Flow of river water into a Karstic limestone aquifer. 1. Tracing the youngfraction in groundwater mixtures in the Upper Floridan Aquifer near Valdosta, Georgia

The quality of water in the Upper Floridan aquifer near Valdosta, Georgia i s affected locally by discharge of Withlacoochee River water through sinkho les in the river bed. Data on transient tracers and other dissolved substan ces, including Cl-, H-3, tritiogenic helium-3 (He-3), chlorofluorocarbons ( CFC-11, CFC-12, CFC-113), organic C (DOC), O-2 (DO), H2S CH4, delta(18)O, d elta D, and C-14 were investigated as tracers of Withlacoochee River water in the Upper Floridan aquifer. The concentrations of all tracers were affec ted by dilution and mixing. Dissolved Cl-, delta(18)O, delta D, CFC-12, and the quantity (H-3 + He-3) are stable in water from the Upper Floridan aqui fer, whereas DOC, DO, H2S, CH4, C-14, CFC-11, and CFC-113 are affected by m icrobial degradation and other geochemical processes occurring within the a quifer. Groundwater mixing fractions were determined by using dissolved Cl- and delta(18)O data, recognizing 3 end-member water types in the groundwat er mixtures: (1) Withlacoochee River water (delta(18)O = - 2.5 +/- 0.3 part s per thousand. Cl- = 12.2 +/- 2 mg/l), (2) regional infiltration water (de lta(18)O = -4.2 +/- 0.1 parts per thousand, Cl- = 2.3 +/- 0.1 mg/l), and (3 ) regional paleowater resident in the Upper Floridan aquifer (delta(18)O = -3.4 +/- 0.1 parts per thousand, Cl- = 2.6 +/- 0.1 mg/l) (uncertainties are +/- sigma). Error simulation procedures were used to define uncertainties in mixing fractions. Fractions of river water in groundwater range from 0 t o 72% and average 10%. The influence of river-water discharge on the qualif y of water in the Upper Floridan aquifer was traced from the sinkhole area on the Withlacoochee River 25 km SE in the direction of regional groundwate r how. Infiltration of water is most significant to the N and NW of Valdost a, but becomes negligible to the S and SE in the direction of general thick ening of post-Eocene confining beds overlying the Upper Floridan aquifer. ( C) 1998 Elsevier Science Ltd. All rights reserved.