Heat oil in a large saucepan over medium-high heat. Add leek; sauté 5 minutes or until tender. Add potato and vegetable broth; bring to a boil. Reduce heat, and simmer 25 minutes or until potato is tender. Place 3 cups of potato mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Return pureed mixture to pan; bring to a simmer. Stir in arugula, salt, and pepper; cook 2 minutes or until arugula wilts.