Nitrate sources and factors controlling its distribution in agricultural
lands and groundwater in two small catchments in southern Quebec
Rachel Thériault1, Christine Rivard2*, Martine Savard2, René Lefebvre1
1
: INRS, Centre Eau Terre Environnement,
2
: Commission Géologique du Canada,
* : crivard@nrcan.gc.ca
Nitrate issues have been identified in many aquifers located in rural areas worldwide.
Agriculture is one of the main economical activities in the Montérégie Est region (QC,
Canada) and the main rivers are heavily impacted by nitrate. However, groundwater does
not seem to be affected by this contamination. This study was carried out as part of a
regional aquifer assessment and it focused on two small catchments in which intensive
agricultural activities were mainly related to corn and soybeans crops. This study
investigated nitrogen exchanges between the vadose zone and the bedrock aquifer in
order to better understand processes contributing to low nitrate concentrations in
groundwater of this region.
Fieldwork included soil sampling, surface and groundwater sampling, groundwater-level
measurement, as well as soundings and drilling at 9 sites. Soils were sampled at three
depths (30, 60 and 90 cm) during three field campaigns. Laboratory work included
analyses for nitrate concentrations, as well as oxygen (18O) and nitrate (15N) isotopes to
identify nitrate sources and characterize chemical processes. Numerical simulations were
also run with the 1D hydrological model Agriflux to estimate magnitudes and temporal
distributions of water and nitrate fluxes resulting from infiltration. Actual nitrogen loads
provided by farmers were available to compare with measured concentrations.
Nitrate was detected in only a few wells and isotopic values exhibited large disparities,
which suggest isolated contamination from local permeable bedrock formations or poor
well sealing. Elevated nitrate concentrations in surface water during the spring melt and
after heavy rainfalls suggest a large contribution from soils and agricultural drains.
Values of 15N and 18O show that denitrification can occur in soils where drainage is
moderate or poor, contributing to lowering nitrate concentrations. Modelling results,
coupled with percentages obtained from another study carried out in the same watershed
that considered drains, showed that 32% of precipitation contributes to runoff, while 29%
flows through soil drains and 39% infiltrates into soils. However, nitrate losses could not
be determined accurately. The presence of poorly permeable soils also prevents
groundwater from being contaminated because it favours subsurface lateral flow.
Identification of nitrate sources in soils is more difficult than in water, because isotopic
values can be affected by several processes, especially in poorly drained soils.
Nevertheless, isotopes offer the possibility of detecting these biogeochemical processes
and improving the understanding of the nitrogen cycle in agricultural lands.
Keywords: nitrate, isotopes, groundwater, soil, southern Quebec, modelling.