Behaviour of Components From Spilled Oil on Their Way Through the Soil
Authors: Dietz Daniel N. 
Published: December 2007

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Oil Spillage on Land
With the growing concern about the environment, a quantitative knowledge of the consequences of oil spillage becomes increasingly important. In the absence of such knowledge, wide safety margins tend to be allowed, resulting in sometimes wastefully expensive preventive and remedial measures. In many cases it is hard or next to impossible to excavate spilled oil, and then the consequences should be made predictable and alternative remedial measures should be available. The problem is being studied in the U.S. as well as in Europe. Not all groups of experts working on it are known to each other and this article is a plea for some additional contact. The movement of oil in soil, first downwards, then spreading over the ground water until the oil is distributed to an immobile saturation in a so-called "oil pancake" is normally of limited extent; and in any case it is predictable with a reasonable degree of accuracy. In the last few years attention has therefore shifted and it seems to be the consensus that the most important and elusive aspect is no longer the movement of the oil, but the problem of what happens to ground water polluted by contact with the oil. A well defined stream tube of ground water will thus carry soluble oil components. If not diverted by judiciously placed and operated salvage wells, this stream will follow its natural course. Its quality becomes of major importance if and when it emerges in a well, drain, or channel for a drinking-water supply. From the moment of its contact with oil the water may have been under way any length of time, but ground-water movement being slow, travel times are measured in years, often scores of years. The question arises whether the polluted ground water will have cleansed itself by biodegradation within a time span of the very general order of 20 years. Biodegradation depends on environmental conditions. Information from experts on underground transport mechanisms, on microbiology and others, has been combined to arrive at the following definition of these conditions.1. Aerobic reactions will exhaust all conceivable supply of free oxygen in a very early stage, so that immediately downstream of an oil pancake anaerobic conditions will prevail.2. Results of anaerobic degradation in oil-polluted ground water have been observed and are significant.3. Ground water flows practically without mixing. This means that a volume of polluted water, once under way, is not diluted by adjacent pure water, nor does it receive free oxygen or nutrients from adjacent water.4. A microbial population wig adapt (positively or negatively) to the environment in a time span that is negligible compared with travel times of ground water. Experiments are in progress on a number of sand columns that have an oil pancake near the top.
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