Urban Soils and Backyard Gardens: Potential Contaminants and Remediation Techniques
Associate Professor and Chair
Department of Environmental Science and Policy
University of Southern Maine
Gorham, ME 04038
An unfortunate byproduct of urbanization and industrialization has been the contamination of soil with toxic heavy metals. One area of major concern is the urban residential yard. It has been found that soils in urban areas are contaminated with higher levels of heavy metals than rural areas. One consequence of this contamination is the potential health risk from consumption of home-grown vegetables and fruit in urban back yard environments. Inhalation risks from contaminated dusts generated while gardening is also a concern. Therefore, heavy metal contamination in urban soils is an environmental problem that needs an environmentally safe and economically feasible solution.
In particular, urban environments often have an increased median level of lead in soil (greater than 400 mg lead/kg) due to higher concentration of industries, age of and automobile traffic. Lead is deposited in soil from anthropogenic sources such as lead-based paint, automobile emissions (prior to the banning of leaded gasoline in 1986), and past industrial emissions. Due to lead's immobility, it usually remains near the soil surface which increases the risk of an exposure. Children are more vulnerable to lead exposure due to mouthing behaviors, peaked interest in their surroundings, their developing nervous systems, and higher absorption from the gastrointestinal tract than in adults.
Lead poisoning is the number one environmental health threat to children in the U.S. Lead poisoning has been linked to developmental delays and neurotoxic effects in children, and reproductive difficulties in women. Lead poisoning research suggests that the primary sources of lead exposure for most children are: deteriorating lead-based paint, lead contaminated dust, and lead contaminated residential soil. Considerable attention has been given to lead paint hazards in homes; less attention has been paid to lead contaminated soil that surrounds these homes. Generally this has been because of the more significant contribution to lead poisoning in children made by deteriorating lead paint and leaded dust on the interiors of homes. However, as lead poisoning rates and average childhood blood lead levels decline, lead exposure from soil, and from produce grown in these soils, may be a more significant portion of the exposure for children. Therefore, the soil in yards of inner city homes is potentially a major contributor to lead poisoning in youth, and is largely unregulated.
While agricultural soils have a median lead content of 11 mg/kg, and natural soil levels are below 50 mg/kg, urban environments often have levels > 1000 mg/kg, and reported values as high as 50,000 mg/kg. The USEPA Final Rule on lead established a soil lead hazard at 400 mg/kg for bare soil in play areas and an average of 1200 mg/kg for bare soil in the remainder of the yard. In Europe the lead standards for residential soils range from 0 to 150 mg/kg, and in Canada the soil guideline for children is 140 mg/kg.
Although one option to combat contaminated garden soil is to simply cover over the problem, laying down a textile barrier and bringing in new soil, our research investigates whether we can solve the problem, rather than bury it. The primary goal of our research is to decrease soil lead concentrations using phytoremediation, a process by which plants are used to extract heavy metals from the soil. Phytoremediation is a plant system technology that offers a low cost in-situ management approach to cleaning up heavily contaminated soils by using plants that are hyperaccumulators. Hyperaccumulators are plants that have the capability of accumulating metals into the tissue at levels up to 100-fold greater than measured in nonaccumulator plants. Once the plants have reached maturity they are harvested thereby removing some of the metal from the system.
There are approximately 400 plant species from 45 plant families that are known to hyperaccumulate metals. Several studies have experimented with Brassica carianata, Brassica juncea and Thalaspi caerulescens as hyperaccumulators and leafy vegetables are known to accumulate heavy metals. At the present time, however, there are no known hyperaccumulators for the most common contaminant in urban soil, lead. Lead is not known to be an essential element for plants, but many studies have shown that various plant species have the ability to absorb lead in the roots and translocate lead to the shoots.
We have used spinach (Spinacea oleracea), sunflowers (Helianthus annus), and Indian mustard (Brassica juncea) as phytoaccumulators for lead. We sampled the garden soils before any planting took place, and then again after the plants were harvested, and analyzed them for total and plant-available soil lead using EPA methods. We also analyzed the roots and shoots of harvested plants. We found that in moderately acid soils (pH 5-6.5) with low organic matter, phytoremediation removed at least 100 mg/kg lead after one growing season.
Other courses of action for lead in your soil depend on the level of lead contamination. Anything that can be done to eliminate bare soil will help reduce the amount of lead you are exposed to. For example, installing raised bed gardens and use clean topsoil is an option. This will involve buying lumber (not pressure treated) and topsoil. A woven or plastic liner placed at the bottom of the bed can also help prevent existing soil from touching plants. Wood-framed play and picnic areas can be constructed and filled with woodchips. Crushed stone may be used in high traffic areas. Other bare soil areas should be fertilized and seeded, or covered with mulch or woodchips.
If lead levels are high gardens should be relocated. Play, pet, and picnic areas should also be relocated in highly contaminated areas. If this is impossible, these areas will need to be covered by installing a permanent barrier. Laying concrete is an example of a permanent barrier. The soil is completely covered and can not resurface. As a last resort the soil can be removed. While this can be costly, it will remove the problem completely. A contractor will need to be contacted.
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