The paper “Long-term Changes in water chemistry as a result of mine flooding in closed mines of the Pittsburgh coal basin, USA” by J J Donovan, B R Leavitt and E Werner was presented at this week’s Sixth International Conference on Acid Rock Drainage held in Cairns, Australia.
The paper looked at an 18-year investigation, which took post-flooding water chemistry data from the Montour mine in Pennsylvania. The mine was closed in 1980 and flooded to within 38 metres of land surface within almost 5 years.
The study described the three phases of chemical behaviour in post-flooding discharge the mine went through in the 18 years after flooding.
For the first two years the water displayed an early low-pH phase of high acidity and iron, in which gypsum is at saturation and likely precipitating in the mine.
The second phase from two to four years exhibited a transition period with a declining iron but sustained gypsum saturation.
The final phase from four to 15 years showed a late high-pH phase, in which gypsum becomes undersaturated and control of calcium concentrations shifts to calcite equilibrium control. In this period, most constituents gradually decline in concentration, including iron and acidity.
“In below-drainage flooded pyritic coal mines, early strongly acid-sulfate conditions under gypsum saturation, followed by later re-equilibration with calcite under Alkaline conditions, is a widespread occurrence in flooding of below-drainage mines with calcite in suitable quantity in the overburden,” the study said.
“Similar net-alkaline mine discharge chemistry is observed in long-closed (20 years plus) mines of this region and coal seam that are nearly fully-flooded.”
The researchers said data obtained from the Montour mine represented a well-documented instance in which mine-water chemistry has been sampled systematically over time since the beginning of full flooding to show the transition from net-acidic to net-alkaline conditions.
“The ramifications of this phenomenon are that long-term fluxes of iron and metals, as well as general water chemistry, may not be adequately predicted from observations of mine-water chemistry either in situ, prior to flooding, or ex situ in pumped overflow from flooded mines, after such flooding is complete.”
“It may take years—perhaps greater than a decade, depending on mine-specific factors—before the evolution of mine water chemistry has stabilised and approached the long-term values of pH, iron and major ion chemistry that will prevail over many years following mine closure.”