It sounds like you've discovered an unexpected pipe in your yard. Based on the information you've provided, it's difficult to say for certain what the pipe is or where it's coming from. It's possible that the pipe is an abandoned well, or it could be a pipe from an old irrigation system, or it could be a pipe from a nearby property that is connected to your well or water supply line. It's also possible that the pipe is a part of an old septic system.
It's likely that the pipe is not connected to your current water supply or septic system, and that the water you observed was a temporary leak. The smell of sulfur may indicate that the water in the pipe is from a natural source, such as a spring or underground aquifer.
Given the complexity of the situation and the fact that you live in a rural area with a community well and septic tank, I would recommend consulting with a licensed plumber or a water management company to help you determine the source and purpose of the pipe. They will be able to perform a thorough inspection of the pipe and provide a diagnosis and a solution. It's also important to keep in mind that this pipe might be a liability and a potential hazard, so it's best to have a professional to check it out, as it might be dangerous or illegal to have this pipe on your property.