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Retardation of Fluorobenzoate Tracers in Highly Weathered Soil and Groundwater Systems

Seaman, John C.
Soil Science Society of America journal 1998 v.62 no.2 pp. 354-361
coastal plain soils, alluvial soils, coarse-textured soils, benzoates, labeling techniques, groundwater, ionic strength, bromides, organic matter, mineralogy, sediments, Southeastern United States
Fluorobeuzoate (FBA) compounds have been used extensively as hydrologic tracers based on the assumption that they are not significantly sorbed or degraded in most soil and groundwater environments. However, previous batch studies have indicated that such compounds may be readily sorbed by organic matter and hydrous Fe oxides, such as goethite. This study was conducted to evaluate the transport properties of FBA tracers within the highly weathered sediments of the southeastern Coastal Plain. Repacked columns of coarse-textured alluvial soil and aquifer sediments were leached with tracer solutions (pH ≈5.0) containing various fluorobenzoates with negative logs of the acid dissociation constants (pKₐs) ranging from 3.83 to 2.85. Bromide was included in many of the solutions to evaluate the influence of ionic strength and competition for anion-exchange sites on FBA mobility. Despite possible differences in the mechanisms of sorption, retardation within surface and subsurface materials increased with increasing pKₐ of a given FBA compound. Retardation was attributed to partitioning to the organic fraction for the surface soil and to sorption by Fe oxides for the subsurface sample. For the transport of mixed-benzoate solutions through surface soil, 2,6-di-FBA, 2,3,4,5-tetra-FBA, and Br⁻ yielded transport velocities consistent with those observed for ³H; however, 3,4,5-tri-FBA and 3,4-di-FBA were significantly retarded, most likely due to partitioning to the organic fraction. For the subsurface sample, all of the FBA tracers and Br⁻ were retarded compared with ³H, with FBA retardation increasing with pKₐ. At higher ionic strengths, retardation was reduced in the subsurface sample due to competition with Br⁻ for anion sorption sites.