Physicochemical controls on nonconservative anion migration in coarse-textured alluvial sediments

Three sandy subsurface materials and a sandy surface soil (Orangeburg Series) from the Upper Coastal Plain, were used to assess the influence of mineralogy and surface chemistry on the determination of physical transport parameters using ionic tracers. The day mineralogy of the surface soil consisted primarily of kaolinite, hydroxy-interlayered vermiculite, and gibbsite, while the dominant clay mineralogy of the subsurface materials consisted of kaolinite, goethite, and mica (illite). Repacked columns of the four differing strata were leached with tritiated (approximately 200 pCi mL-1) Bromide solutions, either MgBr2 or KBr, of varying ionic strengths (0.1-0.001 N). Pore-water velocities estimated by bulk density and mass flux were consistent with those estimated from tritium breakthrough. In contrast, Br- breakthrough differed drastically within the four materials and was altered by experimental conditions (sample drying, carrier cation, etc.). The retardation for 0.001 N KBr varied from 0.94 for the surface soil to 2.15 for the subsurface materials and increased with increasing Fe oxide content. For the subsurface samples, Br- was retarded to a greater degree in the presence of Mg2+ compared to K+. In contrast, oven drying the sample reduced the degree of Br- retardation observed for the subsurface materials. These results indicate that retardation can vary dramatically within materials of similar texture, mineralogy, and origin. The apparent conservative behavior of an ionic species under a given set of conditions (mineralogy, pH, ionic strength, scale size, predominant counter ion) does not automatically ensure that transport will remain conservative as those conditions are altered by changes in experimental design or unforeseen circumstances encountered at the field scale.