Dissolved Gases as Partitioning Tracers for Determination of Hydrogeological Parameters VIJAY M. VULAVA,* EVAN B. PERRY, CHRISTOPHER S. ROMANEK, AND JOHN C. SEAMAN Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, The University of Georgia, Drawer E, Aiken, South Carolina 29802 In this study, dissolved Kr and SF 6 gases were used to determine various hydrogeological parameters of laboratory columns under water-saturated and partially saturated conditions as a function of the flow velocity. The dissolved gases behaved conservatively in saturated columns but were significantly retarded in unsaturated conditions as a direct function of the Henry’s law constant (K H ) and the ratio of column pore spaces filled with air and water (V g / V w ).Lower aqueous diffusion coefficients for SF 6 compared to that for Kr also resulted in significant rate-limited mass transport across gas-water interface. This effect was exacerbated at higher flow velocities as was indicated by the asymmetric shape of breakthrough curves, more so in the case of SF 6 . A nonequilibrium advective-dispersive transport model accurately described tracer breakthrough and was used to estimate parameters such as final V g / V w under partially saturated conditions and partitioning rates. Internally consistent model results were obtained for both dissolved gases despite the wide range in physical properties (e.g., K H and aqueous diffusion coefficients), suggesting that dissolved Kr and SF 6 may be used in conjunction to delineate and validate aquifer characteristics simultaneously from a single pulse injection of the tracer. 1. Introduction Tracers are typically injected into groundwater to obtain information concerningthe direction and/orvelocityofwater movement in aquifers and the potential transport of con- taminants. Ideally, such tracers track the movement of groundwater through porous media without altering the physicalor chemicalconditions ofthe aquifer.As such,they should be (1)intrinsicallystable and nonreactive with porous media, (2) present in extremely low background concentra- tion, (3) detectable at low concentration, and (4) similar to native groundwater in density. Inorganic and organic anions such as Cl - , Br - , and fluorinated benzoicacid derivatives(e.g.,pentafluorobenzoic acid) are typically considered “conservative” or nonreactive under most conditions (1-3), but under certain conditions theyexhibit significant retardation in field (4)and laboratory studies (5-11). Seaman et al. (6, 8, 12) showed that most anionic tracers are highly retarded in aquifer sediments of the Atlantic Coastal Plain because of sorption onto variably charged Fe-oxides. In these studies, water enriched with tritium ( 3 H) was used to confirm the retardation of anionic tracers. However, health and regulatory concerns make it difficult to use radioactive tracers in most field applications, and hence other solutes are needed for use as conservative tracers. Dissolved gases may be used to determine parameters such as groundwater velocity, aquifer porosity, and disper- sivity in site-characterization studies. Nonreactive and non- polar gases do not react with most porous media during transport nor do they influence significantly the physico- chemical properties of groundwater. They are safe to use in porous media sensitive to changes in solution chemistry or ionicstrength.Contraryto manyionictracers,dissolved gases such as helium (He), neon (Ne), krypton (Kr), and sulfur hexafluoride (SF6) can be measured analytically over several orders of magnitude. Despite these advantages, relatively few studies have demonstrated the use ofdissolved gases as conservative hydrologic tracers (13-20). Recent attention has focused primarilyon the use ofdissolved anthropogenic gases, such as SF6 and chlorofluorocarbons (CFC), to determine gas-transfer rates between atmosphere and groundwater (21-25). Because the atmospheric concentra- tion ofthese gases has increased, these gases have also been used to date groundwater ages (26, 27). The widespread use ofdissolved gases as tracers has been hampered because ofperceived difficultiesin deployingand recovering the tracer in the field. In addition, factors such as temperature differences between the groundwater and tracer solution and the potential exsolution of gases have limited their use. With the increasing sophistication of analytical techniques, quantitative recovery and analysis of dissolved gases has become considerably simpler and more accurate. Sample extraction via sparging and sonication followed by headspace analysis using gas chromatography coupled with an electron capture detector or a quadrupole mass spectrometer provides a superior analytical tool for the analysis ofmost gases that have some relevance as tracers (13, 28-30).These techniques yield extremelylow detection limits; hence, some tracers may be injected at relatively low concentration and detected long distances from the point of origin. Sulfurhexafluoride,an anthropogenicgasused in electric insulating equipment (27), is nonpolar, inert, and has an extremelylowambient background concentration (<3pptv) and slight aqueous solubility (∼35 mg/L, 1 atm pure gas at 25 °C). In some laboratory column and field applications, SF 6 has been shown to exhibit transport behavior similar to that ofsolute tracers such as Cl - and Br - (17, 19, 31).However, SF6 was found to be retarded in the presence of air pockets or residual nonaqueous phase liquids in the porous media (14, 19, 31, 32). Noble gases, such as He (15, 20, 33) and Kr (13, 34), have also been used as tracers in both laboratory and field tracer studies with mixed results. Both gases are relativelyinert and naturallypresent in low concentration in groundwaters. Table 1 lists relevant properties of Kr and SF6 for comparison.The most relevant propertyfortracerapplication is the Henry’s law constant (KH,dimensionless)which relates aqueous and air phase concentrations of a gas (35) *Correspondingauthor phone: (865)974-9976;fax: (865)974-2368; e-mail: vulava@utk.edu.Presentaddress: DepartmentofGeological Sciences, The University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996-1410. K H ) c g c w (1) Environ. Sci. Technol. 2002, 36, 254-262 254 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 2, 2002 10.1021/es0107024 CCC: $22.00  2002 American Chemical Society Published on Web 12/12/2001