Leachates in landfills

DEFINITION: Liquids produced when fluids dissolve the constituent elements in landfill wastes

Some of the potentially toxic materials found in landfills include mercury, lead, cadmium, chromium, arsenic, many organic compounds, and even pathogenic organisms. Because leachates may contain high concentrations of such materials, care is taken in modern landfills to prevent leachates from escaping and contaminating groundwater supplies.

Leachate fluids can vary a great deal depending on the composition of material in the landfill, the age of the leachate, and the speed of the addition and removal of water in the landfill. The flow of water depends on the amount of rainfall, the permeability of the garbage and surrounding rocks—how readily water flows through the material—the kind of liner placed at the base of the waste material, and whether a cover has been placed over the landfill to prevent water inflow.

Composition

Underground water moves slowly and may take years to flow through a landfill. The water thus has a lot of time to dissolve materials such as motor oil, paint, batteries, chlorinated hydrocarbons, pesticides, and other industrial wastes. Many of these materials have long been banned from landfills in the United States, but tens of thousands of old landfills still contain such materials. In fact, many old waste disposal sites were simply depressions in the ground in which almost anything was placed. The US military was one of the worst offenders in creating poor landfills in the past, as military facilities generate more than 500,000 tons (453,592 metric tons) of waste per year.

The composition of leachates varies from landfill to landfill, and even within a single landfill. The upper concentrations of certain constituents in some leachates can greatly exceed the maximum concentration of those constituents allowed in drinking water. For example, lead concentrations have been found to exceed 5 milligrams per liter in some leachates; drinking water in the United States is required to have less than 0.015 milligrams of lead per liter (0.000002 ounces per gallon). Zinc concentrations have been found to exceed 1,300 milligrams per liter (0.17 ounces per gallon) in some leachates; drinking water is required to have less than 5 milligrams of zinc per liter (0.00067 ounces per gallon).

The concentrations of constituents in the leachate in a given landfill may also change over time. For example, large amounts of carbon dioxide are initially produced in the landfill by organisms reacting with biodegradable waste. The carbon dioxide reacts with water to produce acidic waters, so that inorganic materials can more readily dissolve. The initial acidic leachate gradually becomes less acidic over time as it dissolves the inorganic materials. The total amount of dissolved materials in leachates has been as high as 40,000 milligrams per liter (5.34 ounces per gallon)—most drinking water has less than 500 milligrams per liter (0.067 ounces per gallon).

Several toxic and carcinogenic organic constituents have also been found in landfills, such as dioxin compounds. Dioxins are a group of chlorinated hydrocarbons that are not very soluble in water, but they concentrate in the fatty tissues of animals. They are chemically stable, so they concentrate upward in the food chain. Dioxins are toxic and can cause problems with the immune and reproductive systems; they may also cause some cancers. Other toxic organic constituents that may be present in landfill leachate include certain alcohols, chlorobenzene, acetone, methylene chloride, and toluene.

Management

The goal of leachate management is to prevent leachate from escaping landfills and polluting groundwater supplies. Methods have been developed to achieve this goal and to collect and treat leachates to remove dangerous impurities.

Modern landfills are lined with impermeable and durable materials that prevent leachates from moving into the groundwater. Some of these liners consist of clay minerals that are compacted; others are geomembranes, such as polyethylene sheets. The clay minerals used to line landfills are fine-grained minerals formed naturally during the weathering of coarser-grained minerals. Some of the clay minerals used, such as smectites, may also adsorb many of the metal ions from leachate, so the metals do not move with the fluid.

Polyethylene liners are useful in landfills because they are durable and easy to install, in addition to being relatively impermeable to most fluids. Such liners can, however, be torn or otherwise damaged by some materials placed into landfills. The most effective system for lining landfills to prevent the movement of leachates out of the landfills and into groundwater combines the two kinds of liners, alternating layers of compacted clay minerals with polyethylene sheets.

Landfills are ideally placed in impermeable sedimentary rocks, such as mudrocks—which also contain abundant clay minerals—that are above the water table rather than in permeable materials such as sand or sandstone. A dry climate is also desirable, but this may not be possible since transportation costs to move material to a landfill may be too high if the landfill is too far from the communities that use it.

Modern landfills also install systems to collect leachate and move it to sites where the worst impurities and contaminants can be removed through chemical and biological treatment. Sometimes, leachate may be recycled through a landfill again so that bacteria can further reduce certain impurities. The leachate may also be placed into an oxygen-rich lagoon so that other bacteria may oxidize some of the dissolved materials contained in the leachate. Chemical treatment of leachate can further reduce some kinds of undesirable materials. For example, the precipitation of calcium or sodium hydroxide solids can remove many dissolved metals.

Bibliography

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