I have simply removed a large branch from and on a near my neighborhood county park trail so a bicyclist does not run over and wreck. I am suspect of picking up some trash, such as a partially filled water bottle with cap on, as some terrorist could have planted it. Invasive plants - personally don't go there.
Any serious issues, like a drainage problem with water covering many sections of the same trail, emailed
parkmail@fairfaxcounty.gov . I even attached digital pictures of the problem (picture worth 1000 words) which helped them to see how serious problem was and more quickly address the problem which they did.