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Dynamics of groundwater‐derived nitrate and nitrous oxide in a tidal estuary from radon mass balance modeling
Author(s) -
Wong Wei Wen,
Grace Michael R.,
Cartwright Ian,
Cardenas M. Bayani,
Zamora Peter B.,
Cook Perran L. M.
Publication year - 2013
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2013.58.5.1689
Subject(s) - submarine groundwater discharge , groundwater , estuary , hydrology (agriculture) , groundwater discharge , baseflow , nitrate , surface water , environmental science , oceanography , groundwater flow , chemistry , geology , aquifer , streamflow , drainage basin , geotechnical engineering , cartography , organic chemistry , environmental engineering , geography
We monitored submarine groundwater discharge (SGD) into the Werribee Estuary, Australia, using both chemical and physical methods. SGD occurred at hotspots where 222 Rn persisted through a 12 month survey period. A significant correlation between 222 Rn and ( r 2 = 0.8, p < 0.01), as well as between 222 Rn and N 2 O ( r 2 = 0.6, p < 0.01) at a 222 Rn hotspot, and much higher and N 2 O concentrations in groundwater relative to surface water suggest that elevated and N 2 O concentrations in the estuary were derived from SGD. Two sampling campaigns over 24 h revealed that variations of 222 Rn,, and N 2 O were controlled by tide‐induced hydraulic‐head gradient fluctuations and, possibly to a much lesser extent, by tidal pumping and density‐driven convection. A two‐box 222 Rn mass‐balance model was used to calculate the rate of SGD into two different layers across the pycnocline of the estuary. A higher total groundwater discharge rate of 0.12 ± 0.09 m d −1 was observed in the surface layer during ebb tide compared with 0.10 ± 0.08 m d −1 during flood tide. Fluxes of groundwater‐derived and N 2 O were higher than the riverine flux at baseflow by more than 30 fold and 20 fold, respectively. SGD derived fluxes exceeded the mean annual riverine and TN fluxes by a factor of ∼ 5 and ∼ 3 respectively. SGD‐derived N 2 O fluxes were 170 µmol m −2 d −1 , which are among the highest N 2 O fluxes observed in estuaries.