My house was built in 1915, last year I discovered an old cistern buried under a small cement slab. This is very close to the back of my house and right where I'm laying a stone patio, so I called city maintenance and got a hold of an old manhole frame and cover. I cemented/mortared the frame over the opening then installed the cover.
From what I can tell, looking at the top only it is made from brick and lined with hydraulic cement. I haven't gotten the nerve to climb inside yet, but it appeared dry, so that tells me it's probably still in decent condition. There are some broken chunks of concrete and rock in the bottom, but my guess is it's about 7-8' in diameter and about that deep. It is split in half with a brick wall.
I am also in the process of installing a shallow sand point well, as the water table is about 15' down around my house. I was going to use that for irrigation, but now I'm thinking of diverting my gutters to the cistern and devising a way to somewhat filter larger particles out, so a pump wouldn't get clogged by debris, then dropping in a pump and hooking it to my irrigation system. I figure I might also run a line from my sand point well to fill the cistern if it gets too low from lack of rain, maybe some sort of float switch that would turn the sand point on if the cistern gets too low.
Any thoughts on this? How safe are these old cisterns? It's obviously not caving in, but what if I fill it with water? Also, any idea what the city would say? I hate to call and ask them, because then they'll know I have it and they might want it filled.