Abandoned Mine Lands (AML)

Intro

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.

Hazards

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!

Case Studies

During the fall of 1993, the 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 (XRF) 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.

Additional mine or mining district reports: