CAS sheets will have soluability and density. Ultimately, it can contaminate a phenomenal amount of water as the soluability of oil is very low and it can continue to dillute beyond the saturation soluability. You should probably look to the EPA site for the MCLG or MCL and calculate the volume of water contaminated to that concentration by the volume spilled.
Of course, if the oil adheres to soil particles (EPA site will mention this) then you can assume that x% remains in the vadose zone.