Landfill hydrology refers to the movement, distribution and quality of fluid (leachate) within wastes in a landfill.
Flow can occur both within the saturated zone (below a water table) and also within the unsaturated zone which in most landfills is likely to represent the vast majority of the waste in the site. Some moisture in landfills is transmitted within the gas phase (saturated landfill gas) and there is an important interrelationship between the gas and liquid phases at a number of levels.
Understanding the nature of liquid flow through landfill is important for a number of practical reasons particularly in relation to the management of leachate within a site. Leachate will normally contain compounds that are considered potential contaminants if released untreated into the natural environment. In modern engineered sanitary landfills, leachate levels will usually be controlled by pumping from leachate collection systems and treated to relevant standards prior to discharge to surface waters.
In historic sites where the drainage system may be absent altogether or in sites where it is performing inadequately (perhaps through poor design or clogging) other leachate control systems will have to be used. Leachate control systems very often rely on vertical wells, but trenches and horizontal wells also play their part.
Leachate control and management can represent a significant cost to operators, and in regions of the world which experience high seasonal rainfall methods to prevent leachate generation, especially within the operational phase, are important.
The presence and movement of moisture in landfill also affects the biodegradation of the organic fraction of the waste, leading to the production of landfill gas and enhanced landfill settlement. Leachate recirculation is often used to enhance landfill gas generation, and an understanding of landfill hydrology is important for this purpose. In particular issues such as landfill heterogeneity (including layering and daily cover) and anisotropy will exert major controls over the even distribution of moisture.