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Assessing land-ocean connectivity via Submarine Groundwater Discharge (SGD) in the Ria Formosa Lagoon (Portugal): combining radon measurements and stable isotope hydrology

Rocha, C.; Veiga-Pires, C.; Scholten, J.; Knoeller, K.; Gröcke, D.R.; Carvalho, L.; Anibal, J.; Wilson, J.

Assessing land-ocean connectivity via Submarine Groundwater Discharge (SGD) in the Ria Formosa Lagoon (Portugal): combining radon measurements and stable isotope hydrology Thumbnail


Authors

C. Rocha

C. Veiga-Pires

J. Scholten

K. Knoeller

L. Carvalho

J. Anibal

J. Wilson



Abstract

Natural radioactive tracer-based assessments of basin-scale Submarine Groundwater Discharge (SGD) are well developed, but because of the different modes in which SGD takes place and the wide range of spatial and temporal scales under which the flow and discharge mechanisms involved occur, quantifying SGD while discriminating its source functions remains a major challenge. Yet, correctly identifying both the fluid source and composition is critical: when multiple sources of the tracer of interest are present, failure to adequately discriminate between them will lead to inaccurate attribution and the resulting uncertainties will affect the reliability of SGD solute loading estimates. This lack of reliability then extends to the closure of local biogeochemical budgets, confusing measures aiming to mitigate pollution. Here, we report a multi-tracer study to identify the sources of SGD, distinguish its component parts and elucidate the mechanisms of their dispersion throughout the Ria Formosa – a seasonally hypersaline lagoon in Portugal. We combine radon budgets that determine the total SGD (meteoric + recirculated seawater) in the system with stable isotopes in water (2H, 18O), to specifically identify SGD source functions and characterize active hydrological pathways in the catchment. Using this approach, SGD in the Ria Formosa could be separated into a net water input and another involving no net water transfer, i.e. originating in seawater recirculation through permeable sediments. The former SGD mode is present occasionally on a multiannual timescale, while the latter is a permanent feature of the system. In the absence of meteoric SGD inputs, seawater recirculation through beach sediments occurs at a rate of ~ 1.4 × 106 m3 day−1, implying the entire tidal-averaged volume of the lagoon is filtered through local sandy sediments within 100 days, or about 3.5 times a year, driving an estimated nitrogen (N) load of ~ 350 t N yr−1 into the system as NO3−. Land-borne SGD could add a further ~ 61 t N yr−1 to the lagoon. The former source is autochthonous, continuous and responsible for a large fraction (59 %) of the estimated total N inputs into the system via non-point sources, while the latter is an occasional allochthonous source, so more difficult to predict, but capable of driving new production in the system.

Citation

Rocha, C., Veiga-Pires, C., Scholten, J., Knoeller, K., Gröcke, D., Carvalho, L., …Wilson, J. (2015). Assessing land-ocean connectivity via Submarine Groundwater Discharge (SGD) in the Ria Formosa Lagoon (Portugal): combining radon measurements and stable isotope hydrology. Hydrology and Earth System Sciences Discussions, 12(11), 12433-12482. https://doi.org/10.5194/hessd-12-12433-2015

Journal Article Type Article
Acceptance Date Oct 19, 2015
Online Publication Date Nov 27, 2015
Publication Date Nov 27, 2015
Deposit Date Jan 19, 2016
Publicly Available Date Mar 28, 2024
Journal Hydrology and Earth System Sciences Discussions
Electronic ISSN 1812-2116
Publisher European Geosciences Union
Peer Reviewed Peer Reviewed
Volume 12
Issue 11
Pages 12433-12482
DOI https://doi.org/10.5194/hessd-12-12433-2015

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