Envlron. Sci. Technol. 1993, 27, 1193-1200
Soil-Borne Mobile Colloids As Influenced by Water Flow and Organic Carbon Daniel 1. Kaplan, Paul M. Bertsch,' Domy C. Adrlano, and Wllllam P. Mlllert
Division of Biogeochemistry, Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802 Paucity of understanding mechanisms relevant to the generation of subsurface mobile colloids is a major limitation to our current knowledge of colloid-facilitated contaminant transport. To evaluate the roles of natural organic materials and pore water velocity on mobile colloid generation, colloids generated from 14-m3 lysimeters containing reconstructed soil profiles were collected and characterized. Colloids generated during low flow rates were 1030% less abundant, contained a t least 65% more iron oxides and gibbsite, were 80% smaller, and had 40% greater electrophoretic mobility than colloids generated during higher flow rates. Quartz, kaolinite, and hydroxyinterlayered vermiculite were enriched by at least 32% in colloidsgenerated during faster flow rates. Mobile colloid surface charge was greatly enhanced by organic carbon (OC) coatings. Concentrations of OC associated with mobile colloids were higher than or equal to the OC concentrations existing in the bulk soils from which the mobile colloidswere derived. The profound effects of pore water flow rate and OC on mobile colloid generation introduces complexityto this potentially critical, yet poorly understood, component of subsurface contaminant transport. Introduction
Understanding the conditions conducive to the generation of mobile colloids in the subsurface environment is becoming increasingly important in light of the growing number of reports on colloid-facilitated transport of contaminants (1-3). Trace metals (41, radionuclides (46), and organic contaminants (7-9),which are typically immobile due to strong binding to soil particles or low water solubility, may move to or through the subsurface environment in association with mobile colloids. Although the concept of colloid-facilitated transport is often invoked to account for anomalies between predicted and observed transport of these contaminants, little field or experimental verification of this potentially important phenomenon is available. Additionally, most current approaches to predicting contaminant transport ignore this mechanism not because it is obscure or because the mathematical algorithms have not been developed (2, IO),but because little information is available on the occurrence, the mineralogical properties, the physicochemical properties, or the conditions conducive to the generation of mobile colloids. Stable colloid suspensions are generally believed to be generated in the subsurface environment by (1) the formation of colloidal precipitates resulting from supersaturated geochemical conditions; (2) the release of colloidal particles from the matrix (the immobile phase) by dissolution of iron oxide and CaC03 cementing agents; (3) the dispersion of particles by lowering electrolyte 'Department of Crop and Soil Sciences, University of Georgia, Athens, GA 30602. 0013-936X/93/0927-1193$04.00/0
0 1993 American Chemical Society
content or raising pH of the aqueous phase which increases particle-particle repulsive forces; and (4) the vertical transport of inorganic and organic materials from surface soils to underlying strata (3). The mechanical energy of moving water may also generate colloidal material by imposing a shear stress on the matrix, resulting in the release of particles into suspension. The extent to which moving water generates mobile colloids in the subsurface environment is largely unknown. In a survey of 35 southeastern US. ultisols, Miller and Radcliffe (11)reported that a majority of soils required some form of mechanical energy before dispersing. Most of these soils contained more than 50% of their total clay in a form that could be dispersed in water after 24 h of being shaken. In fact, much of the dispersion occurred within 1 h of shaking, suggestingthe energy input necessary for dispersion was not excessive and that perhaps the movement of pore water through a soil profile would provide sufficient energy to mobilize some of this waterdispersible clay fraction. Kaplan (12) reported that the clay fractions dispersed from bulk soil samples from ultisol profiles after 16 h of shaking in water was similar in mineralogy and size distribution to the mobile colloid fraction. By increasing well pumping rates, particles that are otherwise stationary in the aquifer may become suspended (13-16). This has significant ramifications regarding groundwater monitoring because if contaminants are sorbed to these particles, then the groundwater sample is no longer representative of the contaminant composition actually mobile within the aquifer. The effect of pumping rate or pore water flow velocity on colloid generation has received only little attention (13, 15). Additionally, changes in colloid properties produced by varying pore water flow rate have not been investigated. The well-drained sandy soils, frequent intense rainfall, and the low ionic strength of the soil solution, which are characteristic of the Upper Coastal Plain of the Southeast, may make the subsurface environment of this region especially susceptible to the generation of mobile colloids. To evaluate the importance of these conditions, mobile colloids generated from lysimeters containing soil profiles typical of this area were collected and characterized. Particular attention was directed at the changes in colloid properties with changes in pore water flow rate because preliminary studies revealed distinct differences in color and light-scattering intensities of mobile colloid suspensions collected during and following rain events. Materials and Methods
Lysimeter. Five 3 m X 3 m X 1.5 m (length X width X depth) lysimeters were used in this study. The lysimeter
bottoms were equipped with a zero-tension drainage port and contained 5 cm of coarse sand (1.5 mm) over 5 cm of pea gravel (10 mm) to facilitate water drainage. The Ap and E horizons of a Blanton sand (loamy, siliceous,thermic Grossarenic Paleudult) collected from a forested area on Environ. Sci. Technol., VoI. 27, No. 6, 1993
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Table I. Selected Chemical and Physical Properties of the Blanton Soil Profile in Lysimeter 1 AP
E
depth, m 0-0.2 0.2-1.3 PH 6.9 5.3 volumetric water content,a cm3 cm3 0.38 0.42
