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| Titel |
Shallow groundwater denitrification capacity at three contrasting hydrogeological environments in Ireland |
| VerfasserIn |
Mohammad Mofizur Rahman Jahangir, Paul Johnston, Mohammad Ibrahim Khalil, Karl Richards |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032243
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| Zusammenfassung |
| Denitrification may be regarded as the dominant nitrate removal or attenuation process in
shallow groundwater. A major concern arising from the denitrification process is that it not
only serves as a natural pathway for excess NO3- removal but its intermediate product, N2O,
is a potent greenhouse gas. A groundwater monitoring network was established on grazed
grassland at research farms at Johnstown Castle (JC) and Solohead (SH) on tillage at Oak
Park (OP) in Ireland, to investigate the denitrification capacity and N2O:(N2O+N2) ratio
within the shallow groundwater zone. The geology of this zone at the 3 sites was i) JC: sand
and gravel intermixed with clay, ii) SH: silt and gravel intermixed with dense clay
and iii) OP: dense gravel with interbedded clays. Average groundwater table was
respectively 3.2, 2.4 and 4.0 m below ground level. Ten piezometers of 50 mm ID
with 2 m screen sections were installed at 3.5-6.0 m below ground level using a
rotary air drilling method at the three sites. Groundwater sampling was carried out
monthly for 6 months (February to July, 2009) using a bladder pump following the
USEPA low flow sampling procedures. Dissolved groundwater N2O was separated
by degassing groundwater in a sealed serum bottle (160ml) using high purity He
(water:He = 3:1) and the collected headspace equilibrium gas was analyzed on a
Varian gas chromatograph. The N2/Ar ratio, measured using a Membrane Inlet
Mass Spectrometer, was used to estimate the denitrified N2 concentrations. The
mean NO3-N concentrations were 7.0, 2.5 and 11.0 mg L-1 in JC, SH and OP,
respectively. Ground water dissolved N2O concentrations ranged from 0.01 to 0.05,
0.01 to 0.06 and 0.002 to 0.06 mg L-1, with corresponding mean values of 0.03,
0.02, 0.02 mg N.L-1 in JC, SH and OP, respectively. The mean values for total
denitrification (N2O+N2) were 1.94, 1.03 and 0.38 mg N.L-1, which accounted for 22,
29 and 3% losses of total NO3- -N in JC, SH and OP, respectively. The higher
denitrification measured on the grassland compared to tillage is most likely related
to the hydrogeological conditions e.g. DOC, DO and redox-potential rather than
land-use. More importantly, the mean N2O:N2O+N2 ratios, being 0.01, 0.02 and 0.05,
respectively in JC, SH and OP indicated that 95-99% of total denitrification was
molecular N2 (98-99% in grassland and 95% in tillage farm). With respect to temporal
changes, denitrification losses were higher in May in grassland systems and in April in
tillage farming systems. Shallow groundwater denitrification appeared to be an
important process in reducing NO3- , with low N2O concentrations indicating
low losses of this greenhouse gas to the atmosphere upon discharging to surface
water. |
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