Possible contamination of garden

Asked July 19, 2016, 2:31 AM EDT

my neighbour cut up roof from my garage, plywood plus asphalt shingles, with saw in middle of my garden. We picked up as much as we could but some of it got mixed into the soil.
Recently I have read somewhere that heavy metals and asbestos are used in shingles manufacturing process.
I did grow raspberries, cherry tomatoes, one big Guyanese "spinach" plant, one "crawling" spinach plant in respective distance from the messy area but some roots may have reached the messy, potentially contaminated area. Dust was probably blown into the wider area. I am seriously alarmed about it. Me and my wife have eaten produce but not much at all. We are not serious gardeners. Few caps of cherries, few caps of raspberries, few leaves of greens, actually very few only once since it didn't taste good.
We also had old asparagus plant which was exposed to some dirt from shingles. After couple of years we ate few asparagus spears and they did taste little bitter. We did wash them well and didn't use broth. I googled asparagus and I find out that it does taste little bitter some times, depending on weather and growing conditions, so I ate it. This messy event took place in 2008.
Can knowledgeable person help me out please.
I have read that heavy organic petroleum products do not get absorbed readily by plants. I hope that is the case since heavy metals can by harmful. Also I wouldn't want asbestos dust blowing around. Shingles were manufactured after 1960, probably before 1976, purchased in Ontario, Canada.
I thank you for reading my question and thank you kindly for expert response. If you need any more info please ask.
Thanks again
Peter Zap

Outside United States canada horticulture soil and fertility issues

1 Response

Not knowing the actual amount of shingle materials that were left behind, the specific composition of the shingles, the type of soils (high or low drainage for example), proximity of edible plants, and other conditions, it is impossible to determine precisely the potential contamination and potential for any subsequent uptake into edible parts of plants. However, given you state that the debris was largely picked up and that the scattering took place 8 years ago, I'd say you have very little to be worried about.

As for asbestos, some brands of shingles did contain asbestos in the period you cite as the date of the shingles... but not all, so there is a good chance your shingles may not have contained asbestos anyway. More significantly, asbestos concerns relate to airborne particles. While tearing up the roof, that's when the danger existed if they contained particles that would have been released during their break up. Any further danger, if there were any, long disappeared on the wind. There may be minute quantities of particles in the soil, but the chances of breathing in dirt particles during normal activities is also small.

As for the petroleum materials in the shingles (asphalt), again there are factors that suggest little need for concern, given the small amounts of debris that were left behind. First is the limited ability of heavy compounds in the shingle pieces to leach through the soil; they tend not to move easily in soil. That means that unless a plant's roots come into direct contact with the shingle pieces, transfer of questionable compounds is highly unlikely. The second factor is the ability of plants to actually take up the compounds, which is highly variable depending on species, and whether the compounds are stored in the plant parts one might consume. So there are too many IFs that have to work out in order for health worries... if there are contaminated shingle bits, if a plant comes in close contact, if there is transfer, if the plant uptakes the compounds, and if the plant stores the compound in the part we eat... and of course the amount of any "contaminated" vegetable (if all those IFs actually came true) would be so small one would have to eat very large quantities to have health concerns.

You are right that petroleum compounds are generally not taken up readily. And any dust that was blown around 8 years has long since been mitigated. I wouldn't be concerned, but there are some things you can consider if you still have worries:

  • You can have soil test done of the area you are concerned about. These are available through the agricultural office in your region and usually cost only a few dollars. These include references to a large number of components and would give you an idea of your soil.
  • Avoid planting root crops in the area specifically where shingle bits were deposited; plant other kinds of vegetables in that area that you consume the fruit or leaves for example (areas outside the very immediate area of the deposit are not a concern).
  • You can build raised beds over the potentially contaminated area, fill the beds with new soil and plant into those. Most annual vegetables do not have particularly deep roots, so a 12" depth of new soil over the old soil would be lots to avoid almost any potential contact with the shingle bits.
All in all... enjoy your garden.