When tilling my back yard to prepare for a lawn, the tiller leaked gasoline in several places where it was parked overnight. I did not notice the leaks until I filled the tiller up one evening, and it was empty the next morning. About a liter spilled. I dug out the contaminated soil where it was parked that night and scattered it thinly in another part of the yard so that the gasoline would evaporate. I dug down until I could no longer smell gasoline in the soil, and I found that the gasoline had percolated to a depth of about 3 feet in poorly draining clay soil. The method work for that area, but I had no way to know where else the tiller had leaked.
I planted lawn seed, and it seemed to thrive everywhere. However, in about six months, yellowish-brownish patches began to appear in several areas where I believe the tiller had been parked overnight. It looked like the lawn was dry and dying in these patches, but heavy watering seemed to make the problem worse. Since gasoline floats on water, I surmised that the watering was causing the gasoline to rise from a lower level.
I did not wish to dig up a large area to a depth of 3 feet, and so, I came up with the idea that diluted laundry soap might mix with the gasoline and take it back down to a lower level where microbes would eventually destroy it. I mixed 1 capfull of Woolite per gallon of water in a five gallon bucket and poured the solution over an area of about 4 square feet. I watered that area heavily, and after about 3 weeks, the grass began to recover. I treated it a second time. In another three weeks, that patch was fully dark green and thriving. It has been growing fine for six months.
To treat the other areas, I filled the bottle on a MiracleGrow garden hose sprayer with Woolite and broadcast it over an area of about 100 square feet, again watering heavily every day for about a week. After about three weeks, the brownish-yellowish patches began to dissapear, and I gave the area a second treatment. My problem now seems to be cured.