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Colloidal transport: the facilitated movement of contaminants into groundwater.

The term "groundwater pollution" typically brings to mind contaminants--pesticides, solvents, and excess nutrients, among others--that are either soluble in water or are fluids themselves, percolating downward through the soil. However, another significant class of contaminants is that of colloids.

Colloids are microscopic particles between 0.005 and 5 [micro]m, similar in size to the particles in tobacco smoke (see figure 1). Colloids move by what is often referred to as colloidal transport.

Often the colloidal particles themselves are contaminants of concern, such as colloid-sized pathogenic bacteria and viruses (sometimes referred to as biocolloids) or aggregations of toxic elements. In other cases, mobile colloidal mineral or organic matter particles are simply a convenient transport mechanism for contaminants that would be otherwise immobile in soils. In either case, particles this small can filter through soil with varying degrees of ease and can often reach groundwater.


The acceleration of movement of normally immobile contaminants due to linking up with mobile colloids (figure 2) is referred to as colloid-facilitated transport. Some of the initial discoveries of facilitated contaminant transport came as an unpleasant surprise, especially because the contaminants in question happened to be radioactive.

Plutonium was among several elements found to be moving from radioactive waste areas at a number of nuclear laboratories and test sites (McCarthy and Zachara 1989; McCarthy and McKay 2004). Despite predictions of then-current transport equations that such elements could move no more than a few millimeters through the soil before being immobilized by soil particles, the elements instead migrated anywhere from tens of meters to kilometers from the point of release. This was because the elements were traveling as colloidal particles; instead of bonding to large soil particles that would have immobilized them, the elements had bonded to particles small enough to be mobile themselves.

This phenomenon has been found to occur for a number of other contaminants. For example, colloidal or dissolved organic matter has been shown to greatly accelerate the movement of toxic "heavy" metals from a range of sources, including land-applied wastes.

Most existing research on how such colloids move through soil is based on saturated transport, with mechanisms based on soil or bedrock pores that are completely filled with water, such as occurs in aquifers. However, most contaminant sources are at or near the soil surface in the unsaturated (or vadose) zone. The presence of air in the soil matrix in the unsaturated zone greatly complicates the transport process.

In fact, McCarthy and McKay (2004) recently stated that "In many ways it is fair to say that, with respect to the vadose zone, we don't really even know all the questions, let alone understand the answers." This makes research on transport through unsaturated soil a high priority.


As for biocolloids, it has long been known that pathogenic microorganisms could move through soil to some extent, as evidenced by the required separation distances between septic systems and groundwater wells. However, there is an increasing level of concern about biocontamination of groundwater (as well as surface waters), due to increasing demand on groundwater resources, increasing use of land for waste treatment and disposal, and increasing numbers of human pathogens occurring in manures, such as E. coli O157:H7.

Gerba and Smith (2005) recently summarized some of the factors of concern:

* Over 300 million tons of animal manure are spread annually in the United States (this figure excludes manure from grazing animals). Over 150 transmissible pathogens have been found in manures to date. Furthermore, most outbreaks of microbial-induced illness from water contamination have been tied to farm animal sources.

* In terms of human sources, the fluid discharge from septic systems is estimated to be 15 billion liters per day. An additional five million tons of wastewater sludge (biosolids) is spread annually in the United States, with varying degrees of treatment to reduce human pathogens.

As such, there is a perceived need to better understand the factors that control the mobility of biocolloids in soil.

As with inorganic colloids, most research on biocolloid mobility has been done on saturated transport, with little known about transport in unsaturated soils. However, biocolloids also present additional complications: unlike inert, nonliving colloids, viruses and especially bacteria respond to their environment in ways that change their mobility. These responses include exuding extracellular substances and/or growing cell wall structures in order to form aggregations termed biofilms that enable them to "stay put." In other situations, they may take measures to increase their mobility.

Another significant limitation is that most existing research involves so-called "indicator organisms" (such as E. coli or other fecal coliforms) as opposed to the actual pathogens of concern, which may behave differently in soils.


Current research underway at Cornell University, the University of Tennessee, and elsewhere is focused on better understanding what controls the mobility of both inorganic colloids and biocolloids in unsaturated soils.

The majority of prior research in these areas has involved "black box" testing, where the colloids of concern are applied to a test soil column, which is then leached with water. The mechanisms that cause colloid mobility or retention are then inferred from the emergence patterns of colloids washing out from the bottom of the test column. Unfortunately, the mechanisms involved are complex, making this approach inadequate for understanding the transport processes.

In contrast, work in our groups is focused on directly observing the movement and retention of colloids with microscopic visualization techniques. Algorithms to count the number colloids in microscope images allow quantitative descriptions of retention processes, and mathematical models are being developed to simulate colloid transport and retention processes.

Visualization of synthetic microspheres (used as easily seen surrogates for both inorganic as well as biological colloids) is showing the importance of the interfaces between solids, air, and water in unsaturated soil in retaining colloids (figure 3). Bacteria that have been genetically altered to fluoresce are being used to help us not only observe but actually quantify when and how the bacteria attach to surfaces. The goals of this work are to better understand what causes and controls the mobility of colloids in soil.


Gerba, C., and J. Smith. 2005. Sources of pathogenic microorganisms and their fate during land application of wastes. Journal of Environmental Quality 34:42.

McCarthy, J., and L. McKay 2004. Colloid transport in the subsurface. Vadose Zone Journal 3:326.

McCarthy, J., and J. Zachara. 1989. Subsurface transport of contaminants. Environmental Science and Technology 23(5):496.

Brian K. Richards, Tammo S. Steenhuis, Yuniati Zevi, and Annette Dathe work in the Department of Biological and Environmental Engineering and Anthony G. Hay works in the Department of Microbiology at Cornell University in Ithaca, New York. John F. McCarthy works in the Department of Earth and Planetary Sciences/Center for Environmental Biotechnology at the University of Tennessee in Knoxville, Tennessee.
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Title Annotation:FEATURE
Author:Richards, Brian K.; McCarthy, John F.; Steenhuis, Tammo S.; Hay, Anthony G.; Zevi, Yuniati; Dathe, A
Publication:Journal of Soil and Water Conservation
Geographic Code:1USA
Date:May 1, 2007
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