Leaching is the process by which a substance dissolves out of a solid material and into a surrounding liquid. It happens when water, acid, or another fluid passes through or over a material, pulling soluble chemicals along with it. This single mechanism shows up everywhere: in your garden soil, in a gold mine, in a plastic container sitting in the microwave, and in the landfill down the road.
How Leaching Works
At its core, leaching relies on a few basic physical processes. A chemical trapped inside a solid material diffuses toward the surface, dissolves into the surrounding liquid, and is carried away. The speed and extent of this process depend on temperature, the acidity or alkalinity of the liquid, the surface area of the solid, and how soluble the chemical is in the first place.
Higher temperatures give molecules more energy to break free. Acidic or alkaline conditions can weaken the bonds holding chemicals in place. And smaller particles leach faster than larger ones because they expose more surface area to the liquid. These principles hold whether you’re talking about nutrients washing out of soil or synthetic chemicals migrating out of a plastic cup.
Leaching also divides into two broad categories. Abiotic leaching is driven by physical and chemical forces: heat, sunlight, wind erosion, pH changes. Sunlight, for example, generates reactive oxygen species on the surface of plastics, aging the material and freeing trapped chemicals. Bioleaching involves living organisms, typically bacteria or fungi, that release enzymes to dissolve and extract materials from a solid. Mining operations use bioleaching bacteria to pull metals from low-grade ore, and your own digestive tract uses acids and enzymes that can leach chemicals from packaging you inadvertently swallow.
Leaching in Soil and Agriculture
Farmers deal with leaching constantly. When rain or irrigation water moves through soil, it dissolves nitrogen, phosphorus, and other nutrients and carries them downward, away from plant roots and toward groundwater. Nitrate is especially vulnerable because it doesn’t bind tightly to soil particles. Studies using nitrogen isotope tracking have shown that fertilizer nitrate can leach directly through soil within days of application, long before crops have a chance to absorb it. In the weeks that follow, additional nitrogen converts to nitrate through microbial activity and continues to drain.
The environmental cost is significant. Nitrate-laden groundwater can contaminate drinking water supplies, and when leached nutrients reach rivers and lakes, they fuel algal blooms that starve aquatic life of oxygen. Phosphorus, while less mobile than nitrate, leaches readily through waterlogged or poorly drained soils, compounding the problem. Depressional areas in fields, where water pools, lose disproportionate amounts of both nitrogen and phosphorus, especially after fall and spring fertilization when the soil’s natural ability to process excess nitrogen through denitrification is weakest.
Leaching From Plastics
Plastic containers can release hormone-disrupting chemicals into food and drinks, and temperature is the key trigger. In lab simulations using distilled water in polypropylene (PP) cups, no detectable levels of endocrine-disrupting chemicals appeared at refrigerator or cool temperatures (4 to 10°C). But once the water temperature reached 40°C (about 104°F), five different chemicals showed up, including BPA and DEHP. At boiling temperature (100°C), DEHP concentrations from PP cups climbed to roughly 1,243 nanograms per liter, more than double the amount released at 40°C.
Not all plastics behave the same way. General-purpose polystyrene (GPPS) cups released no BPA at any temperature in the same study. The type of food or drink matters too. Acidic and alkaline substances accelerate the breakdown of chemical bonds in plastic, increasing migration. This is why reheating tomato sauce in a plastic container, for example, poses more leaching risk than storing plain water in the same container at room temperature.
Even something as simple as a cup of tea can be affected. Plastic teabags made from nylon or PET, when steeped at brewing temperature, release an estimated 11.6 billion microplastic particles and 3.1 billion nanoplastic particles into a single cup. These are particles, not dissolved chemicals, but they illustrate how aggressively hot liquid interacts with plastic materials.
Leaching From Cookware
Metal cookware also leaches, particularly when cooking acidic foods like tomato-based sauces or dishes with vinegar. In tests using a 4% acetic acid solution (roughly comparable to vinegar) boiled for two hours, steel cookware released the most iron: about 51 mg/L. Non-anodized aluminum came in at roughly 22 mg/L of iron, while anodized aluminum released only about 5.6 mg/L. Copper cookware leached the least iron and negligible amounts of nickel and chromium.
For most people, the iron leached from cast iron or steel pans is harmless and can even contribute to dietary intake. The concern is more about chromium and nickel. Steel cookware released about 2.9 mg/L of chromium and 3.3 mg/L of nickel in the same acid test. Anodized coatings make a real difference: anodized aluminum cookware leached less than half the chromium and nearly zero nickel compared to its non-anodized equivalent when cooking meat for an hour.
Industrial and Mining Leaching
In mining, leaching is a deliberate extraction technique. Crushed ore is piled onto lined pads, and a chemical solution is dripped through it to dissolve the target metal. For gold and silver, the leaching agent is typically a dilute sodium cyanide solution, usually between 0.05 and 0.1 percent concentration. The metal-laden liquid (called “pregnant solution”) collects at the bottom and is processed to recover the precious metals. Oxide ores take 4 to 24 hours to leach, while harder sulfide ores require 10 to 72 hours.
Cyanide is extremely toxic, so containment is heavily regulated. Heap leach pads sit on plastic or asphalt liners. Colorado guidelines require double liners with leak detection systems between them. The U.S. Bureau of Land Management requires fencing around all active cyanide areas, bermed tanks, and overflow ponds sized to handle runoff from a 100-year storm event. The solution’s pH is kept above 10 at all times, because if it drops into acidic territory, cyanide converts to hydrogen cyanide gas, which is lethal at low concentrations. Biological treatment is also used: certain bacteria, such as strains found naturally at mine sites, can break down cyanide into less harmful compounds during the cleanup phase.
Landfill Leachate
When rainwater filters through a landfill, it picks up a complex cocktail of dissolved substances from decomposing waste. This liquid, called leachate, typically contains heavy metals, halogenated organic compounds, pharmaceuticals, plasticizers, and infectious microorganisms. It can also carry endocrine disruptors and other emerging contaminants that weren’t well understood when the waste was originally buried.
Engineered landfills use liner systems and collection pipes to capture leachate before it reaches soil and groundwater. But in many parts of the world, open dumpsites operate without any liner, collection, or treatment system. The result is uncontrolled leaching of hazardous substances directly into surrounding freshwater ecosystems, contaminating both drinking water sources and aquatic habitats.
Leaching in the Kitchen
Cooking itself is a form of leaching. When you boil vegetables, water-soluble vitamins, especially vitamin C, dissolve into the cooking water and get poured down the drain. Boiling broccoli retains only about 53% of its original vitamin C. Spinach keeps roughly 40%, and potatoes about 50%. Chard loses virtually all of it.
Steaming consistently preserves more. Steamed broccoli actually tested above 100% vitamin C retention (likely because steaming softened the cell walls, making vitamin C more extractable during measurement), while steamed potatoes retained about 84% and zucchini held onto 89%. The difference comes down to contact: steaming exposes vegetables to water vapor rather than submerging them, so fewer vitamins dissolve out. If you do boil vegetables, using the cooking liquid in soups or sauces recaptures some of what leached out.