PROCESSES CONTROLLING MG-CALCITE COLLOID PRECIPITATION WITHIN MARAMEC SPRING, MISSOURI

Water emanating from Maramec Spring (St. James, MO) upwells from a depth of >60 meters and consistently exhibits a milky blue color under normal flow conditions. The color arises as a result of light scattering from colloidal particles. Spring water samples collected on four different dates displayed variable Ca (16.5 to 48 mg/L), Mg (11.5 to 27 mg/L), and alkalinity values (97 to 144 mg/L). On-site processing of water through 0.02 µm filters did not significantly change these concentrations. Measured pH values for 16 sampling dates were variable at the zone of upwelling, averaging 7.04 ± 0.14 pH units while water temperature was 13.6 ± 0.6^oC. An increase of 0.30 ± 0.13 pH units occurs over the 1400 meter long course of the outflow stream. Unstable pH readings at the point of upwelling, a pH rise in outflow water, and positive CO₂ gas flux measurement all reflect a system evolving CO₂ into the atmosphere.

Both XRD and SEM-EDS techniques were utilized to examine colloidal particles collected both by on-site vacuum filtration centrifuging spring water samples. Organic debris, diatoms, quartz, smectite, and illite were detected as well as unidentified sulfur-bearing phases. XRD analysis also revealed the presence of both dolomite and high-Mg calcite (~29.7° two-theta, CuKα). The dolomite likely is derived from the erosion of the Gasconade Formation, which hosts many springs and caves in the region. SEM examination of micron-sized particles of Mg-calcite did not reveal any corrosion pits or impact features. The particles displayed such delicate features that they undoubtedly were precipitated in and remained suspended by the turbulent spring water. Approximately 100 carbonate particles were hand separated under a microscope, digested in acid, and analyzed chemically to reveal a composition of ~8 mole% MgCO₃. Modeling of the Maramec Springs system data using MINTEC and PHREEQ software yielded negative saturation indices (SI) of -0.9 to -0.7. Unrealistically large increases in alkalinity, temperature, and a_Ca are required to reach calcite saturation (SI=0). Shifting the pH from 7.0 to 7.8, however, would induce saturation by converting HCO₃^- into CO₃^2-. How the Maramec system circumvents this transformation is unresolved but suggests a reaction involving Ca²⁺ directly with HCO₃^- ions.