Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 146

6.3 Microbiologic analyses
These analyses concern a wide spectrum of micro-organisms, varying in
size depending on the width of the pores or fractures through which water
passes. In groundwater analyses, micro-organisms include viruses, bacteria,
protozoans, and small multicellular organisms of < 150 µm (Zwahlen,
Editor-in-chief, 2004). Their analysis can be done after in vitro culture
or not.
Micro-organisms living in biofi lms play a role in trapping various
types of bacteria, which can be released under certain conditions (for
example during fl oods). Biofi lms therefore increase microbes' survival.
During transport, micro-organisms are affected by their own intrinsic
characteristics, but also by their environment (soil, sediment, unsaturated
zone, discharge). The transport mechanisms affecting colloids (particles of
0.001 to 1 µm, kept in suspension by Brownian motion) can also apply to
viruses and bacteria (Figure 81), which are of similar size. Based on the size
of the empty voids in an aquifer, bacteria and protozoans (but not viruses)
are likely to be physically fi ltered out, or subjected to sedimentation, if the
fl ow velocity is low enough.
Figure 81 Size of micro-organisms compared to pore size in aquifers (West et al ., 1998 in
Zwahlen 2004).
The lifespan of micro-organisms (including pathogens) is determined
by temperature (cold is favorable), the type of organism, its predators,
water chemistry, and particularly, the organic carbon content and shaliness
(adsorption). The presence of indigenous bacterial populations can result
in a lower survival rate for allogenous microbes.