CALL FOR PROPOSALS:

ORGANIZERS

  • Harvey Thorleifson, Chair
    Minnesota Geological Survey
  • Carrie Jennings, Vice Chair
    Minnesota Geological Survey
  • David Bush, Technical Program Chair
    University of West Georgia
  • Jim Miller, Field Trip Chair
    University of Minnesota Duluth
  • Curtis M. Hudak, Sponsorship Chair
    Foth Infrastructure & Environment, LLC

 

Paper No. 7
Presentation Time: 10:30 AM

NATURAL GROUNDWATER ACIDIFICATION BY INCREASING CO2


MACPHERSON, G.L.1, TSYPIN, M.A.2, ROBERTS, J.a.3 and CHING, G.B.2, (1)Dept. of Geology, Univ of Kansas, 1475 Jayhawk Blvd, 120 Lindley Hall, Lawrence, KS 66045, (2)Department of Geology, University of Kansas, 1475 Jayhawk Blvd, 120 Lindley Hall, Lawrence, KS 66045, (3)Geology, University of Kansas, Multidisciplinary Research Building, 2030 Becker Dr, Lawrence, KS 66047, glmac@ku.edu

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.
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