USE OF NDIR SENSORS FOR REAL-TIME MONITORING OF CO2 LEVELS IN COAL MINE DRAINAGE DISCHARGE

The chemical weathering of limestone in abandoned coal mines by both carbonic and sulfuric acids can lead to aqueous concentrations of dissolved CO₂ much higher than those predicted to be in equilibrium with the atmosphere. After water is discharged from a mine portal, dissolved CO₂ degasses rapidly as a function of distance and topography and becomes more aerated in the process. The accurate monitoring of CO₂ in such environments by conventional methods, such as alkalinity titration, is difficult due to the geochemical instability of the water during sample processing. Earlier work in our laboratories showed that a volume expansion method used in the beverage industry worked well in determining CO₂ in mine waters under field conditions, but it still suffered from the need to collect grab samples and transfer them to a carbonation meter, a step that results in the loss of some CO₂. Additionally, the ability to collect CO₂ data remotely to determine natural fluctuations over time is desirable. Here we report on the preliminary use of a non-dispersive infrared (NDIR) CO₂ sensor enclosed in a gas-permeable membrane to make measurements directly in the discharge of an abandoned bituminous coal mine in southwestern PA. Results showed that this method was superior to both alkalinity titration and volume expansion as a method of CO₂ detection in this environment. Long-term measurements in the fluctuation of dissolved CO₂ were possible, especially in waters nearest the portal, where the active precipitation of iron did not interfere with gas transfer across the synthetic membrane covering the NDIR sensor. Additional examples of the benefits of this analytical approach will be presented.