The formation of Ca-Cl-rich groundwaters in the Dry Valleys of Antarctica: Field measurements and modeling of reactive transport

Abstract

Ca-Cl-rich brines have been found in shallow subsurface flows, groundwater systems, lakes, and ponds throughout the Dry Valleys of Antarctica. The apparent abundance of Ca-Cl-rich waters near the surface is unusual compared to global surface water compositions and a number of theories have been proposed to explain the genesis of these brines. We show that an ice-cemented soil developing on fluvial sediment in Taylor Valley also contains Ca-Cl-rich brine. The distribution of soluble ions, exchangeable cations, and stable isotopes down to 2.1m depth in the soil suggests that CaCl2 was formed by cation exchange reactions during downward reactive transport of Na-Cl-rich brine from the soil surface. To explore the implications of exchange reactions for the formation of Ca-Cl-rich brine, Ca-Na and Ca-Mg exchange properties were measured in 1mM, 0.1M, and 4.75M solutions. Low-temperature reactions and brine transport were modeled in PHREEQC by incorporating FREZCHEM Pitzer parameters and solubility products into PHREEQC. Modeling shows that by freezing soils in equilibrium with Dry Valley surface waters, a strong Ca-Mg enrichment of the soil solution is caused by the exchange of aqueous Na+ with exchangeable Ca2+ and Mg2+. Ca-Mg enrichment also occurs as Na-Cl-rich brine from the soil surface advects into ice-cemented soil. By modeling this process in the borehole soil, trends in ion distributions with depth can be predicted. Brine compositions from cation exchange reactions are consistent with Ca-Cl-rich brine compositions in the Dry Valleys, although additional water–rock interaction is proposed to account for the low Mg2+ concentrations in Don Juan Pond. Furthermore, the amount of CaCl2 that can be produced by exchange reactions is consistent with estimated amounts of CaCl2 in groundwaters beneath Don Juan Pond. This suggests that cation exchange reactions can explain the Ca-Cl-rich composition of the enigmatic Don Juan Pond and other brines in the Dry Valleys.