As urban wildfires like Eaton and Palisades have devastated sections of Los Angeles, many residents are grappling with fears about the toxins left behind in smoke and ash. Recent soil tests conducted by the L.A. Times Plants in eight burned areas revealed a concerning range of contamination levels, prompting discussions on effects long after the flames were extinguished.
Results from the tests exhibited significant variance across different properties. For example, two locations showed lead contamination surpassing the state limit of 80 milligrams per kilogram of soil—more than twice the acceptable threshold—while other burn locations within the same neighborhoods exhibited lower lead levels between 23 and 40 parts per million (ppm).
Moreover, many urban areas, including Los Angeles, have long histories of soil pollution from various industrial, agricultural, and household products. While the fires likely exacerbated contamination, identifying how many toxins were already present in the soil poses a challenge. Landscape designer Shawn Maestretti of Studio Petrichor and a wildfire survivor expressed, “We’ve been taking soil tests for years before the fires, and more often than not, we found high levels of [toxins like] lead, aluminum, and arsenic. The pollution had already been there. You’ve always been playing in it.”
The approach to soil remediation in the aftermath of wildfires is now the subject of expert discussion. Soil scientists indicate that while scraping and hauling may be one method to remove toxic soils, its scalability may not be sustainable. Danielle Stevenson, an environmental toxicologist leading the nonprofit Centre for Applied Ecological Remediation, mentioned that there’s a growing focus on in-situ remediation methods—treating pollution on-site without removing the soil altogether. Classes and training sessions on bioremediation for concerned residents are now being offered, empowering them to explore alternatives.
For gardeners, particularly those whose homes remain, the suggestion is to invest in soil testing. Understanding soil health can be done affordably, with testing surveys costing less than $200 for heavy metals, nutrients, minerals, and pH levels. Numerous soil testing companies are available throughout Southern California.
The L.A. Times Plants soil tests provided a crucial snapshot of the current state of urban soils following the wildfires. Testing was conducted in Altadena, Pacific Palisades, and Malibu, with samples collected at various depths to provide a holistic view of contamination. The testing laboratory, Wallace Laboratories in El Segundo, conducted these analyses.
Initially, the results were inconsistent, but a significant trend became apparent: high pH levels, indicating increased soil alkalinity, common due to ash’s alkaline nature. Southern California’s soils are already predisposed to high alkalinity, according to Garn Wallace, a biochemist at Wallace Laboratories. While some plants thrive in acidic conditions (sub-7 pH), sensitivity to alkalinity can prove detrimental to plant growth once pH exceeds 8, restricting essential nutrient uptake.
In the analyzed tests, two properties registered pH levels of 8.06 and 8.16. Five others fell within a range of 7.70 to 7.98. To combat high pH levels, adding gypsum is recommended, while caution is advised against materials that can raise alkalinity even further, such as crushed concrete.
Lead contamination was notably high at two sampling sites, with levels around 180 ppm—more than double state health standards. The first site was a burned home along the Pacific Coast Highway, and the second belonged to an older home in Altadena.
Heavy metals like lead are persistent in soil and generally found concentrated within the top six inches. Acceptable levels of lead vary by state, with a recent federal reduction of the limit from 400 ppm to 200 ppm, largely due to findings that even trace amounts are harmful. Plants can absorb lead into their tissues, presenting a danger in consuming root vegetables or leafy greens grown in contaminated soils.
Potential sources for the lead and other heavy metals found in urban gardens could include long-past uses of lead-based paints, which were banned in 1978, and fertilizers that historically contained arsenic and lead. Some fertilizers, such as certain forms of Ironite, have faced scrutiny and bans in other countries.
In addition to lead, concern extends to the presence of copper and zinc in the soil, as both can be detrimental to plant health. Wallace advises a careful threshold for copper levels (between 0.03 to 0.05 ppm as ideal, with anything over 20 ppm being toxic), whereas zinc levels should remain at around 1 to 1.5 ppm, as values much higher may harm various plant types.
While elevated zinc levels pose little risk to human health until levels reach around 23,000 ppm, those concerned with healthy plant growth should avoid planters made with copper or galvanized metal, as they can leach contaminants into the soil. Additionally, using fertilizers with unnecessary “micro-nutrients” should be carefully considered unless evidence from soil testing indicates a deficiency.
In response to the ash fallout, there are no simple solutions. The Army Corps of Engineers has been tasked with cleaning properties within burned areas by removing 6 inches of soil, dumping it in designated landfills. Yet this method possesses drawbacks, as removing this upper layer also extracts rich topsoil critical for plant development—often leaving behind nutrient-deprived subsoil.
Maestretti and fellow landscape designer Leigh Adams are determined to protect the soil they spent decades improving around their Altadena homes, which were decimated by fire. Describing their investments in soil health, Adams noted, “I’ve been building my soil for 38 years. I don’t want them taking that away.” Adams plans to rebuild her home but aims to dedicate much of her property to bioremediation to address toxins using methods such as adding mulch and fungi, while employing absorbent minerals like zeolite to contain lead.
Their approaches highlight a shift from traditional soil remediation to innovative, regenerative tactics. Amid their recovery efforts, both designers have noticed that their native plant landscapes, despite the fire, are thriving thanks to techniques like hugelkultur—permaculture principles that involve building raised beds with wood and compost to enhance moisture retention.
Parker Davis, a landscape designer with Plant Community, shared his experience reclaiming land around residences for formerly incarcerated individuals. Faced with soil lead levels over the state limit, the team could not afford to scrape and replace the contaminated soil; thus, they resorted to a method called sheet mulching.
By covering the soil thoroughly with wet cardboard and adding around six inches of wood chips, followed by planting sunflowers and corn—known for their ability to extract toxins—Davis’s team successfully lowered lead levels from 112 ppm to 37 ppm by mid-2024. “It’s like making it a somebody-else’s-problem type thing,” Davis remarked regarding the scraping method of soil cleanup, advocating for ongoing soil health rather than quick fixes.
For gardeners wanting to effectively manage soil toxicity, Lynn Fang, a soil scientist, suggests that interested residents conduct metals testing, particularly for areas adjacent to homes previously coated in lead paint. Her guide for effective bioremediation spans multiple layers with careful consideration to build on existing soil health strategies. Here’s a concise step-by-step plan:
Begin with a layer of sheet mulching, using wet cardboard piled with 4 to 6 inches of wood chips.
Sprinkle the mulch with spent oyster mushroom block substrate, which aids in binding and immobilizing metals like arsenic and lead.
Incorporate zeolite and potentially biochar to enhance metal retention while boosting soil fertility.
Cover everything with additional wood chips and ensure thorough watering throughout the process.
Grow phytoremediation plants like daikon radishes, sunflowers, and corn, but avoid consuming the produce and discard contaminated plant materials appropriately afterward.
For edible crops, construct raised beds over the top of the layered mulch, ensuring they are deep enough (at least 1 to 2 feet) to keep roots out of contaminated soil while adding good quality soil and compost.
Lastly, test the soil periodically to monitor changes in toxicity levels and track progress toward safer gardening conditions. As Angelenos grapple with the legacy of wildfires, through innovative solutions and community engagement, there remains hope for cultivating a cleaner, healthier urban landscape.
image source from：https://www.latimes.com/lifestyle/story/2025-04-24/how-to-fix-contaminated-soil-los-angeles-fires
