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.
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