A poster near a recycling station mentions that one liter of cooking oil contaminates 1 million liters of water and suggesting cooking oil recycling. If I spill cooking oil in the dirt of the backyard, does it also contaminate groundwater pockets 300 yards below surface?
I question the blanket assertion of 1 liter of oil contaminating 1 million liters of water. That's 1 part per million.
talks in detail about certain volatile fractions of various petrochemicals. Some of these have recommended safe levels of 30 ppb (parts per billion) But most common sources of oil have very small fractions of these in the first place.
In a typical oil spill on soil, most of of the oil is adsorbed onto the surface of dirt particles. As long as the oil isn't applied to a level that it fills all the void space, it will generally be harmless. In large amounts, it effectively seals the surface, keeping water out, and preventing air exchange with the atmosphere.
Observational tests: Check where old cars are parked on gravel driveways. If oil was seriously toxic to plant life, you would have patches where there was no growth at all. Yes, sufficient leaks can do this, but it's not common.
Another observation: Tracks leading into old farmsteads will show compaction while the weeds in the middle thrive.
Cooking oil, as an organic, is readily decomposed by bacteria in the soil. Indeed, many of the components in fuel and motor oil are also eaten by bacteria.
Cooking oil is an organic substance and typically causes no real harm as it's biodegradable. Vegetable oils are compostable, in small quantities. Are you sure it said cooking oil, mineral or petroleum oils can contaminate water. If you do spill cooking oil in the back yard, oatmeal, sawdust can be used to absorb some of it. Then you can just burn it in the fire pit.