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Phillip, That's a physics question, not a math question! Factors that would affect the answer are porosity, slope, whether the surface attracts or repels nonpolar nonionic fluids such as diesel, and surface roughness (at an appropriate scale that depends on other factors). For very porous surfaces (deep gravel) the answer might be a few square inches  on the top. If we assume a nonabsorbing, nonporous medium, I would guess that the answer could range from a few tens of square meters for a very rough surface (for which surface tension would trap significant quantities of fluid in dents and cracks on the millimeter scale) to square kilometers or more (as a film on water). Professionals in fuel delivery or environmental cleanup could probably give you a fairly good answer based on experience. Try the Yellow Pages. And please don't experiment <grin>. Good hunting!  


Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences. 