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PARTICULATE TRANSPORT IN GROUNDWATER PART II - BACTERIA

An On-Line Version of a Column First Published in the:
National Environmental Journal Jan./Feb. 1995 Vol. 5 No. 1 Pages 25-26

by: David B. Vance

The transport of bacteria in groundwater systems is of concern from both ends of the mobility spectrum. Mobility is essential for the injection of bacteria using the bioaugmentation approach to saturated zone in-situ bioremediation. Conversely, there is high concern with mobility, and a desire for immobility, with regards to pathogens from septic systems or other sources. Bacteria range in size from 0.2 to 5 microns and viruses from 0.005 to 0.1 microns. This is within the size range considered colloidal. Therefore, bacteria transport is effected by many of the same processes as those for colloids. In the last issue, this column discussed colloidal transport in groundwater, this is focused on the transport of bacteria.

As with colloids in groundwater, factors effecting bacteria transport include mechanical and adsorptive processes. However, bacteria are live organisms and have other unique qualities that impact their transport properties. These include: surface hydrophobicity; reactive groups on the surface of the bacterial cell wall; and coatings which can be "sticky".

Mechanical Processes

Removal of bacteria can occur solely by straining within the aquifer matrix. The size of the pore space in relation to bacteria size is important enough to repeat the following information from the last column: in 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. As general rules, bacteria should be half the size of the pore entrance for adequate success in passage, and if the average bacteria size is greater than 5% of the grains (not the pore size) within the porous matrix, straining becomes an important removal mechanism.

In addition to reduction of hydraulic conductivity through the accumulation of bacterial cells as described above, other processes unique to bacteria can mechanically reduce transport efficiency. Bacteria may excrete extracellular polymers, low solubility metabolic precipitates, or gaseous products (such as nitrogen, methane or carbon dioxide) that can potentially block pore passages.

Adsorption Processes

Increasing ionic strength of groundwater increases the capability of bacteria to adhere to soil surfaces by increasing the availability of ions to act as bridges between the surfaces of the cell and soil particles, and by decreasing the thickness of the electric double layer (see last issues column for a more detailed description of this phenomena). Increased ionic strength also enhances the ability of bacteria to aggregate, forming a larger overall particle more likely to be captured in pore spaces. For example, transport efficiency of bacteria through clean sands has been shown to be 2 to 3% in water with 750 ppm of total dissolved solids (TDS) versus 70 to 100% in deionized water.

Bacteria have an overall negative charge on the surface of their cell wall. This is primarily due to the presence of peptidoglycan the structural backbone of the bacterial cell wall, which is rich in carboxyl and amino groups. Teichoic acids are a phosphate rich component of bacterial cell walls, which also help contribute to the presence of a negative charge. So that conversely to the above, bacterial adsorption to positively charged surfaces (such as those presented by iron and other metal oxyhydroxides) is at its maximum under conditions of low ionic strength. In a manner similar to that for inorganic anions, the sorption of bacteria to oxyhydroxides is also pH dependent. As a consequence, to evaluate bacteria mobility, it is important to know the specific chemical character of the soil matrix. In some circumstances, due to soil chemistry, high levels of bacterial adsorption will occur irrespective of the manipulation of ionic strength in the surrounding groundwater. This is good news for those concerned with septic systems, but bad for those wishing to inject bioaugmentation bacteria.

The bacterial cell wall also contains varying amounts of lipids that are responsible for hydrophobic behavior. Hydrophobic bacteria have the tendency to adsorb to surfaces in the groundwater system due to repulsion from the polar water molecule. The effect of electrostatic repulsive force decreases with increasing hydrophobicity of a bacterial species.

Biological Processes

Attachment of bacteria to a surface is a two stage process. First, the initial adsorption due to electrostatic or hydrophobic forces takes place. This adhesion is reversible, given adequate shearing from groundwater flow. Second, is the irreversible binding through the cellular production of exopolymers that anchor the cell to the surface. The production of these exopolymers is stimulated by the initial adsorption and has been observed to increase 5-fold once a cell has attached itself to a surface.

Bacterial motility may be important under conditions of static flow or chemotaxis. Motility is the ability of bacteria move through the use of appendages such as flagella or pili. Chemotaxis is the movement or orientation of a bacteria cell along a chemical concentration gradient. By this mechanism porous media can be penetrated by bacteria that literally grow through the pore space. Growth rates have been observed in cores at ranges up to 0.01 to 0.05 cm/hr. However, pore space is occupied by bacterial biomass which ultimately prevents the transport of required nutrients to the growth area.

Starvation of cells will reduce their size to less than 0.3 microns, therefore reducing overall filtration effects and allowing for penetration of a finer grained matrix. However, in response to the starvation stress, many bacteria will increase the stickiness of their cell walls to improve the chance of adhesion to a surface.

In general, bacterial adsorption in uncontaminated water is driven by electrostatic forces. Bacteria adsorption rates are high in groundwater at a pH of 6 or lower. Bacteria transport mobility increases above pH 6.0, significantly so above pH 7.5. In contaminated groundwater the situation is much more complex, and is specific to the site, contaminant properties, and the bacterial species present. In the presence of contamination bacteria metabolic processes play a much more dominant role in transport properties, overwhelming physical/chemical effects.

Coarse grained groundwater flow systems or large aperture fracture flow systems (such as karst) have the capacity to support substantial bacterial transport. Bacteria injection is possible and pathogen transport is probable under such conditions. However, bacteria transport in finer grained groundwater systems is more problematic. The high TDS associated with septic systems (or other potential sources of pathogens) will enhance the tendency for immobilization. Bacterial injection for bioaugmentation may be mechanically impossible due to straining and will require careful assessment under marginal porosity conditions.

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