What it is
Using living organisms to clean contaminated soil and water.
Two main branches. Phytoremediation uses plants. Mycoremediation uses fungi. Bacteria do background work in both. The contaminant gets broken down, absorbed, or locked into a form that no longer leaches.
The science is young. The first commercial mycoremediation trial ran at a Battelle facility in Washington State in 1998, where Paul Stamets used oyster mushrooms (Pleurotus ostreatus) to digest diesel-soaked soil. Hydrocarbon concentrations dropped from 20,000 ppm to under 200 ppm in eight weeks.
Why it works
Fungi eat carbon chains. White-rot fungi produce extracellular enzymes (peroxidases, laccases) that evolved to break down lignin, the toughest polymer in wood. The same enzymes pull apart petroleum, PAHs, dioxins, and many pesticides. The fungus does not know the difference. A carbon-hydrogen bond is a carbon-hydrogen bond.
Plants pull metals up. Hyperaccumulator species concentrate heavy metals into above-ground tissue. Indian mustard (Brassica juncea) lifts lead. Sunflower (Helianthus annuus) takes up uranium, caesium, and arsenic. Alpine pennycress (Noccaea caerulescens) accumulates zinc and cadmium at concentrations that would kill any normal plant. You harvest the biomass and dispose of it as concentrated waste, or smelt it for the metal.
Roots host the cleanup crew. The rhizosphere (the 1 to 2 mm of soil hugging each root) carries 10 to 100 times the microbial density of bulk soil. Those microbes degrade contaminants the plant cannot touch directly. Willow, poplar, and reed canary grass all run aggressive rhizosphere chemistry and are first picks for fuel spills and solvent plumes.
Match the tool to the contaminant
Test the soil first. Without knowing what is in it, you are guessing.
Hydrocarbons (diesel, oil, PAHs). Oyster mushrooms on a straw and wood-chip substrate. Inoculate at 5 to 10 percent by volume, mix into the top 20 cm, keep moist. Willow and alder buffer the edges with deep roots.
Heavy metals (lead, cadmium, zinc, arsenic). Indian mustard, sunflower, alpine pennycress, vetiver grass. Lead is the slowest because it binds tightly to soil. Three to five seasons of cropping and removal can drop topsoil lead from 1,500 ppm to under 400 ppm.
Pesticides and PCBs. White-rot fungi (Phanerochaete chrysosporium, Trametes versicolor). Trials at the Olympia bioremediation site cut atrazine residues by 90 percent in 12 weeks.
Radionuclides (caesium, strontium). Sunflower and Indian mustard. Used at Chernobyl in 1990s trials, lifted up to 95 percent of bioavailable caesium-137 in the rhizosphere zone.
Salinity. Salt-tolerant phytoremediators. Atriplex, Sesbania, vetiver. Mangroves at coastal sites. See mangroves and wetland restoration.
Run it
Take a representative soil sample to a certified lab. You need contaminant identity, concentration, pH, and texture. Without numbers you cannot dose or pick species.
Set a baseline before you plant. Soil tests, photos, and groundwater samples if relevant. Without a baseline you cannot prove the cleanup worked.
Plant or inoculate densely. For phytoremediation, 8 to 12 plants per square metre of the target species. For mycoremediation, blend spawn through the contaminated layer and cover with 10 cm of fresh wood chips to hold moisture.
Harvest carefully. Hyperaccumulator biomass is hazardous waste, not compost. It needs landfill or specialised incineration. The same goes for fruiting bodies on metal-contaminated soil. Do not eat oysters from a remediation site.
Re-test annually. Most sites need two to five seasons. Heavy metals at high concentrations can take a decade. Track the trend, not the single number.
When it goes wrong
No reduction after a season. Wrong species, wrong substrate, or contaminant not bioavailable. Re-sample to confirm starting concentration. Check soil pH, since metal uptake collapses outside species-specific pH ranges.
Fungi dry out. White-rot needs 60 to 70 percent moisture. In summer, water weekly. Cover with cardboard or a tarp during heat waves.
Contaminant moves off-site. Heavy rainfall can leach metals into groundwater before plants take them up. Berm the perimeter. Use swales to slow runoff. Test downstream water.
Edible crops planted too soon. Treated ground may still hold contaminants the lab missed. Run two clean seasons of non-food test crops before any food production. See soil testing.