Managing and Treating Contaminated Soil

Treating contaminated soil

The management of a contamianted soil depends on the site use (garden, play area, wildlife habitat) and the soil lead concentration. Lead reference values and screening limits can help determine a management approach. Mangaement and treatment options can be broken into two categories: interim control measures (those that will not last without human management) and abatement (will last >20 years without intervention).

INTERIM CONTROL MEASURES

Mulching

Strawberries planted into plastic mulchThis easy and inexpensive management approach can significantly reduce incidental ingestion by 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 garden weeds.

Combine mulching with a practice that reduces plant uptake like adding organic matter or liming.

Vegetative coverHedges

A thick vegetative cover like sod, or a planting that creates a barrier such as a hedge can reduce incidental ingestion. The thicker the cover, the better the protection. 

Organic matter 

Applying compost Organic matter binds to soil lead, reducing plant uptake. Cultivate in 30-90% organic matter (OM) by volume into the soil. If you were tilling to a depth of 6 inches, add 2-6 inches of compost. Organic matter works well to reduce plant uptake but is less effective in reducing human and animal bioavailability. If humans or animals are using the site, combine OM treatment with another approach. Organic matter breaks down over time and is not an effective treatment for lead unless it OM levels are maintained: add OM every other year. 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 a practice that reduces incidental ingestion such as mulching.

Liming Soil pH effect on nutrient availability

Liming has little impact on exposure from incidental ingestion of soil, but it does reduce plant uptake. 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.

ABATEMENT

Phosphorus Treatment

Triple superphosphate fertilizerPhosphorus fertilizer chemically bonds to lead in soil and immobilizes it, reducing both plant uptake and absorption from incidental ingestion. While lead concentrations do not change, the lead is in a new insoluble form which is not taken up by plants or by humans.

Add 7.5 lb phosphate per 100 square feet of soil and incorporate to a depth of 6 inches [1,2]. For example, a garden bed 4 feet wide and 8 feet long is 4ft x 8ft = 32ft^2. To treat this area, we need 7.5lb x 32/100 ft^2 = 2.4lbs phosphate. Next, determine the phosphate content of your amendment. For example, triple super phosphate (TSP) is 0-45-0, meaning it contains 45% phosphate by weight. In every 1lb of fertilizer, there is 0.45lb of phosphate. Divide the phosphate requirement by the phosphate fraction of your fertilizer. To get 2.4lbs of phosphate from TSP, we need 2.4/0.45=5.3lbs TSP.  

[1] Stehouwer, R. (2018, December 23). Lead in Residential Soils: Sources, Testing, and Reducing Exposure. Retrieved from https://extension.psu.edu/lead-in-residential-soils-sources-testing-and-....
[2] Stanforth, R., & Qiu, J. (2001). Effect of phosphate treatment on the solubility of lead in contaminated soil. Environmental Geology, 41(1-2), 1-10. doi:10.1007/s002540100262

Capping and raised beds

Raised beds with linersCapping contaminated soil with tested, clean material reduces 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 all other cases. Plant uptake may slowly increase soil lead levels in the clean material over time. Before capping, consider treating with phosphorus fertilizer or lining the contaminated soil with a root-impenetrable layer such as landscape fabric. Landscape fabric and weed cloth found at the garden center or hardware store will let water drain out of the bed but are not penetrated by roots. 

Remove and replace

Removing and replacing soil at the Coast Guard Yard Superfund site.Replacement is the most effective strategy for reducing the risk from 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 highly contaminated soils, but otherwise consider other 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. 

Dilution 

Garden tillerContaminated soil can be mixed with clean material to dilute contaminant concentrations. This reduces both incidental ingestion and plant uptake of lead. Contaminated soil can be mixed either with subsoil by deep tillage if only the surface material is contaminated, or by incorporating 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 reapplication must be done periodically (see "Organic Matter" above).

Stabilization 

Soil stabilization and solidificationPrimarily performed on large sites, stabilization (aka solidification) is a permanent solution to heavy metal contamination that costs less than removal. It involves mixing soil with cement, binding the contaminated soil into a solid block of material that will no longer produce dust or leach metals, thereby reducing incidental ingestion. For more information, see USEPA's A Citizen's Guide to Solidification and Stabilization

Sealing Sealing soil with brick pavers in a garden

Covering the soil with pavement is a permanent way to reduce incidental ingestion of contaminated soil. Bricks, pavers, concrete, or asphalt are all suitable materials.