A major historical problem with waste disposal, whether during the bad old days of primitive dumps or even in early landfill designs, was the issue of leachate. Specifically, leachate refers to water that has percolated through a solid and leached out some of the elements. Landfill leachate, whether it originates as precipitation or is produced through natural organic decomposition processes, passes through the waste heap and picks up a myriad of contaminants, ranging from metals and heavy metals to toxic organic compounds, pesticides, acids, salts, and more. If the leachate is not contained and soaks directly into the soil, it may cause severe damage to local ecosystems and contaminate drinking water sources such as nearby wells, rivers and lakes. Leachate also causes unpleasant odors, and can promote air pollution, explosions, fires, and dangerous land settling. Modern sanitary landfills use sophisticated systems to contain and collect leachate, which include limiting stormwater incursions, draining and collecting leachate within the system, and monitoring groundwater for leaks.
Left to itself, fresh rainwater falls onto masses of garbage tucked in a landfill and joins a stream of various types of liquids already contained in the waste itself or naturally produced by decomposing organic waste. Each type of liquid mixes and picks up contaminants as they filter through layers of waste in route to the bottom layer. This mix of newly contaminated rainwater, chemicals, and decomposition liquid make up highly hazardous leachate.
Subtitle D regulations require that owners/operators of municipal solid waste landfills must construct and maintain:
- A control system for run-on (water running onto the landfill site from surrounding areas) to prevent flow onto the active faces of a landfill. It must be adequate to manage peak discharge during a 25-year storm event.
- A control system for run-off (water that lands directly onto the landfill surface) that can handle, at minimum, the full volume of water resulting from a 24-hour 25-year storm.
Designing a system to meet these standards requires a thorough knowledge of the hydrologic cycle in nature, since precipitation in landfills may go four different ways. It may become surface runoff, evaporate, infiltrate into the cover material and be extracted by plant transpiration, or infiltrate into the refuse, where it may eventually become leachate.
The first priority in managing leachate, then, is removing surface water from the landfill. The longer water remains on the surface of the landfill, the more it will infiltrate and become part of the leachate stream. In daily landfill operations, cell covers and caps are applied to prevent water from entering the cells full of garbage and instead divert it to perforated pipes around the perimeter. Those pipes drain to stormwater swales or ditches located around the landfill. In turn, the drainage ditches carry stormwater to collection/settling ponds. Time spent in these ponds permits any suspended particles of soil or other matter to settle out, at which point the water is tested for contamination by leachate. If none is found, the stormwater is allowed to flow off-site and back into the environment.
As stormwater flows off and around the landfill surface, it’s important to ensure that the precipitation doesn’t infiltrate into the landfill from that point. It’s a good idea to construct the swales with as much slope as possible, even using down chutes where feasible, aiming for the fastest travel into the detention basins. It’s a good idea to line the swales with a smooth geosynthetic liner to mitigate erosion and encourage the water to travel swiftly off the landfill surface and to the detention ponds. If stormwater tends to pond in any areas, those areas should be re-graded as soon as possible to eliminate the issue.
