• For best viewing, keep the sun behind you, holding the aircraft at an altitude of 500 to 1,000 feet (150 to 300 meters) while you observe the slick at about a 30-degree angle.
• Different observers at the same site can reach different conclusions about how much of the slick has been dispersed. To make it easier for observers to achieve consensus during a response, use standard reporting criteria and be sure that all observers have been trained to use a common set of guidelines. (Check our Open Water Oil Identification Job Aid to learn about reporting standards.)
• Be sure to report the presence of marine mammals, turtles, and birds in the area of dispersant application. These species can be adversely affected by contact with dispersants.
• You will find that observation is especially difficult under low-contrast conditions (e.g., twilight or haze).
• As a monitoring observer, you will not make operational decisions, such as deciding how much dispersant to apply, or when or where to apply it. These decisions are made by operational units.

• Some things in and on the water, such as suspended solids or algal blooms, can resemble dispersed oil.
• The appearance of oil slicks and subsurface plumes is affected by factors such as oil characteristics, time of day (different sun angles), weather, sea state, and the rate at which oil disperses.
• Once dispersant has been applied to a slick, you may observe color changes in emulsions (mousse), produced as the demulsifying action of the dispersant reduces the water content and viscosity of the emulsion. Demulsification also can produce changes in the shape and size of a slick: as demulsification pushes water out, the slick may appear to shrink in area.

• A visible subsurface plume of dispersed oil indicates that an application of dispersant is working. When no plume is visible in the water column, it is difficult to determine whether the dispersant is working but does not definitively mean that the dispersant is not having an effect.
• Subsurface plumes of dispersed oil typically have a cloudy appearance, and can range in color from brown to white. Some are not visible. When dispersant is working, oil remaining on the water surface also may change color.
• In some cases, you may not be able to see a subsurface plume even when one is present. When the water is turbid, you may not be able to see a plume. Sometimes, remaining surface oil and sheen can mask oil dispersing under the slick, making it hard for you to see the plume.
• A subsurface plume may not form instantaneously once dispersant has been applied to a slick. In some cases, such as when oil is emulsified, it can take several hours for a plume to form. In other cases, a visible plume may not form (you then may wish to use sampling to learn whether dispersion has occurred).
• A change in the appearance of a treated slick, compared with an untreated slick, might indicate that the dispersant is working. However, a difference in appearance alone is not enough of an indication to be sure that the dispersant is having an effect.
• A clear stretch of water left in the wake of a dispersant application vessel can appear to represent successful dispersion of oil. However, areas of clear water can be created as the vessel wave breaks a path through the oil, either physically parting the oil or mechanically dispersing it. Mechanically dispersed oil will recoalesce and float to the surface.
• When initially applied, dispersants sometimes have a herding effect on oil, making a slick appear to be shrinking when, in fact, the dispersant is "pushing" the oil together. This effect results from the surfactant action of dispersants, which exerts a horizontal spreading force on thin oil films. This effect can cause parts of an oil slick to seem to disappear from the sea surface for a short time.

• Because dispersed oil plumes are often highly irregular in shape and variable in oil thickness, it can be difficult to accurately estimate dispersant efficiency.
• The appropriate dispersant application dose depends on the thickness of the oil in a slick. But slicks are generally not uniform in thickness, and it's not always possible to distinguish among thicker and thinner parts of the same slick. For this reason, it's possible to apply too much dispersant to some parts of a slick ("overdoses") and too little to other parts of the same slick ("underdoses"). Underdosing can cause a dispersant application to be unsuccessful, while overdosing adds to the expense of treatment. Because over- and underdosing lead to variations in the effectiveness of application, be sure to note these variations when you're able to observe them.

Dispersion: The breaking of an oil slick into small droplets that are mixed into the water column by breaking waves, other sea surface turbulence, and the action of chemical dispersants.

Emulsification: The formation of a water-in-oil mixture. Different oils exhibit different tendencies to emulsify, and emulsification is much more likely to occur under high energy conditions (winds and waves, oil well blowouts). A water-in-oil emulsion is frequently called mousse. Demulsification is the breaking of an emulsion.

Plume: Oil that is dispersing into the water column as a cloud of small droplets.

Sheen: Sheen is a very thin layer of oil (0.0003 mm or less) floating on the water surface. It is the most common form of oil seen during the later stages of a spill. Sheens vary in color according to their thickness, ranging from rainbows for the thicker layers, to grays, silvers, and almost transparent for the thinnest layers.

Slick: Oil spilled on the water that absorbs energy and dampens out surface waves, so that it appears smoother—or "slicker"—than the surrounding water.  

More Information about Dispersants

Dispersants: A Guided Tour: Take our guided tour to get a basic overview of this important oil spill countermeasure.

Spill Containment Methods: In addition to dispersants, learn about other ways responders deal with spilled oil.

Dispersant Mission Planner: Find out about this tool that spill responders and planners can use to assess dispersant application system performance.

SMART: Special Monitoring of Applied Response Technologies (SMART) is a cooperatively designed monitoring program for in situ burning and dispersants.

Dispersing Oil Near Shore in the California Current Region [PDF, 1.4 MB]: A 2001 paper summarizing our current knowledge of dispersants, dispersing oil, the need for preapproval, methods used to simulate spill responses, and ecological risk assessment.

ITOPF: Dispersants: The International Tanker Owners Pollution Federation Limited (ITOPF) provides this presentation on dispersants.  

More Information about Job Aids

Job Aids for Spill Response: Learn more about the job aids that OR&R has created to help oil spill responders complete their response tasks.
 

Questions: Contact us with any questions, comments, or suggestions related to NOAA Job Aids.