Managing and Treating Contaminated Soil
TREATING CONTAMINATED SOIL
The approaches below can help homeowners, community gardeners, and urban food producers* reduce their exposure to lead, arsenic, and cadmium in highly polluted soils. Several techniques are listed here: choose those which work best with your budget, site type (garden, play area, wildlife habitat, etc), and maintenance preferences: interim control measures like applying mulch are not permanent and need continuous maintenance to be effective, while abatement measures are designed to be low-maintanence and last at least 20 years. Make sure your practice or combination of practices addresses both risk pathways: incidental ingestion and plant uptake. Combine soil management with good hygiene practices to further lower exposure.
*For information about soil treatment of industrial and commercial sites, see USEPA's Remediation Technologies for Cleaning Up Contaminated Sites or your state's environmental protection agency.
INTERIM CONTROL MEASURES
These measures are not permanent and need to be maintained in order to be protective. For example, organic mulches should be periodically reapplied. Despite their maintenance needs, these practices generally improve soil quality, have additional benefits for crop production, and are inexpensive, making them good options for those able to maintain them.
This easy and inexpensive management approach can reduce incidental ingestion of lead, arsenic, and cadmium in dust and soil adhered to produce. Mulch with at least 2 inches of organic material, or use a synthetic cover such as plastic mulch, landscape fabric, outdoor carpet, paver stones, or playground mats. Replace mulches as needed. Mulches have the additional benefits of suppressing weeds in the garden bed. This is also a good strategy in high-traffic walkways which are prone to bare soil spots.
Combine mulching with a practice that reduces plant uptake like adding organic matter or liming.
A thick vegetative cover like sod, or a planting that creates a barrier such as a hedge can reduce incidental ingestion lead, arsenic, and cadmium in dust. The thicker the cover, the better the protection. Use this strategy in areas where you do not grow edible crops, such as ornamental plantings and low-traffic walkways.
Organic matter (OM) binds to soil lead and cadmium to reduce plant uptake. OM addition may increase plant uptake of arsenic in arsenic-polluted soils. Do not use this technique for arsenic-polluted soils. For the initial treatment, cultivate in at least 30% OM by volume into the soil (if you were tilling to a depth of 6 inches, add 2+ inches of compost). OM breaks down over time, so reapply at least 15% OM by volume every other year (1 inch/ 6 inch garden bed). For more information on materials and application methods, see Washington State University's factsheet "Organic Soil Amendments in Yards and Gardens: How Much is Enough?"
Combine OM addition with liming to further reduce plant uptake and a practice that reduces incidental ingestion.
Liming has little impact on lead and cadmium exposure from incidental ingestion of soil, but it does reduce plant uptake of lead and cadmium. Liming may increase plant uptake of arsenic in polluted soils. Do not use this technique for arsenic-polluted soils. Lime the soil to around pH 7 using the recommendations from a nutrient test. As soil pH may shift overtime, especially with fertilizer and organic matter additions, test every 3 years to maintain near pH 7.
Combine liming with a practice that reduces incidental ingestion such as mulching.
These measures are more permanent than interim control measures and do not need continuous maintenance. They tend to be more expensive than interim control measures, but are a good option for those with short-term funding and operations with multiple or changing caretakers.
Capping and raised beds
Capping contaminated soil with tested, clean material reduces lead, cadmium, and arsenic exposure from incidental ingestion and plant uptake. Contaminated soil should be covered with at least 8 inches of material if used for food production, and 4 inches in other cases. Plant uptake or dust deposition may slowly increase soil contaminant levels in the clean material over time; test for contaminants every 3 years. Before capping, consider treating lead-contaminated soils with phosphorus fertilizer (as long as they are not arsenic contaminated), or lining lead, cadmium, or arsenic-contaminated soil with a root-impenetrable layer such as landscape fabric. Landscape fabric and weed cloth found at the garden center or hardware store allows water to drain out of the bed but is not penetrated by roots.
Remove and replace
Replacement is the most effective strategy for reducing the risk from lead, arsenic, and cadmium-contaminated soil as it eliminates risk from incidental ingestion and plant uptake. It is also the most expensive and destructive. This is often the best option for extremely contaminated soils, but otherwise consider alternate management options. Contaminated soil should be removed to a depth of 6 inches and replaced with tested, clean soil or soil-like material. Contact your state environmental protection agency to learn the disposal requirements for the removed soil.
Contaminated soil can be mixed with clean material to dilute lead, arsenic, and cadmium concentrations. This reduces both incidental ingestion and plant uptake of lead, cadmium, and arsenic. Contaminated soil can be mixed either with subsoil by deep tillage if only the surface material is contaminated, or with clean soil or soil-like materials such as dredge. Organic matter addition is also a form of dilution, but organic matter breaks down over time so this is an intermim control method and reapplication must be done periodically (see "Organic Matter" in the interim control measures above).
Covering the soil with pavement is a permanent way to reduce incidental ingestion of lead, arsenic, and cadmium-contaminated soil. Bricks, pavers, concrete, or asphalt are all suitable materials.