Lead contamination for the home gardener

I've been wanting to write this post for some time as this is an issue that has come up a lot for me - What does one do if their soil is contaminated with lead (and they happen to be helplessly obsessed with vegetable gardening)?  It can be hard for home gardeners to make an informed decision as to the safety of their yard for cultivation.  Figuring out specific vegetable species to avoid is also difficult to find information about.  I hope this post can be a resource for home gardeners on the following:

1. How to figure out if you have lead in your soil

2. Just how much lead is too much lead

3. What vegetable species should be avoided

4. What can be done to mitigate lead contamination



For the sake of all you busy, hard working folk out there who just want to get to the facts without the explanations, the most important points are in bold.



1. How to figure out if you have lead in your soil

Send a soil sample to UMass Soil Testing Laboratory and order a "routine test" with "organic matter": http://soiltest.umass.edu/

A simple soil test which can cost as little as $15 can tell you the levels of lead in your soil.  There are many laboratories which will analyze your soil for lead, but the best option in my opinion is the UMass Soil lab.  They have always been quick, they are inexpensive, and it's a relatively simple process to send them your sample.

In order to have your soil analyzed by UMass you will first need to collect soil samples.  You can have the lab analyze multiple samples at a time which will allow you to divide your yard into sections you can test individually.   This can really help you pinpoint which areas of your yard have lead contamination, as it may only be a certain portion.  Say for example you suspect your backyard is contaminated but that your front yard is fine, it would be best in this circumstance to test these areas separately. It's important to note that the soil within three feet of a house that had exterior lead paint generally has a much higher lead content than other parts of a home garden.  Take this into account when sampling.  You might want to do one sample around the perimeter of the house, and have your other samples not include this area.

Umass has guidelines for how to properly collect your soil samples here: Sampling Instructions.  You're basically going to use a clean shovel to collect soil from at least 12 random spots in your sample area. The soil is collected from a depth of 6-8 inches.  I generally put the soil directly into a ziplock bag.  You can also use a bucket or any other container.  You need the sum of your samples to be at least a cup worth of soil.  Try to avoid getting roots or other large pieces of biomass in your sample.  Before you send in your sampple you have to let the soil dry out before you ship it. It will distort some of the nutrient tests if you seal a bag of soil when it is wet.  I sometimes spread the soil out on a clean baking pan and put it in an oven at the lowest temperature possible to hasten the drying.

Once your soil sample is dry, mix it well and then place it in a ziplock bag which is labeled with the area you sampled from.  Try to use a clean shovel or other instrument rather than touching the soil with your hands.

Print out this order form from Umass for a basic soil test.  The basic test will tell you many metrics in addition to lead.  This includes: pH, exchangeable acidity, extractable nutrients (P, K, Ca, Mg, Fe, Mn, Zn, Cu, B,  S), extractable lead (Pb), extractable aluminum (Al), cation exchange capacity, and base saturation.  Don't worry if you don't understand what all of those mean, they are not that complicated!  Knowing your soil organic matter levels in addtion to these metrics can be helpful when it comes to mitigating the risks of lead contamination.  For this reason it's a good idea to opt for the $6 organic matter test as well on the order form.

Place your completed order form, your labeled samples in ziplock bags and a check into one of the small boxes available at your local post office.  Then send the package off to the UMass lab.  The results should be emailed to you in under a month.


2. Just how much lead is too much lead for home vegetable gardening.

If you have anything over 100 ppm you should inform yourself about how to minimize the risk of lead exposure from home vegetable gardening.  If closer to 400 ppm you should definitely be taking steps to reduce lead bioavailability and use mulch, additional soil and possibly raised beds to limit contact with contaminated soil.  You should also be selective as to which crops you grow.  Cultivating directly in soil with lead over 400 ppm is a bad idea.

Exposure to lead is a dangerous health risk, especially to children.  Take soil lead contamintaion seriously.

Once you've reached this step you should have some test results that show you the levels of lead in your soil.  The information you are looking for is total estimated lead.  Extracted lead is merely the amount of lead that can be measured by the testing procedure.  Total estimated lead though extrapolates from the extracted lead measurement to tell you the actual amount of lead you have in your soil.  The measurement will either be in ppm (parts per million) or mg/kg (milligrams per kilogram).  These are both the same measurement as there are one million milligrams in a kilogram.  If your test only shows extracted lead here is a table (from the very useful UMass Lead testing and recommendations page) to find out the total estimated lead in your soil:

Lead Level	Extracted Lead	Estimated Total Lead
	--------------------------------- mg/kg or ppm ---------------------------------
Low	less than 22	less than 299
Medium	22 to 126	300 to 999
High	127 to 293	1000 to 2000
Very High	greater than 293	greater than 2000

Natural levels of lead in soils range from 10 - 50 ppm.  The EPA says that children should not be exposed to bare soil with levels higher than 400 ppm. The danger with bare soil is that children might ingest this soil or breath in dust from the contaminated soil.  At 400 ppm, eating only 1/4 teaspoon of soil per week would put children 6 years or younger over the threshold of safe lead blood levels.  The EPA has deemed soils with lead levels higher than 1200 ppm unsafe for all humans.  Some states are more strict than the EPA.  The University of Massachusetts says 300 ppm is the point at which children and pregnant women should avoid contact with bare soil. Minnesota is even more stringent with a limit of 100 ppm for bare soil where children are present.

So we know that if people are going to be inhaling dust or ingesting soil, 400 ppm is certainly too high for children and pregnant women and 1200 ppm is too high for adults. But how much lead is too much if one wants to grow vegetables?  If growing vegetables entails kicking up lots of dirt and inhaling dust than anything near 400 ppm is too much.  Likewise if one grows root crops that are not peeled or have soil residue on them, than anything near 400 ppm is too much.  Growing vegetables which are prone to absorb lead means that 400 ppm is again too much.  So there are a lot of reasons why it's best to only cultivate if soil lead levels are under 400 ppm.

There are other factors at play though when evaluating the risk of growing vegetables in soils with slight lead contamination. Two soils with the same amount of lead might pose very different levels of risk.  The two main variables are the bioavailability of the lead in your soil and the species of vegetable you are growing. If one's soil has a high pH, with high levels of organic matter and phosphate, then the lead is less bioavailable and there is a lower risk for lead uptake into vegetables.  Avoiding species of vegetables which uptake more lead than others will also lower ones risk.

In a study I will link to below, tomatoes grown in soil with 3470 ppm lead (ridiculously high) had undetectable levels of lead in their fruits.  However the leaf tissue on these plants had 22 ppm lead and the root tissue 715 ppm.  I wanted to use this tomatoe to illustrate that technically you could grow something in a very contaminated soil and because you were selective about the species you were cultivating you would technically be ok.  Personally I would feel like a little leery eating those tomatoes.

Although we have shown a case where tomatoes did not pose a threat to human health in 3470 ppm lead, swiss chard in soil with 910 ppm lead had 24 ppm lead in the leafy parts of the plant we eat.  We can note here that the swiss chard and tomato leaf tissue had a similar amount of lead, but the difference between these two scenarios is that we eat the fruit of the tomato, not the leaf tissue, and the tomato was grown in soil with over three times the amount of lead.

So we're beginning to see why it's hard to come up with a single number at which point crops become unsafe: Different plants uptake different amounts of lead.  In addition, depending on the plant we'll eat the roots, the leaves or the fruit. Root tissue is more likely to contain lead, followed by leaves and then fruit.

The other variable that makes it hard to declare a specific ppm of lead unsuitable for vegetables is bioavailability. Depending on your soil there are different levels of lead "sorption"- this is when the lead is bound to clays or organic matter in the soil and is not available to the plants. The more organic matter (and certain clays) your soil has the more chances there are to lock up lead.  The higher the pH, the more sites there are on clays and organic matter for lead to be locked up and made unavailable.  If a soil has a low pH of 3 or 4 lead is very very bioavailable for plants to pick up.  Of course, if your soil pH is 3 you probably won't be able to grow much of anything in the first place!  But at a pH of 5.8 you could still grow vegetables and your plants would uptake more lead than if your soil pH was 7.

Given all the different variables at play here, it can be hard to know if a soil is safe for growing vegetables solely based on the measure of estimated total lead.  Certainly if a soil has 3,000 ppm lead we know this soil is dangerous to work with.  But if your soil has only slightly elevated levels of lead, say 200 ppm, the following variables play a large role in deciding if soil is safe to cultivate:
-What crops you are growing
-The bioavailability of lead to plants
-If you are exposed to large amounts of dust

If you have over 100 ppm, you can do several things to minimize your risk.  Check out parts 3 and 4 for more informaiton.


3. What vegetable species should be avoided.

Cilantro, Mint, Lemon balm, Epazote, Rhubarb, Swiss Chard, Beets, Mangels, Good King Henry, Orach, Spinach, Carrots, Radish, Onions, Garlic, Leeks, Potatoes, and Turnips should not be cultivated on soil with mild lead contamination.  These vegetables are more likely to absorb lead into edible portions.

Root vegetables in general have higher levels of lead than leafy vegetables.  Leafy vegetables generally have higher levels of lead than fruiting vegetables. 

Root crops in general are problematic for two reasons.  First, unless they are peeled, it is difficult to fully cleanse them of soil (which in our scenarios contains lead).  Second, plants grown on soils with lead tend to have much more lead in their root tissues than in their shoot tissue or fruits.

Leafy greens are safer than root crops as leaf tissues have lower amounts of lead than a plant's root tissue. However if rain, watering, or dust gets contaminated soil on leaves this is a pathway to lead exposure.

The safest type of vegetables to eat are those with fruiting bodies. Plants move very very little lead into fruiting bodies.  These include peppers, eggplants, tomatoes, squash, melons, grapes, berries and tree fruits.  Grains such as corn, wheat, rye, barley, oats, also have a low chance of receiving lead from the plants they grow on.

What about specific species though?  There are some plants which have a tendency to uptake much more lead than others.  This article, Lead levels of edibles grown in contaminated residential soils: a field survey, is a great resource which examines levels of lead absorption in different types of vegetable.  From the results of this survey, the following vegetables should be avoided when cultivating on lead contaminated soil:

Cilantro, Mint, Rhubarb, Lemon balm, Epazote, Swiss Chard (and by extension Beets and Mangels, which are the same species), Carrots, Radish, and Onions

Potatoes were not included in this study but other members of the genus Solanum had high root concentrations of lead so it would probably be best to avoid this crop as well.

Turnip was also not included but other members of the same genus contained high levels of root tissue lead.

Spinach, Good King Henry, and Orach are closely related to Swiss Chard and Epazote so they would seem to have a high chance of heavy lead uptake.

Leeks, Garlics, Chives and other members of the genus Allium should be avoided as another member of this genus, Onion, was shown to readily uptake lead into the edible part of the plant.

There was one instance of cucumbers having high levels of lead, although cucumbers from six other contaminated plots had undetectable levels in the fruiting body.  I have not added this to the list of plants to avoid.

Two members of the Lamiaceae family, Lemon Balm and Mint, seem to easily uptake lead, but another member, Basil did not.  As such it is hard to predict how other Lamiaceae herbs like Oregano, Thyme and Sage would uptake lead. The Basil which did not uptake detectable levels of lead in the study was only in 280 ppm lead so perhaps at higher levels it would behave more like Lemon Balm and Mint.

Unfortunately this field survey leaves out a number of different vegetables and food crops. It would also have been nice to see the pH and organic matter content of the soils to get an idea of the lead bioavailability the plants were exposed to. The survey is the most informative article I have seen on rates of lead absorption in different vegetables though. By reading the article you can see which types of vegetables are indeed safer for growing on mildly contaminated soils.


4. What can be done to mitigate lead contamination.

Take steps to minimize contact with soil such as laying down mulch or a new layer of soil, and prevent dust inhalation by keeping the soil moist.  Adding phosphorus, organic matter and raising the pH of your soil will decrease the bioavailability of lead to vegetable plants.  For soils with over 400 ppm consider growing in raised beds with bottoms that roots cannot penetrate. 

The University of Massachussetts has a great webpage on soil lead with a helpful section on Good Gardening Practices to Reduce Lead Exposure.  If you have lead contamination it's definitely worth checking out their recommendations.

Other great resources can be found here:
Lead in Garden Soils
Lead Contaminated Soils: Minimizing Risks
Gardening on Lead and Arsenic Contaminated soils

There are two ways that the home gardener should aim to minimize exposure to lead: reducing physical contact with contaminated soil and reducing bioavailability of lead to vegetables.

Reducing phycical contact with contaminated soil simply entails putting a barrier between yourself and the soil.  You can acheive this by laying down landscaping fabric, mulch, or new soil on top of the contaminated soil.  If you want to cultivate the soil and there is less than 400 ppm in the soil, what I prefer to do is bring in a few cubic yards of compost and raise the soil level a a few inches. You should also try not to work with the soil when it is extremely dry to prevent dust inhalation.

Reducing bioavailability to vegetables involves three parts: raising soil pH to 7, raising levels of soil organic matter, and adding phosphorus.  The higher the pH, the more lead is bound to certain clays and organic matter, making it less likely plants will take up the lead.  To increase soil pH, add a liming agent like calcium carbonate  or calcium magnesium carbonate (also known as lime and dolomite lime).

Phosphorus in soil will bind with available lead to form lead phosphate, which has very low solubility and is unlikely to be absorbed by plants.  Rock phosphate, bone meal and fish emulsion are usually good sources of phosphorus.  I prefer fertilizers that will break down and release their nutrition gradually.  This decreases losses to leaching.  Pollution of groundwater from excess phopsphorus fertilizer is a serious problem so don't over apply.  If you're soil test shows that you have more than 50 ppm phosphorus you should not apply any additional phosphorus fertilizer.

Lastly aim to get your organic matter levels to 4 or 5%.  Not only will you be immobilizing soil lead, but you will also have an extremely healthy and productive soil once you hit this level of organic matter.  Add manures, organic fertilizers and, most importantly, compost to your soil and your organic matter levels will slowly rise.

Lead is a naturally occuring element in soils.  Often times in urban settings soil lead levels become more elevated than naturally occuring levels.  This can pose a health risk to those wishing to cultivate vegetables.  Keep in mind the following points though and you should be safe:

Don't cultivate on soil with over 400 ppm
Pay close attention to the different types of vegetables that you should not grow (Part 3.)
Take steps to minimize contact with the contaminated layers of soil and reduce lead bioavailability (Part 4.)


Epilogue

If you happened to have eaten vegetables grown on soil with heavy lead contamination or toiled in contaminated soil (I've done both- a lot!)  all is not lost.

First, if you think your child have been exposed to high levels of lead, a simple test can be performed to determine the level of lead in their blood.  Contact your primary physician for more information.

If you're like me and you think you've probably come in contact with lead through your gardening activities there are several ways you might be able to reduce the levels of lead in your body. Eating cilantro has been shown to chelate and remove lead from the body in some studies, while other studies show that it at least protects the body from absorbing lead.  If you recall from Part 3, cilantro was a plant to avoid cultivating due to it's propensity to absorb lead.  Interestingly, of all the plants surveyed in the article I linked to, cilantro had the highest ratio of shoot tissue to root tissue lead, 1:1.6.  In addition to cilantro, chlorella has been shown to help the body eliminate mercury and other heavy metals.

Other than cilantro and chlorella, there are several other foods which are being examined for their ability to help the body eliminate heavy metals.  Just eating healthy is a good start though as research shows that a diet high in vitamins and minerals can help protect against lead absorption.  There are very powerful pharmaceutical chelating agents (EDTA, DMSA etc.) which can be used to remove lead from the human body, but these are not without side affects since they can remove many important elements from the body such as magnesium in addition to heavy metals.

If you're interested in reading the studies yourself, below you'll find various journal articles on foods which support heavy metals elimination.

Studies which examine the role of cilantro in absorption and elimination of heavy metals:

Preventive effect of Coriandrum sativum (Chinese parsley) on localized lead deposition in ICR mice.

Significant mercury deposits in internal organs following the removal of dental amalgam, & development of pre-cancer on the gingiva and the sides of the tongue and their represented organs as a result of inadvertent exposure to strong curing light (used to solidify synthetic dental filling material) & effective treatment: a clinical case report, along with organ representation areas for each tooth.

Effect of cilantro on plasma lead levels and some hematological parameters in rats

Effect of Coriandrum Sativum L. extract on blood and urine lead concentrations in 3-7 year old children
In this study, although blood and urine lead concentrations decreased in children given cilantro extract, similar results occured in children given a placebo. This study of course casts some doubt on cilantro's use in eliminating heavy metals.  The conclusion reads as follows: " According to the results of this study, it seems that Coriandrum Sativum is not effective in lead elimination. Increasing renal lead elimination in both groups of children may be due to other factors like improvement of
nutrition following the education at the beginning of this study."

A study which examines the role of Chlorella in eliminating methylmercury:

Enhanced elimination of tissue methylmercury in Parachlorella beijerinckii-fed mice

Studies examining the role of different vitamins and minerals in heavy metals absorption and elimination:

Intake of magnesium and toxicity of lead: an experimental model

Maternal Blood Lead Concentration, Diet During Pregnancy, and Anthropometry Predict Neonatal Blood Lead in a Socioeconomically Disadvantaged Population

Effects of Micronutrients on Metal Toxicity

Selection of Nutrients for Prevention or Amelioration of Lead-Induced Learning and Memory Impairment in Rats

Preventive and therapeutic role of vitamin E in chronic plumbism

Beneficial effect of combined administration of some naturally occurring antioxidants (vitamins) and thiol chelators in the treatment of chronic lead intoxication

Vitamin C modulates lead excretion in rats.

Lastly, here's a review (that I wish I had found much earlier!) which gives a brief overview of the current research on heavy metals absorption and elimination using different foods and pharmaceuticals:

Chelation: Harnessing and Enhancing Heavy Metal Detoxification—A Review


It's important to remember what this research is NOT saying.  Don't think that because you eat cilantro every now and then, you now have a license to work in 1,000 ppm lead soil.  Stay safe when you're vegetable gardening and make sure you understand the lead situation in your soil.


I hope this post helped answer some questions you might have had about growing vegetables in soil with lead.  If you have further questions, feel free to post a comment!