How to fix polluted soil after L.A. fires without scraping

Angelenos are understandably worried about the toxins deposited by smoke and ash from the Eaton and Palisades urban wildfires in January, but after L.A. Times Plants tested soil around eight burn areas, we discovered some uncomfortable truths:

— The results vary considerably. For instance, at two burned homes the lead levels were more than double the state’s limit of 80 milligrams of lead per kilogram of soil (or 80 parts per million). But other burned homes in the same communities had lead levels far lower, between 23 and 40 ppm.

— The soils around most urban areas, including Los Angeles, have been polluted for decades from chemicals used in industry, agriculture, paints, gasoline and even residential fertilizers. While the fires likely made things worse, it’s hard to know what toxins already existed in the soil.

“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,” said landscape designer Shawn Maestretti of Studio Petrichor, who lost his Altadena home. “The pollution had already been there. You’ve always been playing in it.”

— Scraping and hauling is the fastest way to remove contaminated soils, but given the scale of what needs to be done, it may not be sustainable, said soil scientists.

“That’s why people are starting to turn to methods where remediation can be done in place; where you don’t remove the soils, you just remove the toxins from the soils,” said Danielle Stevenson, an environmental toxicologist who runs the nonprofit Centre for Applied Ecological Remediation, which is offering classes and other training for people who want to try bioremediation.

So what’s a gardener to do, especially one whose home did not burn or won’t be scraped for some time by federal officials? It’s a good idea to test your soil — a survey of heavy metals, nutrients, minerals and pH levels costs less than $200 — and consider all the options for repairing it, soil scientists said. There are multiple soil testing companies throughout Southern California.

Below, we walk through our results, explain how to interpret your own and offer expert suggestions on soil repair.

Interpreting our results| If you don’t scrape |Step-by-step removal guide

Contaminant breakdown

L.A. Times Plants tested soil in burn-area gardens prior to scraping to get a snapshot of toxins in the soil and whether they existed at levels harmful to plants or people — which aren’t always the same. We used a hand trowel to collect samples in Altadena, Pacific Palisades and Malibu in late March. The sites were a combination of properties with burned or standing structures. We mixed each property’s soil — collected from four inches deep in multiple spots within a five- to 10-foot radius — in a plastic bag. This is in line with what a property owner may do to collect their own samples. Wallace Laboratories in El Segundo conducted the tests.

The results were all over the map, but at least one finding was consistent: All but one of the properties showed evidence of high pH, or increased alkalinity, likely because ash is highly alkaline.

pH levels

Southern California soils already tend toward high alkalinity, said biochemist Garn Wallace of Wallace Laboratories, which has tested soil for 35 years. Some plants, such as blueberries, azaleas and camellias, prefer acidic soils, i.e., soils with a pH below 7. Soil experts differ on the perfect number between acidity and alkalinity, but the sweet spot is generally believed to be between 6.5 and 7.2, Wallace said, and some scientists believe plants in SoCal can tolerate alkalinity as high as 7.9.

Once the pH levels get above 8, however, plant growth suffers because the roots can’t get the nutrients they need to survive.

In our tests, two yards had pH readings of 8.06 and 8.16; five others had alkalinity ranging from 7.70 to 7.98.

The best remedy for high pH, Wallace said, is to add gypsum to your soil and avoid things that can make the soil more alkaline, such as crushed concrete. Sometimes people add crushed concrete to large planters so they don’t need as much soil, or wash out concrete mixers into the soil, but those ingredients will raise pH to toxic levels for plants, he said.

Lead

We found very high lead levels at two properties, one in Malibu from a burned home’s planting strip along Pacific Coast Highway and one in the backyard of a well-landscaped 100-year-old home in Altadena. The soil from both sites had lead levels around 180 parts per million, more than double the state health standard.

Lead is one of many heavy metals that stay indefinitely in the ground, usually in the top six inches of soil, and levels accumulate over time, said mineralogist Aaron Celestian, curator of mineral sciences at the Natural History Museum of Los Angeles County. Acceptable lead levels vary by state and the federal limit recently decreased from 400 ppm to 200 ppm because of increasing evidence that lead in any amount is harmful to humans. Many plants absorb the toxin into their tissues, which makes it dangerous to eat root vegetables or leafy greens growing in lead-laced soil.

The metals around that 100-year-old house could have come from multiple sources, Wallace said. The home likely had at least one layer of lead-based paint, which was used for decades on homes until it became illegal in 1978.

But another possible source could have been from repeated applications of fertilizers containing lead and arsenic that were widely used over the last century, Wallace said. For instance, an old formulation of the fertilizer Ironite, popular for creating deep green lawns, was ultimately banned in Canada in 1997 and the subject of lawsuits in the U.S. Even so, there are still fertilizers and amendments on the market that include “micro-nutrients” such as zinc, manganese and copper that over time can accumulate in the soil, Wallace said.

Copper and zinc

Copper and zinc are highly toxic to plants, Wallace said. “The optimum levels for copper are .03 to .05 ppm, and anything over 20 is way too high. Ideally, zinc should be at 1 or 1.5 ppm [for plants]. Anything over 30 ppm is harmful to woody plants like trees and roses; over 50 ppm is toxic to herbaceous plants such as vegetables.”

High zinc levels aren’t considered toxic to humans until they reach about 23,000 ppm, Wallace said. But if you want healthy plants, avoid planters made of copper or galvanized metal because they can leach copper and zinc into the soil. He also advises avoiding fertilizers with micro-nutrients unless you have tested your soil and have evidence that it’s lacking certain metals or minerals.

Scraping alternatives

For those who have opted in, the Army Corps of Engineers is cleaning up properties where structures burned by removing 6 inches of soil in a burn area and transporting it to landfills set aside for toxic soils.

The problem is, when you remove 6 inches of soil from a yard, you’re likely removing some or all of the top soil, where plants germinate and get the nutrients they need to grow. After scraping you’ll likely be left with subsoil, layers of clay or sand, that lack the life-giving nutrients plants require.

Furthermore, while scraping 6 inches may quickly remove some toxins from a property, it’s no guarantee the soil is clean.

Maestretti and Leigh Adams of Studio Petrichor are vigilantly trying to protect the soil they’ve long been nurturing around their Altadena homes, both of which were destroyed in the Eaton fire. “I’ve been building my soil for 38 years,” Adams said. “I don’t want them taking that away.”

Both Adams and Maestretti plan to have the area where their houses stood cleaned up, but not the rest of their yards. Maestretti’s landscape of native plants is growing lushly now, despite the fire. He credits their health to the wood-chip mulch around his property and hugelkultur mounds — logs covered with wood chips and compost — that attract and retain moisture.

Adams lost her 3,000-square-foot home as well as a studio, smaller rental and garage. She plans to rebuild a much smaller home and devote the rest of her property to bioremediation to clean up toxins using amendments such as mulch, fungi, an absorbent mineral known as zeolite, which encapsulates lead, making it inaccessible to people and plants, and a variety of plants such as sunflowers and corn.

“When you scrape, you’re exposing those toxins to the air, and just moving the problem around, from one toxic site to another,” said Adams. “Rather than throw away the soil we soiled, why not regenerate it? Unless you’re planning to farm on that soil, the best bet is to put down mulching and observe.”

Landscape designer Parker Davis took this route when his nonprofit, Plant Community, helped another nonprofit, Victory Starts Now, reclaim land around two of the group residences it created for formerly incarcerated people in the Vermont-Slauson neighborhoods of South L.A.

The residents wanted to grow food, he said, but the soil had lead levels around 112 ppm, well over the state limit. There wasn’t money for scraping and replacing the soil, but he did have eager helpers, so in the fall of 2022, they sheet-mulched the properties — completely covering the ground with wet cardboard and then covering the cardboard with about 6 inches of wood chips.

In the spring of 2023, they used a technique called phytoremediation by growing plants such as sunflowers and corn, which studies have shown will pull toxins from the soil. They also planted fruit trees because studies have shown that the trees will pull toxins into their woody parts without affecting the fruit, Davis said. They crumbled up spent blocks of mushroom spawn to further aid in breaking down the toxins.

When they tested their soil again in June 2024, they were thrilled to see the lead levels had dropped to 37 ppm, Davis said, making them feel safe enough to add soil and compost to deep raised beds on the property to grow food.

They are still careful about the food they grow. They avoid root vegetables, which have the greatest chance to absorb harmful toxins from the soil, but vegetables such as tomatoes, beans and broccoli are considered safer because the toxins have to go through so many layers of protection, from the roots to the stem to the fruit.

Davis said he was never a fan of using scraping to fix contaminated soils because “it’s like making it a somebody-else’s-problem type thing.”

Step-by-step guide to toxin removal

Soil scientist Lynn Fang conducts soil testing around Los Angeles through her consulting business, SoilWise. She too has seen lots of heavy metals in local soil over the years.

“At the very least, do a metals test near your home,” she said, especially if you have a home that could have used lead paint or even the copper and zinc found in some paints today. Even if the house didn’t burn, the paint could have dripped into the soils around the house and contaminated the soil nearest the structure, she said.

If you’re worried about your soils, she said, and can wait at least a year or two for results, here’s her recipe for bioremediation and planting food on contaminated ground, which closely follows what Davis did. It’s all about creating layers:

— Start by sheet mulching with layers of wet cardboard covered thickly by wood chips, at least 4 to 6 inches.

— Sprinkle the mulch with spent oyster mushroom block substrate, to help the organic matter bind with and immobilize metals like arsenic and lead, so they don’t move higher toward the surface. The mushroom crumbles don’t have to be super thick.

— Add the mineral zeolite and/or a charcoal-like substance known as biochar to help capture and hold the heavy metals, while improving soil fertility and water retention. Celestian, who used zeolite to reduce lead on a project in Vernon in 2022-23, estimated a 20-by-20-foot yard would need about 50 pounds, “but any amount will help.” It took anywhere from a couple of months to almost a year for lead to reach target levels at the project sites. “The level of zeolite dusting is similar to how you would dust a countertop with flour if you were rolling dough,” he said, noting he purchased zeolite in 25-pound tubs from Home Depot.

— Add more wood chips on top and make sure all the ingredients are well watered.

— Grow plants known to pull toxins from the soil, such as daikon radishes, sunflowers and corn, but do not eat any of the food produced by these plants, and do not add them to your compost. Instead, pull up the plants at the end of the season and put them in the garbage. You don’t want to contaminate community compost piles with toxic plants.

— If you want to grow food for eating, try adding raised beds on top of all that mulch. Make sure the beds are at least 1 to 2 feet deep, to keep the roots out of contaminated soil. Add soil and plenty of compost, and just to be safe, avoid root vegetables until tests show the lead levels are safely lowered.

— Test your soil as often as you can afford, to chart your progress.

Stevenson, who runs the remediation nonprofit, said it’s important to also test the plants you’re using, to ensure they’re pulling toxins out of the soil. Otherwise, the mulch and other organic amendments are just diluting the soil readings, and once they decompose, the lead will still be waiting in the soil.