The Basics of Oil Spill Cleanup - Chapter 5

CHAPTER 5 Detection, Analysis, and Remote Sensing of Oil Spills Special instruments are sometimes required to detect an oil spill, especially if the slick is very thin or not clearly visible. For example, if a spill occurs at night, in ice, or among weeds, the oil slick must be detected and tracked using instruments onboard aircraft, satellites, or spacecraft. This technology is known as remote sens-ing. There are also surface technologies available to detect and track oil slicks. In addition, samples of the oil must often be obtained and analyzed to determine the oil’s properties, its degree of weathering, its source, or its potential impact on the environment. This analysis, as well as tracking and remote sensing technologies, are discussed in this chapter. THE IMPORTANCE OF ANALYTICAL AND DETECTION TECHNOLOGIES In the past, when an oil spill occurred, the location and extent of the spill, the potential behaviour of the oil, and its impact on the environment were often not immediately known. Today, technology is available to provide much of this information. Laboratory analysis can provide information to help identify an oil if its source is unknown and a sample is available. With a sample of the source oil, the degree of weathering and the amount of evaporation or biodegradation can be determined for the spilled oil. Through laboratory analysis, the more-toxic compounds in the oil can be measured and the relative toxicity of the oil at various stages of the spill can be determined. This is valuable information to have as the spill progresses. ©2000 by CRC Press LLC Photo 39 Oil must often be analyzed in the laboratory to determine its origin and charac-teristics. (Environment Canada) SAMPLING AND LABORATORY ANALYSIS Taking a sample of oil and then transporting it to a laboratory for subsequent analysis is common practice. While there are many procedures for taking oil samples, it is always important to ensure that the oil is not tainted from contact with other materials and that the sample bottles are pre-cleaned with solvents, such as hexane, that are suitable for the oil. The simplest and most common form of analysis is to measure how much oil is in a water, soil, or sediment sample. Such analysis results in a value known as total petroleum hydrocarbons (TPH). The TPH measurement can be obtained in many ways, including extracting the soil, or evaporating a solvent such as hexane and measuring the weight of the residue that is presumed to be oil. The oil can also be extracted from water using an oil-absorbing and water-repelling solid. The oil is then analyzed from this substrate by a variety of means, including measuring the amount of light absorbed in certain selected narrow bands. Still another method is to use enzymes that are selectively affected by some of the oil’s components. A test kit that uses colour to indicate the effect of the oil on the enzymes is available. A more sophisticated form of analysis is to use a gas chromatograph (GC). A small sample of the oil extract, often in hexane, and a carrier gas, usually helium, are passed through a small glass capillary. The glass column is coated with absorbing ©2000 by CRC Press LLC materials and, as the various components of the oil have varying rates of adhesion, the oil separates as these components are absorbed at different rates onto the column walls. The gases then pass through a sensitive detector. The system is calibrated by passing known amounts of standard materials through the unit. The amount of many individual components in the oil is thereby measured. The components that pass through the detector can also be totalled and a TPH value determined. While it is highly accurate, this value does not include resins, asphaltenes, and some other components of the oil with higher molecular weight that do not pass through the column. Photo 40 Taking a sample from a thin sheen can be difficult and results in more water than oil. (National Oceanic and Atmospheric Administration) A typical chromatogram of a light crude oil with some of the more prominent components of the oil identified is shown in Figure 11. One type of detector used on a gas chromatogram is a mass spectrometer (MS). The method is usually called GC-MS and can be used to quantify and identify many components in oil. The mass spectrometer provides information about the structure of the substance so that each peak in the chromatogram can be more positively identified. This information can then be used to predict how long the oil has been in the environment and what percentage of it has evaporated or biodegraded. This is possible because some of the components in oils, particularly crude oils, are very resistant to biodegradation, while others are resistant to evaporation. This difference in the distribution of components then allows the degree of weathering of the oil to be measured. The same technique can be used to “fingerprint” an oil and positively identify its source. Certain compounds are consistently distributed in oil, regardless of weathering, and these are used to identify the specific type of oil. ©2000 by CRC Press LLC Figure 11 Chromatogram of a light crude oil. FIELD ANALYSIS Analysis performed in the field is faster and more economical than analysis done in a laboratory. As analytical techniques are constantly improving and lighter and more portable equipment is being developed, more analytical work can be carried out directly in the field. Test methods are now available for measuring physical properties of oil such as viscosity, density, and even flash point in the field. Test kits have also been developed that can measure total petroleum hydrocarbons directly in the field. While these test kits are less accurate than laboratory methods, they are a rapid screening tool that minimizes laboratory analysis and may provide adequate data for making response decisions. DETECTION AND SURVEILLANCE Detection and Tracking Buoys and Systems As oil spills frequently occur at moorings and docks, buoys and fixed-point monitoring systems have been developed to ensure rapid response at these sites. These systems detect the oil on water and transmit a radio signal to an oil spill response agency. Fluorescence is one method used to detect oil in these systems. An ultraviolet light is focused on the water surface and any oil that is present fluoresces, or absorbs the ultraviolet light and re-emits it as visible light. This fluorescing phenomenon is relatively unique to oil and provides a positive detection mechanism. ©2000 by CRC Press LLC Photo 41 Field analysis is becoming more common with the development of test kits. This chemist is measuring the amount of oil in soil. (Environment Canada) In another detection method, an oil sorbent is used that changes in physical properties when it absorbs oil and thus triggers a device. An example of this would be a sorbent that loses it strength when oil is absorbed. The sorbent is placed in contact with a spring and a switch, which is activated when oil enters the sorbent. This type of device is not effective for fast response. Other detection units are triggered by the differential light reflection or absorption properties of oil. Photo 42 Spill-tracking buoys are used to track actual spills or to assess trajectories before a spill occurs. (Environment Canada) ©2000 by CRC Press LLC ... - tailieumienphi.vn