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PARTICULATE TRANSPORT IN GROUNDWATER, PART I - COLLOIDS
An On-Line Version of a Column First Published in the:
National Environmental Journal Nov./Dec. 1994 Vol. 4 No. 6 Pages 21-22
by: David B. Vance
Evaluation of mass transport of entrained material in groundwater flow systems is typically focused on two primary areas, dissolved organic or inorganic species, and flowing free phase liquids. However, while generally not as dominant overall, the movement of small particles in groundwater can cause problems at the low concentrations required by regulatory limits. In addition, the proponents of in-situ bioaugmentation, through injection of bacteria, must rely on successful particulate mass transport through the saturated soil matrix. This column will be focused on colloids, a special class of material with properties that lie between that of the dissolved state and the solid (or liquid) state. The next column will address the issue of bacteria transport.
Historically the term colloid is applied to particles with a size range of 0.001 to 1 micron (1 micron is 0.001 millimeter or approximately 0.00004 inches). The surface area per unit mass is very high for colloids, which has great effect on their mass transport behavior. The sources of colloids in groundwater include the following:
- Detached soil, mineral, or contaminant particles.
- Colloids formed from solutes undergoing geochemical precipitation due to changes in redox conditions from mixing with injected or percolated surface water.
- Emulsions of fine droplets from free phase hydrocarbons.
- Agglomerations forming micelles seeded by macromolecules such as humic acids.
- Colloids introduced directly into the groundwater from landfills or other surface sources.
On a mass basis, colloid concentrations in groundwater range from 1 to 75 mg/L. Colloid density in natural groundwater systems can range to upper limits o f 1010 particles per liter in igneous fractures and 1012 particles per liter in sandy aquifers.
Contaminants can be transported as colloids resulting in unexpected mobility of low solubility material. Colloids can also act as adsorbents for contaminants which are then transported with the colloid. If contaminants have been adsorbed to colloids it is important to remember that the transport behavior is determined by the physical/chemical properties of the colloid, not the physical/chemical properties of the contaminant. In cases where colloids are formed in-situ, contaminants can be incorporated (or occluded) into the colloidal particle as it forms.
Mechanical Colloidal Processes
Particles larger than 2 microns in the low flow conditions common in groundwater systems are subject to removal by sedimentation (settling under the influence of gravity). Below 0.1 microns the effects of adsorptive process are much more pronounced. As a result, colloids and particles in the range of 0.1 to 2.0 microns are likely to be the most mobile in groundwater. Although particles at the middle of the colloid range are overall more mobile, larger particles may travel through formations more rapidly due to size exclusion. In that process the particle travels a reduced path length through the soil matrix since it is excluded from the smaller pore spaces. Colloids or other particles can be mechanically removed by the soil matrix. The key parameter to this process is the pore entrance size, which is a function of grain size. For fine to coarse grained silts pore entrance size ranges from 0.7 to 7 microns, for fine to coarse grained sands from 24 to 240 microns, and for fine to coarse grained gravels 720 to 7,200 microns.
Mechanical removal of particles occurs most often by straining, a process in which particles can enter the matrix, but are caught by the smaller pore spaces as it traverses the matrix. If within the soil matrix there is groundwater flow through heterogeneity’s, a surface mat may form at the interface when particles are too large to enter the finer grained matrix at all. The best example of this is along the walls of fractures through fine grained sediments.
Adsorptive Colloidal Processes
The primary forces that influence colloids include: electrostatic repulsion and attraction; London- van der Walls attraction; and brownian motion. Electrostatic forces are familiar. London-van der Walls attraction is a weak (but still effective) form of chemical bonding. Brownian motion is due to molecular collisions between a particle and the surrounding fluid matrix, it becomes apparent when particle size reaches a few microns. The effect predominates colloids 0.1 microns or smaller, the smaller the size the higher the velocity that can be imparted due to brownian motion.
Adsorptive interactions of colloids may be effected by
- the ionic strength of the groundwater;
- ionic composition;
- quantity, nature and size of the suspended colloids;
- geologic composition of the soil matrix; and
- flow velocity of the groundwater.
In most instances however, the mobility of a colloid is dependent upon groundwater chemistry rather than forces due to advective flow. Higher mobility occurs at lower overall concentrations of total dissolved solids (TDS). Higher levels of TDS encourages deposition of colloids.
The reasons for this behavior deserves some explanation. Surfaces in an aquifer matrix in general have a net negative charge due to the predominance of silica in the minerals of the matrix. This charge on the matrix surfaces and the colloids in the groundwater system has a configuration that is described as an electric double layer. The first layer forms due to the collection of positive ions on the exposed negatively charged interfacing surfaces. Anions in solution then form the second diffuse layer around the first to counter the resulting positive surface charge.
As the ionic strength of the groundwater increases the thickness of the double layer decreases. When a negatively charged colloid approaches a negatively charged grain within the groundwater matrix (both with double electrical layers) mutually repulsive forces increase. Conversely, if the two surfaces can approach past the repulsive maximum, attractive London-van der Waals forces will take over, overcome the repulsive forces, and the colloid is attached to the matrix surface. The high velocities imparted to colloids smaller than 0.1 microns due to brownian motion provides the mechanism for overcoming the electrostatic repulsion of the double layers. The process is delicately balanced such that the reduction of the electrical double layer thickness through increased ionic strength is also required.
The result of this colloidal behavior is beneficial with regards to typical surface sources such as landfills. The high ionic strength of leachate will serve to provide optimum conditions for the immobilization of entrained colloids. Conversely, the injection of low ionic strength water has the potential to enhance colloid mobility and even mobilize previously adsorbed material.
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