When the Ground Becomes Airborne, So Do the Metals

Mar 18
4 min read

How Grading Activities Re‑Mobilize Arsenic and Other Metals in Post‑Fire Soils

Wildfire recovery is rarely linear. Even after the ash settles and the first rounds of soil testing are complete, the landscape continues to shift, literally. As rebuilding begins, heavy equipment moves in, foundations are cleared, and lots are graded. What many residents don’t realize is that these activities can unintentionally re‑mobilize contaminants that were previously locked in the soil, including arsenic, chromium, cobalt, copper, and lead. This isn’t speculation. It’s a well‑documented environmental pathway supported by decades of research and regulatory guidance.

How Metals Become Airborne During Grading

Metals do not evaporate. They move by attaching to fine soil particles the clays, silts, and ash fractions that are easily lofted into the air during disturbance.

Across multiple studies, researchers have shown that:

Arsenic, lead, chromium, copper, and cobalt are enriched in PM₁₀ and PM₂.₅ dust generated by soil disturbance
Mechanical grading dramatically increases dust emissions, especially in dry, post‑fire landscapes
Fine particles travel farther and carry higher concentrations of metals than coarse soil
Wildfire ash contains elevated metals and is extremely prone to re‑entrainment

These findings are consistent across environmental health literature, wildfire ash studies, and construction‑site monitoring.

Once airborne, metal‑bearing dust can:

Travel across property lines
Deposit on previously cleaned or treated soils
Enter homes through open windows or HVAC systems
Re‑contaminate areas that had already shown improvement

This is the same mechanism by which lead dust spreads during demolition and chromium dust spreads during industrial soil disturbance. Arsenic behaves no differently.

Metal‑by‑Metal: What the Research Shows

Arsenic (As)

Strongly binds to fine particles
Enriched in wildfire ash
Detected in PM₁₀ during soil disturbance
Easily re‑entrained during grading

Lead (Pb)

One of the most studied airborne metals
Highly concentrated in fine dust fractions
EPA and OSHA identify dust inhalation as a primary exposure pathway
Readily transported during construction and demolition

Chromium (Cr)

Cr(III) binds to clays; Cr(VI) binds to even finer particles
Wildfire ash studies show chromium enrichment in fine ash
Air monitoring detects chromium in dust downwind of disturbed soils

Cobalt (Co)

Binds to iron/manganese oxides in fine soil fractions
Occupational studies show cobalt in airborne particulates during excavation
Mobile in PM₁₀ generated by grading

Copper (Cu)

Enriched in wildfire ash and organic‑rich fine particles
Frequently detected in construction‑site dust
Easily re‑mobilized in dry, disturbed soils

Bottom line: All five metals follow the same airborne pathway. If the soil contains them, grading will mobilize them.

Why This Matters in the Eaton and Palisades Burn Zones

Post‑fire soils in these areas are uniquely vulnerable:

Ash deposition introduced or concentrated metals
Vegetation loss removed natural soil stabilization
Surface soils are dry, fine, and easily disturbed
Rebuilding has triggered widespread grading and excavation

USDA and USGS guidance confirm that post‑fire soils are among the most dust‑prone landscapes, and that mechanical disturbance dramatically increases particulate emissions.

This means that even properties that were initially treated—showing improved soil structure and reduced contaminant mobility—remain at risk if neighboring lots undergo aggressive grading without adequate dust controls.

Why Later Soil Tests May Show Higher Metal Levels

A common question from homeowners is: “Why are my recent soil results higher than the samples collected right after the fire?” The answer is straightforward: Re‑contamination from airborne particulates is not only possible—it is expected.

This is due to:

Natural variability in regional metal concentrations
Ongoing disturbance of untreated soils nearby
Fine ash and dust being re‑entrained during grading
Wind‑driven transport of metal‑bearing particulates

This does not indicate treatment failure. It reflects ongoing disturbance in the surrounding landscape.

What Homeowners and Rebuilders Can Do

Maintain mulch or ground cover
Water exposed soils during dry, windy periods
Use HEPA vacuums indoors
Request dust controls from contractors
Re‑test soils during active construction phases

These steps help reduce re‑contamination and protect soil and human health.

Conclusion: Higher Levels Are a Predictable Outcome

Based on the scientific literature and federal guidance, it is reasonable and scientifically consistent to expect that arsenic, chromium, cobalt, copper, and lead levels in some soils may be higher now than they were immediately after the Eaton and Palisades fires.

The combination of naturally variable metals in regional soils, post‑fire vulnerability, and intensive grading and construction creates a predictable environment for airborne transport and re‑deposition of metal‑bearing dust.

Understanding this dynamic helps communities interpret soil results accurately, advocate for proper dust controls, and continue moving toward long‑term recovery with clarity rather than confusion.

Reference List:

U.S. Environmental Protection Agency (EPA). (2020). Fugitive Dust Emission Factors for Construction Activities.
U.S. Environmental Protection Agency (EPA). (2019). Exposure Factors Handbook: Particulate Matter Chapter.
U.S. Forest Service (USFS). (2016). Soil Disturbance Field Guide.
U.S. Geological Survey (USGS). (2017). Wildfire Ash: Characteristics and Environmental Impacts.
Pereira, P., et al. (2021). Wildfire ash as a source of potentially toxic elements in the environment. Environmental Research.
Wang, X., et al. (2020). Metal(loid) enrichment in wildfire ash and implications for human exposure. Science of the Total Environment.
Huggins, F., et al. (2011). Arsenic in airborne particulate matter: sources, transport, and health effects. Journal of Hazardous Materials.
U.S. Geological Survey (USGS). (2014). Arsenic in Groundwater and Soil of the United States.
Lanphear, B., et al. (1998). Lead-contaminated house dust and urban soil: a review of exposure pathways. Environmental Research.
U.S. Environmental Protection Agency (EPA). (2021). Lead in Dust: Technical Review.
Hsu, S., et al. (2016). Chromium in airborne particulate matter from soil disturbance. Atmospheric Environment.
U.S. Geological Survey (USGS). (2017). Wildfire Ash and Chromium Mobility.
International Agency for Research on Cancer (IARC). (2006). Cobalt and Cobalt Compounds.
Gonzalez, M., et al. (2019). Cobalt in PM₁₀ from soil disturbance and industrial activity. Environmental Pollution.
Amato, F., et al. (2014). Sources and transport of copper in urban airborne particulate matter. Atmospheric Chemistry and Physics.
U.S. Geological Survey (USGS). (2017). Post‑Fire Ash Chemistry and Copper Mobility.