Because there was oil, diesel fuel and gasoline in the storm surge your arborvitae could be coated with them and sufficating. It may be not breathing properly. Try using a garden hose attached sprayer filled with DAWN dish detergent/water mixed. Thoroughly "LATHER" the arborvitae so that you clean off as much oils as possible, then rinse it off just as thoroughly. This may help. At this point it may be too late, but anything you do may still save it.