Caustic soda
from the EPA's Mid-Atlantic Integrated Assessment:
Caustic soda is used "in remote locations where electricity is not available and flows are low. Caustic soda is gravity fed as a liquid into AMD. It dissolves, disperses, and raises pH very quickly. Because it is denser than water, it should be applied to the surface of a pond where it will then sink. Caustic soda is contained in tanks that can hold up to 8,000 gallons. A line is run from the bottom of the tank and carries the caustic soda to a ditch or pond. Valves control the rate of flow. In the winter, caustic soda can freeze, so the tank may require a heater (which will need to be replaced often because of the corrosive nature of caustic soda) or be buried underground. If it is buried it must comply with strict EPA regulations. Another option is adding potassium hydroxide to the soda, making it solid but also freeze-proof. As a solid, the soda is dispensed as flakes or beads via large drums."