Identification of thermodynamic controls defining the concentrations of hazardous elements in potable ground waters and the potential impact of increasing carbon dioxide partial pressure

摘要

Concern has been expressed that carbon dioxide leaking from deep storage reservoirs could adversely impact water quality in overlying potable aquifers by mobilizing hazardous elements present in the aquifer rocks to the extent that their concentrations might exceed Maximum Contaminant Levels (MCLs). To evaluate this issue, 38,000 ground water quality analyses from aquifers throughout the United States, each containing one or more analyses for As, Ba, Cd, Hg, Pb, Sb, Se, U or Zn, were retrieved from the Unites States National Water Information System (NWIS). The analyses were used to calculate the saturation indices (SIs) of all identified and thermodynamically characterized minerals containing the listed elements as essential components. These minerals were initially selected through literature surveys to establish whether field evidence supported their postulated presence in potable water aquifers. SI frequency histograms were plotted to evaluate whether these minerals are at saturation in NWIS ground waters (i.e., they show modes at SI ≈ 0). Mineral assemblages meeting the criterion of thermodynamic saturation were assumed to control the aqueous concentrations of the hazardous elements at initial system state as well as at elevated CO_2 partial pressure caused by the ingress of leaking CO_2. The impact on the identified mineral solubilities of increasing CO_2 partial pressures was then predicted over the range from –4 to +1 (i.e., 10~(-4) ≤ P(CO_2) in bar ≤ 10). Under reducing conditions (characteristic of most ground waters), the most serious problem resulting from intrusion of CO_2 into shallow groundwater may arise through enhanced dissolution of pyrite and solubilization of arsenic. At the highest P(CO_2) assumed in our study, Ba, Pb and Zn may also approach or exceed regulatory concentration limits. Of the remaining elements, the MCLs of Cd, and Sb are unlikely to be exceeded, and Hg, Se and U concentrations are unaffected by CO_2 intrusion.