Effect of iron diagenesis on the transport of colloidal clay in an unconfined sand aquifer

Abstract

The role of Fe diagenesis in the transport of clay colloids was investigated in the Cohansey Sand, an Fe(III) oxide-coated quartz arenite that covers most of the New Jersey Coastal Plain. Based on our past work, we hypothesized that clay had been transported into the sediments, that the clay distribution was controlled by attachment to surface Fe(III) oxides and that anoxic water infiltrating from a swamp had dissolved Fe(III) oxides and released clay colloids into flowing groundwater. Sediment cores were collected from upland and swamp terrains and the composition and distribution of the clay-sized and heavy mineral fractions were examined by X-ray diffraction, optical and electron microscopy, separations and elemental analyses. Throughout the upland core and below 6.1 m in the swamp core, oxidized soil and sediment layers contained goethite and > 15 μmol g − surface Fe (extractable by Ti(III) reduction), but the swamp-reduced soils and sediments lacked goethite and contained <5 μmol g − surface Fe. The clay-sized content of the oxidized sediments was roughly double that of the reduced sediments. Electron microscopy revealed that coatings on the quartz grains had the appearance of infiltrated clay particles. The relationship between clay and surface Fe content indicated that the onset of reducing conditions below the swamp remobilized clay colloids by dissolving Fe(III) oxide cement. Surface Fe(III) oxides were derived from weathering of ilmenite and pseudorutile, Fe-Ti oxides found in the heavy mineral fraction. In the oxidized sediments, Fe was transported from the Fe-Ti oxide grains to quartz surfaces, where it was deposited as surface Fe(III) oxides mixed with kaolinite. Thus, the weathering of Fe-bearing minerals and the formation and dissolution of secondary Fe(III) oxides influenced the mobility of colloidal clay in the Cohansey Sand.