First, let's look at the symbol of mud. Lawrence describes the horses at the ranch as living in "fine black mud." One thing we know about mud is that it's very easy to get in stuck in. One could argue that the mud symbolizes how Mabel feels trapped now that her father has died, and that she—unlike her brothers—doesn't know what to do with her life.
Mabel's sense of hopelessness is further highlighted by the symbol of water. It's telling that Lawrence describes the water in the pond as "dead," indicating that he's using it as a symbol of death. Indeed, Mabel's attempt to drown herself in the pond would appear to confirm this. Yet as well as death, water can also be said to symbolize rebirth in the story. Water is used to baptize Christian believers, both young and old, symbolizing their new life in Christ. And after her failed suicide attempt, Mabel undergoes a rebirth of her own, albeit without any overt religious connotations. By the close of the story, Mabel appears to have been born to a new life, with the prospect of a stable, loving marriage to Jack.