Abandoned Mine Lands

Abandoned mine lands 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 our state and contributed to the development of the nation 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 themselves. Across the state, there are an estimated 23,000 abandoned mine sites on both public and private land.

image: acid mine adit water

Water draining from an adit (mine tunnel) that is discolored due to high amounts of metals and other chemical constituents.

The Colorado Geological Survey 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 during this program and helped to identify sites that warranted further investigation and characterization. Several of these mine site characterization reports are available from the CGS bookstore.


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 can see on the photo to the right exiting a mine adit. 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 can exist at abandoned mine sites. Mine openings such as tunnels and shafts can be 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. Abandoned mine sites present a variety of hazards to a site visitor and the rule of thumb is Stay Out and Stay Alive!


mine waste rock dump

The light colored rock pile in the center of the picture is a waste rock dump. These are rocks from the mine that did not contain enough ore to be economically valuable. The waste rock often contain minerals such as pyrite that, when exposed to air and water, oxidize to form acidic water.

image: The Pennsylvania Mine with numerous waste rock dumps surrounding a wooden structure

The Pennsylvania Mine with numerous waste rock dumps surrounding a wooden structure.


U.S. Forest Service Abandoned Mine Lands Inventory Map

Case Histories

During the fall of 1993, the Colorado Geological Survey (CGS) inventoried mines in the Cinnamon Gulch area of the Dillon Ranger District, White River National Forest, near the town of Montezuma. This project was part of an eight-year, Statewide inventory of abandoned mines on USFS-administered lands in Colorado. In September 2000, the Forest Service requested a watershed characterization study for Cinnamon Gulch, and more detailed studies on five mines in the area.

Field work for this study included a visit to each site to see if major changes had occurred since the inventory work in 1993. Although water samples were collected at some of the sites in 1993, additional samples and water tests were collected in 2001. In-stream samples were collected from some of the receiving streams in efforts to bracket selected mines or groups of mines and better quantify impacts to the watersheds. In addition, waste-rock piles on some of the mines were sampled on a grid pattern to assess their potential environmental effects. Waste-rock samples were analyzed for gold, silver, mercury, paste pH, acid neutralization potential, and potential acidity. Samples are also analyzed using X-Ray fluorescence to determine a suite of major, minor, and trace elements. A total of 39 water samples were collected from the Cinnamon Gulch watershed over two sampling events in 2001. During the high-flow sampling event in July, 19 water samples were collected, and during the low-flow event in October, 20 water samples were collected.