NOBLE GAS AND TRITIUM - HELIUM-3 DATA FROM MUNICIPAL WELL FIELDS IN SHELBY COUNTY, TENNESSEE, USA: IMPLICATIONS FOR GROUNDWATER RECHARGE PROCESSES TO THE MEMPHIS AQUIFER THROUGH WINDOWS IN THE UPPER CLAIBORNE CONFINING UNIT

Recharge to the partially confined Memphis aquifer in Shelby County, Tennessee, occurs, in part, through “windows” in the upper Claiborne confining unit. Windows are discrete zones of hydraulic connectivity in an otherwise continuous confining unit where leakage from an overlying or underlying aquifer can recharge a confined aquifer. In the present study, we use noble gas and 3H -3He data along with other hydrogeologic data to evaluate residence time and origin of leakage water in several municipal well fields that pump from the Memphis aquifer in southwestern Tennessee. Over the past five years, noble gas and 3H -3He data have been obtained from production wells in five municipal well fields in the Memphis area. The noble gas samples were collected utilizing diffusion samplers within flow-through cells attached to wells pumped continuously for 24 hours following at least 24 hours of purge. Tritium samples were taken from raw water collected after 48 hours of pumping. The samples were analyzed at the University of Utah Dissolved Gas Laboratory. Because of analytical error and assumptions regarding sources of 3He, the residence times have as much as 10% error. A replicate age determination from one production well was within 9% of the original value. Noble gas data indicate varying quantities of excess air. Accounting for excess air, recharge temperatures between 10 and 15°C were estimated using the closed-equilibrium model of Aeschbach-Hertig et al. (2000). Tritium-bearing waters (3H > 0.1 TU) have R/Ra of helium isotopes that range from 0.97 to 1.58. Non-tritium bearing waters have R/Ra values as low as 0.90, indicating minor accumulation of terrigenic 4He. Assuming negligible mantle He contributions, 3H-3He residence times range from 10 to 61 years. Based on estimated tritium loading in the Memphis area, all of these ages reflect mixtures of as much as 25% modern water with pre-modern water. In all cases, production well waters with modern-water ages were produced from the upper part of the Memphis aquifer and within 3 km of an identified window location. The noble gas and 3H -3He results constrain the degree and location of inter-aquifer leakage to the Memphis aquifer near municipal well fields as well as provide calibration parameters for numerical ground-water modeling.