Landfill Capping Safety
Jul 24, 2012

Landfill Capping Safety

I’ve heard that some cleanups of landfills and other toxic sites just put a cap over the contamination! Is this safe? Shouldn’t all the hazardous waste be removed instead?

In almost all cases, properly designed and monitored caps on landfills can allow the site to be reused safely. Since 1988, landfills in the US have been regulated under the Resource Conservation and Recovery Act (RCRA). This legislation requires new landfills to be designed with protective liners under the layers of trash, to have careful plans for their operation, and to set aside funding to monitor and maintain the landfill for at least 30 years after its closure. Older landfills, which were not required to have these same protections, must install monitoring wells and protective underground layers after the fact.
To begin, it’s important to understand what exactly a landfill cap is. While different methods of construction exist, all landfill caps contain a solid layer that is placed over the top of a landfill to prevent water from getting into the landfill. This layer may be thick, water-resistant clay, a thin layer of impermeable plastic, or both. Keeping water out of the landfill is important because when water trickles through the waste, it can pick up hazardous chemicals. This water is called “leachate.” If leachate escapes from the landfill, it can contaminate groundwater and spread potentially harmful chemicals through the soil.
Of course, not even the best protective layers can keep out all water, and some water forms inside the landfill itself when trash decomposes. As a result, it’s important to have some way to remove the leachate that does get through. When a landfill is closed, a geologist specializing in landfill design will evaluate where the leachate will flow when leaving the landfill. Depending on how much leachate is expected to leave the landfill, different systems to capture it can be used. The most advanced actively pump leachate out of the ground for treatment, while passive systems add a layer of rock that directs the leachate out of the ground, allowing it to drain naturally. In older landfills which have no underground liner in place to prevent leachate from seeping out, slurry walls or other barriers may be buried along the sides of the landfill.
Caps have other features to manage potential hazards. When the waste in a landfill decomposes, it emits a mix of chemicals, primarily carbon dioxide and methane (natural gas). While neither is directly harmful to health, methane can be explosive in high concentrations, something anyone who owns a gas stove knows. Other chemicals which may be emitted in small concentrations by decomposing waste, such as VOCs or benzene, can directly harm people who come into contact with them. Although these emissions are usually quite small, monitoring their release is very important to protect the health of people who will use the cleaned site. To prevent this, a layer of rock that is easy for gases to travel through directs the methane and hazardous gases to collection stations and monitoring wells. Here, it is either vented off, or collected to be burned as fuel.
Despite the concerns regarding landfill gases and leachate, even old, unregulated landfills have been safely redeveloped. One example is Fresh Kills, NYC’s landfill on Staten Island, which operated for 50 years as the largest landfill in the United States. Today, it is being redeveloped into parks, sports fields, wildlife protection, and hike and bike trails. Meadows and shrubs are usually planted in a layer of topsoil as the last layer of the cap, preventing erosion and protecting the deeper layers from cracking. Open spaces are the typical use for redeveloped landfills because as the waste breaks down over time, there is a risk that the ground will sink down and shift slightly. However, modern compacting technology can even allow buildings to be safely built on top of old landfills – for instance, see the Boulevards at South Bay, which aims to shopping centers, residences, restaurants, and more on a former Los Angeles municipal landfill.
The key to ensuring safety at any redeveloped landfill is good engineering that caps the site safely and adequate monitoring for contaminant release. All caps should be designed with the details of the specific site in mind – what sort of waste it contains, the climate of the area, and what future uses will be. If careful planning and appropriate precautions are taken, what was once a dangerous eyesore can become a much-needed community space.
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