Opencast mining, also known as surface mining or strip mining, has disadvantages that include disturbance of the environment and local ecosystems, exposure of radioactive elements and contamination of freshwater sources. The use of heavy machinery and blasting techniques also create environmental pollution.
The presence of pits and ponds around mines contributes to groundwater contamination. Rock blasting creates fractures in the rock, allowing mine drainage leakage from abandoned mines into groundwater aquifers. Contaminated groundwater typically has a higher pH and total hardness, and contains dissolved sulfate. Radioactive elements produced from rock slurries can also pollute groundwater by leaking into fractured bedrock. Surface mining also contributes to air pollution through the release of toxic compounds. These particulates react with water vapor to produce acid rain.
Surface coal mining, for example, has contaminated more than 5,000 miles of streams in the eastern United States, impacting aquatic ecosystems and public drinking water supplies. Although surface mining reduces production costs and poses fewer health dangers to miners compared to underground mining, surface mining also requires blasting into rock and changing the landscape, increasing the risk of mountainside erosion, mudslides and compaction of the soil. Some states, such as Oklahoma, require the reclamation of old surface mining locations in order to restore their usability.