Summary #
Understanding how coal was mined helps identify where AMD contamination originates. The three main historical mining methods — surface mining, underground mining, and auger mining — each expose sulfur-containing rocks to water in different ways, creating different AMD formation patterns. Knowing the mining history of a site is a practical first step in any AMD assessment.
Mining Methods and AMD #
Knowing how coal was mined can provide important clues about where to look for contamination sources. All past mining approaches sought to remove coal from beneath sometimes thick layers of soil and rock called overburden. Many coal-mining practices exposed sulfur-containing rocks and minerals to weather and, in some cases, to groundwater flow.
There are two broad approaches to mining: removing the overburden to reach the coal (surface or strip mining), or extracting the coal while leaving overlying material in place (underground or auger mining). Underground mining has accounted for approximately 70 percent of Appalachian mining production historically. Surface mining, underground mining, and augering were all common throughout the Appalachian region.
Surface Mining #
Surface mining was practiced where coal beds were at or near the surface of eroded hillsides. Operators removed overburden using excavating equipment to expose coal outcrops. The amount of material disturbed depended on the thickness and quality of the coal seam, but typically extended well beyond the near-surface zone. Overburden fill left in drainage zones can become a source of acid formation if sulfur-containing materials (pyrites) are present and exposed to water.
Underground Mining #
Underground mining — also called deep mining — involves tunneling through relatively soft coal beds to reach the seam. Vertical, horizontal, and sloping mine works were constructed to access coal and manage air circulation. Many old, abandoned underground mines are honeycombed with miles of tunnels capable of collecting large amounts of water, known as the mine pool. As this water passes through overburden and tunnels, it can react with pyritic acid-forming materials and generate strong concentrations of acid and dissolved heavy metals.
Auger Mining #
Auger mining involves drilling coal seams horizontally from a vertical highwall using a large auger. This practice often accompanied contour mining. Augers could penetrate more than 200 feet horizontally into a seam, removing as much as 60 percent of the coal. Waste material from the drilling was sometimes placed back into auger holes to prevent surface subsidence. Auger holes can be a source of contaminated drainage if rain percolates through overlying layers and enters the holes.
Related Pages #
- What is AMD?
- How AMD is Formed
- Background Search (Watershed Tools — Watershed Assessment)
- Identify Discharges (Watershed Tools — Watershed Assessment)
Source and Last Reviewed #
Source: Coal Mine Drainage Prediction and Pollution Prevention in Pennsylvania, PA DEP: http://www.dep.state.pa.us/dep/deputate/minres/districts/CMDP/main.htm
Last reviewed: 2026-03 | Links may require verification — originally published pre-2010.
Tags: amd, assessment, practitioner, pa, monitoring
