We think of oil as being a single substance, but there actually are many different kinds of oil. Kinds of oil differ from each other in their viscosity, volatility, and toxicity. Viscosity refers to an oil's resistance to flow. Volatility refers to how quickly the oil evaporates into the air. Toxicity refers to how toxic, or poisonous, the oil is to either people or other organisms.
When spilled, the various types of oil can affect the environment differently. They also differ in how hard they are to clean up. Spill responders group oil into four basic types, which you can see here, along with a general summary of how each type can affect shorelines.
Type 1: Very Light Oils (Jet Fuels, Gasoline)
- Highly volatile (should evaporate within 1-2 days).
- High concentrations of toxic (soluble) compounds.
- Localized, severe impacts to water column and intertidal resources.
- No cleanup possible.
Type 2: Light Oils (Diesel, No. 2 Fuel Oil, Light Crudes)
- Moderately volatile; will leave residue (up to one-third of spill amount) after a few days.
- Moderate concentrations of toxic (soluble) compounds.
- Will "oil" intertidal resources with long-term contamination potential.
- Cleanup can be very effective.
Type 3: Medium Oils (Most Crude Oils)
- About one-third will evaporate within 24 hours.
- Oil contamination of intertidal areas can be severe and long-term.
- Oil impacts to waterfowl and fur-bearing mammals can be severe.
- Cleanup most effective if conducted quickly.
Type 4: Heavy Oils (Heavy Crude Oils, No. 6 Fuel Oil, Bunker C)
- Little or no evaporation or dissolution.
- Heavy contamination of intertidal areas likely.
- Severe impacts to waterfowl and fur-bearing mammals (coating and ingestion).
- Long-term contamination of sediments possible.
- Weathers very slowly.
- Shoreline cleanup difficult under all conditions.
More Information about Oil Types
The figure at right shows the weathering processes affecting oil spills:
Adsorption (sedimentation): The process by which one substance is attracted to and adheres to the surface of another substance without actually penetrating its internal structure.
Biodegradation: The degradation of substances resulting from their use as food energy sources by certain micro-organisms including bacteria, fungi, and yeasts.
Dispersion: The distribution of spilled oil into the upper layers of the water column by natural wave action or application of chemical dispersants.
Dissolution: The act or process of dissolving one substance in another.
Emulsification: The process whereby one liquid is dispersed into another liquid in the form of small droplets.
Evaporation: The process whereby any substance is converted from a liquid state to become part of the surrounding atmosphere in the form of a vapor.
Photo Oxidation: Sunlight-promoted chemical reaction of oxygen in the air and oil.
The various types of oil differ in how they weather (chemically or physically change when exposed to the elements). Most crude oil blends will emulsify quickly when spilled, creating a stable mousse that presents a more persistent cleanup and removal challenge. Even in high winds, usually over 70% of a Fuel Oil No. 6 spill will persist as floating or beached oil for a week or longer. On the other hand, over 90% of the diesel in a small spill in the marine environment is either evaporated or naturally dispersed into the water column in time frames of a couple of hours to a couple of days.