A TALE OF TWO DISCHARGES: CHEMISTRY AND VARIABILITY IN SHALLOW AND DEEP MINE DISCHARGES FROM THE PHILLIPS MINE, UNIONTOWN PA

A well-known consequence of coal mining is the production of coal mine drainage (CMD), which can negatively impact local water quality. Dissolved inorganic carbon (DIC) occurs when sulfuric acid in mine water drives the dissolution of carbonate minerals in the rock strata. Depending on the pH of the discharging water, the DIC can be degassed as carbon dioxide (CO₂) or exported downstream as bicarbonate (HCO₃^-).

Most CMD research does not address DIC emissions. This study examines two CMD discharges from the Phillips Mine in Uniontown, Pennsylvania. One is a shallow mine discharge and the other is a deep mine discharge. The objective of this study was to compare the chemistry in the two discharges. Direct measurements of the DIC, CO₂, pH, alkalinity, and discharge were taken along with major ions analysis. The samples were collected on a ~3-week schedule between March and December 2024. Over that time, 14 samples were collected from the deep discharge and 11 from the shallow discharge. The shallow discharge was dry between November and December.

DIC and CO₂ were present in all samples. The shallow discharge contained lower concentrations of DIC and CO₂ (averages DIC 7.6 mM, CO₂ 4.9 mM) than the deeper discharge (averages DIC 9.8 mM, CO₂ 5.2 mM). The average concentration of CO₂ in the mine water was up to 320 times greater than would be expected from the equilibrium with current atmospheric CO₂ pressures. The maximum concentration of CO₂ in the two discharges was similar (5.5 – 5.6 mM).

The shallower discharge was also chemically more variable over time. The relative standard deviations for the shallower discharge were 11% for CO₂ and 18% for DIC; the deeper discharge had a relative standard deviation of 6.5% for CO₂ and 8.9% for DIC. The difference in variability is attributed to the shallower mine having more influence from the surface activities and recharge while the water flowing through the deeper mine has more time to reach thermal and chemical equilibrium.

By quantifying the DIC and CO₂ fluxes, this study captures the seasonal variability in CMD locations concerning the depth of the discharging waters. Understanding the extent of CO₂ emissions regarding the seasonal variability and depth of mine water contributes to the evolving understanding of the regional carbon cycle of coal mining communities.