Water Quality - Hydrogeology: Objectives, Methods, Applications - page 91

is put into circulation when meteorological conditions are unfavorable, for
example, when a 30 mm rainfall follows a fertilizer application. In addition,
if the phenological stage of the culture is not nitrogen-fi xing, the stock
accumulated in the soil will all be intact when it rains. So the spreading
of liquid cow manure during a snowy period in the Haut-Doubs results
in a pollution spike in the underlying karst groundwater reserves, while
the same input on a prairie in full growth is signifi cantly dampened. The
solution is to increase the storage capacity of farmers' tanks, so that they can
store their winter waste, and to educate them in environmentally friendly
practices. In addition, the soil reacts with the molecules initially used on
cultures, and certain phytosanitary products can transform into something
else (for example, glyphosate into amino-methyl-phosphonic acid—AMPA,
formerly atrazine into desethyl-atrazine—DEA).
4 CONTAMINATION CAUSED BY HUMAN ACTIVITY
The over-exploitation of groundwater resources, that is, the mining of a
discharge higher than the normal recharge fl ux, can cause the infl ow of
highly mineralized water (salty, fl uoridated). Due to this phenomenon, the
wells exploiting the infra-Toarcian aquifer in Deux-Sèvres must not draw on
the base of the aquifer, which has a fl uoride content of about 7.5 mg·L -1 .
5 SELF-PURIFICATION
Self-purifi cation is a set of hydraulic, physical, chemical, and microbiological
processes, with the effect of reducing the concentration and/or mass of an
injected contaminant.
Contaminants can behave in several ways (Figure 49):
• the concentration does not signifi cantly decrease as the contaminant
moves downslope. In this case it is called a conservative (or conserved)
contaminant, and the transfer occurs through convection (or advection).
Example: the chlorides contaminating catchment systems in the Alsace
aquifer downstream of potassic slag heaps;
• the concentration decreases, but the total mass of contaminant remains
the same: only hydraulics played a role, through diffusion, dispersion,
and dilution (Figure 50);
• a special case of the preceding behavior is the presence of a delay
between the passage of the water and the passage of the contaminant.
This delay is due to sorption/desorption events;