Protection of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 195

Analyses at the catchment point measure the “health” of the water
resource to be potabilized. This quality at the source is a function of the
potential risks over the catchment basin, as well as of the natural “fi ltration”
conditions of the aquifer. The aquifer fi lters through two principal processes:
mechanical fi ltering of particles too large to pass through the pore spaces
(Figure 81); fi ltering of bacteria as they die if the transit time is long enough.
Environments with small pore spaces and slow circulation velocities (silt,
clayey sand) retain bacteria. In addition, advection takes enough time
(months) that it is longer than micro-organisms' half-life (DT 50 ), resulting in
their death. In open environments (gravel channels, fractures in hard rock,
karst conduits) however, the transit time (hours or days) is not enough for
the microorganisms to die off, and the fracture “porosity” is too large to
fi lter them out.
However, other analyses are performed after disinfection, and these
then serve to measure the treatment effectiveness. Finally, analyses are done
on the water reaching consumers (bars, communities, individuals), serving
to minimize the risk of contamination in the distribution network.
These three analysis locations are important; the one at the resource
helping to manage activity in the recharge area, the one after treatment
guaranteeing the distributed product's innocuity, and the one at the fi nal
destination helping to determine the appropriate management of public
and private infrastructure (reservoir cleaning, sewage leaks). Treatment
accidents can cause catastrophes, as in the cryptosporidiosis outbreak
in Milwaukee (USA), which resulted in 104 deaths among the 403,000
people contaminated in 1993. That was a case involving surface water, but
7 groundwater contaminations were reported between 1984 and 1997 in
Great-Britain, in the USA, and in Japan.
Certain bacterial contaminations, such as Legionella , are a result of unsafe
use of hot water (aerosols in showers, cooling towers), which results in
bacteria proliferating in the less used branches of household piping.