When the car is being lowered on top of Sara in the desert, Sara reaches out with her right arm, which is the one that then gets pinned underneath the car. Throughout the remainder of this episode, it is Sara's left arm that is pinned/broken as a result.
It rained all night when Sara was trapped under the car, but the next day, the ground was still cracked and dry. There also were no mud anywhere, or puddles Sara could have found water at.