Radiocarbon dating of dissolved inorganic carbon in groundwater from confined parts of the Upper Floridan aquifer, Florida, USA

Geochemical reaction models were evaluated to improve radiocarbon dating of dissolved inorganic carbon (DIC) in groundwater from confined parts of the Upper Floridan aquifer in central and northeastern Florida, USA. The predo minant geochemical reactions affecting the C-14 activity of DIC include (1) dissolution of dolomite and anhydrite with calcite precipitation (dedolomi tization), (2) sulfate reduction accompanying microbial degradation of orga nic carbon, (3) recrystallization of calcite (isotopic exchange), and (4) m ixing of fresh water with as much as 7% saline water in some coastal areas. The calculated cumulative net mineral transfers are negligibly small in up gradient parts of the aquifer and increase significantly in downgradient pa rts of the aquifer, reflecting, at least in part, upward leakage from the L ower Floridan aquifer and circulation that contacted middle confining units in the Floridan aquifer system. The adjusted radiocarbon ages are independ ent of flow path and represent travel times of water from the recharge area to the sample point in the aquifer. Downgradient from Polk City (adjusted age 1.7 ka) and Keystone Heights (adjusted age 0.4 ka), 14 of the 22 waters have adjusted C-14 ages of 20-30 ka, indicating that most of the fresh-wat er resource in the Upper Floridan aquifer today was recharged during the la st glacial period. All of the paleowaters are enriched in O-18 and H-2 rela tive to modern infiltration, with maximum enrichment in delta O-18 Of appro ximately 2.0 parts per thousand.