Lead in Urban Soils
Lead is a widespread problem in America’s urban areas. Years of driving with leaded gasoline, using lead paint on our houses, and running our water through pipes joined with lead solder have seriously contaminated our soils.
Background concentrations of lead in agricultural soils average 10 parts per million. In urban soils, however, lead levels typically are much higher. The Centers for Disease Control estimate that some 21 million pre-1940s homes contain lead paint. When 125 inner city gardens were tested in Boston in 2000, 82% of them had lead levels above the reportable limit of 400 parts per million (ppm). We have banned leaded gas and lead in paint, but the element does not migrate easily nor is it taken up in plants or degraded by biological activity.
In Syracuse, New York soil sampling conducted by university researchers found high levels of lead and arsenic in five out of six community gardens in low-income and minority neighborhoods where residents grew much of their own fresh vegetables dfuring the summer months. The gardens were located on plots where abandoned homes had been razed. Lead paint from the demolished hhouses dontaminated the soil, and lead-rich exhaust from passing traffic built u in the soil over decades.
Where to Look for Lead
Visual assessments of painted structures help to identify areas around the property that are high risk for causing lead poisoning, but they cannot be used to confirm lead contamination of soil. You can look for deteriorated paint on all painted building components, especially any exterior walls, windows, or trim damaged from a roof or plumbing leak. Also look on surfaces that experience friction or impact like doors, windows, floors, and trim areas. In addition, look for chipped paint on the yard around the house. The next step will be to check if there are areas of bare soil or thin grass that are greater than 9 square feet. The special risk areas for soil are drip lines – within 2 feet of the house, play areas, gardens (in native soil) and uncovered walkways.
If the house/structure was built before 1978, and you see deteriorated paint or there is bare soil or thin grass in special risk areas, you should test your soil for lead. Testing is especially important if there are children under the age of six living in the house and if there is or will be a garden in native soil.
Suggested Sampling Materials:
- 1-quart Ziploc-style bags (one per composite sample, usually 1-4 per yard)
- Permanent marker (to mark bag)
- Record Sheet, Map Sheet (see samples Appendix B and C)
- Auger, shovel, trowel or similar tool
- Rag or paper towels
- If windy/risk of creating dust: Respirator (3M / HEPA filter)
Step 1 – Identify Potential Risk Areas
With input from resident and/or owner, identify areas that are most likely to be a risk based on the following high risk factors:
- High use, especially by children (play areas, gardens, walkways)
- Bare soil
- Proximity to house (especially the “drip zone” within 3 feet of the house, aka “drip line”)
- Visible chipping paint or known former structures
Choose the areas to be tested and mark them on a Map (optional), drawn in the context of the property, streets, and compass heading (mark North on the map). Give each area a sample name/number (ex. #1 = Drip line).
Step 2 – Collect Composite Samples
Within each possible risk area chosen, collect 6-8 samples (evenly spread out in the area) in this way:
- Make a hole with auger, shovel, trowel or similar tool. The hole should be thin and approximately 6 inches deep. Take some soil from each depth of the hole either by scraping the tool along the side of the hole, or poking out a column of soil from the core if using a bulb tool or something similar. Mark each hole with an “X” on the Map Sheet. Wear gloves and, if windy, respirator to prevent inhaling dust.
- Put all 6-8 soil samples in one bag, avoiding insects and large pieces of debris such as sticks, stones, bark, etc. Total soil should be approximately one cup. Wipe off sampling tool between composite sample sets (not individual samples).
- Label bag with sample name/number, address of site, name of organization and date.
DEPTH ANALYSIS: A useful tool to find out lead concentrations at different depths: take a separate sample from 2-4 different depths (for example: 1 inch deep, then 3 inches deep, then 6 inches deep).
Step 3 – Document Area. Complete the map and record sheet, making sure sample names/numbers match up and marking important structures, notes and other relevant details such as type of siding, use of spaces, etc.
Each lab requires different sample preparation and bag labeling. For labs that require dry samples, dry them by placing soil in sun on a piece of dark flexible material or newspaper in an area with little or no wind. Return to bag when dry, being careful not to mix up the samples. Debris can be removed at this stage as well. Send samples, with a list of samples, to soil testing lab. You may want to retain a “copy” of each sample in case of a lab or mail error.
Where can you send your soil samples?
One recommended lab for quick turn-around and inexpensive testing, useful if just the lead concentration is needed, is Environmental Health Services Lab: http://www.leadlab.com/
If you need more information about the soil composition in addition to the lead concentration you can contact your local extension service. One such recommended lab is University of Massachusetts Soil and Plant Tissue Testing Lab: http://www.umass.edu/soiltest/
Interpreting the Soil Test
Look for the “total concentration” or “total estimated lead” or similar number on the lab report. Results are measured in µg/g or mcg/g or most commonly: Parts Per Million: PPM.
0-400 PPM Recommended options:
The EPA deems these levels safe for gardening and play. At levels of 200 and above, some groups advise using compost amendment and/or phytoremediation.
400-2000 PPM Recommended options:
–Build raised beds or containers gardens for immediate gardening
–Compost amendment (in addition to diluting toxic concentrations, studies have shown high phosphorus compost amendment reduces bio-availability of lead)
–Cover with 6 inches of clean soil, then stabilize or create a barrier using the following: perennial plants, wood chips, landscaping fabric, crushed stone, patio, stepping stones, etc.
2000+ PPM Recommended options:
- Get children who have come in contact with infected area tested (blood lead level tests done at most doctor’s offices and health clinics).
- Block off or cover area.
–Some groups recommend permanent coverings (see above) for this level.
–Build raised beds or container gardens for immediate gardening
–Some groups recommend excavation or burying on site with proper safety precautions. This can be very costly, especially for disposal, and safety precautions are extensive.
- As densely as possible, plant hyper-accumulators, such as scented geraniums (others include mustard greens, Indian mustard, sunflowers, collards or spinach), to accumulate lead into the roots, stems and leaves of the plant. After the growing season, safely dispose of the plants (if you put into the municipal waste stream, ensure that your area has good lead protections on incinerators, landfills, etc.) Note: this technique is very slow, and depending on the lead concentrations and soil conditions remediation can take several growing seasons. It is advisable to combine these phytoextraction techniques with other lead-safe landscaping techniques in most cases.
- Stabilize the soil by planting plants that grow soil-retaining root systems such as shrubs or ground-covers to reduce foot-traffic access and dust-creation. This process is call “biostabilization.”
Advantages of Phytoremediation:
- Does not disrupt ecosystems
- Low-tech, accessible
- Metals can be reclaimed
Disadvantages of Phytoremediation:
- Remediation is confined to depth of roots
- Leaching into groundwater is not prevented
- Time consuming (studies suggest 300 ppm can be removed in 7 to 10 years)
Compost and Soil Amendments
- Add 6-12 inches compost to your garden to dilute and bind up the lead.
- You can reduce the amount of lead that is available to be absorbed into people’s bodies by adding phosphorus to the soil (in the form of rock phosphate) forming pyromorphite crystals.
- Some cities and towns have free or inexpensive municipal composting programs.
Other Landscaping Techniques
Capping with clean soil: Add 6-12 inches of clean screened loam on top of contaminated areas, then stabilize the new soil. Stabilization techniques include bio-stabilization (lawns, perennial garden beds, bushes, spreading ground covers such as pachysandra) or installing a hardscape (patios, walkways, crushed stone / peastone beds with sturdy edging).
Drainage: All our suggested hardscaping techniques are water permeable (as opposed to paving, for example) but most require drainage to be taken into account, especially when capping with clean soil is used, as to not have the capped soil wash away. Lawns that are the low-points of the yard often require a buried drainage pipe (also called French Drain) which is installed by digging a trench with at least a 1% slope, lining it with landscaping fabric, installing a drain pipe (ideally a 4 inch rigid plastic perforated drain pipe, flexible corrugated pipe can be used but is harder to clean out), then surrounded by crushed stone, and covering with landscaping fabric to divert water from undesired areas (like towards foundations or low lawns).
Edging: usually the most challenging aspect of hardscaping work, especially in the case of lead-safe landscaping where disturbance of native soil must be minimized. Some digging to set edging (blocks, plastic edging, rot-resistant lumber) is often unavoidable, but the more you can use existing edges or build up clean soil to retain hardscaping base or material the better. For all projects that include digging, remember to call dig safe (dial 811), use respirators (3M HEPA filter), coveralls, boot coverings, and other dust prevention such as tarps and wetting soil before displacement.
Rain gardens: also a great design element that address drainage and flooding issues. More info on Rain Gardens here: http://www.raingardennetwork.com/build.htm
Raised Beds and Containers: See below on how to make raised garden beds. Container gardens are also a good immediate option for gardeners wanting to safely grow within one season. Get creative with your containers! Recycled bathtubs, bins, mini swimming pools make great container garden receptacle, as long as they have a way to drain excess water.
Construction Guide for Raised Beds
Want to grow this season and worried about your soil being contaminated or not good enough quality? You can make a raised garden bed for about $100 and fill it with fresh compost!
Wood: 4x4s in 3ft lengths or longer (our most common combinations: for 10ftx4ft bed that is approximately 1ft deep, we use six 10ft and three 8ft – cut in half – 4×4 timbers). Make sure to get naturally rot-resistant (black locust is great, cedar works) or alternatively pressure treated wood (ideally sodium silicate), especially avoiding those that contain Arsenic (most often in the form of Chromated Copper Arsenate – CCA) as you don’t want to be putting toxins near your food crops! Some use 2×6 inch boards, but we’ve found that, though less expensive, they have half the life span.
- Spikes: These are 6inch long nails. 30 spikes needed for each one foot deep bed. You can also use 6 inch screws such as “timberlocks” with a strong power drill or using pre-drilled holes in the timbers.
Compost: (soil made from composted organic matter such as yard waste) Make your own from food/yard waste, purchase, or look for free or inexpensive municipal composting programs.
Landscaping Fabric: To create a barrier under the bed so that water can go through but the plant’s roots cannot. One small roll will be plenty, most hardware stores carry this.
Tools: Four-to-six pound sledge hammer for spikes, shovels for soil, scissors or utility knife to cut the landscaping fabric, gloves, eye protection and a circular saw if you need to cut the timbers to length.
- Find a flat place that gets lots of sun. Gather materials (see above). Cut lumber and landscaping fabric to desired lengths.
- Pin the landscaping fabric on the ground with fabric staples, then place the first level of boards on top in the shape and location desired. Notice how each piece of wood is touching the end of only one other piece (i.e. you do not want the end pieces touching the ends of both side pieces, etc.)
- Hammer the spikes into the ends of the wood horizontally – connecting them to the other pieces in the rectangle – and four spikes into the ground to hold the fabric bed in place. For hills, it is recommended to use re-bar that go through pre-drilled holes in wood and are pounded into the ground at least 1 foot deep.
- Lay the second layer remembering to rotate the wood so that no connection is directly above the one below it (see image above).
- Hammer the second layer vertically down into the first layer with spikes every 2-3 feet. Some horizontal spikes into the ends of the timbers are useful to keep tight corners.
- Repeat with a third layer, remember to rotate the layout again.
- Fill the bed with soil.
- Plant your organic vegetables!
You can excavate very small gardens 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.