Landfill Leachate
Adapted from materials by the Conservation & Environmental Studies Center, Burlington County, NJ
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English composition
social studies
earth science
CONCEPT: All landfills produce some leachate. Whether the leachate contaminates groundwater depends on how the landfill is built, as well as on characteristics of the site.
OBJECTIVE: To become aware that everything we do on and to the surface of the earth affects our water.
MATERIALS: Local land use maps, soil maps, land use planning documents, solid waste reports (see agencies listed below for sources of these documents).
KEYWORDS: leachate, permeability, aquifer, water table
BACKGROUND: Groundwater, the source for wells and springs, supplies drinking water to over half of the people in this country and over 90 percent of the residents in rural areas. Of the population served by public water supplies, close to 40 percent rely on groundwater. Geological formations which yield significant amounts of groundwater are called aquifers. The top of the groundwater layer is called the water table.
Although traditionally groundwater has been assumed to be free from contamination, numerous discoveries in recent years of toxic chemicals in well water have proven this assumption to be false. Groundwater contamination from chemical dumpsites tends to attract the greatest public attention, but a number of other sources including landfills, septic systems, pesticides, and underground storage tanks also can be significant sources.
Water percolating through landfills produces leachate, which may contain undesirable or toxic chemicals. Modern sanitary landfills are constructed to prevent leachate contamination of groundwater or surface waters. The bottom of the landfill is lined with impermeable layers, and the leachate is collected and treated before being released to the environment.
Factors affecting the composition of landfill leachate include:
Landfill material. Is it biodegradable or non-biodegradable? Is it soluble or insoluble? Organic or inorganic? Liquid or solid? Toxic or nontoxic?
Landfill conditions. The pH, temperature, degree of ongoing decomposition, moisture content, climate, and landfill age.
Characteristics of entering water. The pH, temperature, and amount.
Soil characteristics under the landfill. Permeability, depth and thickness of geologic strata, and mineral content.
The risk of groundwater contamination by any leachate that is not caught by collection systems is determined by the following factors:
Depth of the water table. If the water table is low (far below the ground surface), water will become partially filtered as it percolates downward through the soil. If the water table is high (close to the ground surface), contaminants can enter the groundwater directly, without filtration by soil.
Concentration of contaminants. A high concentration of contaminants in leachate will make groundwater pollution more likely.
Permeability of the geologic strata. Highly permeable geologic strata allow water to quickly percolate through, receiving little filtration along the way. Strata consisting of relatively impermeable materials such as silt and clay impede the downward percolation of water.
Type of geologic strata. Some earth materials, such as clay, are more effective at filtering out contaminants, not just because they are impermeable but because chemicals can bind to their particle's surfaces.
The toxicity of the contaminants. Leachate is produced when water filters downward through a landfill, picking up dissolved materials from the decomposing wastes. Depending on characteristics of the landfill and the wastes it contains, the leachate may be relatively harmless or extremely toxic. Generally leachate has a high biochemical oxygen demand (BOD) and high concentrations of organic carbon, nitrogen, chloride, iron, manganese, and phenols. Many other chemicals may be present, including pesticides, solvents, and heavy metals.
The direction of groundwater flow. Groundwater moves slowly and continuously through the open spaces in soil and rock. If a landfill contaminates groundwater, a plume of contamination will occur. Wells in that plume will be contaminated, but other wells, even those close to the landfill, may be unaffected if they are not in the plume.
Have the students write a report about the sanitary landfill your community uses, including how the leachate is collected and treated, and whether any groundwater or surface water contamination problems have been found. This could be a group project. There are many old town landfills, so individuals or small groups could address different landfills.
As part of the research for this project, the students should contact some of the following organizations:
- the local department of solid waste
- the New York State Dept. of Environmental Conservation
- the local planning board
- the local health department
- the local Environmental Management Council
- the regional office of the U.S. Geological Survey, for information on aquifers and groundwater
- the Soil Conservation Service, for local soil data
The students' research should address questions such as the following:
1. Where are the local landfills? (Plot them on a map.) How long will they continue to take waste?
2. Does the landfill have a leachate collection system? If so, how is the leachate treated? What other measures have been taken by the landfill operators to prevent groundwater contamination?
3. Have any water quality problems (either groundwater or surface water) been identified in the area surrounding the landfill?
4. Is there an aquifer near the landfill? If so, is it used for any community water supplies? Where are the nearest wells?
5. What is the depth of the water table below the landfill? Are there any impermeable layers between the landfill and underlying groundwater?
6. What water quality regulations apply to the landfill operations? What types of water quality testing are performed?
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