The reason for that is because Firefox and Chrome recognize that word as wrong. Although, the built-in dictionaries for RadSpell find that word as correctly spelled. Thus, the browser highlights the word as incorrect (the curly red underlining), but the AjaxSpellchecker of RadEditor does not (it is not highlighted in yellow).
As this is a correctly spelled word, the problem is with the spelling dictionary used by the browser and does not relate to the RadSpell dictionaries. So, to fix that, use the native, browser context menu (when the curly red underlining appears) and choose add to dictionary. This will add the word to the dictionary of the browser. Further information about browser's spelling feature is available in this article for Firefox—https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker
, and this one for Chrome—https://support.google.com/chrome/answer/95604?hl=en
If the case is different, please, provide more details what is your exact concern with the word leachate.
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