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Defining reactions and mass transfer in part of the Floridan aquifer
Author(s) -
Plummer L. N.
Publication year - 1977
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/wr013i005p00801
Subject(s) - dolomite , aquifer , groundwater recharge , calcite , geology , groundwater , gypsum , dissolution , polk , geochemistry , water table , mineralogy , hydrology (agriculture) , chemistry , geotechnical engineering , paleontology , biochemistry
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 σ 13 C and 14 C 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 CO 2 entering the groundwater, σ 13 C = −22.5, is near that measured for soil gas CO 2 in the vicinity of the recharge area (Rightmire and Hanshaw, 1973(, the mechanism)s) by which soil CO 2 enters the saturated zone is not well defined. Down the hydraulic gradient from the vicinity of Polk City, isotopically light dolomites (σ 13 C = − 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 CO 2 , presumably soil zone CO 2 . Further south between Fort Meade and Wauchula the Floridan aquifer becomes essentially closed to CO 2 . Beyond Wauchula to Arcadia, oxidation of lignite via sulfate reduction adds additional carbon to the water composition. 14 C 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 14 C ages are in reasonable agreement with flow velocities estimated from hydrologie considerations. The corrected 14 C 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.