Protection of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 202

When subjected to effective rainfall episodes, the soil is leached (Figure
106), and these nitrates migrate into the groundwater. Springs and wells
can, as a result, present small spikes in their nitrate content.
Figure 106 Anthropogenic nitrogen in water resources.
Three major anthropogenic sources of nitrates have increased the nitrate
concentration in groundwater: organic nitrogen from domestic wastewater
(sewage), organic nitrogen from livestock farming operations (liquid
excrement, slurries, manure), and mineral nitrogen from nitrogen-based
fertilizers (ammonium nitrate).
Today, a signifi cant percentage of domestic wastewater is treated
in wastewater treatment plants, in some cases equipped with a tertiary
treatment setup, and point-source nitrogen pollution from domestic sources
is decreasing. The same cannot be said for agricultural sources, from which
diffuse nitrogen surpluses fl ow into surface waters and/or infi ltrate into
groundwater. In regions with irrigated agriculture (Figure 107), the soil
produces two nitrate pulses per year (bottom line), one during the natural
infi ltration of spring rains, and the other during the summer, during the
peak of irrigation. The natural signal is no longer perceptible once the
minimum anthropogenic nitrogen content is greater than the maximum
natural nitrogen content (upper line).
The density of livestock (pigs, poulty) in certain farming regions
(Brittany in France) causes regional “structural” organic nitrogen surpluses,
as the spreading of manure is too intense over the surface of the plots in
question. As a result, in certain drainage basins, the concentration of organic
nitrogen being spread is greater than 300 Kg·ha -1 , when it is not meant to
exceed 170 Kg·ha -1 . This legal limit, the maximum allowance, does not,
however, take into account the agronomic particularities of each plot (soil
grade, thickness, and texture).