Liquid crude oil or petroleum products that are spilled on land can either remain in the soil within the general area of the spill or be carried or leached away by gravity or rain, eventually ending up in a river or lake, or into a groundwater aquifer. Factors such as the quantity and the physical properties of the spilled petroleum, the local weather conditions, the characteristics of the soil and the amount and type of vegetation at the area of the spill, the land topography and elevation of the area, and so on all have an impact on whether the oil “plume” will stay in place or move downwards to lower elevations or deep into the lower soil layers.
A localized oil pollution plume confined to a limited area is usually easier to manage than one that has managed to sink deeper into the soil or to flow towards lower-lying areas or into bodies of water. Localized petroleum contamination of the soil is typically addressed by any combination of mechanical, chemical, and biological treatment methods. The choice of methods depends mostly upon the physical properties (e.g., viscosity) of the spilled petroleum product.
If the spilled petroleum is relatively highly viscous, such as tar-sands oil or liquid asphalt, shoveling by men or machine can remove the bulk of the plume if it has formed into a pool of thick sludge or molasses-like mass of oil. Oil-transfer pumps can also effectively remove liquid asphalt especially with the addition of hot water and/or steam. These pumps are also useful for the less viscous heavy crude oil or standard crude oil, as well as the relatively lighter petroleum fractions such as kerosene, diesel, gasoline, etc. Oil transfer pumps are more robust than ordinary liquid pumps and usually employ solid-debris crushing or cutting mechanisms to prevent blockages.
Once the bulk of the spilled liquid petroleum has been removed, the remaining oil can be mopped up with use of sorbent mats, pads, pillows, socks, or loose granular, fibrous, or powdered materials of natural (such as Norwegian Pine tree bark, recycled paper, clays, and silica) or engineered (such as graphene, polyurethane, and polypropylene) oil-absorbent materials.
Oil molecules can still remain trapped between soil particles or even saturate the cells of underbrush plants as well as the spaces between clay minerals. Such tightly bound oil cannot be effectively removed by mechanical means unless the affected soil layer itself is dug up and placed into a large machine (such as a truck-mounted cement mixer) and then put on a wash cycle with water and appropriate detergents. Obviously such an approach requires a lot of man-machine hours and can be costly, depending upon the volume of soil that needs to be washed. Alternatively, chemical surfactants, solvents, and emulsifying agents can be used to dissolve or detach the oil stains in situ from rocks, soil, and clay particles. Water is then used to disperse the released oil stains; and the resulting oil-rich sludge can also be collected. But since inorganic or synthetic organic chemical substances can also become environmental pollutants, biodegradable chemical agents or bioactive agents, in turn, seem to have attracted more attention recently.
Examples of biodegradable soil-remediation agents include chicken manure, plant-extract-based dispersants, and oil-munching microbes. Microbial agents include natural or genetically engineered non-pathogenic bacteria and fungi. Pseudomonas putida is an example of a genetically engineered bacteria that can be used to degrade certain types of petroleum products.