Modeling nitrate fluxes at the catchment scale using the integrated tool CAWAQS

Area of Science:

• Environmental Science
• Hydrology
• Water Quality Management

Background:

• Intense agricultural activity in the Grand Morin basin leads to significant nitrate fluxes.
• In-stream nitrate concentrations showed a concerning average of 6 mg N L(-1) between 1991-1996, with a rising trend.
• Understanding nitrate transport and fate is critical for managing water quality in agriculturally impacted catchments.

Purpose of the Study:

• To quantify nitrate fluxes within the Grand Morin basin under agricultural pressure.
• To identify and assess the primary mechanisms responsible for nitrate loss at the basin scale.
• To compare in-stream observations with physically based simulations to understand discrepancies.

Main Methods:

• Conducted in-stream water quality observations over five years (approx. 250 sampling days).
• Employed the physically based CAWAQS (CAtchment WAter Quality Simulator) model for simulations.
• Performed a mean annual mass balance analysis at the basin scale.

Main Results:

• Simulated and observed nitrate concentrations differed, indicating significant nitrate losses within the basin.
• Denitrification in wetlands, alluvial plains, hyporheic zones, and benthic river processes were identified as key loss pathways.
• Annual mass balance revealed 40% of infiltration flux removed via river network, 40% stored in aquifers, and 20% lost to denitrification.

Conclusions:

• Denitrification plays a vital role in reducing nitrate loads in the Grand Morin basin.
• The study highlights the importance of integrated hydrological and biogeochemical processes in managing agricultural impacts on water quality.
• Findings underscore the need for strategies that enhance natural denitrification pathways to mitigate nitrate pollution.