Limestone channels
The simplest method to passively treat acidity in AMD is to drop limestone fragments into its stream channel. Alternatively, AMD can be rerouted into a ditch lined with limestone. Either way, the limestone dissolves as water flows over it, adding alkalinity and increasing pH.
Operations & Maintenance Considerations
If the AMD contains dissolved metals, the limestone is likely to become "armored" (coated) with as the metals drop out, reducing its efficacy. Since the channel is not enclosed, armoring can be remedied by stirring up the limestone or adding fresh limestone. If the channel is on a steep slope, the abrasion of the water against limestone can prevent some armoring; in this case there may be a trade-off, because quickly-moving water is in contact with the limestone for less time.