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Water Treatment: Surface Water: Passive Treatment
Systems: Anoxic Limestone Drains
Anoxic limestone drains (ALDs) are used in conjunction with a settling
pond, wetland or ditch in order to improve metal removal capabilities.
This passive system uses trenches of buried limestone which raise
the pH and alkalinity of the acid mine drainage (AMD). Limestone
is a cheap and effective way to generate alkalinity, but it must
be utilized under the right conditions or its effectiveness is limited.
ALDs create the "right" conditions where limestone can generate
alkalinity for long periods of time. In an anoxic (meaning insufficient
or low oxygen) drain, limestone does not get coated with iron oxides
and continues to dissolve (unlike using limestone for aerobic chemical
treatment as discussed above). It is important to realize that ALDs
simply raise the pH and add alkalinity to acid water. Once the water
exits the drain, sufficient area must be provided for metal oxidation,
hydrolysis and precipitation to occur. The type and size of the
area, be it a ditch, settling pond, or wetland, depend on the metals
in the water. Many times, allowing flow through an ALD before entrance
into a wetland can improve metals removal since the additional alkalinity
added to the acid mine drainage from the ALD is favorable for the
chemical and biological processes that precipitate metals in wetlands.
For more information on anoxic limestone drains (ALDs), see Skousen
and Zimkiewicz (1996) and Hedin
et al. (1994). Also, check out the following websites:
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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