TY - JOUR
T1 - Flow of river water into a karstic limestone aquifer - 2. Dating the young fraction in groundwater mixtures in the Upper Floridan aquifer near Valdosta, Georgia
AU - Plummer, L. N.
AU - Busenberg, E.
AU - Drenkard, S.
AU - Schlosser, P.
AU - Ekwurzel, B.
AU - Weppernig, R.
AU - McConnell, J. B.
AU - Michel, R. L.
N1 - Funding Information:
Appreciation is extended to the many landowners who permitted the use of their wells for the collection of groundwater samples. We gratefully acknowledge the support and assistance of Art Daniel, Valdosta City Engineer, and the City of Valdosta, Ga, during the course of the investigation. We also appreciate the assistance and cooperation of (the late) Raymond Sutton, Water Plant Superintendent, in sampling many of the City of Valdosta municipal wells. The authors thank Frank Creasy and William Garner, well contractors in the area, for their assistance in locating wells and providing well–construction data. Assistance in field sampling by Gary R. Buell, Alan M. Cressler, Barbara M. Dawson, Howard H. Persinger, Jr. (USGS, Atlanta, Ga), Mark S. Reynolds, (USGS, Albany, Ga), and Stephanie D. Shapiro (USGS, Reston, Va) is gratefully acknowledged. Richard E. Krause (USGS, Atlanta, Ga) provided considerable advice and consultation during the course of this investigation. We thank Tyler B. Coplen (USGS, Reston, Va) for providing the stable isotope data. Gerhard Bönisch (L-DEO) provided some of the 3 H (by He ingrowth) determinations. R. L. Van Hoven, R. A. Williams, P. K. Widman, J. E. Wayland and G. C. Casile (USGS, Reston, Va) performed the chemical and CFC analyses of water. Eric C. Prestemon (USGS, Reston, Va) and David L. Parkhurst (USGS, Denver, CO) assisted with some of the calculations in this report. We thank Andrew G. Stack, Paul Taney (USGS, Reston, Va) and Willis G. Hester and Eric A. Steinnagel (USGS, Atlanta, Ga) for assistance in constructing and drafting the illustrations in this report. Stephanie D. Shapiro (USGS, Reston, VA), Brian G. Katz (USGS, Tallahassee, FL), A. White (USGS, Menlo Park, Ca), R. Wanty (USGS, Denver, CO), Y. Kharaka (USGS, Menlo Park, CA), and D. K. Solomon (Univ. Utah) critically read earlier versions of the manuscript and significantly improved the final report. We acknowledge the support of S. Drenkard by the Deutsche Forschungsgemeinschaft (German Science Foundation, Dr 277–1). The Lamont-Doherty Earth Observatory Tritium/Helium Laboratory was established through a donation from the W. M. Keck Foundation. This work was supported by Columbia University through the Strategic Research Initiative. Lamont-Doherty Earth Observatory of Columbia University contribution No. 5802.Editorial handling:—Y. Kharaka
PY - 1998/11
Y1 - 1998/11
N2 - Tritium/helium-3 (3H/3He) and chlorofluorocarbon (CFCs, CFC-11, CFC-12, CFC-113) data are used to date the young fraction in groundwater mixtures from a karstic limestone aquifer near Valdosta, Georgia, where regional paleowater in the Upper Floridan aquifer receives recharge from two young sources the flow of Withlacoochee River water through sinkholes in the river bed, and leakage of infiltration water through post-Eocene semi-confining beds above the Upper Floridan aquifer. In dating the young fraction of mixtures using CFCs, it is necessary to reconstruct the CFC concentration that was in the young fraction prior to mixing. The 3H/3He age is independent of the extent of dilution with older (3H-free and 3He(trit)-free) water. The groundwater mixtures are designated as Type-I for mixtures of regional paleowater and regional infiltration water and Type-2 for mixtures containing more than approximately 4% of river water. The fractions of regional paleowater, regional infiltration water, and Withlacoochee River water in the groundwater mixtures were determined from Cl- and δ18O data for water from the Upper Floridan aquifer at Valdosta, Georgia The chlorofluorocarbons CFC-11 and CFC-113 are removed by microbial degradation and/or sorption processes in most allaerobic (Type-2) groundwater at Valdosta, but are present in some aerobic Type-I water. CFC-12 persists in both SO4-reducing and methanogenic water. The very low detection limits for CFCs (approximately 0.3 pg kg-1) permitted CFC-11 and CFC-12 dating of the fraction of regional infiltration water in Type-I mixtures, and CFC-12 dating of the river-water fraction in Type-2 mixtures. Overall, approximately 50% of the 85 water sam pies obtained from the Upper Floridan aquifer have CFC-12-based ages of the young traction that are consistent with the 3H concentration of the groundwater. Because of uncertainties associated with very low 3H and 3He content in dilute mixtures, 3H/3He dating is limited to the river-water fraction in Type-2 mixtures containing more than about 10‰ river water. Of the 41 water samples measured for 3H/3He dating, dilution of H and low -He concentration limited 3H/3He dating to 16 mixtures in which 3H/3He ages are defined with errors ranging from ±2 to ±7.5 a (1 σ). After correction for dilution with (assumed) CFC-free regional infiltration water and regional paleowater in the Upper Floridan aquifer, adjusted CFC-12 ages agree with 3H/3He 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-I mixtures indicate that travel times of infiltration water through the overlying Post-Eocene semi-confining beds exceed 35 a. The CFC and 3H/3He dating indicate that the river fraction in most groundwater entered the groundwater reservoir in the past 20 to 30 a. Few domestic and municipal supply wells sampled intercept water younger than 5 a. Calculated velocities of river water in the Upper Floridan aquifer downgradient of the sinkhole area range from 0.4 to 8.2 m/d. Radiocarbon data indicate that ages of the regional paleowater are on the 10 000-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.
AB - Tritium/helium-3 (3H/3He) and chlorofluorocarbon (CFCs, CFC-11, CFC-12, CFC-113) data are used to date the young fraction in groundwater mixtures from a karstic limestone aquifer near Valdosta, Georgia, where regional paleowater in the Upper Floridan aquifer receives recharge from two young sources the flow of Withlacoochee River water through sinkholes in the river bed, and leakage of infiltration water through post-Eocene semi-confining beds above the Upper Floridan aquifer. In dating the young fraction of mixtures using CFCs, it is necessary to reconstruct the CFC concentration that was in the young fraction prior to mixing. The 3H/3He age is independent of the extent of dilution with older (3H-free and 3He(trit)-free) water. The groundwater mixtures are designated as Type-I for mixtures of regional paleowater and regional infiltration water and Type-2 for mixtures containing more than approximately 4% of river water. The fractions of regional paleowater, regional infiltration water, and Withlacoochee River water in the groundwater mixtures were determined from Cl- and δ18O data for water from the Upper Floridan aquifer at Valdosta, Georgia The chlorofluorocarbons CFC-11 and CFC-113 are removed by microbial degradation and/or sorption processes in most allaerobic (Type-2) groundwater at Valdosta, but are present in some aerobic Type-I water. CFC-12 persists in both SO4-reducing and methanogenic water. The very low detection limits for CFCs (approximately 0.3 pg kg-1) permitted CFC-11 and CFC-12 dating of the fraction of regional infiltration water in Type-I mixtures, and CFC-12 dating of the river-water fraction in Type-2 mixtures. Overall, approximately 50% of the 85 water sam pies obtained from the Upper Floridan aquifer have CFC-12-based ages of the young traction that are consistent with the 3H concentration of the groundwater. Because of uncertainties associated with very low 3H and 3He content in dilute mixtures, 3H/3He dating is limited to the river-water fraction in Type-2 mixtures containing more than about 10‰ river water. Of the 41 water samples measured for 3H/3He dating, dilution of H and low -He concentration limited 3H/3He dating to 16 mixtures in which 3H/3He ages are defined with errors ranging from ±2 to ±7.5 a (1 σ). After correction for dilution with (assumed) CFC-free regional infiltration water and regional paleowater in the Upper Floridan aquifer, adjusted CFC-12 ages agree with 3H/3He 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-I mixtures indicate that travel times of infiltration water through the overlying Post-Eocene semi-confining beds exceed 35 a. The CFC and 3H/3He dating indicate that the river fraction in most groundwater entered the groundwater reservoir in the past 20 to 30 a. Few domestic and municipal supply wells sampled intercept water younger than 5 a. Calculated velocities of river water in the Upper Floridan aquifer downgradient of the sinkhole area range from 0.4 to 8.2 m/d. Radiocarbon data indicate that ages of the regional paleowater are on the 10 000-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.
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U2 - 10.1016/S0883-2927(98)00032-8
DO - 10.1016/S0883-2927(98)00032-8
M3 - Article
AN - SCOPUS:0032211427
SN - 0883-2927
VL - 13
SP - 1017
EP - 1043
JO - Applied Geochemistry
JF - Applied Geochemistry
IS - 8
ER -