MINERALOGY & CHARGE PROPERTIES OF
DISPERSIBLE COLLOIDALS

Introduction

The transport of contaminants derived from agricultural and industrial operations to both surface and groundwater is increasingly acknowledged as a major environmental problem. Past studies dealing with contaminant migration to subsurface environments have considered soil or groundwater as essentially a two-phase system where the contaminants could partition either between the mobile aqueous phase or the immobile solid phase (McCarthy and Degueldre, 1992; McCarthy and Zachara, 1989; McDowell-Boyer, 1986). Based on this approach it was predicted that many contaminants would be relatively immobile, as many are either sparingly soluble or possess a high affinity for the solid phase. However, recent evidence suggests that under certain conditions contaminant transport to and/or within groundwater can be significantly enhanced via colloid association, where the colloidal phase itself is undergoing transport. Contaminants such as radionuclides, pesticides and other organics, oxyanions such as arsenate and phosphate, and hazardous metals may migrate via colloid-facilitated transport mechanisms, and currently significant research efforts are focusing on the existence and transport of colloid associated contaminants and microbes to and within groundwater systems (Coles, 1982; Gounaris, 1993; Kaplan, 1994; Kaplan et al., 1995; Kaplan et al., 1994; McCarthy and Zachara, 1989; Newman et al., 1993; Nylan, 1985; Penrose et al., 1990; Puls and Powell, 1992; Roy and Dzombak, 1997; Seta and Karathanasis, 1997; Vinten, 1983). Current approaches to predicting contaminant movement in the soil environment have often neglected the potential for colloid-facilitated transport, however, because little information is available on the physicochemical characteristics of mobile colloids or the processes controlling their generation and transport in soil and ground water systems.

The effect of colloid assisted transport mechanisms on water quality is not limited to the subsurface, as it is generally well recognized that non-point sources of nutrients and pesticides can enter streams and lakes associated with particulates eroded from agricultural lands. The hydrologic continuum between upland fields and adjacent streams is a pathway for water with suspended and dissolved materials to move from the terrestrial to the aquatic environment, with the associated hazards for contamination of surface waters from attendant nutrients and pesticides. Both the volume and quality of water moving along this hydrologic continuum, either by direct runoff or subsurface movement, is largely determined by colloid behavior in the upland. The occurrence of impermeable surface seals, which inhibit infiltration of rainfall, coupled with the formation of stable colloid suspensions at the soil surface under raindrop impact, are important sub-processes in this continuum. The quantity, mineralogy, and dispersibility of soil colloids, along with hydrologic factors, determine the amounts, types, and stability of colloids entering the aquatic system.

Bulk clay fractions (< 2 µm) of many soils in the Southern Region have been extensively characterized, primarily for the purposes of soil characterization. Far fewer investigations have specifically addressed the mineralogical or physicochemical processes regulating the generation of stable colloidal suspensions (SCS) within soils and other unconsolidated geologic materials, or on the mechanisms operative in their transport through the vadose zone. While a significant body of literature has addressed the theory and application of colloid stability and transport through well-defined, homogeneous matrices, the processes governing the generation of stable colloidal suspensions in intact soil and geologic materials has received less attention. Little information on the mineralogical or qualitative organic composition of the readily dispersive clay mineral fraction in soils of the Southern Region has, heretofore, been available. Thus, it has not been clear how the extensive data base on bulk-clay mineralogy for soils throughout the region may be related to the potential mobility of soil colloids. Differential dispersivity of highly charged clay minerals representing a relatively small fraction of the total clay may be an important mechanism. However, in other situations surface charge modification of some fraction of the more predominant lower charged minerals or the specific organic/oxide/phyllosilicate arrangement within the complex mineral assemblage may be the primary control of differential dispersion processes. If the role of colloids from soil and other unconsolidated geologic materials on surface and groundwater quality are to be better understood and predicted, then specific mineralogical and surface chemical characterization of the dispersive clay mineral fraction and the relationship to bulk clay mineralogy and surface chemistry will be required.

To address these gaps in our knowledge and provide critical information required for the development of models to predict the susceptibility of soils in the Southern Region to produce mobile colloids, the S-207 Regional Project ìMineralogy & Charge Properties of Readily-Dispersible Colloidal Fractions From Selected Soils and Sediments of the Southern Regionî was initiated on October 1, 1991.

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Last Modified: September 30, 1998

Document Prepared by:
North Carolina Agricultural Research Service
North Carolina State University