IONIC TRACER MOVEMENT THROUGH HIGHLY WEATHERED SEDIMENTS

A highly-weathered, sandy aquifer material from the Upper Coastal Plai n region of the southeastern U.S.A. (Aiken, South Carolina) was used t o determine the impact of ionic strength and solution composition on t he 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 similar t o 1.5 g cm(-3)) were leached at a constant rate (similar to 0.25 cm mi n(-1)) with a given tracer solution. For comparison, tritium (similar to 200 pCi mL(-1)) was included in leachate of selected columns and se veral of the experiments were replicated in columns of acid-washed san d. Pore volume estimates based on tritium breakthrough were consistent with those calculated from the bulk density of the repacked matrix. I n contrast, solute breakthrough for the sandy geologic material was de pendent on concentration, as well as cation and anion type. At low ion ic strengths (0.0005-0.010 M) that are analogous to conditions that ma y be encountered in field-scale transport experiments, neither the cat ion nor the anion acted conservatively, yielding systematically high e stimates of column porosity or low estimates of flow velocity. At the higher ionic strengths (similar to 0.10 M), solute breakthrough was es sentially conservative regardless of ionic composition. The impact of cation valence and concentration on Br- breakthrough was determined us ing MgBr2 and KBr solutions of varying concentrations (0.001-0.1 N). B romide breakthrough was substantially delayed for concentrations below 0.10 M and was delayed to a greater extent in the presence of a dival ent cation (Mg2+) than in the presence of a monovalent cation (K+). Fa ilure to recognize these interactions in the field could lead to a fal se interpretation of Br displacement in terms of physical interactions , i.e. flow velocity, dispersivity, etc.