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Douglas Harper's Etymology Dictionary
leachate

1920, from leach + -ate (1).

Wiktionary
leachate

n. The liquid produced when water percolates through any permeable material.

Wikipedia
Leachate
This article is about landfill leachate; for use in mining see Heap leaching or In situ leach.

A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed.

Leachate is a widely used term in the environmental sciences where it has the specific meaning of a liquid that has dissolved or entrained environmentally harmful substances that may then enter the environment. It is most commonly used in the context of land-filling of putrescible or industrial waste.

In the narrow environmental context leachate is therefore any liquid material that drains from land or stockpiled material and contains significantly elevated concentrations of undesirable material derived from the material that it has passed through.