The effects on the hydrology in the region where mountaintop mining takes place is not well understood—how does this mining activity affect the movement and storage of water?

The EPA, in a 2011 report, explains that stream ecosystems are affected by mountaintop mining in five principal ways:

• Springs are permanently lost when buried under valley fill.
• Elevated levels of chemicals are found downstream.
• Degraded water quality in streams can be lethal to organisms.
• Selenium levels are elevated in the water, toxic to fish and birds.
• Fish communities are degraded.

One way hydrology changes is through the reclamation process. Heavy equipment compacts the soil, which inhibits water infiltration and natural succession. That is, the compacted soil doesn’t soak in water the same way as it did before when it was natural forest. Native trees do not grow well in this compacted soil, where rainfall runs off faster. Even when grasses are planted immediately during reclamation, runoff is increased in these areas.

Increased runoff can lead to more frequent downstream flooding. Some fill areas that have large volumes of crushed rock can actually increase watershed storage. But how and to what extent these areas control runoff are not clear.

Furthermore, groundwater samples from mined areas after reclamation have been found to contain more mine-derived chemicals than water from unmined areas. These chemicals (lead, aluminum, chromium, manganese, and selenium) would otherwise remain sealed up in the coal and rock. Water flowing through the valley fills picks up these chemicals and flows into the streams. Researchers have linked declines in stream biodiversity to an increase in these contaminants, affecting the freshwater species found there.