5.1 Aquifer Storage of Nitrogen
The central area of the Seine basin is characterised by the presence of large aquifers
within sedimentary rock formations (Fig. 1), with decadal groundwater residence
time. Nitrate concentrations of these aquifers monitored since the beginning of the
twentieth century in several locations (Fig. 11a) show a significant increase from
the beginning of the 1960s. The MODCOU model coupled with STICS [20, 21]
simulates this evolution in the main aquifer formations (Fig. 10b) and provides a
picture of the current level of N contamination in several aquifers at a rather fine
resolution (Fig. 10c). The drinking water standard of 11 mgN/l is exceeded in many
places.
The model also calculates the recharge of the aquifer formations (infiltration from
agricultural, forested and urbanised soils of the basin) and its N concentration and
the exfiltration from the aquifer to the river network for the period from 1970 to
2015. As a long-term average, about 56% of the total water runoff of the Seine
watershed flows through aquifers, forming the base flow of the river network (with
water ages about 10 years), while the rest forms the surface or sub-surface flow
rapidly (weeks) reaching rivers. Although no denitrification process is taken into
account within the aquifers, the model calculations show that the N flux associated
with the base flow is 55% lower than the N flux contributing to the recharge of
aquifers. This large budget default can be explained by two processes: (1) water
extraction both for irrigation and drinking water provision, currently accounting for
about 1.2 Gm
3 /year, i.e. 13% of aquifer recharge, and (2) long-term storage of nitrate
in the groundwater and the non-saturated zone. Both processes together reduce by
more than half the amount of N transferred from watershed soils to the hydrosystem.
5.2 Riparian Processes
Before they reach the river bed, flows of superficial and phreatic water coming
from the watershed, with their nutrient concentration determined by land use and
agricultural practices as discussed above, have to cross a more or less extended
riparian area where biogeochemically active superficial soils, often rich in organic
matter, are in contact with the river water table. These soils have a significant
denitrification capacity, as well as a propensity to reduce iron oxides, thus possibly
releasing adsorbed phosphates. Unless the watershed area is equipped with tile
drains, by-passing the riparian zone (as is the case in some areas), the flow of nitrate
effectively reaching the river is therefore reduced by the denitrification capacity of
the riparian wetland. Billen et al. [7] estimated the extent of riparian denitrification
in the Seine watershed at 150 kgN/km
2 /year. A more recent study, based on the
coupling of Riverstrahler with STICS-MODCOU, yields a significantly higher
figure of 270 kgN/km
2 /year, i.e. 18% of the flux of nitrate coming from base and
sub-surface runoff. As expected, this riparian retention mostly occurs in large
The Seine Watershed Water-Agro-Food System: Long-Term Trajectories of C. . .
105
The central area of the Seine basin is characterised by the presence of large aquifers
within sedimentary rock formations (Fig. 1), with decadal groundwater residence
time. Nitrate concentrations of these aquifers monitored since the beginning of the
twentieth century in several locations (Fig. 11a) show a significant increase from
the beginning of the 1960s. The MODCOU model coupled with STICS [20, 21]
simulates this evolution in the main aquifer formations (Fig. 10b) and provides a
picture of the current level of N contamination in several aquifers at a rather fine
resolution (Fig. 10c). The drinking water standard of 11 mgN/l is exceeded in many
places.
The model also calculates the recharge of the aquifer formations (infiltration from
agricultural, forested and urbanised soils of the basin) and its N concentration and
the exfiltration from the aquifer to the river network for the period from 1970 to
2015. As a long-term average, about 56% of the total water runoff of the Seine
watershed flows through aquifers, forming the base flow of the river network (with
water ages about 10 years), while the rest forms the surface or sub-surface flow
rapidly (weeks) reaching rivers. Although no denitrification process is taken into
account within the aquifers, the model calculations show that the N flux associated
with the base flow is 55% lower than the N flux contributing to the recharge of
aquifers. This large budget default can be explained by two processes: (1) water
extraction both for irrigation and drinking water provision, currently accounting for
about 1.2 Gm
3 /year, i.e. 13% of aquifer recharge, and (2) long-term storage of nitrate
in the groundwater and the non-saturated zone. Both processes together reduce by
more than half the amount of N transferred from watershed soils to the hydrosystem.
5.2 Riparian Processes
Before they reach the river bed, flows of superficial and phreatic water coming
from the watershed, with their nutrient concentration determined by land use and
agricultural practices as discussed above, have to cross a more or less extended
riparian area where biogeochemically active superficial soils, often rich in organic
matter, are in contact with the river water table. These soils have a significant
denitrification capacity, as well as a propensity to reduce iron oxides, thus possibly
releasing adsorbed phosphates. Unless the watershed area is equipped with tile
drains, by-passing the riparian zone (as is the case in some areas), the flow of nitrate
effectively reaching the river is therefore reduced by the denitrification capacity of
the riparian wetland. Billen et al. [7] estimated the extent of riparian denitrification
in the Seine watershed at 150 kgN/km
2 /year. A more recent study, based on the
coupling of Riverstrahler with STICS-MODCOU, yields a significantly higher
figure of 270 kgN/km
2 /year, i.e. 18% of the flux of nitrate coming from base and
sub-surface runoff. As expected, this riparian retention mostly occurs in large
The Seine Watershed Water-Agro-Food System: Long-Term Trajectories of C. . .
105