I got this estimate from a news article and it seems most of the fuel oil etc. was already pumped out long ago. What they worry about now is essentially rotting food. So let's focus on that aspect.
"Costa Concordia had only just set off on a Mediterranean cruise when it ran aground in the waters by Giglio Island, leaving behind a week’s worth of food for more than 4,000 people. The grocery list of items trapped inside the ship right now includes 11,000 eggs, 17,000lbs of beef, 5,500lbs of cheese and more than 1,000 gallons of milk, according to the Daily Telegraph, among a host of other now-rotten foods. Experts fear this toxic stew .. is just waiting to spew out of the crippled cruise liner..."
So if I sum up all the food matter I think we have ~30 tons at most of biodegradable organic matter. Let's assume dilution even in a hypothetical tiny volume the enclosing box size of the ship itself. 290 m x 36 m x 15 m
That's 156×103 m3.
That gives an organic matter concentration of ~190 ppm. I'm roughly assuming this leads to a BOD of 190 ppm which I think is a reasonably conservative assumption. (In 2 years a lot of the BOD is probably consumed away anyways)
Isn't that level of Chemical Oxygen Demand almost below what Governments allow you to dump into a river directly after treatment?
I must be doing something wrong, I feel, because they are spending 500 Million Euros on that salvage project after all. I'd love to be corrected.
PS. I agree there can be other environmentally damaging considerations but this rotting food point is what intrigues me.