How does one even begin do fix something like that. Can you turn it off?
1- Very carefully.
2- No, you work on it leaking.
Actually, I spray water on it till it builds a block of ice that acts like a plug. Then I can access the leaking joints. The cold weather causes the flanged joints and valve packings to leak. Happens every year. And of course I don't trust anybody else to fix it.
Note the pretty light blue coloring on the concrete slab. That's actually liquid oxygen pooled on the ground. Below -290 degree. If the liquid spraying out hits any part of your body, it's toast. The concrete actually absorbs the LOX. Freezes your shoes right to the ground if you stand in one place.
Fun, fun, fun!
I got that one repaired, now on to a broken 24" diameter concrete water line. Repair parts won't arrive till after 7pm tonight.