Water Quality - Hydrogeology: Objectives, Methods, Applications - page 87

C HAPTER A11
Water Quality
The quality of groundwater is the result of a natural mineral content
(biogeochemical background noise), to which are added anthropogenic
substances (pollution).
1 NATURAL CHARACTERISTICS
Groundwater acquires its physical and chemical properties as it passes
through different sections of the water cycle: atmosphere, soil, surface
water, unsaturated zone, aquifer.
The atmosphere carries, among other things, chlorides and sulfates,
sodium and potassium from marine aerosols, but also compounds carried
as atmospheric pollution (sulfates, nitrates, hydrocarbons), originating
from the burning of fossil fuels (coal, petroleum): thermal power stations,
domestic heating, cars, or dust from bare soil (cultivated or desert zones).
The soil, as well as weathered surface terrains (weathering mantles,
epikarst), is where evaporation can act, concentrating conservative tracers
from precipitation. The available water capacity (AWC) is not only a
reservoir for water, but also for chlorides, nitrates, sodium, potassium,
which evaporation concentrates in the soil and that the next effective
precipitation will lixiviate towards the unsaturated zone.
The soil is also the fi rst interface between water and minerals: calcium
or magnesium carbonates and sulfates are dissolved there. Silicates weather
into clay, releasing calcium, magnesium, sodium, and potassium.
These dissolutions and hydrolysis reactions are possible thanks to the
presence of carbon dioxide in the soil produced by biologic activity, for
example, respiration by microfl ora and microfauna (bacteria, roots).
In the soil, the decomposition of organic material produces CO 2 , nitrates,
potassium, and humic and fulvic acids. These last two mineralize over
time (CO 2 ), thus creating the potential for carbonate dissolution and for