CALCIUM CARBONATE PRECIPITATION INDUCED BY CO2 DEGASSING AND pH CHANGES AT MARAMEC SPRINGS, MISSOURI

Suspended white colloidal material, composed of calcium carbonate (CaCO­­­­­­_3­­­­), is observed within waters of Maramec Spring located near St. James, Missouri. The purpose of this project is to identify the colloidal precipitate, precipitation mechanism(s), and overall aquatic chemistry of the spring.

In situ measurements were made and solution samples collected at the spring over a period of several years. Temperature, pH, Eh, Ca- and Mg-hardness, conductivity, alkalinity, and CO_{2 ­­}flux measurements were periodically made at preselected sampling locations at the spring conduit and an additional seven sites located progressively further downstream. Measurements were made during spring and summer, with water temperature remaining a relatively constant 13 to 15°C throughout the sampling stretch. The stream pH showed slight increases downstream in all twelve samplings with an average increase ~0.30 pH units (maximum 0.51 and minimum 0.12). The Ca- and Mg-hardness, conductivity, and alkalinity values were relatively constant; however, they differed from one sampling date to another in response to local rainfall events. CO_{2 ­}flux was measured on one occasion at three sampling locations (a few days following a heavy rain). Flux values, in μmoles CO₂/m² . sec, are 17.5 ± 3.3 (conduit site), 8.5 ± 3.9 (85m from the conduit), and 2.6 ± 6.1 μmoles (670m from the conduit). The high variability at the last site likely results from aquatic plant influence.

Water samples were filtered on site and brought back to the laboratory for ICP-OES analysis (unfiltered, <5.0 μm, <0.45 μm, and <0.02 μm samples). Water samples show a decrease in Si, Fe, Al, and Mn concentrations with decreasing filter size; most notably between the 5.0 μm and 0.45 μm filters. In contrast, Ca, Mg, K, and Na values were unaffected by filtering. Acid digestion of the trapped particulates and analysis of the resulting solution shows enrichment in Ca, Si, Mg, and Fe, and Ca/Mg ratios between 1.2 to 3.0. Isolated particulates have a rhombohedral morphology and effervesce when immersed in an acid solution, suggesting a calcite composition.

We conclude that the precipitation of calcite is induced via CO₂ degassing, pH increase, and decrease in the solubility of Ca²⁺ and Mg²⁺ in solution. This degassing is most likely the result of a change in pressure due to upwelling.