Spills and leaks of petroleum hydrocarbons such as gasoline, diesel, motor oils, and similar materials have caused widespread contamination in the environment. Cleanup is often expensive and slow. However, under the proper conditions at some sites, natural attenuation can contribute significantly to remediation of some components of the contamination.

As the bulk hydrocarbon moves through the subsurface, some of the liquid may be trapped in the soil or sediment pores; some may evaporate; some may become sorbed to the surface of the soil particles and some may dissolve in the ground water.

Since petroleum hydrocarbon liquids are less dense than water, the liquid may float, rising and falling as the water table rises and falls. This process can create a smear zone of residual saturation.

As the dissolved plume moves, the concentration of the dissolved hydrocarbons is lowered by dispersion and dilution. Microorganisms may degrade hydrocarbons that are dissolved, volatilized or sorbed.

For more information, view the referenced Fact Sheet at https://clu-in.org/download/remed/pet-hyd.pdf; it makes for easy reading.