Heres a recipe for success: Well-drained loam. Adequate nitrogen. Plenty of organic matter to provide good soil porosity. Yepjust the ticket to remediate an oil spill.
In a former life I cleaned up petroleum spills for a living. Although bulk-storage facilities as well as tanker semi-trucks and train cars were occasionally the source of oil or gasoline spills, it was a real eye-opener to learn how often faulty home-heating oil tanks or even the odd leaky auto gas tank led to significant soil and water contamination.
When three-fourths of soil is oil you may wonder why cleaning up an oil spill even matters. More than once I heard grumblings to the effect that oil is a natural product that comes out of the ground, so it may as well go right back in.
Granted, theres a certain logic to that, but with precious few exceptions, oil is found deep in the ground essentially the other end of the earth from the one we live on in zones far removed from drinking water aquifers. Im not a livestock expert, but Im pretty sure horse apples are associated with one specific end of a horse, and real apples are the only kind that should go in the other end. Its a coarse analogy, though a fair one. Oil comes from in the ground but should not go on the ground.
Dealing with contaminated water is complex and obscenely expensive. So its important to deal with oil-soaked dirt which can be remediated by gardening (Really; more on that in a minute) before rain leaches dissolved-phase petroleum into groundwater.
I can hear some collective head-scratching out there (or maybe thats everyone closing their newspapers), so Ill clarify that petroleum does actually dissolve in water. It does so to a small degree; were talking in the parts-per-million range, but its more than enough to make your hot chocolate taste and smell like high-test gas.
In the aftermath of a big spill, petroleum-laced soil can be trucked to an incinerator facility or hazardous-waste landfill. These are costly options, but sometimes there is little choice. For the homeowner or farmer with a modest-size spill, though, its possible to garden the oil away.
Soil microbes are the engines that drive this gardening process, also called bioremediation. While some oil does vaporize, the vast majority is broken down by native bacteria and fungi already present in the soil. They adapt to run on oil and use hydrocarbons for food, leaving only carbon dioxide and water behind. Special lab-developed microbes do work, but often dont fare well in real-world situations. Native microbes do the job just fine.
Oxygen is also critical. Oil breaks down infinitesimally slowly in anaerobic (without oxygen) conditions. Ive seen the removal of buried fuel tanks that had not been used in 30 or more years, and the stench of petroleum as they came out of the ground was overwhelming. On the other hand, well-managed gardens of contaminated soil can be clean in one or two years.
Organic matter exponentially increases the surface area that microbes can colonize, and it helps with porosity to allow oxygen in. Finally, a little nitrogen at the beginning of the process gives the microbes a boost to get going.
Because of leaching potential, 8-10 of affected soil is spread on a foot of clean sand on heavy plastic such as bunker-silo covering. The edges need to be bermed up to prevent runofff. Soil amendments such as compost or manure are worked in, and the garden is then rototilled several times per season. Lab analysis, or in some cases field instruments, mark progress until at some point petroleum is no longer detected. Then the soil can be used for any purpose at all so long as it stays on the owners property.
Sowing cover crops like buckwheat can help speed up remediation because fungi colonize the root systems, which provide more surface area. Studies show that plants do not take up petroleum the way they do heavy metals, which is good news for anything feeding on vegetation grown in a bioremediation unit.
I should say that the New York State Department of Environmental Conservation requires all petroleum spills to be reported to its 24-hour Spill Hotline (1-800-457-7362) within 2 hours of discovery. Just telling you the law. Bioremediation is approved by the DEC in many instances, and they can help monitor these gardens for you and offer advice on tending them. See http://www.dec.ny.gov/regulations/4636.html for more details.
Paul Hetzler is a horticultural and natural resource educator with Cornell Cooperative Extension of St. Lawrence County.