U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C

Equilibrium Vapor Method to Determine the Concentration of Inorganic Carbon And Other Compounds in Water Samples

by Ronald J. Baker, Arthur L. Baehr, and Matthew A. Lahvis

ABSTRACT

The total concentration of dissolved carbon dioxide, bicarbonate, and carbonate (C_T) in water is a fundamental parameter in studies of natural and contaminated systems. A new method (the equilibrium vapor method) for determining C_T that involves the measurement of carbon dioxide concentration in the headspace of a water sample by gas chromatography and calculation of C_T by using the equilibrium relations of the carbonate system is presented. The method is most accurate when sample pH is low (near the alkalinity titration endpoint, about 4.3) when the standard titration method is least accurate or inapplicable. The method is also advantageous over the standard titration method when the water contains other alkaline species, such as salts of organic acids, which can occur as metabolites of microbial activity. The presence of such species can result in overestimation of C_T with the standard titration method because they contribute to alkalinity. An additional advantage of the equilibrium vapor method is that aqueous concentrations of volatile compounds, such as hydrocarbons, methyl-tert-butyl ether (MTBE), and dissolved gases, such as oxygen, nitrogen, methane, and carbon disulfide, can be calculated from the headspace analysis of a single sample by using Henry's law.

Ground-water samples from two gasoline-spill sites were analyzed to demonstrate the method and compare the results with those obtained by using the standard titration method. The two methods provided comparable estimates of C_T for samples with pH values above 5.0. For low-pH samples, C_T was obtainable only with the equilibrium vapor method. Filtration and acidification of samples did not affect the accuracy of the equilibrium vapor method for samples from these sites; however, acidification of samples from sites with significant amounts of carbonate minerals may result in overestimation of C_T. The concentrations of gasoline hydrocarbons in several samples also were determined.