Natural Protection of Spring and Well Drinking Water Against Surface
Microbial Contamination:
I. Hydrogeological Parameters
THE ISSUE
One of the multiple barriers that can be claimed for human health
protection
is source water sequestration. In particular, some groundwaters
may be so
sequestered and not under the influence of surface water directly
so as to pre-
clude microbial intrusion. In order to verify the sequestered
nature of these
groundwaters, hydrogeological parameters were established.
RESEARCH STRATEGY
A practicing hydrogeologist (Mr. John Robertson, Hydro Geologic]
and prac-
ticing medical microbiologist (Dr. Stephen Edberg, Yale University)
collaborat-
ed to develop hydrogeological parameters that would ensure
the integrity and
sequestration of the source water.
MAJOR FINDINGS AND SIGNIFICANCE
The fate and transport of microbes in groundwater are controlled
by physi-
cal/chemical characteristics of the microbe and of the
groundwater/aquifer
media. Key characteristics of the microbe include size,
inactivation (die-offl
rate, and surface electrostatic properties. Key properties
of the
groundwater/aquifer system include flow velocity, aquifer
grain (or pore) size,
porosity, solid organic carbon content, temperature,
pH and other chemical
characteristics of water and mineral composition. Because
of size and surface
electrical properties, viruses are much more mobile in
groundwater than
Cryptosporidium and Giardia (which are about 100 times
or more larger than
viruses). The inactivation or die-off rate is usually
the most important factor
governing how far microbes in groundwater migrate from
a few hours to a few
weeks. Examples of maximum reported migration distances
of microbes in
groundwater include: bacteria, 600 meters in a sandy
aquifer; viruses, 1,000 -
1,600 meters in channeled limestones and 250-408 meters
in glacial silt-sand
aquifers; no reports were found which confirm the extent
of migration dis-
tances for Cryptosporidium and Giardia. Investigations
by the EPA have indi-
cated that distances of 210 to 325 meters away from septic
tanks are necessary
to achieve a reduction in virus concentrations of an
11 order magnitude.
Robertson, J. B., and Edberg, S.C., Natural Protection
of Spring and Well Drinking Water
Against Surface Microbial Contamination: I. Hydrogeological
Parameters, Critical Reviews in
Microbiology, 23(2):143-178 (1997).
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