Gardening on contaminated urban soils
Gardening on vacant lots in urban and suburban areas is becoming more and more popular. There are many benefits to this, provided some basic precautions are taken. Those who raise food on these soils would be well advised to have the soil tested for potentially harmful contaminants. If any contaminants are found, we must find ways to minimize any potential health concerns.
What are the potential contamination problems? Urban soils have sometimes been found to contain toxic levels of heavy metals including lead, arsenic, cadmium, mercury, zinc, nickel, and copper. Contamination may have come from paint, gas or oil, waste incineration, lead pipes, specific industries, and so forth.
It would be helpful to know what practices could be used to minimize potential health risks posed by contamination of garden produce on these soils.
To address this problem, in 2009 we studied lead uptake by several types of vegetables at an urban garden site in Kansas City with “average” levels of lead. In 2010, we tested at a different area on this same site known to have high levels of lead. Lead is one of the more common contamination problems on urban soils, and a contaminant that can cause serious health concerns if levels are too high.
We studied two general approaches to minimizing lead contamination at this site: soil treatments (to reduce plant uptake) and food preparation methods.
Figure 1. Chammi Attanayake, Agronomy graduate student, at the urban garden site in the Washington Wheatley area of Kansas City. Photo courtesy of Ganga Hettiarachchi, K-State Research and Extension.
Soil amendment treatments
One of the most readily available organic amendments for urban gardeners is leaf compost. We tested the effect of compost as a soil treatment on the bioavailability of lead in this soil. Compost was added at 28 kg per square meter and mixed with the top 15 cm of soil, representing a compost:soil ratio of approximately 1:3 (v/v). Control plots with no compost were maintained. Soil pH in compost-added and no-compost-added soils was in the neutral range; furthermore, compost addition improved the concentration of soil organic carbon and cation exchange capacity.
A leafy vegetable, a fruiting vegetable, and a tuber/root vegetable were grown to assess lead uptake of vegetables in the presence and absence of compost treatment. In 2009, Swiss chard, tomatoes, and sweet potatoes were grown. In 2010, Swiss chard, tomatoes, and carrots were grown.
In plants, lead tends to be held in roots. It transfers only slowly to shoots or fruits. As a result, root vegetables such as carrots or sweet potatoes could potentially have the highest concentrations of lead. Leafy vegetables, such as Swiss chard, are next. Fruits such as tomatoes are the least likely to contain lead absorbed from the soil.
To determine lead concentrations in the vegetables we tested, the edible portions of the plants were harvested as each plant type reached maturity at the end of the growing season. Also, representative soil samples were collected before adding compost, at planting, and at harvesting from each plot.
Our study found that adding compost at the rate we used diluted the overall lead concentration in the soil by anywhere from 29 to 52 percent. This reduced the amount of lead absorbed by the vegetables. Also, the phosphorus in compost (and iron oxides in some compost materials) can help hold lead in the soil, which reduces its availability to plants. Finally, compost increased the soil fertility and resulted in larger vegetables, diluting the concentration of lead in them.
In analyzing the edible portions of the vegetables (using the laboratory washing method described below), we found no relation between the concentration of lead in the soil and the concentration of lead in the vegetables.
In 2009, on the soil with relatively low levels of lead, the addition of compost had no effect on the lead concentrations in either Swiss chart, tomatoes, or sweet potatoes. This is most likely because the lead concentration in the produce samples was low, close to the background lead concentration levels.
In 2010, on the soil the relatively high levels of lead, the addition of compost reduced the lead concentration in Swiss chard by 59 percent and in carrots by 20 percent. Since the lead concentration in the soil and produce was high enough in this situation, we could see the compost treatment effectiveness. On the other hand, the lead concentration in tomatoes was unaffected by the addition of compost, even at this location. Fruits, such as tomatoes, have very low lead concentrations under almost any conditions, so it is not surprising that the compost treatment did not affect lead uptake in tomatoes.
The concentration of lead in the vegetables does not necessarily reflect total lead uptake by that vegetable. In 2010, one of the main reasons for the lower concentration of lead in Swiss chard and carrots in the compost-treated soil is that the vegetables were much larger in the treated soils. This diluted the concentration of lead in the vegetables, while the absolute amount of lead in the vegetables was not significantly different between the compost-treated and untreated soils.
This result indicates that an increase of total biomass of vegetables could be an effective means of reducing potential lead transfer to humans.
Vegetable cleaning methods
We tested the effect three methods of cleaning vegetables on lead concentrations in 2010.
- Laboratory cleaning. Vegetables were rinsed with tap water, then rinsed with deionized water, then with sodium lauryl sulfate solution, and finally rinsed again with deionized water. The objective was to remove all soil particles from produce surfaces.
- Kitchen cleaning. Vegetables were rinsed only with tap water, to mimic the washing procedure in a home kitchen. This removed all visible soil particles from the produce.
- Peeling. After kitchen cleaning, a portion of the carrots were peeled.
The various cleaning methods made a significant difference in lead concentrations of Swiss chard and tomatoes but not for carrots. Swiss chard cleaned with the kitchen cleaning method contained 2.6 to 4.6 times greater lead concentrations than that cleaned with the lab cleaning method. Similarly, kitchen-cleaned tomatoes had 3.0 times greater lead concentrations than lab-cleaned tomatoes.
Vegetables can be contaminated with lead not only because of absorption by roots, but also by surface contamination where lead-contaminated soil particles become attached to the plant surface or get embedded in the waxy outer layer of plants. The sodium lauryl sulfate used in the lab cleaning method is an anionic surfactant that can solubilize a large portion of the cuticle barrier on these plant parts. As a result, the lab cleaning methods may have effectively removed particles embedded in the plant surface by solubilizing the cutin lipid cover. That’s one reason the lab-cleaned tomatoes and Swiss chard showed lower lead concentrations.
In contrast, cleaning methods did not significantly affect lead concentrations in carrots. This can be explained by the absence of cuticle lipid layer on the roots. Peeling also did not statistically change lead concentrations in carrots. When peeling, we removed a very thin outer layer of the carrots. Synchrotron-based X-ray fluorescence mapping has shown that the concentrations of lead in the peel and the phloem of the carrot are low compared with the concentration of lead in the xylem.
Bioaccessibility of lead in soils
Ingestion of lead directly from contaminated soil and dust is considered as one of the major exposure pathways of lead for children. What percentage of the lead in soil and dust is “bioaccessible” -- meaning soluble in the simulated gastrointestinal (GI) environment and available for absorption by humans?
In this study we measured the level of bioaccessible lead in the soil to understand the direct ingestion risk. Bioaccessible lead in our soil was low (<10% of total lead). This indicates that the potential for lead absorption on this site through direct ingestion of soil or dust would be low. The addition of compost further reduced the concentration of bioaccessible lead in the soil, mainly through dilution.
Urban and suburban soils should be tested often for nutrient and pH levels. Testing for micronutrients and heavy metals, such as lead, might also be advisable in any circumstance in which contamination may have occurred. One example is soil close to an older house where flakes of lead-based paint may have contaminated the soil.
If the soil test does indicate lead contamination, it might be a good idea to avoid growing root crops such as carrots in the garden. For leafy vegetables, remove the lower or outer leaves since they will be the leaves most likely to be contaminated with soil particles.
Also, if necessary apply lime to keep the soil close to neutral in pH, or possible slightly alkaline. Lead becomes more available for plant absorption under acidic soil conditions.
Be sure to wash vegetables thoroughly with high-quality water and soap. If soil particles are hard to wash off, peel the vegetables or remove the parts contaminated with soil.
Adding leaf compost to the soil may be a good idea as well, as long as the compost does not itself contain high levels of heavy metals. In our study, the addition of compost diluted initial total soil lead concentrations, indicating that the continuous addition of compost would lower total lead concentration in soils significantly. Compost addition also plant lead concentration and bioavailable lead concentrations in the soil.
In addition, compost addition helps maintain good soil nutrient status in soils. Maintaining good soil fertility and thereby increasing biomass production diluted lead concentrations in the vegetables. The highest concentrations of lead in edible portions were found in root/tuber crops, followed by leafy and fruiting vegetables.
In our study, lead concentrations of the edible portions of vegetables, except carrot, were below the maximum allowable limits of lead established by the Food and Agriculture Organization and the World Health Organization.
The abstract of this study published in the Journal of Environmental Quality is online at: http://dx.doi.org/doi:10.2134/jeq2013.07.0273
Ganga Hettiarachchi, Associate Professor, Soil Chemistry
Chammi Attanayake, Agronomy Graduate Student