MadSci Network: Earth Sciences

Re: Is the effluent from acid mine drainage 'hard' or 'soft' water?

Area: Earth Sciences
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Date: Thu Jul 24 08:40:09 1997
Area of science: Earth Sciences
ID: 869627535.Es
Dear Michelle:  There are many different kinds of mine drainage, but you 
specifically asked about acid mine drainage.  The word acid here refers to 
the familiar pH or hydrogen ion activity scale, in which liquids can be 
acidic, neutral, or basic (alkaline).  Hard water can get that way in 
several different ways, but the chief culprit is dissolved calcium 
carbonate (of which limestone is composed).  Water carrying a lot of 
dissolved calcium carbonate is alkaline, not acidic.  So hard water is the 
opposite of acidic, and acidic water cannot be hard.
	If acid mine drainage encounters a stream containing hard water, then 
the combined flow will have some intermediate pH, depending on the volume
of flow in each of the two components and the pH of the two components.  If 
the acid mine drainage enters a stream carrying a large quantity of hard 
water, then the result will be water that is slightly less hard. However, 
it will no longer be acidic!  The acid will have been neutralized.  In 
fact, lime (calcium carbonate) is often added to acidic water bodies to 
reduce acidity.  Conversely, if a large quantity of very acidic mine 
drainage enters a tiny stream, even if the stream carried hard water, the 
result might be acidic.  But it won't be hard.  
	If acidic mine drainage flows over a limestone terrane, then the acid
water will dissolve the limestone.  Given enough limestone and enough time, 
the acidity can be neutralized in this way.Given a subsequent excess of 
limestone to be dissolved, the water can become hard (but as above, it will 
no longer be acidic). 
	You can find articles about this subject in the (defunct) journal 
Northeastern Environmental Science, published by the Northeastern Science 
Foundation in Troy, New York.  I believe they published 1 or 2 special 
issues on acid precipitation.  Any geochemistry textbook or introductory
physical geology textbook will also provide useful information.
	Please do not hesitate to ask again if this response raises further
questions.  However, I suspect that any remaining confusion can be cleared
up by looking at a textbook as suggested above.
		David Kopaska-Merkel

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