Chemical behaviour revealed in mine flooding

A STUDY into the results on water evolution in flooding of the Montour mine in the northern Pittsburgh coal basin has turned out some interesting findings on mine water chemistry.

Angie Tomlinson

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.”

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