Got Arsenic?

Arsenic in Soil After a Wildfire

Wildfires can leave behind ash and debris that temporarily change the chemistry of soil. One of the most common questions after a fire is: “My soil shows arsenic. Should I be worried?” The short answer is: Not usually. Understanding two numbers used by the California Department of Toxic Substances Control (DTSC) helps explain why.

1. DTSC Screening Level

What it is: A very conservative, risk‑based number used as an early warning flag to decide whether soil should be looked at more closely.

Arsenic screening level: ~0.11 mg/kg. This number is far below what naturally occurs in most California soils. 301 Organics uses the most conservative screening level published by the DTSC at the 0.032 mg/kg level (Human Health Risk Assessment (HHRA) Note Number 3, DTSC-modified Screening Levels (DTSC-SLs), April 2025) for added margin of safety, especially when exposure to children or pets are a concern. It is a cancer endpoint with the lowest screening level found.

What it means for homeowners: Exceeding the screening level is normal and expected. It does not mean your soil is unsafe. It simply means the soil contains more arsenic than the theoretical model assumes and may need additional context.

2. DTSC Background (“Safety”) Level

What it is: A number based on real soil samples collected across Southern California. It reflects what is naturally present in local soils—even in areas untouched by fire.

Typical Southern California background arsenic: 2–12 mg/kg

DTSC does not require cleanup below natural background levels.

What it means for homeowners: If your soil results fall within this natural range, they are considered normal and not a health concern, even though they are much higher than the screening level.

Why These Numbers Don’t Match

The screening level is a theoretical “better safe than sorry” trigger. The background level reflects real‑world soil conditions.

This is why almost every property in Los Angeles County “exceeds” the screening level but is still considered safe.

What This Means for Your Property

• Exceeding the screening level is common and not a danger signal.

• Background levels are what matter for real‑world decisions about safety and cleanup.

• Fire ash can temporarily raise metals, but most levels return to normal with rain, compost, and soil rebuilding.

• After debris removal and soil stabilization, gardening and rebuilding are safe.

• Adding compost, mulch, and plants helps bind metals and restore healthy soil.

Key Takeaway

If your arsenic results fall within the natural Southern California range, DTSC considers your soil safe and not requiring cleanup, even if the numbers look high compared to the screening level. The screening level is a trigger for management and treatment, not necessarily a danger line.