Does anyone remember what the original post was?
I think H2O's point was that if the well case is not properly sealed to the bedrock then contaminates carried by surface water can run down beside the case and quickly enter the well. Though I have seen this situation happen before, cloudy water was not what showed up, coliform bacteria was. There are countless studies done by the EPA regarding ground water pollution form various sources. Though under ideal conditions it is thought that 10' of clean sand is capable of filtering most pollutants, if the sand is continually subjected to those same pollutants it will eventually become saturated to the point where the pollutants will be carried into the aquifer. So the question as to how much gasoline can be dumped and how far from the well would depend on a lot of variables such as soil type, saturation level, proximity to the well case as well as the depth of the aquifer.