Abandoned mine structures in the Pike-San Isabel National Forest, Colorado. Photo credit: Colorado Geological Survey

Abandoned Mine Lands (AML)

Abandoned mine lands (AML) are those lands, waters, and surrounding watersheds contaminated or scarred by the extraction, beneficiation or processing of coal, ores, and minerals. Abandoned mine lands include areas where mining or processing activity is determined to have ceased.

Colorado’s abundant mineral wealth helped drive the economic development of the state and contributed to the development of the United States as a whole. Unfortunately, the mines that hosted these minerals have left a legacy of hazards ranging from environmental issues like water quality degradation and increased sedimentation, to physical hazards associated with the mines excavations themselves. Across the state, there are an estimated 23,000 abandoned mine sites on both public and private land.

The CGS has been involved in characterizing and understanding various aspects of abandoned mine lands in Colorado. An inventory of environmental degradation and physical hazards associated with abandoned mines on United States Forest Service property was conducted by CGS from 1991 to 1998. In all, 18,382 mine-related features were identified and documented during this program and helped to identify sites that warranted further investigation and characterization. The CGS published a number of these mine site characterization reports.


Environmental Degradation

Acid mine drainage (AMD) is water that is discharged from mining or mine-related operations, which contains high levels of dissolved metals and sulfates in conjunction with pH values less than 4.5 (acidic). AMD is formed from the reaction of various minerals (principally pyrite) with oxygen and water. AMD can degrade the water quality of streams and water supplies, often to the point of causing harmful effects to the aquatic life of the stream.

Sedimentation and Sediment Contamination

Surface runoff can carry AMD-originated silt and debris down-stream, eventually leading to stream clogging. Sedimentation results in the blockage of the stream and can cause flooding of roads and/or residences and pose a danger to the public. Sedimentation may also cause adverse impacts on fish.

Air Pollution

Air pollution can occur from piles of earth materials left at an abandoned mine. Windblown dust can have significant impacts on nearby residents and wildlife. The toxicity of the dust depends on the proximity of environmental receptors and the composition of the material. In particular, arsenic and lead are contaminants of concern.

Physical Hazards

Various physical hazards may exist at abandoned mine sites. Mine openings such as tunnels and shafts are imminent hazards because they present an opportunity for falls as well as the potential for collapse onto adjacent areas. Dangerous gases may exist in mine workings and can quickly overcome an unsuspecting visitor. Historic structures and equipment can present a hazard as these features are often unstable and prone to collapse. The rule of thumb is

Stay Out and Stay Alive!

Following are publications that examine particular abandoned mine instances from around the state as selected by the USFS and researched by the CGS: