Behavior of fluorobenzoic acid groundwater tracers in a highly fractured shale saprolite

Sorption and transport of fluorbenzoic acid (FBA) tracers were examined in a highly weathered and fractured clay-rich saprolite formation under both acidic and neutral conditions. The fluorobenzoic acids are a group of 14 organic acids with negative log dissociation constants (pKa) from 2.71 to 3.83. In order to separate the physical transport factors of advection, dispersion and diffusion from the chemical processes of sorption and hydrophobic partitioning, laboratory batch tests and column transport studies were carried out. All of the investigations were carried out at the lab-scale rather than the field-scale; so that hydraulic and geochemical conditions, which can effect transport, were carefully controlled and monitored. An equivalent porous media (EPM) model was used to evaluate the transport processes and to determine whether tracer retardation could be predicted based on distribution coefficients measured in the batch experiments. The results of the batch and sorption studies show that the degree of sorption was directly related to the pKa of the isomer used and the pH of the pore fluid, but was not dependent on the amount of organic carbon present (<0.1%). When multiple FBA isomers were present in the batch sorption tests, there appeared to be some competition for sorption sites, but the effects were small compared to differences caused by experimental variation in pH of the pore water solution. An intrinsic partitioning coefficient of the neutral FBA species (K^d = 2.50 L/kg) was derived for the saprolite used in this study. The results of the column transport experiments show that at neutral pH, none of the FBA isomers were retarded relative to a nonreactive halide tracer (Br). Under slightly acidic conditions (pH=5.2) significant sorption and retardation was observed with the two isomers with the lowest pKa of the three isomers tested. Transport modeling investigations showed that it was not possible to accurately simulate breakthrough curves for the retarded FBA isomers solely by varying the retardation coefficients or by using values derived from the batch sorption tests. However, the FBA breakthrough curves can be successfully simulated using a dual solute transport approach, where the reactive neutral species and the nonreactive anionic fractions are modeled separately.