HR: 0830h
AN: H21D-0868    [PDF]
TI: Inter-aquifer Dynamics in and Near a Confining Unit Window in Shelby County, Tennessee, USA
AU: * Gentry, R W
EM: rgentry@utk.edu
AF: The University of Tennessee, Civil & Environmental Eng. 62 Perkins Hall, Knoxville, TN 37996-2010 United States
AU: McKay, L D
EM: 37932
AF: The University of Tennessee, Earth & Planetary Sciences, Knoxville, TN 37996-1410 United States
AU: Larsen, D
EM: dlarsen@memphis.edu
AF: The University of Memphis, Department of Earth Sciences, Memphis, TN 38152 United States
AU: Carmichael, J K
EM: jkcarmic@usgs.gov
AF: U.S. Geological Survey, 640 Grassmere Park Suite 100, Nashville, TN 37211 United States
AU: Solomon, D K
EM: dksolomon@mines.utah.edu
AF: The University of Utah, Geology & Geophysics 135 S. 1460 E., RM 719, Salt Lake City, UT 84112-0111 United States
AU: Thonnard, N
EM: nthonnar@utk.edu
AF: The University of Tennessee, IRIM 10521 Research Drive, STE 300, Knoxville, TN 37932 United States
AU: Anderson, J L
EM: jlandrsn@memphis.edu
AF: The University of Memphis, Ground Water Institute, Memphis, TN 38152 United States
AB: An interdisplinary research team is investigating the interaction between the surficial alluvial aquifer and the deeper confined Memphis aquifer in the Memphis area, Shelby County, Tennessee. Previous research has identified a window in the clay-rich, upper Claiborne confining unit that separates the two aquifers near a closed municipal landfill in east-central Shelby County, an area undergoing rapid urbanization. For this investigation, a combination of environmental tracers (tritium/helium-3), major and trace ion geochemistry, hydraulic response testing, measurement of hydraulic gradients, and groundwater flow modeling is being used to quantify recharge of young water from the alluvial aquifer through the window to the Memphis aquifer. The research will provide results to better understand how windows were formed and how they influence recharge and water quality in otherwise confined parts of the Memphis aquifer downdip of its outcrop/subcrop area. Examination of continuous core samples and geophysical logs from wells installed for the study using Rotasonic drilling methods confirmed the existence of a sand-dominated window that may be as much as 1 km in diameter in the upper Claiborne confining unit. The upper Claiborne confining unit is 15 to 20 m thick in most of the study area and is overlain by a 10 to 12 m thick alluvial aquifer. The window is interpreted to have formed as a result of depositional and incisional processes in an Eocene-age deltaic system. Hydraulic gradients of several feet exist vertically between the alluvial and Memphis aquifers within the window, indicating downward flow. Groundwater age-dates from tritium/helium-3 analyses indicate that groundwater in the window at the depth of the base of the surrounding confining unit (approximately 30 m) has an apparent age of 19.8 years, which confirms the occurrence of downward flow. Young groundwater age dates (less than 32 years) also were obtained from wells in the Memphis aquifer at confined sites downgradient of the window, suggesting that a plume of young water is spreading outwards from the window and mixing with the older Memphis aquifer water. Preliminary inverse modeling of the site using a genetic algorithm coupled with a central finite difference flow model indicates a probable steady-state downward flux of about 12,000 m$^{3}$/d through the window. Collection and analysis of additional groundwater samples are planned to examine geochemical conditions in the confining unit and in the Memphis aquifer upgradient of the window. These analyses will aid in developing a final conceptual model and in subsequent numerical modeling of mixing of the young recharge water with the older Memphis aquifer water.
DE: 1040 Isotopic composition/chemistry
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1884 Water supply
SC: Hydrology [H]
MN: 2003 Fall Meeting