The Groundwater Environment

In the groundwater within the crust, there are a number of physical and chemical factors which would begin to stress any incumbent microorganisms at deeper and deeper depths.

 

This would change the nature of the dominant microbial species. However, at shallow depths where the physical factors are within the normal range for surface dwelling species, these species may continue to remain dominant. As the groundwater environment becomes deeper there are a number of factors which will influence the microbial species dominance. These factors are increased in temperature, pressure and salt concentration together which decreases in oxygen availability and the Eh potential.

 

In general, geothermal temperature gradients show average increase in ordinary formations of 2.13 ºC per 100 meters of depth. Groundwater at a depth of 1,000 meters would therefore be expected to have a temperature elevation of 21.3ºC and, for 2,000 meters the rise would be on average 42.6ºC which would take the environment up into the range where thermophilic organisms would flourish but not the surface dwelling microorganisms (mesophiles). These gradient temperatures rises would be influenced by the presence of hot rock masses or unusually high or low hydraulic conductivity.

 

Hydraulic pressures will also be exerted to an increasing extent at greater depths particularly below the interface between the unsaturated (vadose) zone and the saturated (water table).

 

In general, the soil-water zone will extend 0.9 to 9.0 meters from the surface and will overburden the vadose zone. Generally in igneous rocks, the bottom of the groundwater could be at depths of 150 to 275 meters while in sedimentary rocks depths could reach 15,900 meters. In the latter case, extremely high hydraulic pressures could be exerted upon any incumbent microflora. Hydraulic pressures will be very much influenced by the hydraulic conductivity of the overburden and the degree of interconnectiveness between the various related aquifers within the system. In oceanic conditions, these factors are not significant and pressures can reach 600 atmosphere (8,820 p.s.i. or 1,280 kPa). At these higher pressures, the temperature at which water would convert to steam would also be elevated so that microorganisms could exist and flourish at superheated temperatures provided that the water was still liquid. For example, water will not boil until the temperature (ºC) is reached for the pressure shown (in brackets, p.s.i.): 125ºC (33.7); 150ºC (69.0); 200ºC (225.5); 250ºC (576.6); 300ºC (1,246); and 350ºC (2,398). There would therefore appear to be a strong probability that there may be a series of stratified microbial communities within the crust which are separable by their ability to function under heavier pressures in more concentrated salts at higher temperatures under extremely low (-Eh) redox potentials (e.g., -450 mL).

 

For these microorganisms to survive and flourish in extremely deep sub-surface environments, the geologic formations must provide openings in which the water (and the organisms) can exist. Here the microbes will, in all probability, grow attached to the surfaces presented within the openings. Typical openings include intergrain pores (in unconsolidated sandstone, gravel, and shale), systematic joints (in metamorphic and igneous rocks, limestone), cooling fractures (in basalt), solution cavities (in limestone), gas-bubble holes and lava tubes (in basalt) and openings in fault zones. All of these openings provide surfaces large enough to support such attachment if the cells can reach the site. Such a restriction would be relative to the size of the microbial cells in relation to the size of the openings.

 

Porosity is usually measured as the percentage of the bulk volume of the porous medium that is occupied by interstices and can be occupied with water when the medium is saturated. For coarse to fine gravel, the percentage porosity can range from 28 to 34% respectively.

 

Porosities for sand tend to range from 39 to 43%. Fine and medium grain sandstone possess porosities from 33 to 37% respectively.

 

A tightly cemented sandstone would have a porosity of 5%. However rocks tend to be porous structures with pore sizes large enough to accommodate bacterial cells which, in the vegetative state, have a cell diameter of between 0.5 and 5 microns. When these cells enter a severe stress state due to starvation or environmental anomalies, many bacteria will shrink in size to 0.1 to 0.5 microns and become non-attachable.

 

Such stressed survival cells are referred to as ultramicrobacteria and are able to pass through porous structures for considerable distances.

 

Recovery is dependent upon a favorable environment being reached which would allow growth and reproduction. These ultramicrobacteria, therefore have the potential to travel through porous media and remain in a state of suspended animation for very prolonged periods of time. Even normal vegetative cells (e.g., those of Serratia marcescens) have been shown to pass through cores of Berea sandstone (36.1 cms) and limestone (7.6 cms) during laboratory studies. There is, indeed, a growing body of knowledge supporting the ability of bacteria to invade and colonize porous rock formations. Viruses have also been recorded as being able to travel through porous structures.

 

Fractures in rocks tend to have much larger openings than pore structures and can therefore form into potential groundwater flow paths which could act as focal sites for microbial activities. This heightened activity would result in part from the passage of groundwater containing potentially valuable dissolved and suspended chemicals (e.g., organics, nutrients, oxygen). In addition, the flow paths would allow the transportation of the microbial cells and also the dissolution of waste products. Microbial mobility through fracture flow paths in rocks can be very fast. In 1973, bacteria were recorded to travel at rates of up to 28.7 m/day through a fractured crystalline bedrock.

 

Microbial events can include corrosion initiation, bioimpedance of hydraulic or gaseous flows (e.g., plugging, clogging), bioaccumulation of chemicals (e.g., localized concentration of heavy metals, hydrocarbons and/or radionuclides within a groundwater system), biodegradation (e.g., catabolism of potentially harmful organic compounds), biogenesis of gases (including methane, hydrogen, carbon dioxide, nitrogen which can lead to such events as the fracturing of clays, displacement of water tables and differential movement of soil particles), water retention within biofilms (which, in turn, can influence the rates of desiccation and/or freezing of soils). These events occur naturally within the biosphere as microbiologically driven functions within and upon the crust of the planet.

 

  1. MIF (microbially induced fouling) where the microorganisms begin to generate a confluent fouling of the surfaces of the porous media particularly at the reductionoxidation interface. As these biofilms expand and interconnect, so the transmissivity of water through the system becomes reduced (plugging).
  2. MIA (microbially induced accumulation) will occur at the same time and will involve a bioaccumulation of various ions such as metallic elements in the form of dissolved or insoluble salts and organic complexes.
  3. MGG (microbial generation of gases) with the maturation of the biofilm and extension of anaerobic growth, gas formation is much more likely. Gases may be composed of carbon dioxide, methane, hydrogen and nitrogen and may retain within the biofilm as gas filled vacuoles or in the dissolved state. Radical gas generated biofilm volume expansion could radically reduce hydraulic transmissivity through the biofouled porous structures.
  4. MIC (microbially induced corrosion) poses a major threat to the integrity of structures and systems through the induction of corrosive processes. Where the biofilm have stratified and/or incorporates deeper permanently anaerobic strata there develops a greater risk of corrosion. Such corrosive processes may involve the generation of hydrogen sulfide (electrolytic corrosion) and/or organic acids (solubilization of metals).
  5. MIR (microbially induced relocation) occurs where the biofilm growth and biological activities associated with these microbial activities causes a shift in the redox potential or the environmental conditions in such a way that there is a relocation of the biofilm. Here, the biofilm begins to sheer and slough away carrying some of the biomass and accumulates in the flow. Relocation may also be a managed function when theenvironmental conditions are artificially manipulated.