To assess the amount of lead in a gardener’s soil, we soil test in their gardens. To date, we have completed soil tests on 113 gardens within the Dudley Street community and 12 from the Bowdoin and Draper Street neighborhoods. 82% of these gardens tested have lead concentrations above the MA-DEP reportable limit of 400 ppm. The results of these soil tests are maintained in a database of gardens, started in 2000. Soil testing is a process of collecting soil from the garden and then bringing the soil to a lab at Wellesley College where we can scan it for lead content. After testing the soil we return the results to the gardener and help them analyze them. Often parts of a garden are significantly more contaminated than other sections. Along the drip line of a house is a common place for high lead contamination. In these cases, we advise gardeners to grow in the less contaminated areas, and if they are to grow near the drip line they should try and grow plants such as tomatoes or squash that do not accumulate lead in the same way that leafy greens would.
When a garden is severely contaminated with lead we consider how we can remediate, or clean, the garden. We use four different types of remediation depending on the level of contamination and the size of the garden.
To date, we have excavated one very small garden with extremely high levels of lead by replacing the top three feet of contaminated soil with compost and clean soil. Due to the high costs and intense labor of the excavation process, the opportunity to use this technique is very limited.
Throughout the year and as part of our City Farm Festival we advocate using compost as a means of amending and building up soil. Compost neutralizes the pH of the soil, making it less bioavailable to plants. If enough compost is used (2 feet at least), plants can grow in healthy soil. This was the method we used on our Langdon and West Cottage farms.
Building raised beds is an alternate method to deal with contaminated soil. We build raised beds on contaminated gardens and fill them with new compost and soil. These beds are built with untreated lumber (we use cedar) and secured into the ground with stakes. We use landscape fabric to prevent roots of the plant from coming in contact with contaminated soil.
In the summer of 2003, we began experimenting with the potential for mustard greens to pull up lead in five lots in Roxbury and Dorchester. Five gardens with lead levels above the EPA’s established safety level were selected to participate in the study. To do the experiment, each gardener agreed to section off a fourth of their lot to enable us to grow mustard greens for lead uptake. That year, we also partnered with Dan Brabander, professor of Geochemistry at Wellesley College, to help conduct these experiments and increase our database of soil tests.
The experiment continued in 2004 with three gardens all above the MA-DEP reportable limit of 400ppm. Mustard greens, sunflowers and collards were planted to test several known “hyperacculumators” as a viable method of remediation.
In the summer of 2005, we grew sunflowers and mustards in seven gardens for a third phytoremediation experiment. The results from three years of research showed that plants alone do not accumulate lead fast enough for phytoremediation to serve as a practical remediation method. Using these results, plants would decrease lead in soil only by 300 ppm over the course of 7-10 years. Without the use of an added amendment to increase the mobility of lead, such as a chelating agent, plants are unable to make a significant impact on the lead content in soil. On average, the mustards accumulated 46 µg/g of lead in its plant tissue, sunflowers 47 µg/g, and collards 7 µg/g in the leaf and 57 µg/g in the root.
These results invite further studies to improve on the phytoremediation experiments for next year. We have discussed the use of acidic fruits, such as oranges or lemons, incorporated into the soil in order to lower the ph, thereby increasing the lead mobility to facilitate lead accumulation in plants. Another possibility is using a different type of hyperaccumulator such as geraniums. On the other end of the spectrum, we will also be experimenting with phosphorus which will help compost bind the lead in the soil, thereby making the lead less bioavailable to plants.