The main component of petroleum and petroleum products is hydrocarbons. Hydrocarbon ratio varies in each petroleum product and can reach 98%.
Hydrocarbons can enter the environment from various sources. In contrast to other pollutant categories, like pesticides, petroleum hydrocarbons are not applied in soil for a specific reason so pollution is actually almost always the result of an accident or misuse. High hydrocarbon levels can be found in:
In soils near facilities like petrol stations. Hydrocarbon pollution consists mainly from low molecular weight hydrocarbons and originates from gasoline, diesel oil and heating oil leaks.
Such points are, for example, airports, where low and middle molecular weight carbohydrates can be found in the refuelling points. In railway stations hydrocarbons of greater molecular weight might appear.
Pollution due to accidents during transportation burdens the environment with low molecular weight hydrocarbons (gasoline) or medium molecular weight (diesel oil).
The second case usually burdens the marine ecosystem, while the first mainly soils, with different molecular weight hydrocarbons but mostly with higher molecular weight compounds.
In soils near refinery areas, pollution appears mainly from higher molecular weight hydrocarbons.
Although hydrocarbons in soil are mainly of anthropogenic origin, there are cases at which soil hydrocarbon pollution originates from natural sources. Such cases are mainly due to leaks from natural petroleum tanks and organic material decomposition. In addition to that, there is some evidence that certain families of higher species of plants have the ability of composing hydrocarbons, which can later move to the ground. Pollution from these sources consists of higher molecular weight hydrocarbons.
These last two sources are of secondary importance and can not actually cause significant soil pollution.