Carbonate Equilibria and Groundwater Sample Collection: Implications for Estimated Average Subsurface Properties in Continental North America

Abstract

The geochemistry of carbon dioxide in oxygenated groundwater may have a profound impact on analytical results obtained from efforts to measure in situ groundwater pH conditions and on the speciation, partitioning behavior and potential transport of inorganic contaminants in aquifers. Based on the observation that an estimate of the mean, national-average groundwater pH changes from 6.65 to 6.83, depending on the delay time before analysis, a mechanism of pH alteration resulting from CO₂ degassing is examined to provide an estimate of the ratio between groundwater and atmospheric partial pressures of CO₂ (P_CO2.GW/P_CO2atm. = 1.5).

The groundwater-degassing P_CO2 estimate is considerably less than literature values of groundwater P_CO2 and estimates of P_CO2 derived from the same data assuming other mechanisms. An examination of the potential sources of error in the degassing P_CO2 estimate suggests significant sample degassing may have occurred prior to measurement in previous field groundwater sample pH determinations. The instability in groundwater sample pH after exposure to the atmosphere may be useful for characterizing the source environments. An estimate of groundwater P_CO2 (4.67E-2 atm.) derived from an average groundwater bicarbonate concentration is within the range of values reported in the literature.

Key Words:

• Groundwater
• carbon dioxide
• sample collection
• pH instability