What is the presumed cause of the fuel contamination in the water treatment plant?

The investigation found an old underground fuel tank that was buried nearby the water treatment plant as the cause of the contamination. At an unknown point of time, the fuel leaked into an area (known as the void) beneath the surge tank of the water treatment plant. Over time, ground water (naturally occurring) mixed with the fuel and rose until the bottom of the surge tank was submerged with the contaminated water.  The surge tank is the only below ground tank that made contact with the contaminated water.

Upon making contact with the contaminated water, volatile petroleum hydrocarbon compounds wicked through the natural pores of the concrete on the bottom of the surge tank and vapourized within the surge tank. As a matter of explanation, the movement of petroleum hydrocarbon compounds moved from an area of high concentration to an area of low concentration through a porous barrier. There is no evidence of a crack of the below ground tanks (including the North Clearwell).

The vapour pressure in the surge tank increased as more vapourized petroleum hydrocarbons wicked through the porous concrete. The petroleum vapour moved to the interconnected tanks, such as the unused tank connected above by an empty pipe.

When the petroleum vapour pressure in the interconnected tanks became sufficiently high, the petroleum hydrocarbons infiltrated the raw water storage tank by petroleum vapour intrusion (definition below). From the raw water storage tank, the contaminated water was transported through the water treatment plant and accumulated within the North Clearwell.