|MadSci Network: Earth Sciences|
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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