Defining reactions and mass transfer in part of the Floridan aquifer

Abstract

This paper examines the observed changes in water chemistry down a hydraulic gradient in part of the Floridan aquifer. Through the use of mass balance relationships and mass transfer calculations several reactions are derived that simulate the observed water chemistry. The calculation of σ13C and 14C activity implied by these reactions limits the number of possible reactions and leads to the following conclusions. The waters of the recharge area near Polk City have an average age of 3200 yr and have formed by congruent solution of dolomite and calcite. Although the calculated isotopic composition of CO2 entering the groundwater, σ13C = −22.5, is near that measured for soil gas CO2 in the vicinity of the recharge area (Rightmire and Hanshaw, 1973(, the mechanism)s) by which soil CO2 enters the saturated zone is not well defined. Down the hydraulic gradient from the vicinity of Polk City, isotopically light dolomites (σ13C = − 3.9 to − 1.5‰) and gypsum dissolve. All reactions south of Polk City are incongruent to a low‐magnesium calcite. The reaction path between Polk City and Fort Meade is partially open to CO2, presumably soil zone CO2. Further south between Fort Meade and Wauchula the Floridan aquifer becomes essentially closed to CO2. Beyond Wauchula to Arcadia, oxidation of lignite via sulfate reduction adds additional carbon to the water composition. 14C ages, corrected for the derived mass transfer reactions, are slightly younger than was previously recognized owing to consideration of incongruent dissolution. The age of Arcadia water (at the farthest point down the gradient) is estimated to be 36,000 yr B.P. Flow velocities derived from 14C ages are in reasonable agreement with flow velocities estimated from hydrologie considerations. The corrected 14C ages and derived mass transfer reaction coefficients allow estimation of apparent rates of reaction in central Florida from field data. The results of this study point to a complex diagenetic history in central Florida in which the position of the freshwater‐saltwater interface appears to determine whether dolomite is a reactant or product mineral.