Make a solution of 1/3 of a cup of laundry detergent, one quart of liquid bleach, and three quarts of warm water. Add the bleach to the water first, followed by the detergent. Apply the mixture to the affected areas, letting it stand for about five minutes. Do not let it dry. You may find it necessary to scrub the stains with a broom or nylon brush. Finally, flush the entire area with water.

If this doesn't work, try using a solution of muriatic acid. Mix one part acid to nine parts water. Apply to the affected areas and flush with water after a couple of minutes. Remember, it's always a good idea to wear rubber gloves and eye protection when working with these types of products.