Flow of river water into a karstic limestone aquifer - 2. Dating the youngfraction in groundwater mixtures in the Upper Floridan aquifer near Valdosta, Georgia

Tritium/helium-3 (H-3/He-3) and chlorofluorocarbon (CFCs, CFC-11, CFC-12, C FC-113) data are used to date the young fraction in groundwater mixtures fr om a karstic limestone aquifer near Valdosta, Georgia, where regional paleo water in the Upper Floridan aquifer receives recharge from two young source s - the flow of Withlacoochee River water through sinkholes in the river be d, and leakage of infiltration water through post-Eocene semi-confining bed s above the Upper Floridan aquifer. In dating the young fraction of mixture s using CFCs, it is necessary to reconstruct the CFC concentration that was in the young fraction prior to mixing. The H-3/He-3 age is independent of the extent of dilution with older (H-3-free and 3He(trit)-free) water. The groundwater mixtures are designated as Type-1 for mixtures of regional pale owater and regional infiltration water and Type-2 for mixtures containing m ore than approximately 4% of river water. The fractions of regional paleowa ter, regional infiltration water, and Withlacoochee River water in the grou ndwater mixtures were determined from Cl- and delta(18)O data for water fro m the Upper Floridan aquifer at Valdosta, Georgia. The chlorofluorocarbons CFC-11 and CFC-113 are removed by microbial degrada tion and/or sorption processes in most anaerobic (Type-2) groundwater at Va ldosta, but are present in some aerobic Type-1 water. CFC-12 persists in bo th SO4-reducing and methanogenic water. The very low detection limits for C FCs (approximately 0.3 pg kg(-1)) permitted CFC-11 and CFC-12 dating of the fraction of regional infiltration water in Type-1 mixtures, and CFC-12 dat ing of the river-water fraction in Type-2 mixtures. Overall, approximately 50% of the 85 water samples obtained from the Upper Floridan aquifer have C FC-12-based ages of the young fraction that are consistent with the H-3 con centration of the groundwater. Because of uncertainties associated with ver y low H-3 and He-3 content in dilute mixtures, H-3/He-3 dating is limited t o the river-water fraction in Type-2 mixtures containing more than about 10 % river water. Of the 41 water samples measured for H-3/He-3 dating, diluti on of H-3 and low He-3 concentration limited H-3/He-3 dating to 16 mixtures in which H-3/He-3 ages are defined with errors ranging from +/- 2 to +/- 7 .5 a (1 sigma). After correction for dilution with (assumed) CFC-free regio nal infiltration water and regional paleowater in the Upper Floridan aquife r, adjusted CFC-12 ages agree with H-3/He-3 ages within 5 a or less in 7 of the 9 co-dated Type-2 mixtures. Tritium data and dating based on both CFC-11 and CFC-12 in Type-1 mixtures indicate that travel times of infiltration water through the overlying Post -Eocene semi-confining beds exceed 35 a. The CFC and H-3/He-3 dating indica te that the river fraction in most groundwater entered the groundwater rese rvoir in the past 20 to 30 a. Few domestic and municipal supply wells sampl ed intercept water younger than 5 a. Calculated velocities of river water i n the Upper Floridan aquifer downgradient of the sinkhole area range from 0 .3 to 8.2 m/d. Radiocarbon data indicate that ages of the regional paleowat er are on the 10000 - a time scale. An average lag time of approximately 10 to 25 a is determined for discharge of groundwater from the surficial and intermediate aquifers above the Upper Floridan aquifer to the Withlacoochee River. (C) 1998 Elsevier Science Ltd. All rights reserved.