You will be surprised how quickly nature deals with spilt oil, in the 80's we used to go to a stunning Spanish beach just up from Gibraltor, passing oil tankers used to clean their tanks and discharge them, the outcome was our sandy beaches were a mess of tar balls. Usually it took only weeks to completely grind down the tar with the waves and sand and it was surprising how quickly it dissappeared from the water line and that further up the beach seems to break down with the sun and elements. Our kids became realy adept at removing it from their feet with the best tar remover of all, olive oil.

Obviously quantities are greater here but I wouldn't be to dispondent about long term spoiling of your waters once the well is capped.

Despite the extensive cleanup attempts, less than ten percent of the oil was recovered and a study conducted by NOAA determined that as of early 2007 more than 26 thousand U.S. gallons (22,000 imp gal; 98,000 L) of oil remain in the sandy soil of the contaminated shoreline, declining at a rate of less than 4% per year.

Both the long- and short-term effects of the oil spill have been studied comprehensively. Thousands of animals died immediately; the best estimates include 100,000 to as many as 250,000 seabirds, at least 2,800 sea otters, approximately 12 river otters, 300 harbor seals, 247 bald eagles, and 22 orcas, as well as the destruction of billions of salmon and herring eggs. The effects of the spill continued to be felt for many years afterwards. Overall reductions in population have been seen in various ocean animals, including stunted growth in pink salmon populations. Sea otters and ducks also showed higher death rates in following years, partially because they ingested prey from contaminated soil and from ingestion of oil residues on hair due to grooming.

Almost 20 years after the spill, a team of scientists at the University of North Carolina found that the effects are lasting far longer than expected. The team estimates some shoreline Arctic habitats may take up to 30 years to recover. Exxon Mobil denies any concerns over this, stating that they anticipated a remaining fraction that they assert will not cause any long-term ecological impacts, according to the conclusions of 350 peer-reviewed studies. However, a study from scientists from the NOAA concluded that this contamination can produce chronic low-level exposure, discourage subsistence where the contamination is heavy, and decrease the "wilderness character" of the area.