Anoxic and Oxic Limestone Drains

Anoxic limestone drains (ALD)

ALD's are ditches filled with limestone gravel. As minewater flows through them, the limestone dissolves, adding alkalinity and increasing the pH. The water needs to have very little oxygen dissolved in it or else iron hydroxides can form, clogging the drain and causing it to fail.

Regardless of the oxygen content, aluminum can also cause ALD's to fail by forming aluminum hydroxides.

Oxic limestone drains (OLD)
OLD's are similar to ALD's, but they are more experimental. Iron or aluminum hydroxides form within them, and hopefully these solids are periodically flushed out by temporarily increasing the pressure or head, then releasing water from the drain rapidly. Dr. Chuck Cravotta of the US Geological Survey has built a successful small-scale OLD in the Anthracite Region.

Schematic cross-sections of anoxic and oxic limestone drains
These components are installed upstream of oxidation ponds or wetlands for waters which have net acidity (acidity > alkalkinity).

Anoxic limestone drain under construction on a tributary of Swatara Creek.
The anoxic (very low dissolved oxygen) mine discharge emanates from the far end of the ditch. The limestone gravel to be added is on the left side of the picture.

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