Growing your own fruits and vegetables is both challenging and rewarding. Many people plant gardens with the expectation that they can control what goes into their food. Gardening, especially in urban areas, has grown in popularity, but unfortunately, it comes with some risks. Contamination with elements present in the air, soil, or groundwater is a concern in both urban and rural gardens, and fruits and vegetables are good accumulators of heavy metals. This blog post details sources of heavy metal contamination and what you can do to help prevent soil contamination and heavy metal exposure from home-grown fruits and vegetables.

How Do Heavy Metals Get into My Garden?

There are many natural and anthropogenic [anthropogenic = originating from human activity] sources of heavy metals. Accumulation of these contaminants in soil typically occurs over a long period of time and may be from multiple sources. Here is a list of common sources of heavy metals.

• Vehicle exhaust
• Coal fired power plants
• Industrial waste
• Mine waste
• Well/irrigation water
• Fertilizers/pesticides
• Paint chips
• Natural events (volcanoes, floods, etc.)

Which Heavy Metals Are Most Concerning?

Lead – Lead is a major contaminant of soil due to its past use in gasoline, plumbing pipes, and paint. Soil around areas of heavy traffic has been shown to contain higher levels of lead, which is concerning for the urban gardener. Lead paint on the exterior of old houses can flake off or be blown into soil. Water coming from leaded pipes and fittings can also contaminate soil. The good news is that the use of lead in gasoline, pipes and paint is now banned in the US.

Cadmium – Green leafy vegetables and grains are the largest source of cadmium in non-smokers, while smokers are at greatest risk from use of tobacco which is a good cadmium accumulator. Cadmium primarily comes from fertilizers and pesticides but can also come from waste water and industrial emissions. Cadmium is readily absorbed by plants [1].

Arsenic – Arsenic is often found in well water used for irrigation. Old apple orchards used lead arsenate as a pesticide, so many old apple orchards are contaminated with both arsenic and lead.

Thallium/Mercury – Coal-fired power plants are the greatest source of anthropogenic thallium and mercury. Soil located around coal fired power plants has been shown to have higher levels of these two heavy metals.

Zinc/Copper/Manganese – Although considered essential elements, zinc, copper and manganese can be toxic at high enough concentrations. Fertilizers, mine waste, and natural sources can result in high soil concentrations of these elements.

Bioaccumulation of these heavy metals is a big contributor to their adverse health effects.

How to Reduce Heavy Metal Contamination in Soil and Produce

Here are a few ways to ensure heavy metal contamination in your garden is reduced or prevented.

• A soil test for heavy metals is the best way to determine what contaminants are present.
• A well water test will help identify a contaminated aquifer or naturally high amounts of heavy metals like arsenic.
• It can be very difficult to remove heavy metals from soil, so using a raised planter with clean topsoil can reduce exposure.
• Plant gardens away from traffic and old buildings. Trees are good barriers that can block wind-blown dust and air pollution [2]. Increased soil contamination has been linked to closer proximity to roads [3].
• Keep soil at a neutral pH of 6.5-7. Lead is not readily absorbed by plants unless the soil is very acidic [4]. Use soil amendments to raise the pH if necessary.
• Plant fruits and vegetables that arise from flowers, as heavy metals tend to accumulate in root, leaf, and stem tissue at a much higher level than in fruits and head or flower vegetables (e.g., broccoli, artichokes, cauliflower, etc.) [5].
• Wash your hands after working in the garden and before washing produce.
• Make sure to thoroughly wash any produce from the garden with cold running water, as lead can be present on the outside of things like root vegetables.
• Limit tilling as air pollution accumulates on the top 1-2 inches of soil.
• Determine which crops are poor accumulators of heavy metals, as crop type has shown to be a better predictor of metal contamination than soil [6].

Conclusions

The goal of this blog is not to scare people away from gardening, but to point out that because something is home-grown does not mean it will be safe for consumption. Taking steps to prevent or reduce heavy metal contamination in the garden will result in healthier produce and reduced exposure to toxins. As a follow-up, ZRT Laboratory offers heavy metal and essential element testing in both blood and urine, which will help identify if recent/past exposure has occurred.

Related Resources

• Blog: Cadmium in Jewelry - Not All That Glitters is Gold
• Blog: Crystal Glassware and Wine - An Unexpected Source of Lead
• Data Sheet: Heavy Metals & Essential Elements Testing in Dried Urine & Dried Blood Spot

References

[1] Mench, M, et al.Evaluating of metal mobility, plant availability and immobilization by chemical agents in a limed-silty soil. Environmental Pollution, 1004;86:279–286.

[2] Säumel I, et al. How healthy is urban horticulture in high traffic areas? Trace metal concentrations in vegetable crops from plantings within inner city neighbourhoods in Berlin, Germany. Environ Pollut. 2012;165:124-32.

[3] Gherardi M, et al. Heavy metals in the soil-plant system: monitoring urban and extra-urban parks in the Emilia Romagna Region (Italy). Agrochimica 2009;53:196–208.

[4] Sauvé S, et al. Solid-solution partitioning of metals in contaminated soils: Dependence on pH, total metal burden, and organic matter. Environ Sci Technol. 2000;34:1125–1131.

[5] Cheng S., et al. Efficiency of constructed wetlands in decontamination of water polluted by heavy metals.  Ecological Engineering. 2002;18:317-325.

[6] Alexander PD, et al. Genotypic variations in the accumulation of Cd, Cu, Pb and Zn exhibited by six commonly grown vegetables. Environ Pollut. 2006;144:736-45.