Mineralogical and physicochemical differences between mobile and nonmobile colloidal phases in reconstructed pedons

Ultisols can be highly dispersive and therefore a potential source for mobile colloids to underlying aquifers. In this study, mobile colloids were collected after a rain from 13.5-m(3) lysimeters containing reconstructed pedons of two Ultisols. The mobile colloids were enriched with kaolinite, Fe oxides, and gibbsite and were more dilute in quartz and hydroxy-interlayered vermiculite (HIV) relative to the total clay fractions of the originating pedons. The colloids were <1000 Nn in diameter and had a bimodal size distribution: the larger colloids (approximate to 760 nm) consisted primarily of quartz and HIV; the much more abundant smaller colloids (approximate to 230 nm) consisted primarily of Fe oxides, kaolinite, and gibbsite. The colloids also had exceptionally high negative surface charges that probably originated from organic coatings (10 g kg(-1)). These organic coatings indicate that the majority of the colloids originated from the surface horizon, the only horizons with measurable amounts of organic C. Soil minerals most likely to enter the mobile phase (Fe oxides, gibbsite, and kaolinite) were readily dispersible and in the <200-nm fraction. The HIV in the reconstructed pedons was readily dispersible but because of its relatively large size (>200 nm), it was removed from the mobile phase during transport, presumably via a straining mechanism. Quartz was neither readily dispersible nor abundant in the <200-nm fraction and, therefore, was not prevalent in the mobile phase. The ability of Ultisols to release colloids that can readily move through a pedon may make these soils and the underlying aquifers especially susceptible to colloid-facilitated transport of contaminants.