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Materials that are "Fighter than air (vapor density less than ambient atmosphere) , will drift upwards into the atmosphere and be driven by the wind in a downwind direction. Heavier than air materials will tend to hug the ground, following the contours of the land from higher to lower elevations or be pushed by the wind movement. B. Land Spills Generally, solids (even in the form of particulates) that spill on the land are the easiest materials to confine. Even if shipping containers rupture, solids ordinarily don't move far. The release area should be closed off to avoid having the materials tracked away from the site on shoes, clothing, or vehicle tires. It is also important not to increase the mobility of the material by the indiscriminate application of water or other liquids. Covering the material with plastic, tarps, or other means can help prevent it from becoming windborne. Liquids spilled on the land may be somewhat more difficult to confine. In some cases, confinement may already be in place. For example, most tank farms have a berm around their periphery for confining major leaks. If a transfer line breaks or if an accident occurs in transporting or loading a liquid, there will be no Hautomatic" containment. On concrete, blacktop, or other hard surfaces, berms can be constructed with dirt, sand, absorbents, or urethane foam packs specifically designed for this purpose. If the spill is on the ground, berms can be constructed by simply mounding the soil itself. In many cases, though, it may be more advantageous to "herd" the liquids by ditches, swales, and berms to an existing low point or construct a catch basin. This allows the material to pool and may make cleanup easier. There are three techniques for controlling spills on the land: • Diversion: The controlled movement of the liquid from one course or area to another where the effects to human health and the environment are substantially reduced. • Diking: The use of a barrier to confine or control the movement of liquids from an area of potential harm. • Retention: The temporary confinement of the liquid in an area (e.g. , in a pond) where it can be absorbed, neutralized, diluted, or pumped out. Determining which of these three techniques should be used to confine a spill of hazardous materials depends on several factors: time; personnel; equipment; supplies and the potential harmful effects of the leaking material. For example, response personnel may determine that diversion, rather than diking and retaining, is more appropriate for controlling the movement of fuel oil that could enter a storm drain. h this situation, response personnel may determine that diversion should be used to control the movement of oil because the oil is flowing toward the storm drain at a rate that will not permit the timely construction of a dike. 56 Site Safety 6 Health Plan Rev./1. 0 October 20, 1997