The four mounds at Fresh Kills are the result of over 50 years (1948 – 2001) of landfilling municipal solid waste. By 1997, North and South Mounds were closed and covered with a thick, impermeable cap. The remaining mounds stopped accepting waste in 2001. East Mound was capped in 2011, and West Mound is expected to be complete by 2021.
A “cap” is made up of multiple layers of soil and impermeable materials that are placed over the landfilled garbage. The cap covers the garbage and its byproducts and prevents them from migrating into the surrounding environment. It also provides a barrier between the garbage and users of the park.
The layers placed over the solid waste are constructed in phases. The essential design goals are hydraulic performance, slope stability and long term integrity or durability of the landfill and its systems. These are achieved by minimizing surface water infiltration, preventing erosion, promoting proper surface water drainage, and separating the waste layer from the environment to protect public health. The final cover also captures and prevents the emission of air–polluting gases. The final cover is made up of about 30 inches of layers, each with distinct functions.
The Soil Barrier Layer covers the garbage and makes sure the hills are stable.
The Gas Vent Layer allows the landfill gas to get captured by underground wells. The gas is sent to a plant where it’s made into gas that people can use to heat and power their homes!
The Impermeable Plastic Liner prevents water from entering the landfill and forming leachate. The liner also prevents landfill gas and its odors from escaping.
The Drainage Layer drains water off the hills to help keep them stable.
The Barrier Protection Material Layer protects the landfill cap and adds a layer of clean soil that is safe for park visitor use.
The Planting Soil Layer is where native grasses and plants grow. These plants prevent erosion and serve as a habitat for wildlife.
Rainwater falling on the top of the final cover must be carefully collected and conveyed off the mound to ensure that it does not destabilize or erode the layers of soil that make up the cover. Stormwater is collected via a network of stormwater swales, channels, and downchutes. These convey the water to detention basins at the base of the mounds. In the quiet waters of the detention basins, suspended sediments settle out, and water is released to the waterway in a controlled flow.