SREL Reprint #2020

Ionic tracer movement through highly weathered sediments

J.C. Seaman1, P.M. Bertsch1, and W.P. Miller2

1Savannah River Ecology Lab, Biogeochemistry Division, University of Georgia, Drawer E, Aiken, SC 29802, USA
2Environmental Soil Science Department, University of Georgia, Athens, GA 30602, USA

Abstract: A highly-weathered, sandy aquifer material from the Upper Coastal Plain region of the southeastern U.S.A. (Aiken, South Carolina) was used to determine the impact of ionic strength and solution composition on the determination of physical transport parameters using ionic tracers. The mineralogy of the clay fraction consisted primarily of kaolinite, goethite and mica. Repacked saturated columns (bulk density ~1.5 g cm-3) were leached at a constant rate (~0.25 cm min-1) with a given tracer solution. For comparison, tritium (~200 pCi mL-1) was included in leachate of selected columns and several of the experiments were replicated in columns of acid-washed sand. Pore volume estimates based on tritium breakthrough were consistent with those calculated from the bulk density of the repacked matrix. In contrast, solute breakthrough for the sandy geologic material was dependent on concentration, as well as cation and anion type. At low ionic strengths (0.0005-0.010 M) that are analogous to conditions that may be encountered in field-scale transport experiments, neither the cation nor the anion acted conservatively, yielding systematically high estimates of column porosity or low estimates of flow velocity. At the higher ionic strengths (~0.10 M), solute breakthrough was essentially conservative regardless of ionic composition. The impact of cation valence and concentration on Br- breakthrough was determined using MgBr2 and KBr solutions of varying concentrations (0.001-0.1 N). Bromide breakthrough was substantially delayed for concentrations below 0.10 M and was delayed to a greater extent in the presence of a divalent cation (Mg2+) than in the presence of a monovalent cation (K+). Failure to recognize these interactions in the field could lead to a false interpretation of Br displacement in terms of physical interactions, i.e. flow velocity, dispersivity, etc.

SREL Reprint #2020

Seaman, J.C., P.M. Bertsch, and W.P. Miller. 1995. Ionic tracer movement through highly weathered sediments. Journal of Contaminant Hydrology 20:127-143.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).