Phytoremediation of Heavy Metals

ArtemisiaVulgaris

Mugwort (Artemisia vulgaris)

Us city slickers have fouled the sandboxes we play in. Find an open field in a big city like Los Angeles or New York and the odds are that it’s a former toxic waste dump. Here in our neighborhood we’ve got a lot of lead and zinc–lead from paint and gasoline and zinc from brake linings.

These heavy metals don’t magically go away. They are elements, and short of an alchemical transformation you have to physically remove them, cover them up, or apply phosphate so that plants don’t take them up as readily.

One promising strategy is phytoremediation, the use of plants to uptake heavy metals. City Atlas youarethecity, in New York, is experimenting with Indian mustard, mugwort, basket willow and sunflowers to remediate a contaminated garden. The results are promising with some metals down 50% in a year. Mugwort (Artimesia vulgaris) did an especially good job with a wide range of contaminants.

I should note that Garm Wallace, who runs Wallace Labs, a soil testing service, says that phytoremediation can take many years to get heavy metals down to a safe level. That being said, breeding plants specifically for heavy metal hyperaccumulation is a technology that could make up for our past transgressions.

Thanks to Michael Tortorello for the tip.

Urban Chickens and Lead

From the One More Thing To Worry About department, the New York Times has an article on lead levels in eggs laid by urban chickens “Worries About Lead for New York City’s Garden-Fresh Eggs.” According to the article, the lead levels found in New York City’s home grown eggs ranged from none to over a 100 parts per billion. Since the FDA does not have an acceptable lead level in eggs it’s difficult to interpret the results. And I have to wonder what unknown problems lurk in industrial eggs.

It’s a reminder that those of us who live in older cities and grow food need to confront the lead problem. Personally, I’d also like to see the Real Estate industry come clean on this issue beyond boiler plate disclosures buried in sales documents. But I’m not holding my breath.

Five Lessons We Learned About Lead in Soil

As regular followers of this blog may recall, we did some soil tests last year that revealed elevated levels of lead and zinc in our backyard. The cause? Most likely, paint from our 92 year old house and nearly a hundred years of auto exhaust and dust from brake linings.

Applying a little alchemy to turn lead to gold, I think the most productive thing I can do is to help get the word out about lead soil and how common this problem is in urban areas. Towards that end I though I would share five things that we learned from our backyard lead crisis:

    1. Buyer Beware. When you are shopping for a house do multiple soil tests. Once you buy the house it’s too late. Real estate contracts in California (and I suspect elsewhere) have been loaded up with disclaimers about lead and old houses. I’m no legal expert, but I suspect it would be difficult to go after the seller given the lead clauses we signed in our ignorance.
    2. The dirt on soil labs. Choose a soil lab that gives detailed results and is willing to do some phone consultation. We used to recommend the cheap tests form UMass, but their results were significantly different than Timberleaf Soil Testing and Wallace Labs, which were more in line with each other. Both Wallace and Timberleaf give you more detailed reports and are both willing to chat on the phone. They cost more but are worth it.
    3. Raised beds.  Not much else to say other than those two words if you want to grow vegetables and have a lead problem. Right now I have a big compost pile going that I made with hay, straw and horse manure. I’ll use this compost along with imported soil to fill the raised beds that I’m going to build this winter. And you can bet that I’m going to test that imported soil first.
    4. Philosophical lesson. The future health of the human species requires us to be a lot more conservative in the use of chemicals. Lead was known to be a problem since the Romans, but we went ahead and used it anyways in paint and in gasoline and the results have been tragic. Nassim Taleb has written eloquently about the need to approach complex systems like nature and the economy with an attitude of humility and admit our ignorance. When we fool around with complex systems we’re asking for trouble.
    5. Rhetorical lesson. There should be a logical fallacy called the “appeal to technology.” It’s the idea that there just has to be a technological solution to every problem. This is a very common fallacy in our age. Some people have suggested that I try phytoremediation, the process of growing plants like sunflowers that accumulate lead. At the end of their growing cycle you pull the sunflowers and send them to a toxic waste dump.  Sadly, this is just not practical in a residential yard. I would have to pull every living thing, including a mature avocado tree, and grow nothing but sunflowers for the next 20 years. Others have suggested mushrooms as a remediation technique, but they don’t grow well in this dry climate.  Lead is an element and it simply doesn’t go away. Once it’s in the ground it’s in the ground. The best way to deal with it is not to use it in the first place. The city of Oakland is trying a more practical solution to deal with lead: applying fish bone meal to lead contaminated soil. Fish bones contain phosphates which bind up lead and make it less bioavailable to plants. Our soil tests indicate that we already have lots of phosphates in our soil, so I’m not sure if adding more would help.

    Of all of these points I think the first is the most important. I’ll repeat it again, if you are shopping for property get a soil test.

    Testing the Lead Testers

    Varian ICP-MS from Wikipedia

    Dear readers,

    Excuses for a technical post here, but we need your scientific expertise. If you have experience in soil laboratory testing techniques, or know someone who does, please send us an email at [email protected] or leave a comment. We’re attempting to reconcile slightly different lead results from three different labs and I’d like to be able to write about soil testing methods. Two of the labs we sent samples off to (UMass and Timberleaf Soil Testing) use inductively coupled plasma mass spectrometry (ICP) to test for lead. Wallace labs uses an extractant, AB-DTPA (ammonium bicarbonate Diethylene Triamine Pentaacetic acid).

    Here’s how Wallace described their lead testing techinique,

    We use AB-DTPA (ammonium bicarbonate Diethylene Triamine Pentaacetic acid).

    It is a gentle extractant and it mimics roots in extracting minerals from the soil. Most often environmental tests are made with boiling acids which are more aggressive than roots. The AB-DTPA method is a standard testing method of the Soil Science Society of America. It is called the universal extractant. It measures the bioavailable or plant available minerals which is expected to be adsorbed by plants. Most of the background heavy metals are occluded and are unavailable to plants. Our testing does not see the occluded metals.

    Total lead is approximately 10 times higher than our AB-DTPA measured lead. We recommend that AB-DTPA lead be less than 30 parts per million for home production of edible produce.

    UMass says,

    We use a modified Morgan solution (dilute glacial acetic acid and ammonium hydroxide) to measure extractable lead (using ICP).  Total Estimated Lead is calculated using a correlation established during a study performed here at UMass that compared total digestion levels to extractable levels using 300-400 soils.

    I divided one soil sample into three parts and sent a portion to three labs. While all three labs indicated the presence of above natural levels of lead, there were enough differences between the tests to warrant a closer look at the techniques. Your assistance would be greatly appreciated and we’ll share what we find out.

    Lead Update

    Our post about a high soil lead level needs an update. I asked my doctor do a blood test to check for lead levels since we’ve eaten plants grown in the backyard and done a whole lot of digging over the past 13 years. The good news is that no lead showed up in my blood.

    In the interest of “testing the testers,” I took one soil sample and split it in three, sending one sample to Wallace Labs, one to the University of Massachusetts and the other to Timberleaf Soil Testing. I’ll report back on what those tests come up with.

    Hopefully that first test with the high lead level was a mistake. I’ve realized that one small lead paint chip in a soil sample could easily throw off the test since we’re checking for something that is measured in parts per million. I’ll admit that this lead issue is definitely a complicated problem that is at the limits of my grasp of scientific methods. I appreciate all of you who have chimed in with advice, prayers and good wishes.

    What We’re Going To Do About That Lead

    White sage, yarrow, rosemary and aloe vera–the kind of plants we’ll be planting more of.

    Let’s assume that we have a lead problem in our backyard. That’s a big assumption at this point because we now have two very conflicting test results. But, for the sake of an argument, let’s say the first alarming test is true, what are we going to do about it?

    These are the options:
    • Radical remediation: Remove all the soil in the yard and replace with new soil.
    • Cover the contaminated soil so that it doesn’t give off dust, and so people can’t come in direct contact with it, e.g. lay sod, cover the yard with concrete or decking, or lay down a thick layer of mulch.
    • Grow ornamental plants only
    • Grow all food in raised beds
    • Attempt phytoremediation (grow plants that uptake lead, pull them and send them to the dump)
    • Move

    It turns out we were already doing some of these things, so we’re just going to keep on going as we were with a few changes. Our yard has always been covered in a thick layer of mulch and we do most of our growing in raised beds. We will stop growing edibles directly in the ground. We’d already planned to redesign the yard to include lots more native and Mediterranean flowering plants. These we can’t eat, but will secure the soil and provide food and shelter for lots of beneficial insects who will aid our food crops. We’re really happy that we’ve always mulched, because it has helped keep the (potentially) contaminated soil in place and has increased bio-activity as the mulch decomposes.

    This raised be would have to be raised higher.

    Most plants don’t take up much lead, it turns out. Our soil phosphate levels are high and the pH is moderate, two factors that further decrease the ability of plants to access lead. The main health concern comes from directly ingesting dirt, and that can be avoided by washing and peeling vegetables and fruits. The recommended wash for lead is a splash of vinegar in water (1 tsp per 1 1/2 qts water).
    I’d be more concerned if we had kids and they played in the backyard. Little kids, of course, are most at risk because they tend to ingest dirt during play. If it turns out there is a lead problem at our house, I plan on knocking on the doors of my neighbors with kids, warning them about what I’ve found and offer to help them do a lead test. In our public speaking and teaching we emphasize the importance of getting a soil test and we’ll keep doing this.
    Phytoremediation, the process of growing plants that concentrate lead, pulling them up every year and disposing of them holds a lot of promise. Unfortunately I don’t see how it would be practical in a residential situation. If the contamination is widespread I’d have to remove every plant and tree and grow nothing but, let’s say, sunflowers, for perhaps several decades. (Sunflowers pull up lots of lead.) In short, phytoremdiation is an intense, long term process.
    I think the most important thing we can do is spread the message that soil is a sacred resource–something that we should not foul up in the first place. More and more we’re coming to realize that soil is everything–our future and our well being is inextricably tied to its health. We know how to treat it right. We’re just not doing it. We know what toxins are, and we’ve know it for a long time. For instance, lead has been known to be a problem for thousand of years. Benjamin Franklin, in a letter dated July 31, 1786, comments on lead poisonings caused by contaminated rum, and the lack of concern about this problem: “You will observe with concern how long a useful truth may be known and exist, before it is generally received and practiced on.”  
    You said it, Ben.

    Get a Soil Test!

    Regular readers have probably already got this message, but right now we can’t repeat it enough. If there’s a lesson with our backyard lead scare , it’s to practice due diligence when beginning a garden –or better yet, when you buy property–and that means getting a soil test from a soil lab. They’re not that expensive, especially when you consider the high cost of remediation, and the well being of your self and your family.

    Test soil for both nutrients and heavy metals when:

    • Buying a house or land
    • Starting to grow food in your yard  
    • Are growing food and have never tested
    • Starting a plot in a community garden or a school garden
    • Buying soil in bulk

    Property does not have to be on the former site of gas station to be suspect. Lead contamination comes not only from lead paint on older houses, but was also deposited all over urban centers and near busy roads via a constant rain of fine particulates from auto emissions.

      It really is a shame that lead testing is not a standard part of the inspection phase of home-buying, especially as this is a pervasive problem in urban areas.

      Do companies that sell bulk soil test that soil for lead? A few phone calls Darren Butler made to local companies indicate that they don’t. So let the buyer beware here too. Wondering about bagged soils? Susan Carpenter, at the LA Times, tested a bunch and found no problems.  She did find a possible lead link to fish fertilizer, however.

      We’ve used all three of these services. UMass is the cheapest by far, but gives the least analysis. However, if you just want your lead level numbers, that’s not a problem.

      Wallace Laboratories
      UMass Soil Testing Service
      Timberleaf Soil Testing

      In our next lead post we’ll let you know what our plan is. And we know there’s interest in all this, so over the week we’ll talk about remediation, raised beds, what’s dangerous, what’s edible, and more. Fun for all, guaranteed!

      Lead Update

      This week I thought I’d do a series of posts about soil and heavy metals beginning with a few more details about the possible lead contamination situation in our backyard.

      Two weeks ago Darren Butler, who is teaching a vegetable gardening series at our house, led a class project where we took four samples from different locations in the backyard, mixed them together and sent them off to Wallace Laboratories, a local soil testing lab with an international reputation. The results came back showing plant available lead levels at 112 parts per million. Note that “plant available” is different than the total amount of lead in the soil. The total amount would be about ten times higher or 1,120 ppm. According to the University of Minnesota Extension Service,

      Generally, it has been considered safe to use garden produce grown in soils with total lead levels less than 300 ppm. The risk of lead poisoning through the food chain increases as the soil lead level rises above this concentration. Even at soil levels above 300 ppm, most of the risk is from lead contaminated soil or dust deposits on the plants rather than from uptake of lead by the plant.

      If the Wallace Labs report is correct, we’ve got a serious problem. It is possible that, in sampling and averaging multiple locations, we hit a “hot” spot where someone may have dumped paint or paint chips. Clearly, we’ll have to set up a grid of tests to see if the problem is isolated.

      I re-did the first test, trying as best I could to take samples from the same locations and sent this second test off to the less expensive UMass soil testing service. The results came back with substantially lower lead levels: 220 ppm, in the “low” range according to most experts, but still higher than I would like. Except for the soil pH, all the other numbers were completely different.

      The next step will be to test the testing services. I’m going to take one sample and split it into three parts, sending one to Wallace, another to UMass and the third to Timberleaf Soil Testing. I hope that two of the testing services agree on something!

      In subsequent posts I’m going to discuss what we’re going to do, phytoremediation (spoiler alert: I don’t think it’s practical in residential situations), and my issues with the real estate industry.

      This week the Center for Disease Control and Prevention is promoting their National Lead Poisoning Prevention Week. Especially if you have kids, get your soil and the interior of your house tested.