Most oil tanks will have some surface contamination around the fill pipe. This is usually the result of small spills during oil delivery. These small spills are likely not to be a major concern. The greater concern is related to the contamination from leaks from the tank and/or the lines leading to the house. In order to detect contamination from such leaks, soil cores have to be taken from areas around and below the bottom of the tank. Soil testing costs range from $150 to $500.
The standard method of testing involves measuring the depth to the bottom of the tank. Once this depth is established, a hole is drilled in the ground next to the tank and away from the likely location of the oil lines. A sample of soil is removed from the bottom of the hole for laboratory testing. Oil contaminated soil will often have a gray color and a strong odor.
Clean soil will test at less then 20 parts per million (ppm) of oil. However, test results between 20 and 200ppm may be an indicator of higher contamination levels at another location. Test results of less than 20 ppm are good news, but only prove that the soil at this specific location was clean. It is therefore very important to have the test sample taken by a skilled professional who will locate the most likely area(s) for any contamination.
Clean up levels depend upon the local jurisdiction. Arnie Sugar, P.G. of HWA GeoSciences Inc. was kind enough to help out with the rules in Washington State: "The new (since 2001) Washington State Department of Ecology (not EPA) 'Method A' cleanup level for heating oil TPH is 2,000 mg/kg (ppm). It went up by a factor of 10, as it became known that the heavier hydrocarbons (including heating oil) have relatively low toxicity and mobility."