When oil leaks into the ground, a sequence of physical, chemical, and biological events is set in motion that dictates the extent and duration of environmental damage. Oil is not a single substance but a mixture of thousands of different hydrocarbon compounds, each with unique properties. The initial movement, subsequent transformation, and ultimate fate of this petroleum mixture depend entirely on its interactions within the subsurface environment, which is composed of soil, water, and air. Understanding how these substances partition between the solid soil matrix, the liquid pore water, and the gaseous phase is important for managing contamination.
How Oil Moves Through Soil
When oil is released onto the ground, it begins to migrate downward through the unsaturated zone, also known as the vadose zone, primarily under the force of gravity. Oil is largely immiscible with water and less dense, which is why it is classified as a Light Non-Aqueous Phase Liquid (LNAPL). As the oil moves through the soil’s pore spaces, a significant portion becomes trapped and immobilized due to capillary forces and the irregular geometry of the soil particles.
This trapped oil is referred to as residual saturation, existing as disconnected droplets within the soil pores. The amount of oil held in residual saturation depends on the soil type, with finer-grained soils generally holding more residual oil than coarser sands. If the volume of the release is large enough, the mobile free-phase oil continues its descent until it reaches the water table.
Upon reaching the water table, the oil’s lower density causes it to accumulate and spread laterally, forming a pooled layer of free product that floats on the groundwater. This layer can penetrate slightly into the saturated zone, displacing water from the larger pores. The fluctuation of the water table, such as from seasonal changes, can smear this oil layer vertically, trapping additional oil as residual saturation in a wider zone.
Chemical Weathering and Soil Interaction
Once the oil is trapped within the soil matrix, it immediately begins to undergo chemical weathering through non-biological means. One of the most rapid processes is volatilization, where the lighter, more volatile hydrocarbon components evaporate into the air-filled pore spaces of the soil. Volatilization rates are influenced by factors like temperature, soil type, and the oil’s composition, with light products evaporating more quickly than heavy crude oil.
Another key process is dissolution, where certain oil components move from the oil phase into the surrounding soil water. This dissolution of water-soluble compounds, such as benzene, toluene, ethylbenzene, and xylenes (BTEX), is a primary mechanism for the spread of contamination. The oil’s chemical structure also changes through adsorption, where hydrocarbon molecules physically bind to the surface of soil particles, particularly to organic matter and clays. This binding can limit the oil’s mobility, but the adsorbed compounds can still be a long-term source of future contamination.
The Role of Subsurface Microbes
Subsurface microorganisms, including bacteria and fungi, break down spilled hydrocarbons through a process known as biodegradation. These microbes utilize the oil’s compounds as a source of carbon and energy, transforming them into less harmful substances like carbon dioxide and water. This natural consumption of contaminants is a form of natural attenuation that occurs at nearly all contaminated sites.
The effectiveness of this microbial breakdown is heavily influenced by the availability of oxygen and nutrients within the soil. Under aerobic conditions, degradation is typically faster and more complete. However, as oxygen is quickly consumed in the core of a significant oil spill, the microbes switch to slower anaerobic processes, utilizing compounds like nitrate or sulfate as electron acceptors. Generally, lighter hydrocarbons are broken down more easily, while heavier, more complex molecules like polycyclic aromatic hydrocarbons are more resistant to microbial action and persist longer.
Contamination of Groundwater
The most significant long-term consequence of an oil leak is the contamination of the underlying aquifer. This occurs through the sustained dissolution of soluble oil components from the free-product layer and the residual oil trapped in the soil above it. These dissolved compounds move with the natural flow of the groundwater, creating a mass of contaminated water known as a groundwater plume.
The dissolved phase plume typically contains the most water-soluble and mobile compounds, such as BTEX, which can travel significant distances from the original spill source. The plume acts as a long-term source of risk because the contaminants are constantly being flushed out from the oil source and carried away by the flowing groundwater. Furthermore, as microbes break down the oil, they can produce metabolites that are more soluble and mobile than the original hydrocarbons, potentially extending the plume’s reach. This combination of a persistent oil source and a mobile dissolved plume means that groundwater contamination can last for decades, presenting a difficult and costly cleanup challenge.