Raised beds over gravel driveway

8 years ago

New house and unfortunately the best place for a vegetable garden is on a large gravel area that previously had cars. The ground is compacted with gravel and weeds and while I don't mind putting raised beds there I am actually worried about the contamination.

Does anyone know how much oil/antifreeze/whatever else would stick around in the area or how far I would need to raise the beds to avoid any leaching from these things? I do live in Northwest Washington and the rain washes a lot of it into the ground/street but the gravel is still stained with it all.

Again, specifically for vegetables - if it were just a flower garden I'd do it and see how it turns out but I don't want to be eating anything leached into the soil. Would it be a good idea to plant flowers for a few years to help break down the contaminates before putting a vegetable garden there maybe? Would a wood bottom to the raised beds be enough to avoid it (though it would rot and breakdown itself over time). Or should I just stick to proper containers or other locations?

Featured Answer

digdirt2

8 years ago

Without extensive soil testing there is really no way to know what contaminants are in the soil there but the odds would favor at least some. So a simple removal of the top 3-4 inches of soil (and the gravel of course) would eliminate most of the issue. Then you can build the beds on the cleaned area and get by with much more shallow beds - 8-10".

The alternative would be really deep beds, something like 18-24" but without tilling or turning at least some of the surface and getting rid of at least most of the gravel could still create drainage problems for you down the line. Same issue with using some sort of permanent wood bottom in the beds - that creates serious drainage problems, ponding and pooling of the water and subsequent root rot.

As a general rule "leaching" happens down, not up, and plant roots don't generally suck up contaminants unless they are water-soluble. So if this area is the only option then either removing the contaminated soil or at least diluting the contaminants with lots of fresh soil mixed in or sticking with containers would seem to be the only options.

Lastly take into consideration which vegetable plants you want to grow. Many, like all the leafy greens, are shallow rooted but if you want tomato plants, peppers, and such they are tap root plants and tap roots can often be several feet long.

Hope this helps.

Dave

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Comments (7)

daninthedirt (USDA 9a, HZ9, CentTX, Sunset z30, Cfa)
8 years ago
There are inexpensive test kits commercially available for oil (DNAPL, LNAPL) contaminants. Not sure how good they really are, but they might be worth a try. Local ag centers do routine soil tests, and I'm sure they'd have suggestions about where to go for professional contaminant testing. Contaminants aside, it would still he highly informative to get one of these routine soil tests done, just to see if there are any serious composition issues.

One has to assume that they eventually leach away, as these pollutants are considered threats to groundwater quality. But it might be simplistic to assume that they just get washed out of the soil entirely.
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gawdly
8 years ago
You could probably put your beds on a heavy tarp, and then allow the runoff to head downhill. That way, if there are any contaminants, they aren't being absorbed by your beds. Do be sure you have the lower edge of the bed raised to allow runoff.
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rgreen48
8 years ago
last modified: 8 years ago
Unfortunately, unless you are putting down something more that a 'tarp', you can forget trying a membrane approach. There is no tarp that will last more than a couple of years. You'd need something along the lines of what is used to line trash dumps or thick rubber roofs.

I agree that the best coarse of action, if you continue to be very concerned, is to either dig out and replace the contaminated soil (if tests show it exists,) or go up with taller raised beds.

I saw one example recently of raised beds built atop milk crates so that the soil didn't even begin until 2 crates up. Whatever material used to line it would have to include drainage. Such beds are treated as containers.

Another option, and I doubt this is within your desired realm of choices, is to plant species known to absorb heavy metals and contaminates. There are a number of varieties used for cleaning up contamination. Although, this approach would take a number of seasons to accomplish the task.

Cost and time wise, renting yourself a bobcat for a weekend (you usually get a free day over weekends) and hauling off the soil seems the cheapest option. I suppose it depends on how large an area is to be cultivated.
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reelcharacter
8 years ago
You should be able to tarp the gravel surface and build up 18 to 24 inches or a little higher with your raised bed. Consider installing a form of sub-irrigation system (Google it to find lots of info and how to). This would also help ensure the bed would not get to dry on you, even if you go away for a few days.

I likely over engineered my raised bed project last year.

http://www.houzz.com/ideabooks/59917769/thumbs/the-2015-raised-bed-project
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Common Cents
8 years ago
You really need to either remove the contaminated gravel and subsoil or you need an impermeable separator between your garden soil and the contaminated gravel and subsoil.
Oil and oil soluble contaminants are lighter than water and will rise into your raised beds with rain or irrigation water if there is no impermeable separation.
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nancyjane_gardener
8 years ago
Before you spend $ on contaminent testing, when I was building my house 25 years ago, the bank appraiser came out and ordered an environmental assessment cause we had a 55 gal drum (full of kindling! He wouldn't come by to look at the drum's contents! GRRRRR) When we called the E.A office, they told us that if there was EVER an abandoned car on the property we wouldn't pass!

So, I would guess that testing for contaminents in that area will turn up positive! Nancy
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Raised beds over gravel driveway

digdirt2

Comments (7)

daninthedirt (USDA 9a, HZ9, CentTX, Sunset z30, Cfa)

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