS be at the level
of the outlet pipe (assuming that at least 1500 gallons of water has
been flushed down the drains) if there is no leak?

You probably have a small crack at around 6" below the out-flow pipe
in the 2nd chamber. It's probably very small, since a large crack
would be obvious. Being small, the only thing that will leak out is
liquid, not solids. The water that leaks out will be absorbed in the
soil, just like from the leach lines.

One thing you can do is fill the tank, and time how long it takes for
the level to drop 6" without any water going into the tank. Obviously
you will have to do this when no one in the house is using water.
Knowing the dimensions of the tank and the time it takes for the level
to drop, you can calculate the leak rate. I'll bet it's very small.

You could probably find the leak by digging around the sides of the
tank to a depth of 8" below the tank top (bottom of the lid).

I wouldn't worry about it unless you see water coming to the surface
or causing a muddy condition.