Mining methods and AMD
Knowing how the coal was mined can provide you with clues as to where to look for the sources of contamination. All past mining approaches sought to remove coal from beneath sometimes thick layers of soil and rock called overburden. Many of the various coal-mining practices exposed the sulfur-containing rocks and minerals to the weather, and in some cases, to groundwater flow.
There were and continue to be two approaches to mining:
- removing the overburden to get at the coal (surface or strip mining) or
- extracting the coal while leaving the overlying material in place (underground or auger mining).
Underground mining has accounted for approximately 70 percent of the Appalachian mining production in the past. Surface (or strip) mining, underground mining, and augering were all common to the Appalachian region.
Surface mining was practiced where coal beds were at or near the surface of eroded hillsides. Operators removed the overburden with various excavating equipment to expose the coal outcrops. The amount of material disturbed by this technique depended on the thickness and quality of coal being mined, but in most situations, resulted in more than just the near surface zone. The overburden fill is left in the area and if located along drainage zones, can become a source for acid formation if sulfur-containing materials (pyrites) are present and exposed to water.
Removing coal through a process of tunneling through the relatively soft coal beds is among one of the oldest, and still most common, mining practice. In this practice (also called deep mining), vertical, horizontal, and/or entire sloping mine works were constructed to provide access to the coal seam and to remove dust and recharge fresh air supplies. Many old, abandoned underground mines are honeycombed with miles of tunnels, capable of collecting huge amounts of water (known as the mine pool). As this water passes through the overburden and the maze of tunnels in the coal, it can react with the various pyritic acid-forming materials (if present) and create strong concentrations of acid and dissolved heavy metals.
Auger mining is the process of drilling out coal seams from a vertical highwall with a large auger. This practice often accompanies the contour mining process. Augers could penetrate horizontally more than 200 feet into a seam, removing as much as 60 percent of the coal. Waste material brought out with the coal was sometimes placed back into the auger holes to prevent subsidence and tension cracks at the surface. Auger holes can be a source of contaminated drainage if rain percolates through the overlying layers and seeps into the holes.
Coal Mine Drainage Prediction and Pollution Prevention in Pennsylvania - a handbook published by PA DEP