Risk and Contaminated Soil

This page provides an overview of contaminant exposure pathways and describes factors that we can modify to reduce risk. Although this page focuses on lead (Pb) contamination, the dose equation and approaches are similar for most heavy metal and metalloid contaminants. See Managing and Treating Contaminated Soil for specific guidelines. 

Soil metal exposure can be calculated with the above equation. This helps explain why children are the most vulnerable to lead poisoning. Children ingest higher rates of soil (SOIL INGESTION RATE). They also absorb more of the lead that they ingest (BIOAVAILABILITY). And because they are smaller than adults, they receive more lead relative to their size (BODY WEIGHT). All these factors contribute to children receiving a higher dose of lead from contaminated soils. In order to reduce the risk of a lead-contaminated soil to children, we can change some factors in the dose equation. A management strategy that addresses multiple pathways and exposure dependents will be more effective than focusing on fewer pathways. 

LEAD (Pb) CONCENTRATION in a soil can be reduced by replacing or capping it with a clean, tested soil; deeply tilling soil to dilute contaminated surface soil with uncontaminated subsoil; and diluting soil by tilling in large amounts of organic materials, tested clean soil, or tested clean soil-like materials such as dredge. 

SOIL INGESTION RATE can be reduced by tackling both intentional and incidental consumption of soil. Children may intentionally ingest soil (pica) while playing in soil. Children may also incidentally ingest soil through hand-to-mouth behaviors, especially toddlers. Observe children's play behaviors. Alternative play surfaces (sand boxes, outdoor carpet, rubber mats...) can reduce soil ingestion. Incidental ingestion primarily occurs in adults when soil dust is filtered out of the respiratory tract and sent to the stomach. Keeping soil moist and covered with plants, mulch, or other surface materials can reduce dust. Soil adhered to food crops can also contribute to soil ingestion. More soil adheres to leafy vegetables and root crops than fruiting vegetable or fruit crops. In any crop, mulching and windbreaks can reduce soil adherence. Finally, plants may take up lead through their roots. Leafy vegetables and root crops appear to accumulate more lead than fruiting vegetables or fruit crops. 

BIOAVAILABILITY: Bioavailability is the portion of soil lead that humans, animals, and plants absorb when they are exposed to contaminated soil. In some soils, soil lead has very low bioavailability, meaning if a child ingested soil, very little lead would be absorbed into the bloodstream. On the other hand, if a child ingested soil with very high lead bioavailability, much of the lead present in the soil would be absorbed into the bloodstream. The availability of soil lead to organisms like humans, animals and plants depends upon the soil makeup and lead source material. However, soil lead can be made less bioavailable through the addition of amendments. Soluble phosphorus fertilizers, like triple super phosphate and monoammonium phosphate, are added to lead-contaminated soils to transform the lead minerals into insoluble lead-phosphates with low bioavailability. Organic matter can also reduce lead bioavailability, though to a lesser degree than phosphorus. Organic matter does a good job reducing the bioavailability of lead to plants, however, as does raising the soil pH >6.5. This will reduce uptake of lead into food crops. Additionally, a well-balanced diet high in minerals like phosphorus, iron, and calcium and low in fat can reduce lead uptake by children.