NATURAL GROUNDWATER ACIDIFICATION BY INCREASING CO2

At the Konza Prairie LTER Site, groundwater CO₂ in a limestone aquifer has been increasing for at least the past 20 years that groundwater chemistry has been monitored. The CO₂ dissolves carbonate minerals, which only partially buffer the decreased pH from higher pCO₂. As a consequence, groundwater is slowly becoming more acid at this tallgrass prairie research site, a location that is minimally influenced by human activities. Assessment of the acidification and increase in CO₂ is critically dependent upon accurate pH measurements along with well-known calculations. We have used a variety of methods in low-yield, shallow (less than 12.5 m) monitoring wells to compare field pH and related parameters, including field measurement in closed containers using pH meter and electrode, deliberate degassing in partially-filled known-volume containers with back calculation to in situ pH, headspace CO₂ measurement, downhole sonde and extrapolation based on seasonal calcite-saturation cycles. Results vary significantly, although all results support the phenomenon of naturally increasing groundwater CO₂ with groundwater pH decrease. Accurate pH measurement is critical to correct determination of mineral saturation indices and dissolved CO₂ speciation; these are also important to chemical weathering studies, some kinds of groundwater contaminant degradation and CO₂ sequestration monitoring. CO₂-dependent calculations are extremely sensitive to pH measurement, and field pH measurement of groundwater is not simple or straightforward, especially in low-yield monitoring wells where pumping and flow-through cells cannot be used.