Re: Where is my septic tank?
Find it with a probe. Not a shovel. Most are pretty close to the surface, so in the summer the grass is just about dead over the septic tank. Look for the brown rectangle.
If you don't want to have to find it again in five years, call your local concrete products supplier. They probably make septic tanks right there. They will have access to a locking (screw down)replacement cover you can add to bring the access to the surface. For about $30.00 you can have a green plastic lid in the lawn instead of a hole every so often.
Waste not, want not.
Use it up. Wear it out. Make it due, or due without.