Water Quality Evolution in Flooded Underground Coal Mines

After below drainage underground coal mines are abandoned, the mines flood to form an aquifer. The size of the voids in these mine aquifers depends upon the mining techniques and the mechanical properties of the coal and roof. In Longwall and full extraction room and pillar mining the roof collapse is immediate. However, where pillars are left in place, and where the roof is well supported large openings can remain for decades. During the flooding process, acid mine drainage (AMD) is liberated from the rock and coal surfaces after years of formation in the mine. New AMD may continue to form as water and oxygen in the mine atmosphere are exposed to pyritic minerals in the coal and overburden. When the mine aquifer is completely flooded, less oxygen can come into contact with the pyritic minerals in the mine and the discharge from the mine aquifer can become net alkaline with decreasing metal loads. In addition, inflow to the mine from recharge can bring alkalinity into the system altering the mine water chemistry. Understanding these processes requires a regional appreciation of mine geometry, interconnectivity and hydrogeology.

This paper summarizes ten years of study at West Virginia University that began with regional mine structural mapping, flooding dynamics and is now investigating the evolution of mine water chemistry. Observations were processed through the TOUGHREACT model by incorporating the results of the coal and overburden geology, mine water, and mine atmosphere analyses.