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 influenc e of mineralogy and surface chemistry on the determination of physical transport parameters using ionic tracers, The clay mineralogy of the surface soil consisted primarily of kaolinite, hydroxy-interlayered ve rmiculite, and gibbsite, while the dominant clay mineralogy of the sub surface materials consisted of kaolinite, goethite, and mica (illite), Repacked columns of the four differing strata were leached with triti ated (approximate to 200 pCi mL(-1)) Bromide solutions, either MgBr2 o r KBr, of varying ionic strengths (0.1-0.001 N), Pore-water velocities estimated by bulk density and mass flux were consistent with those es timated from tritium breakthrough, In contrast, Br- breakthrough diffe red drastically within the four materials and was altered by experimen tal 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, F or 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 mate rials, These results indicate that retardation can vary dramatically w ithin materials of similar texture, mineralogy, and origin. The appare nt conservative behavior of an ionic species under a given set of cond itions (mineralogy, pH, ionic strength, scale size, predominant counte r ion) does not automatically ensure that transport will remain conser vative as those conditions are altered by changes in experimental desi gn or unforeseen circumstances encountered at the field scale.