The mobilization of aluminum in a natural soil system: Effects of hydrologic pathways

Abstract

A two‐component soil water flow model was used in conjunction with an equilibrium speciation model WATEQF to study aluminum mobility in soils of a forested watershed, White Oak Run, in the Shenandoah National Park, Virginia. Soil solution samples, taken from the O, E, B, C₁, and C₂ horizons, were collected from zero‐tension lysimeters designed to collect faster gravitational macropore flow and tension lysimeters designed to collect slower capillary micropore flow. Dissolved aluminum was fractionated into acid‐soluble, inorganic monomeric, and organic monomeric aluminum. Soil water aluminum concentrations decreased with depth indicating that the deep soil is a sink for aluminum. All waters contained significant concentrations of acid‐soluble aluminum and exhibited a negative correlation between pH and the inorganic monomeric aluminum concentrations. Water in the shallow soil showed distinctly different chemical compositions for the two flow types, while C horizon micropore and macropore waters were more similar. Because of its shorter residence time, water flowing in deep soil macropores underwent less extensive neutralization and immobilization of aqueous aluminum than micropore water. The O horizon macropore waters were undersaturated for all hydroxide, silicate, and sulfate mineral phases considered. The C horizon samples from both flow types were near equilibrium with respect to kaolinite and synthetic gibbsite, indicating that mineral solubility controls water chemistry in the deep soil, while organic substances are the key control in the shallow macropore waters.