Quantification of denitrification rate in shallow groundwater using the single-well, push-pull test technique

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Denitrification is a key process for removing nitrate from groundwater.
• Accurate quantification of denitrification rates is essential for managing groundwater nitrate contamination.
• Limited research exists on shallow groundwater denitrification rates using diverse analytical and kinetic approaches.

Purpose of the Study:

• To assess different analytical approaches (reactant vs. product) and kinetic models (zero-order vs. first-order) for quantifying shallow groundwater denitrification rates.
• To compare denitrification rate estimates derived from nitrate reduction versus nitrous oxide production.
• To evaluate the suitability of zero-order and first-order kinetic models in analyzing push-pull test data for denitrification.

Main Methods:

• Conducted single-well push-pull tests at two sites in the Manawatū River catchment, New Zealand.
• Analyzed groundwater samples to measure nitrate reduction (reactant) and nitrous oxide production (product).
• Applied zero-order and first-order kinetic models to estimate denitrification rates.

Main Results:

• Denitrification rates based on nitrate reduction were significantly higher (6-60 times) than those based on nitrous oxide production.
• Zero-order kinetic models yielded rates of 0.05-0.12 mg N L⁻¹ h⁻¹ (WDV) and 0.42-1.07 mg N L⁻¹ h⁻¹ (PNR).
• First-order kinetic models yielded rates of 0.002-0.012 h⁻¹ (WDV) and 0.03-0.09 h⁻¹ (PNR).

Conclusions:

• Nitrate reduction measurements may offer more representative denitrification rates for shallow groundwater systems, especially when comprehensive nitrogen species analysis is impractical.
• Both zero-order and first-order kinetic models can be valid for analyzing denitrification rates from push-pull tests.
• Discrepancies in rate estimates highlight the need for further research into appropriate measurement techniques and understanding the spatial and temporal variability of denitrification.