An Expanded Suite of Tracers for Hydrological Investigations
Several fluorinated derivatives of benzoic acid (fluorobenzoate, FBA) have proven their usefulness as nonreactive tracers in soil and groundwater. In the first part of this research, four previously untested trifluorobenzoates (TFBA) and two tetrafluorobenzoates (TeFBA) were examined for their suitability as water tracers in saturated and unsaturated environments. Negative log acid dissociation constants (pKa) determined by base titration ranged from 2.71 to 3.54 at 298 K. Some sorption of FBAs was observed in batch equilibration experiments with lower pH, high organic C fraction soils. An algorithm was developed for calculating a linear partition coefficient (KD) for each FBA isomer based on pKa, soil organic C, pH and an average organic C-based partition coefficient for protonated FBA (K*oc). K*oc was estimated to be 18,200 +/- 6,300 L/kg when total FBA solution concentration is less than 5 mg/L. Column mobility experiments in sandy media under saturated and unsaturated conditions yielded breakthrough curves for TFBA and TeFBA isomers that were indistinguishable from Br-. The results presented herein suggest that all 16 ring-substituted FBAs can serve as nonreactive water tracers.
In the second part of this research, a method was developed to concentrate FBAs in natural water samples. The technique utilized solid-phase extraction of FBAs followed by elution and analysis by high performance liquid chromatography. Under the best conditions, a five-fold concentration of a suite of FBAs dissolved in a natural water matrix could be achieved with greater than 80% recovery. Attempts to further concentrate FBAs were hindered by limitations of the capacity of the solid-phase extraction material for large volumes of solution and/or by difficulties in eluting retained FBAs.