Blend watermelon in blender until smooth. Pour watermelon into a pitcher and add lemonade concentrate, stirring until combined. Slowly add the chilled soda and stir. Add ice and garnish with lemon slices (optional) and enjoy! Best if served immediately or store leftover sparkling watermelon lemonade covered in the refrigerator. Serves 8.
If you prefer things a little less sweet, consider using sparkling water instead