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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 20, issue 8 | Copyright
Hydrol. Earth Syst. Sci., 20, 3077-3098, 2016
https://doi.org/10.5194/hess-20-3077-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 01 Aug 2016

Research article | 01 Aug 2016

Assessing land–ocean connectivity via submarine groundwater discharge (SGD) in the Ria Formosa Lagoon (Portugal): combining radon measurements and stable isotope hydrology

Carlos Rocha1, Cristina Veiga-Pires1,2, Jan Scholten3, Kay Knoeller4, Darren R. Gröcke5, Liliana Carvalho1,2, Jaime Anibal1,2, and Jean Wilson1 Carlos Rocha et al.
  • 1Biogeochemistry Research Group, Geography Department, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland
  • 2CIMA-Marine and Environmental Research Center, Universidade do Algarve, Portugal
  • 3Institute of Geosciences, University of Kiel, Germany
  • 4UFZ, Helmholtz Centre for Environmental Research Leipzig/Halle, Germany
  • 5Department of Earth Sciences, Durham University, South Road, Durham, County Durham, DH1 3LE, UK

Abstract. Natural radioactive tracer-based assessments of basin-scale submarine groundwater discharge (SGD) are well developed. However, SGD takes place in different modes and the flow and discharge mechanisms involved occur over a wide range of spatial and temporal scales. Quantifying SGD while discriminating its source functions therefore remains a major challenge. However, 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 leads 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 (δ2H, δ18O), 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 two modes, a net meteoric water input and another involving no net water transfer, i.e., originating in lagoon water re-circulated through permeable sediments. The former SGD mode is present occasionally on a multi-annual timescale, while the latter is a dominant feature of the system. In the absence of meteoric SGD inputs, seawater recirculation through beach sediments occurs at a rate of  ∼ 1.4 × 106m3day−1. This implies that the entire tidal-averaged volume of the lagoon is filtered through local sandy sediments within 100 days ( ∼ 3.5 times a year), driving an estimated nitrogen (N) load of  ∼ 350TonNyr−1 into the system as NO3. Land-borne SGD could add a further  ∼ 61TonNyr−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 capable of driving new production in the system.

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We combine radon and stable isotopes in water to determine total submarine groundwater discharge (SGD) in the Ria Formosa and discriminate its component modes. We show that tidal action filters the entire water volume in the lagoon through local beaches 3.5 times a year, driving an estimated 350Ton nitrogen/year into the system. Conversely, fresh groundwater is discharged into the lagoon only occasionally, adding ~ 61 Ton nitrogen/year, but is capable of driving new production in the system.
We combine radon and stable isotopes in water to determine total submarine groundwater discharge...
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