Out of sight, out of mind. We bought this house when it was about two years old, nearly twenty-five years ago. For the most part, the septic system works, so I don't pay any attention to it. But in December, there was an occasional whiff in the air. Indoors. So being promptly on the spot, a month later I called in the specialist, thinking it might need to be pumped, like we had done many years before.
What had been a simple pumping job now became ten times more expensive. First off, I was warned most strenuously not to drive my tractor over the area. He had seen these situations before and even riding mowers were too heavy in many cases, and you do not want to have to fish your tractor out of the septic tank.
It was clear to me that the lid had to be replaced, although he was struggling to come up with cheaper solutions. Unfortunately all of those just were just delays, not fixes. As he dug more and exposed the surface of the lid, it was clear that the formerly flat slab was bowing down in the middle. The concrete's strength was fading away and it was only held in shape by the internal rebar steel rods.
Decades old septic tank lids aren't an off the shelf item. He took dimensions and some company somewhere poured and cast a new one. Wait two weeks. Don't walk on the septic tank.
I asked why the concrete decayed like that. The guy with the experience wasn't sure. Perhaps water softening. Perhaps the chemicals given off in the air gap. All he knew was that the corrosion always happened on the outflow side of the tank. I'm just happy to forget about it. Out of sight, out of mind.