Managing Metals & Bio-accumulation

Got Metals?

If your garden has levels of metals that you feel are unsafe for you and your family, taking action to reduce exposure is your first priority.  The 8 simple actions mentioned previously will help limit your exposure, however you may want to take extra precautions, especially if you have children.  The Childhood Lead Prevention Program in your area may offer education, soil testing, blood testing for children, need-based financial assistance for homeowners, and other services to help manage lead in and around the home, including lead in soil.  However, soil with high levels of lead may be considered a hazard that a landlord may be required to remove or pave over with concrete, if reported to a government agency such as a Lead Prevention Program.

Perhaps the easiest way to get cleaner soil is to create raised beds.  Make
raised garden beds using non-pressure treated wood, and fill with ‘new’ soil or compost that is organic or has been tested by the supplier for heavy metals.  Test your soil every few years to ensure that you have reduced or eliminated the source of the metals. 

In some cases, gardeners may be able to reduce the concentration of metals in their garden.  If you decide to try the strategies listed below, you should still retest your soil yearly until repeat results show the soil is at a level you consider safe.   Depending on where your levels start off and which steps you choose to take, it may take years for the soil to get to the level you want.  If your contamination is extensive, you may want to contact an environmental engineering firm to professionally assess the situation.

  • Try to identify the source of your contamination and take steps to reduce or eliminate exposure.  If your lead level is high, it is likely that nearby homes are releasing dust from lead-based paint that is drifting into your garden.  Fix any flaking paint on your home, talk to your neighbors if they have flaking paint, and contact the Lead Prevention Program if it is appropriate.
  • Add compost and/or soil from a nursery. (Intawongse & Dean, 2006; Gadepalle, 2008)
  • Do not put any plant debris removed from a garden with high concentrations of metals into a home compost bin.  Place your compost in your green waste bin if your city has a curbside composting program, where it will be diluted.
  • Grow plants that have the ability to remove metals from the soil. See the Bio-accumulation section below for more information.

Whichever strategies your choose, be it raised beds or remediation, continue to test your soil regularly. If you cannot afford certified soil testing from an independent lab, the UMass, Amherst lab provides basic household soil testing for $10/sample.  See the Sampling & Labs/Testing sections for more information on collecting samples and soil testing.


Bio-accumulation

Exposure to Metals through Gardening

Heavy metals that find their way into our garden soil come get there a variety of ways.  Some have always existed there,  with others resulting from past activities such as industry (metal refinery), agricultural (pesticides), transportation (railroad), and residential-use (dumping or burning trash).  Some metal particulates, especially lead, are released into the air and eventually settle back down to the ground.  These tiny particles can coat the leaves of our crops and become part of the soil. Once in the soil, the metals can be absorbed through the roots of our crops where they can stay in the roots, or travel up the plant into its stem, leaves, or fruits.

All plants absorb metals, with some plants being experts at absorbing and storing certain metals in their tissues (called bio-accumulation).  Different plants accumulate different metals in different amounts and they store metals differently (Intawongse & Dean, 2006). A lot of research has been done on which plants are best at accumulating certain metals (a field called phytoremediation).  What has been found is that each plant is different and unique in how it deals with metals in the soil.  Here is a list of a few of the difference that have been observed:
  • Some plants absorb certain metals, but not others.
  • Plants are more likely to store certain metals in particular areas (such as roots), with other plants storing the same metal in other areas (such as their leaves or fruits) (Clark, et al., 2006, Grant et al., 1998).
  • The amount of a particular metal in the soil can affect how well a plant absorbs it.
  • If plants are growing in soil that has a mixture of different metals, it may affect how much of the metals are absorbed.
  • The pH of the soil (a measure of acidity) can affect how well a plant absorbs certain metals.
  • The amount of organic matter, such as compost, in the soil can affect the rate at which a plant can absorb certain metals.
If your soil has high levels of a particular metal, you should carefully consider which crops you want to grow.  You may want to avoid growing root crops, for instance, if you have high lead levels.  A few commonly grown crops known to absorb metals include:

  •  Lead: Napa Cabbage1, Onions2,5, Mustard/Turnip Greens6,8, Cilantro3, and various root crops3
  •  Cadmium: Lettuce2,7, Spinach4,7, Mustard/Turnip Greens8, Carrots5, Sunflowers1,7
  •  Zinc: Spinach4, Chard4
  • Arsenic: Mustard Greens2 
(1Gardea-Torresday, et al., 2005; 2Peralta-Videa, et al., 2009; 3Finster, et al., 2004; 4Pillay & Jonnalagadda, 2007;5Sipter, et al., 2008; 6Clark, et al., 2006, 7Grant, et al., 1998; 8Jiang, et al., 2004)

If you want to try to remediate your soil by growing these crops and disposing of them properly, know that may take a few years to see results, you should conduct extensive soil testing, and it is advisable to do extensive research on creating a remediation plan... something with is beyond the scope of this website.

Determining Risk from Exposure

When we harvest and eat crops from our garden that contain metals, we are "exposing" our bodies to the metals.  For most adults in the U.S., it is unlikely that you will be exposed to unhealthy levels of heavy metals from eating your garden produce.  However, pregnant women and children are at higher risk of getting sick from exposure to metals, as are those with weak immune systems or a poor diet. (Farias, et al., 1996; Mahaffey, 1990)  Children under 6 and unborn babies are growing so fast that their bodies absorb metals at a higher rate than adults, and those with a diet low in certain vitamins and minerals can also absorb more metals than is normal.  

It is very difficult to gauge if a particular person or family is at risk for over exposure to heavy metals.  In the field of Public Health, various methods are used to determine if certain levels of exposure could result in health problems.  This is called a risk assessment. When calculating risk related to eating garden produce, many of the different factors mentioned above would need to be considered (i.e., soil metal and nutrient levels, pH, and % of organic matter), in addition to information about a person's diet, weight, age, health and
the types and amounts of each plant that is eaten from the garden.  It would also be important to know if a person has other routes of exposure, such as in the workplace, that would increase their risk of developing health problems.  

All in all, because doing a risk assessment involves a lot of complicated math and soil testing, most gardeners should simply focus on getting their soil tested so they know the amount of heavy metals in their garden and then take simple steps to reduce your family’s exposure.