『Abstract
Nitrate (NO3--N) contamination
of groundwater and associated surface waters is an increasingly
important global issue with multiple impacts on terrestrial, aquatic
and atmospheric environments. Investigation of the distribution
of hydrogeochemical variables and their connection with the occurrence
of NO3--N provides better insights
into the prediction of the environmental risk associated with
nitrogen use within agricultural systems. The research objective
was to evaluate the effect of hydrogeological setting on agriculturally
derived groundwater NO3--N occurrence.
Piezometers (n=36) were installed at three depths across four
contrasting agricultural research sites. Groundwater was sampled
monthly for chemistry and dissolved gases, between February 2009
and January 2011. Mean groundwater NO3--N
ranged 0.7-14.6 mg L-1, with site and groundwater depth
being statistically significant (p＜0.001). Unsaturated zone thickness
and saturated hydraulic conductivity (Ksat)
were significantly correlated with dissolved oxygen (DO) and redox
potential (Eh) across sites. Groundwater NO3--N
occurrence was significantly negatively related to DOC and methane
and positively related with Eh and Ksat.
Reduction of NO3--N started at
Eh potentials＜150 mV while significant nitrate reduction occurred＜
100 mV. Indications of heterotrophic and autotrophic denitrification
were observed through elevated dissolved organic carbon (DOC)
and oxidation of metal bound sulphur, as indicated by sulphate
(SO42-). Land application of waste
water created denitrification hot spots due to high DOC losses.
Hydrogeological settings significantly influenced groundwater
nitrate occurrence and suggested denitrification as the main control.
Keywords: Ksat; Dissolved C; Dissolved oxygen;
Redox-potential; SO42-; Nitrate
retention』
1. Introduction
2. Materials and methods
2.1. Study sites
2.2. Monitoring well establishment
2.3. Groundwater sampling
2.4. Analysis of dissolved CO2 and CH4
2.5. Hydrologic properties
2.6. Hydrogeochemistry
2.7. Statistical analysis
3. Results
3.1. Hydrology
3.2. Hydrogeochemistry
3.3. Groundwater redox chemistry
3.4. Groundwater N dynamics
3.5. Environmental processes controlling the abundances of NO3--N
4. Discussion
4.1. Hydrology vs. nitrate abundances
4.2. Hydrogeochemistry and the abundances of nitrate
4.3. Groundwater redox chemistry
4.4. Groundwater N dynamics
4.5. Nitrate reduction processes and factors
5. Conclusions
Acknowledgements
References