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 colloid s were collected after a rain from 13.5-m(3) lysimeters containing rec onstructed pedons of two Ultisols. The mobile colloids were enriched w ith 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 dia meter and had a bimodal size distribution: the larger colloids (approx imate 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 exception ally high negative surface charges that probably originated from organ ic coatings (10 g kg(-1)). These organic coatings indicate that the ma jority of the colloids originated from the surface horizon, the only h orizons with measurable amounts of organic C. Soil minerals most likel y to enter the mobile phase (Fe oxides, gibbsite, and kaolinite) were readily dispersible and in the <200-nm fraction. The HIV in the recons tructed pedons was readily dispersible but because of its relatively l arge size (>200 nm), it was removed from the mobile phase during trans port, 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 an d the underlying aquifers especially susceptible to colloid-facilitate d transport of contaminants.