The Basics of Oil Spill Cleanup - Chapter 11

CHAPTER 11 Shoreline Cleanup and Restoration Oil spilled on water is seldom completely contained and recovered and some of it eventually reaches the shoreline. It is more difficult and time-consuming to clean up shoreline areas than it is to carry out containment and recovery operations at sea. Physically removing oil from some types of shoreline can also result in more ecological and physical damage than if oil removal is left to natural processes. The decision to initiate cleanup and restoration activities on oil-contaminated shorelines is based on careful evaluation of socio-economic, aesthetic, and ecological factors. These include the behaviour of oil in shoreline regions, the types of shoreline and their sensitivity to oil spills, the assessment process, shoreline protection mea-sures, and recommended cleanup methods. Criteria of importance to this decision are discussed in this chapter. BEHAVIOUR OF OIL ON SHORELINES The fate and behaviour of oil on shorelines is influenced by many factors, some of which relate to the oil itself, some to characteristics of the shoreline, and others to conditions at the time the oil is deposited on the shoreline, such as weather and waves. These factors include the type and amount of oil, the degree of weathering of the oil, both before it reaches the shoreline and while on the shoreline, the temperature, the state of the tide when the oil washes onshore, the type of beach substrate, i.e., its material composition, the type and sensitivity of biota on the beach, and the steepness of the shore. Other important factors are the existence of a high tide berm on the beach, whether the oil is deposited in the intertidal zone, and whether the particular length of shoreline is exposed to or sheltered from wave action. An exposed beach will often “self-clean” before a cleanup crew can perform the task, which can result in the released oil being transported to other beaches or even back to the same beach. ©2000 by CRC Press LLC 102 107 103 108 104 109 105 110 106 111 ©2000 by CRC Press LLC Opposite: Types of Shoreline—Photo 102—Bedrock; Photo 103—Boulder beach; Photo104—Pebble–cobble beach; Photo 105—Mixed sand gravel beach; Photo 106—Sand beach; Photo 107—Mud tidal flat; Photo 108—Sand tidal flat; Photo 109—Marsh; Photo 110—Peat and low-lying tundra; Photo 111—Mangrove shore. The extent to which an oil penetrates and spreads, its adhesiveness, and how much the oil mixes with the type of material on the shoreline are all important factors in terms of cleanup. Cleanup is more difficult if the oil penetrates deeply into the shoreline. Penetration varies with the type of oil and the type of material on the shoreline. For example, oil does not penetrate much into fine beach material such as sand or clay, but will penetrate extensively on a shore consisting of coarse material. A very light oil such as diesel on a cobble beach can penetrate to about a metre under some conditions and is difficult to remove. On the other hand, a weathered crude deposited on a fine sand beach can remain on the surface indefinitely and is removed fairly easily using mechanical equipment. The adhesiveness of the stranded oil varies with the type of oil and the degree of weathering. Most fresh oils are not highly adhesive, whereas weathered oils often are. Diesel and gasoline are relatively non-adhesive, crudes are generally moderately adhesive when fresh and adhesive when weathered, and Bunker C is adhesive when fresh and highly adhesive when weathered. An oil that is not adhesive when it reaches the shore may get washed off, at least partially, on the next tidal cycle. The extent of oil coverage often depends on the stage of the tide when the oil is deposited on the shoreline. At high tide, oil can be deposited above the normal tide line and often spreads over a broad intertidal area. The least amount of oiling occurs when the oil is deposited on the shoreline during the falling tide, although this is less likely to occur as the water is moving away from the shoreline. The nature of the intertidal zone, i.e., its composition and slope, will often dictate the fate of the oil. If large amounts of oil are not retained in the intertidal zone, then the oil will have less impact on the area. The fate of oil on shorelines also depends on the wave regime. Oil can be removed and carried away by energetic waves within days whereas it can remain for decades in sheltered areas. For example, some of the oil spilled from the Arrow in 1970 remains in the sheltered coves of Nova Scotia to this day. Similarly, a significant amount of oil spilled from the Metula in 1974 remains on sheltered beaches in Chile. In both cases, the oil was Bunker C and weathering produced a crust on top of the oil. Under this crust, the oil is still relatively fresh, even after decades. Beaches are a dynamic environment that changes in profile during seasonal storms. This can result in oil being buried on the beach in layers, often as deep as 1 metre, or buried oil can be brought to the surface. Oil stranded on shorelines, especially above the high tide line, weathers with time and becomes more adhesive, viscous, and difficult to remove. If nutrients are present and the oil is crude, limited biodegradation can take place, but this occurs slowly and only 10 to 30% of the oil is removed in one to two years. As oil stranded above the high tide line is above the limit of normal wave action, physical removal can occur only during storm events. ©2000 by CRC Press LLC Photo 112 Photo 113 Driftwood is frequently oiled during spills and is usually cut and burned. Complete removal of the driftwood could result in erosion on some beaches. (Environment Canada) Oil sometimes arrives on the shore in the form of tar balls. (Environment Canada) ©2000 by CRC Press LLC Photo 114 Photo 115 This oil from the Arrow spill has been on the Nova Scotia shore for more than 25 years. (Environment Canada) A hole dug in this sand beach reveals layers of buried oil. (Environment Canada) Another mechanism that can significantly affect the fate of oil on shorelines is the mixing of the oil with beach material. Oil often mixes with sand and gravel on beaches and then weathers to form a hard, resilient material called “asphalt pave- ©2000 by CRC Press LLC ... - tailieumienphi.vn