HR: 08:15h
AN: H41I-02    [Abstracts]
TI: Radium Isotopes as an Indicator of Microbial Activity in a Deep Aquifer at Memphis, Tennessee, USA
AU: * Luo, S
EM: sluo@usc.edu
AF: University of Southern California, Department of Earth Sciences University Park, Los Angeles, CA 90089-0740 United States
AU: * Luo, S
EM: sluo@usc.edu
AF: National Cheng-Kung University, Department of Earth Sciences , Tainan, 701 Taiwan
AU: Ku, T
EM: rku@usc.edu
AF: University of Southern California, Department of Earth Sciences University Park, Los Angeles, CA 90089-0740 United States
AU: Todd, V M
EM: vtodd@usc.edu
AF: University of Southern California, Department of Earth Sciences University Park, Los Angeles, CA 90089-0740 United States
AU: Gentry, R W
EM: rgentry@utk.edu
AF: University of Tennessee, Department of Civil & Environmental Engineering, Knoxville, TN 37996-2010 United States
AU: McCarthy, J F
EM: jmccart1@utk.edu
AF: University of Tennessee, Department of Ecology and Evolutionary Biology, Knoxville, TN 37996-1410 United States
AB: Groundwater samples were collected from three well fields (Shaw, Morton, and Sheahan) at Memphis in southwestern Tennessee, to study the behavior of Ra isotopes ($^{226}$Ra, $^{228}$Ra, $^{224}$Ra, and $^{223}$Ra) in a semi-confined deep aquifer in the area. Elevated activities were observed for each of these radioisotopes ($^{226}$Ra: 0.4-0.8 dpm/L; $^{228}$Ra: 0.8-1.6 dpm/L; $^{224}$Ra: 1.4-2.4 dpm/L; $^{223}$Ra: 0.07-0.1 dpm/L) at the Shaw site where groundwaters are recharged to form the Memphis aquifer, while relatively low activities ($^{226}$Ra: 0.02-0.07 dpm/L; $^{228}$Ra: 0.03-0.1 dpm/L; $^{224}$Ra: 0.04-0.13 dpm/L; $^{223}$Ra: $<$0.003 dpm/L) were found at the Morton site where the deep Memphis aquifer is effectively isolated from the unconfined aquifer at shallow depths. At the Sheahan site, the Ra isotope activities in the aquifer all reach maximum values at a depth of $\sim$100 m below the ground surface. The high activities are found to be associated with high dissolved organic carbon concentrations, suggesting that leakage via plumes of contaminated groundwater from the shallow unconfined aquifer into the deep Memphis aquifer may have taken place. As Mn oxides are a strong scavenger of Ra isotopes in low-salinity groundwaters, our results imply that the elevated Ra isotope activities in these ground waters may be caused by a reduction/dissolution of Mn oxide coatings on the aquifer solids in association with enhanced microbial activities. This implication points to the potential of using the Ra isotopes as a proxy for assessing deep aquifer microbial activities and their influence on the chemistry of groundwater.
DE: 4840 Microbiology
DE: 4860 Radioactivity and radioisotopes
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1615 Biogeochemical processes (4805)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting