I recently purchased some land which previously held many junk vehicles. Aside from random scraps of trash there are also a few spots where oil stains are visible on the soil. Additionally, the wet lands on my property about 200 feet away appear to have something oily floating on top. I plan on getting the water tested, but I am also looking for techniques and strategies for dealing with contaminated land.
I personally am not overly concerned about it but I fear that my wife will not eat anything grown on our land until the issue is remedied.
Well made, finished, microbe/fungi rich compost has been found to bind the heavy metals left over from the Stamets method (where the mycelium break apart the hydrocarbons of petrol into hydrogen and carbon, which they eat, essentially leaving only the heavy metals as residue) in a way that prevents plant roots and tissues from incorporating them, theoretically turning rail yards and other oil soaked bone yards into, at very least, not a toxic wasteland. Id say verdant paradise, but only after some soil tests :p
"It might have been fun to like, scoop up a little bit of that moose poop that we saw yesterday and... and uh, put that in.... just.... just so we know." - Paul W.
My experience has been more with antifreeze contamination than oil. If you have livestock this could be an issue because it is sweet tasting and kills them. I have lost a few sheep when they went into my sawmill and injested antifreeze.
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Oil is organic. Lots of biological stuff eats it. It also works as a fertilizer if concentrations are not too high. Bioremediation is one of the standard ways of dealing with oil contamination that's not too severe; basically the technique is to till the contaminated soils (to aerate it) and plant stuff in it. In a few years all the oil is broken down by biological activity and is gone.
Metals are a bigger problem if present, but there's no automatic correlation between oil contamination and metals. (Sometimes activities that spill one also spill the other, but it depends on what the activity was. If they were smashing a lot of lead-based car batteries into your soil, you could have a problem.)
First, the iridescent sheen on the wetlands is probably naturally occurring. It is very common. If you stir it with a stick the normal wetland sheen will tend to break up more than get pulled and mixed together. Petroleum product sheen usually mixes much easier and stays more liquid.
For the soil there are many options. You could look at excavating the worst of it and disposing. I work on large spill sites and often do this. Usually the cost is $20-40/ton at land farms or landfills near me, not including transport if hired-out. The cost may be different where you are and for smaller quantities, etc. Disposing normally requires a lab analysis of the soil to verify it is non-hazardous by the regs. Hazardous in this connotation is about flammability so the soil has to be almost dripping with petroleum for the lab result to indicate it is hazardous. The quantities involved with junk cars make a hazardous result unlikely. So there are hassles and costs associated with disposal.
For dealing with it on site. If the cars were mostly in one area and you can work that area as is, leave it where it is. If not, you may want to excavate the worst of the soil and work that soil in a convenient place. You could try the fungal approach, however, my guess is the concentrations of petroleum compounds wouldn't be high enough to sustain the colonizing fungi alone and you would then have to add additional food for the fungi. This isn't such a bad thing, but a little more work and fuss.
Normally landfarms just spread the soil out and till it frequently. Some of the more volatile contamination evaporates and the soil bacteria deal with the rest. This is high-till gardening - since you want the surge of soil bacteria activity that comes after tilling. Also, the remediation occurs faster in the soil if you do not add organic material. Bacteria will breakdown the easiest to digest organic material available, and most petroleum compounds are harder to breakdown than plant matter. It isn't a big deal if the soil has a lot of organic matter it may just take a little longer to break down all the petroleum hydrocarbons. Also, the bacteria are already in the soil. You could add effective microorganisms if you want. It might help but it is not necessary. The soil bacteria that will break down petroleum are absolutely everywhere.
If you are concerned about it or curious, you can test the soil. There are many different test methods but in this case I would recommend a total petroleum hydrocarbons test like EPA Method 8015. This method can be broken down further into gasoline range, diesel range, and motor oil range. One soil sample tested for all three ranges would probably be about $100-200 (call a lab near you). A test like this adds up the concentrations of a wide range of hydrocarbons and gives you the total. Compound specific tests are more expensive and are a little more complex to interpret. To make a composite sample grab about five hand fulls of soil (of the most contaminated) and mix together then put the sample in a clean, sealed glass jar, bring to lab on ice. The lab may have other instructions for you and have you fill out a chain-of-custody form, etc.
I have spot in my yard where a previous owner poured gasoline to kill an old tree stump. Twenty years later everything I plant there eventually dies, I surmise when the roots get deep enough to reach that spot.
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3 Plant Types You Need to Know: Perennial, Biennial, and Annual